The hardest OCR Chemistry questions

Answer

C

Mark scheme

Examiner's comment

Candidates found this question difficult with comparatively few obtaining the correct response of C. Many candidates selected B instead, the number of CO2 or O2 molecules, and not the number of O atoms. Good advice is to read the question carefully and to underline any key features.

Answer

C

Mark scheme

Examiner's comment

This part discriminated well, with most able candidates selecting the correct answer of C. A sizeable number selected B, presumably by not considering the 2:1 stoichiometric ratio in the equation.

Answer

A

Mark scheme

Examiner's comment

Candidates found this part difficult with many selecting B, the equation that looked a little different, rather than the correct answer of A (a redox equation). This suggests that many candidates are unaware of the role of ammonia as a base.

Answer

D

Mark scheme

Examiner's comment

As is often the case, candidates find structure and bonding difficult. Many candidates selected silicon (C) instead of the correct response of sulfur (D).

Answer

B

Mark scheme

Examiner's comment

This question proved to be the most difficult of the multiple-choice questions. Most candidates constructed a correct energy cycle using the provided equation. This gave an energy change of –3684.6 kJ mol–1 (A). The question asks for the enthalpy change of combustion for Al2S3, which is half this value (option B). As with question 2, candidates are advised to read the question carefully.

Answer

B

Mark scheme

Examiner's comment

Candidates find questions based on practical procedures difficult and this question proved to be no exception. A similar number of candidates selected each option, suggesting that most guessed. The correct option is B.

Answer

C

Mark scheme

Examiner's comment

This question was a good discriminator with well-prepared candidates usually selecting the correct option of C. Incorrect responses were reasonably evenly split across the other options, suggesting guesses and poor preparation.

Answer

C

Mark scheme

Examiner's comment

This was a difficult question but higher ability candidates selected the correct option of C. The main discriminator was B, which identifies the 1 double bond as Z. CIP analysis is required to show that the double bond is E. This is a good ‘hard’ example for illustrating E/Z isomerism.

Mark scheme

Examiner's comment

This question discriminated well. Most candidates recognised that a COCl2 molecule has a trigonal planar shape. The best answers explained this shape in terms of the three electron regions around the central C atom and their repulsion.

Notes from the mark scheme and report

• 3 electron regions/bonding regions around C
• IGNORE 'electrons repel' (pairs needed)
• DO NOT ALLOW atoms repel
• IGNORE just '3 bonds'

Skills: M4.1 Use angles and shapes in regular 2-D and 3-D structures

Mark scheme

Examiner's comment

Candidates often struggle with questions based on practical work. There were many random responses to this question, with relatively few candidates identifying that solid magnesium phosphate could be obtained by filtration, followed by drying.

Notes from the mark scheme and report

• filter to obtain the solid, then dry
• IGNORE 'leave to crystallise' (already solid)
• IGNORE distil if product or H2 is distilled

Mark scheme

Examiner's comment

Candidates are well-versed with the relationship q = mc∆T and most were able to calculate that 2.926 kJ of energy was released in this reaction. It was also common to see the amount of AgNO3 correctly calculated as 1.28 × 10–3 mol. Candidates were expected to determine the amount of energy released from 1 mol AgNO3 as 229 kJ and finally to multiply this value by 2 for the molar quantities in the equation to match the ‘enthalpy change of reaction’. It was common to see –229 given as the final answer but this was rarely multiplied by 2. The question also required the final answer to be given to an appropriate number of significant figures. Many candidates seemed to be unaware that this reflects the least significant figure provided in the data, in this case 3 significant figures. The exemplar shows a typical response for 3 of the available 4 marks. Many omitted the negative sign in their ∆H value to consider the exothermicity of the reaction. Candidates are also advised to only round at the end of a multi-step calculation. Rounding of intermediate values introduces rounding errors in the final answer. Answer = –457 kJ mol–1

Notes from the mark scheme and report

• 3 SF and – sign needed
• ±229 scores 3 (not doubled for 2 mol AgNO3)
• mass used in q is 25.0 g of solution
• ALLOW ECF throughout

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Most candidates recognised that silver nitrate and chloride ions react together to form a white precipitate, but many did not make the link between this observation and whether any silver nitrate was left unreacted. Many candidates did not give a correct equation, with missing or incorrect state symbols being common. This question discriminated extremely well.

Notes from the mark scheme and report

• state symbols required in equation
• observation must be linked to conclusion (white precipitate = not all reacted)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

This question discriminated well and resulted in a full range of marks. Most candidates were aware that structural isomers have different structural formulae but the same molecular formulae. It was common though for candidates to refer to different arrangements of atoms in space, clearly confusing with stereoisomerism. The best candidates used the structures (as in the question) to show that the common molecular formula was C5H12. Candidates were expected to link the amount of surface contact between molecules with induced dipole–dipole forces or London forces. ‘Contact’ or the name of the intermolecular forces was often omitted. Finally, candidates were expected to link the amount of branching to the strength of the intermolecular forces and the energy needed to change state. Lower ability candidates often let themselves down by being unable to construct a well-reasoned response. There was often a gulf between the clear responses of able candidates and those of lower ability candidates.

Notes from the mark scheme and report

• same molecular formula AND different structural formulae ('same formula' not sufficient)
• C5H12 for all three
• more branching gives less surface contact and weaker London forces
• IGNORE van der Waals
• IGNORE just 'bonds'
• DO NOT ALLOW reference to spatial arrangement

Mark scheme

Examiner's comment

Many candidates correctly drew the structure of compound D but comparatively few were able to construct a correct equation. For this equation, candidates needed to apply their knowledge and understanding of monosubstitution of alkanes to substitution of three H atoms by three Cl atoms. This task proved to be one of the most difficult questions on this paper. The exemplar shows an excellent response. The candidate has drawn a trisubstituted structure that fits the molar mass of 175.5 g mol–1 and a correct equation for its formation. Many attempts at this equation showed H2 as the second product rather than HCl.

Notes from the mark scheme and report

• any trichloro isomer of A
• molecular formulae required in equation (C5H12 + 3Cl2 → C5H9Cl3 + 3HCl)
• no ECF from incorrect structure of D

Mark scheme

Examiner's comment

Part (i) discriminated extremely well and rewarded the well-prepared candidate. Compound F proved to be the most difficult option, with a large variety of responses, many appearing to be guesses. Candidates were much more successful with compounds G and H, although these were sometimes shown in reverse order. A significant number of candidates drew structures containing C=C or C=O bonds in which the carbon atom had five bonds. Candidates should check drawing of organic structures carefully to ensure that all carbon atoms have four bonds. There were some good responses for part (ii), with many clearly shown and correct systematic names.

Notes from the mark scheme and report

• F = alkene, G = aldehyde (distil), H = carboxylic acid (reflux)
• 1 mark if G and H correct but in wrong boxes

Answer

A

Mark scheme

Examiner's comment

This part discriminated well, with most able candidates selecting the correct answer of A. A sizeable number selected B, accompanied by a diagram of an H2O molecule with a 180º bond angle, presumably by ignoring the lone pairs. C2H4 was often shown with a bond angle of 109.5º, presumably as the C=C bond had not been identified, giving a bond angle of 120º.

Answer

D

Mark scheme

Examiner's comment

This part discriminated extremely well. Many scripts showed clear working of the atom economy of each process, the usual result being the correct response of D. Candidates choosing an incorrect process (usually A), often showed no working suggesting the response was a guess. The advice here is obviously to work through calculations before choosing the answer.

Answer

B

Mark scheme

Examiner's comment

This question proved to be the most difficult of the multiple-choice questions. Candidates clearly did not use the clue in the question: ‘until one of the reactants is used up’. Many then responded with C, the volume of NO2 formed from complete reaction of 6 dm3 of O2. D was another common error, which is simply the sum of the volumes of NO and O2 provided in the question. The correct answer of B required candidate to identify that NO is in excess, reacting with 4 dm3 of the O2 to form 8 dm3 NO2 and leaving behind 2 dm3 of O2, and contributing to a total volume of 10 dm3 of gas.

Answer

B

Mark scheme

Examiner's comment

Many candidates found this question difficult with less than half choosing the correct option of B. Options A and C proved to be the main discriminators in almost equal amounts. Although first encountered at GCSE level, energies associated with bond breaking and bond making continue to cause candidates problems at AS and A Level. The discriminator C would have been chosen by candidates who did not recognise that –184.6 kJ is released when 2 mol HCl is formed and that the enthalpy change of formation would be half this value.

Answer

C

Mark scheme

Examiner's comment

This part discriminated well. Although most candidates did select C as the correct structure, many were diverted into selecting option D, the other alternative containing a double C=C bond. In identifying a monomer for an addition polymer, candidates are advised to identify the repeat unit and then to replace the single C–C bond with a double bond to give the monomer.

Answer

A

Mark scheme

Examiner's comment

Candidates found this part difficult. Less than half the candidates selected the correct option A, with ‘B’ being the main distractor. From annotations on scripts, successful candidates often drew rings around parts of the structures which helps in identifying parts of a structure that might fragment.

Answer

D

Mark scheme

Examiner's comment

Less than half the candidates correctly chose option D. This part discriminated well, with less able candidates selecting the incorrect A and C in similar amounts. A small number of candidates selected B.

Mark scheme

Examiner's comment

Most candidates showed a correct, clear ‘dot and cross’ diagram. Lower attaining candidates sometimes used wrong charges, not enough ions or an incorrect number of electrons on N. Covalently-bonded molecules were seen, but rarely.

Notes from the mark scheme and report

• 3 Ca2+ AND 2 N3– with correct charges
• N with 8 electrons (5 of one symbol and 3 of the other)
• circles/brackets not required

Mark scheme

Examiner's comment

N2O is a very unfamiliar molecule for candidates and they found this ‘dot and cross’ diagram far more difficult than diagram for Ca3N2 in 21(d)(i). Information in the question clearly stated that a nitrogen atom is in the centre but many diagrams were drawn with the O atom at the centre. It was also fairly common to see NO2 rather than N2O. Candidates found the bonding of the O atom to the central N atom easier than the double or dative covalent bond between the two N atoms. Many candidates included lone pairs on the central N atom despite this resulting in a non-linear molecule. (The question states that the molecule is non-linear). It was common to see an expanded octet with 10 electrons being involved with the central N atom (a triple and double bond). If correct, this was given, reflecting a candidate’s knowledge at this stage of the course. Candidates are advised to take great care in showing clear symbols for electrons (dots and crosses or other symbols). Parts of the diagram where a dot and a cross cannot be distinguished cannot be credited. This part discriminated extremely well.

Notes from the mark scheme and report

• DO NOT ALLOW ions
• watch for stray electrons on central N
• ALLOW 10 electrons around central N (N=N=O)

Mark scheme

Examiner's comment

This question discriminated well. Candidates were expected to link increasing boiling point down the group with the increasing number of electrons and stronger induced dipole–dipole forces or London forces between molecules that must be broken. To be given all 3 marks, candidates needed to show that intermolecular forces (or forces between molecules) are broken during boiling. This mark proved to be the most difficult. The response shows a typical response that did not include intermolecular forces. Lower attaining candidates often let themselves down by being unable to construct a well-reasoned, relevant response. Many discussed nuclear charge, shielding, atomic radius and the ease of losing or gaining an electron to produce a valid explanation for ionisation energy or electron gain trend but not for different boiling points. The contrast in the clarity of low- and high-attaining candidate responses was particularly pronounced for this question.

Notes from the mark scheme and report

• IGNORE van der Waals/vdW and abbreviations (LDF, IDID)
• IGNORE 'more shells'/shielding
• breaking covalent bonds is a CON for the last mark
• needs idea of forces between molecules

Mark scheme

Examiner's comment

This question was best discriminator of the paper and rewarded the well-prepared candidates who were competent in writing equations. Most candidates were given the 2 marks for the bromide test with silver nitrate and the related equation (usually shown ionically). Many found the test for the ammonium ion more challenging to describe. The alkaline nature of ammonia was well-known and the indicator colour change to blue was often seen. Many candidates omitted the NaOH reagent and tested the compound with indicator, thinking that the ammonium ion itself is alkaline. Few candidates were able to write the equation for the ammonium test. Lower attaining students often outlined electrolysis as a test and many candidates wrote about the carbonate and sulfate tests prior to the halide test.

Notes from the mark scheme and report

• NaOH (NOT ammonia) and heat, damp indicator paper turns blue
• AgNO3 gives cream precipitate
• IGNORE nitric acid
• ionic or full equations allowed
• ALLOW displacement by Cl2 giving orange solution (DO NOT ALLOW precipitate)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

AS LEVEL CHEMISTRY A Examiners’ report H032/01 Summer 2019 series Version 1 H032 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 1 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 2 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 6 Question 5 ............................................................................................................................................ 7 Question 6 ............................................................................................................................................ 7 Question 9 ............................................................................................................................................ 8 Question 10 .......................................................................................................................................... 9 Question 11 ........................................................................................................................................ 10 Question 12 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 11 Question 15 ........................................................................................................................................ 11 Question 17 ........................................................................................................................................ 12 Question 18 ........................................................................................................................................ 13 Question 19 ........................................................................................................................................ 13 Question 20 ........................................................................................................................................ 14 Section B overview ................................................................................................................................. 15 Question 21 (a) ................................................................................................................................... 15 Question 21 (b) .................................................................................................................................. 15 Question 21 (c) ................................................................................................................................... 16 Question 21 (d) (i) ............................................................................................................................... 16 Question 21 (d) (ii) .............................................................................................................................. 17 Question 21 (d) (iii) ............................................................................................................................. 18 Question 21 (d) (iv) ............................................................................................................................. 19 Question 22 (a) (i) ............................................................................................................................... 20 Question 22 (a) (ii) .............................................................................................................................. 20 Question 22 (b) ................................................................................................................................... 21 Question 22 (c) ................................................................................................................................... 22 Question 23 (a) ................................................................................................................................... 23 Question 23 (b) ................................................................................................................................... 24 Question 24 (a) (i) ............................................................................................................................... 25 Question 24 (a) (ii) .............................................................................................................................. 25 Question 24 (b) (i) ............................................................................................................................... 26 Question 24 (b) (ii) .............................................................................................................................. 27 Question 25 (a) (i), (ii) and (iii) ............................................................................................................ 28 Question 25 (b) (i) ............................................................................................................................... 29 Question 25 (b) (ii) .............................................................................................................................. 29 Question 25 (b) (iii) ............................................................................................................................. 30 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 25 (a) (i), (ii) and (iii) Part 25(a)(i) discriminated extremely well and rewarded well-prepared candidates.

Notes from the mark scheme and report

• reflux so primary alcohol A gives the carboxylic acid
• tertiary alcohol B gives NONE
• DO NOT ALLOW 'sticks' in structures

Mark scheme

Examiner's comment

AS LEVEL CHEMISTRY A Examiners’ report H032/01 Summer 2019 series Version 1 H032 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 1 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 2 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 6 Question 5 ............................................................................................................................................ 7 Question 6 ............................................................................................................................................ 7 Question 9 ............................................................................................................................................ 8 Question 10 .......................................................................................................................................... 9 Question 11 ........................................................................................................................................ 10 Question 12 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 11 Question 15 ........................................................................................................................................ 11 Question 17 ........................................................................................................................................ 12 Question 18 ........................................................................................................................................ 13 Question 19 ........................................................................................................................................ 13 Question 20 ........................................................................................................................................ 14 Section B overview ................................................................................................................................. 15 Question 21 (a) ................................................................................................................................... 15 Question 21 (b) .................................................................................................................................. 15 Question 21 (c) ................................................................................................................................... 16 Question 21 (d) (i) ............................................................................................................................... 16 Question 21 (d) (ii) .............................................................................................................................. 17 Question 21 (d) (iii) ............................................................................................................................. 18 Question 21 (d) (iv) ............................................................................................................................. 19 Question 22 (a) (i) ............................................................................................................................... 20 Question 22 (a) (ii) .............................................................................................................................. 20 Question 22 (b) ................................................................................................................................... 21 Question 22 (c) ................................................................................................................................... 22 Question 23 (a) ................................................................................................................................... 23 Question 23 (b) ................................................................................................................................... 24 Question 24 (a) (i) ............................................................................................................................... 25 Question 24 (a) (ii) .............................................................................................................................. 25 Question 24 (b) (i) ............................................................................................................................... 26 Question 24 (b) (ii) .............................................................................................................................. 27 Question 25 (a) (i), (ii) and (iii) ............................................................................................................ 28 Question 25 (b) (i) ............................................................................................................................... 29 Question 25 (b) (ii) .............................................................................................................................. 29 Question 25 (b) (iii) ............................................................................................................................. 30 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 25 (a) (i), (ii) and (iii) Part 25(a)(i) discriminated extremely well and rewarded well-prepared candidates.

Notes from the mark scheme and report

• DO NOT ALLOW butan-3-ol or but-2-ol
• ALLOW butane-2-ol
• IGNORE missing hyphens

Mark scheme

Examiner's comment

AS LEVEL CHEMISTRY A Examiners’ report H032/01 Summer 2019 series Version 1 H032 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 1 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 2 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 6 Question 5 ............................................................................................................................................ 7 Question 6 ............................................................................................................................................ 7 Question 9 ............................................................................................................................................ 8 Question 10 .......................................................................................................................................... 9 Question 11 ........................................................................................................................................ 10 Question 12 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 11 Question 15 ........................................................................................................................................ 11 Question 17 ........................................................................................................................................ 12 Question 18 ........................................................................................................................................ 13 Question 19 ........................................................................................................................................ 13 Question 20 ........................................................................................................................................ 14 Section B overview ................................................................................................................................. 15 Question 21 (a) ................................................................................................................................... 15 Question 21 (b) .................................................................................................................................. 15 Question 21 (c) ................................................................................................................................... 16 Question 21 (d) (i) ............................................................................................................................... 16 Question 21 (d) (ii) .............................................................................................................................. 17 Question 21 (d) (iii) ............................................................................................................................. 18 Question 21 (d) (iv) ............................................................................................................................. 19 Question 22 (a) (i) ............................................................................................................................... 20 Question 22 (a) (ii) .............................................................................................................................. 20 Question 22 (b) ................................................................................................................................... 21 Question 22 (c) ................................................................................................................................... 22 Question 23 (a) ................................................................................................................................... 23 Question 23 (b) ................................................................................................................................... 24 Question 24 (a) (i) ............................................................................................................................... 25 Question 24 (a) (ii) .............................................................................................................................. 25 Question 24 (b) (i) ............................................................................................................................... 26 Question 24 (b) (ii) .............................................................................................................................. 27 Question 25 (a) (i), (ii) and (iii) ............................................................................................................ 28 Question 25 (b) (i) ............................................................................................................................... 29 Question 25 (b) (ii) .............................................................................................................................. 29 Question 25 (b) (iii) ............................................................................................................................. 30 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 25 (a) (i), (ii) and (iii) Part 25(a)(i) discriminated extremely well and rewarded well-prepared candidates.

Notes from the mark scheme and report

• 6 O2 (the O atom in the alcohol must be counted)

Skills: M0.2 Use ratios, fractions and percentages

Answer

D

Mark scheme

Examiner's comment

Candidates found this question difficult. Although argon looked to be the obvious choice, many candidates selected phosphorus (B) or chlorine (C). Candidate annotations alongside the question included atomic number and electron configurations suggesting that many find it difficult to link trends in melting point to the correct chemical concepts.

Answer

B

Mark scheme

Examiner's comment

Candidates found this calculation more difficult than Question 7. From the annotations, most showed working using the relationship between moles, concentration and volume. Options A (18  250/1000) and C (18  250/1000 ÷ 0.450) were the main distractors.

Answer

D

Mark scheme

Examiner's comment

Candidates found this question very difficult. B was the main distractor, obtained by multiplying the number of moles (4) by the Avogadro constant. Only the highest-attaining candidates realised that the question asked for the number of ions and multiplied the answer to B by 3 to obtain option D. The lesson here is to consider carefully any bold text in the question (ions).

Answer

D

Mark scheme

Examiner's comment

Candidates found this question difficult with very many choosing option B rather than the correct option D. Candidates are advised to draw out all bonds displayed when tackling such as question as the answer of B (3) results from considering just the three bonds shown in the skeletal formula and omitting the other 6 C–H bonds.

Answer

D

Mark scheme

Examiner's comment

This question discriminated extremely well with most candidates choosing the correct option. Most candidates showed some working. The key to success was to identify the balanced equation that produced CO2 and H2O in a 3 : 4 molar ratio.

Answer

D

Mark scheme

Examiner's comment

This was a difficult question, requiring candidates to use CIP rules along carbon chains. Higher-attaining candidates rose to the challenge but a variety of responses were seen, suggesting that many candidates guessed.

Mark scheme

Examiner's comment

Candidates had difficulty in naming this compound correctly as 3,3-dimethylbut-1-ene. Many counted an incorrect number of carbons in the chain, numbered substituents from the wrong end (e.g. 1,1,1-) or used insufficient numbering (e.g. 3-dimethyl). Hex-1-ene was a common incorrect answer, presumably as there are six carbon atoms and one C=C double bond in the alkene.

Notes from the mark scheme and report

• DO NOT ALLOW meth or methy for dimethyl
• IGNORE lack of hyphens or added commas

Mark scheme

Examiner's comment

Most candidates drew out the correct repeat unit although some less successful responses showed the molecule instead of the polymer. Missing ‘side’ bonds were very few as were attempts to connect to the CH3 group leaving the C=C intact. Where candidates made an error, it was often from missing out a C or a 3 from the side chain.

Notes from the mark scheme and report

• side bonds required on either side of the repeat unit
• IGNORE brackets and n

Mark scheme

Examiner's comment

Candidates answered this question very successfully. Few candidates mis-wrote the numbers and even fewer gave answers to the incorrect number of decimal places. This type of question has featured on previous examinations and it was pleasing to see how well it was answered, even with more isotopes being included than in previous instances.

Notes from the mark scheme and report

• answer to 2 decimal places (190.47)
• 1 mark ECF if percentages used with wrong isotopes once or one percentage transposed

Skills: M0.2 Use ratios, fractions and percentages; M1.2 Find arithmetic means; M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Candidates processed the empirical formula data well to obtain the correct molar ratio of the elements, with many being able to convert this ratio into an empirical formula. This type of problem is common in chemistry and candidates were clearly well-versed in the method of its solution. The most common error was H:11 rather than H:12. The third mark for water of crystallisation was far harder to obtain. Many candidates identified that 6H2O was present, but the O was often missing in a formula shown as ZnN2•6H2O, or ‘included’ with the water as ZnN2•6H2O2. Many candidates did not seem to know how to proceed here. The mark scheme allowed credit for both the preferred answer of Zn(NO3)2•6H2O and for ZnN2O6•6H2O, as both had extracted the waters of crystallisation from the rest of the empirical formula. A few candidates inverted the moles calculation, a strategy that could not be given marks.

Notes from the mark scheme and report

• inverse fractions score no marks
• correct hydrated formula with no working scores 1 only
• ALLOW ECF from incorrect molar ratios

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Although this question included important clues within the table, these were usually ignored by candidates and this item did not score as well as expected. The bromate(III) was poorly identified, with many candidates missing the oxidation state of the bromine. Many candidates wrote bromide, bromate without (III), bromide (III), bromate (VI) and other oxidation numbers. The phosphate ion was more familiar with many candidates identifying its formula as PO43–. A common error was the inclusion of the wrong number of oxygen atoms in the ion, such as PO5 with various charges.

Notes from the mark scheme and report

• bromate(III)
• PO4 3– (ALLOW PO4 –3)

Mark scheme

Examiner's comment

Many candidates correctly identified that neutralisation is the formation of 1 mole of water, whereas this equation forms 2 moles of water. Significantly fewer were then able to use their answer to the calculation in Question 25 (a) (i) to determine a value for the neutralisation enthalpy as half of that that value. This was a novel question, not used in previous examinations, and many candidates coped with the challenge admirably.

Notes from the mark scheme and report

• neutralisation refers to formation of 1 mol H2O (H2O essential) but equation forms 2 mol
• value = answer to (a)(i) ÷ 2
• IGNORE sign

Skills: M0.2 Use ratios, fractions and percentages

Answer

C

Mark scheme

Examiner's comment

Candidates found this question more difficult than Questions 1–3. The question discriminated well. Most candidates showed the electron configuration in their working. A and D were the main distractors. Rather than the number of p-orbitals occupied, option A (2) is the number of p sub-shells (2p and 3p) and D (10) is the total number of p electrons (6 + 4). The errors may be the result of candidates not understanding the meaning of orbital and sub-shell or perhaps not reading the question closely enough. Underlining ‘p-orbitals’ may have helped candidates.

Answer

D

Mark scheme

Examiner's comment

Candidates found this question demanding. It showed very good discrimination between abilities with most able candidates choosing the correct option D. Option C was chosen by many, the result of working out the moles of NO2(g) without adding the moles of O2. Care is needed when reading the question which asked for ‘volume of gas’, rather than ‘volume of NO2(g). Despite the question stating the volume has been measured at RTP, some candidates chose to use the Ideal Gas Equation instead of the simplified molar volume of 24 dm3 mol–1 at RTP. This approach could still lead to success but would have wasted significant time.

Answer

C

Mark scheme

Examiner's comment

Candidates found this question very difficult with less than half successfully choosing option C. Many candidates chose option D, as shown in Exemplar 1. This candidate’s working is clear and would have been correct had AgNO3 have been the limiting factor. Calculating the moles of Zn shows that this is the limiting factor, producing 0.33 g of Ag for option C.

Answer

D

Mark scheme

Examiner's comment

This question proved to be very difficult with only the highest attaining candidates obtaining the correct response of D. Options A and C were the main distractors but there didn’t seem to be any clear pattern. The many candidates could not identify the correct response suggests that candidates have difficulty in understanding the real meaning of these enthalpy terms.

Answer

B

Mark scheme

Examiner's comment

This question discriminated well, rewarding those candidates with a good understanding of the link between concentration, time and rate. Many candidates drew a tangent to the curve at 200 s, measuring its gradient to get the correct choice of B. Common errors focused on reading of the concentration at 200 s. This was then either matched to option D directly or divided by 200 to give option A.

Answer

D

Mark scheme

Examiner's comment

Candidates find it difficult to identify an intermediate within a synthesis and less than half selected the correct option, D.

Mark scheme

Examiner's comment

The ‘dot-and-cross’ diagrams were usually correct. Errors were commonly the result of missing a lone pair of electrons from one or more fluorine atoms. BF3 was the more challenging diagram suggesting that some candidates were unfamiliar with molecules with less than an octet in their outer shell. Some candidates used two electrons from one of the fluorine atoms to show a double bond around the central B atom to complete the octet.

Notes from the mark scheme and report

• two different symbols needed
• N has one lone pair, B has none
• 1 mark if both correct but F lone pairs omitted
• IGNORE absence of circles

Mark scheme

Examiner's comment

This question discriminated extremely well with many obtaining the correct enthalpy change of –394 kJ mol–1 for 3 marks. There were 3 stages to the calculating: • Use of –824 and 3  –111 to obtain –1157 • Correct incorporation of –25 to obtain –1182 • Division by 3 for 1 mole of CO2 to obtain –394 kJ mol–1 Weak candidates usually gained the first marking point but did not get any further. Incorrect signs were common as was not dividing by 3 for the final marking point. The mark scheme allowed ECF to be applied, provided that there was some correct method in the response. Unfortunately, many responses were very difficult to follow, often shown as numbers almost placed at random in the answer space. This makes it very difficult to identify any logic and to apply ECF. Exemplar 3 serves as a model, clear response, showing what is possible.

Notes from the mark scheme and report

• common errors: –1182 (no ÷3) scores 2
• +394 scores 2
• –320 (no ×3 for CO) scores 2
• ALLOW ECF throughout

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Few candidates successfully answered this difficult application question. Some candidates considered ensuring complete combustion, or not wasting reactants, or various environmental reasons. The question did supply a hint: ‘in terms of equilibrium’ but this was ignored by most candidates. The idea of equilibrium shift to the right was essential.

Notes from the mark scheme and report

• equilibrium shifts to the right/towards SO3

Mark scheme

Examiner's comment

Most candidates were given 1 or 2 marks, with some omitting the idea of surface area or surface contact. Most candidates identified London forces or induced dipole interactions as the relevant intermolecular force. A few candidates gave a general comment in terms of ‘intermolecular’ forces without specifying the type of intermolecular forces. There has been a general improvement in candidate responses to this type of question with fewer candidates than in previous exams suggesting the breaking of hydrogen bonds or covalent bonds.

Notes from the mark scheme and report

• comparisons needed
• longer chain so more surface contact
• stronger/more London forces need more energy to break
• 'branching' is a CON
• IGNORE van der Waals' forces

Mark scheme

Examiner's comment

There were some excellent responses to this question which discriminated extremely well. Unfortunately, there were a significant number of incorrect responses and some less successful candidates had clearly struggled to recall and apply this important material. The identification of the isomers was usually correct, as was the identification of the oxidation products from the primary and secondary alcohols, and the conditions required to produce the organic products. The equation proved to be the hardest requirement with the H2O by-product often being omitted or H2 shown instead. A general point applies to organic structures. Some candidates did not show the structures of the isomers and attempted this question using the molecular formula of C3H8O supplied in the question for both alcohol isomers and no structural formulae. It was then impossible to know which isomer was being reacted and this could cost the candidate a significant number of marks. It is essential in organic chemistry to use unambiguous formulae which can be any combination of skeletal, structural or displayed. Unless a question specifies that a molecular formula is required, candidates should assume that an unambiguous formula is required.

Notes from the mark scheme and report

• propan-1-ol AND propan-2-ol
• distillation for aldehyde AND reflux for carboxylic acid or ketone
• one correct [O] equation (2[O] for the acid)
• ALLOW named products if functional group not given

Skills: PAG5 Synthesis of an organic liquid; PAG7 Qualitative analysis of organic functional groups

Answer

B

Mark scheme

Examiner's comment

This question did discriminate, but the correct option of B was missed by many candidates. D was the main distractor, despite the question assessing basic chemical facts. This multiple-choice question required candidates to choose the option that is not correct, and some candidates may not have read the question closely enough. It is recommended that candidates underline the word not in such MCQs to highlight what is required.

Answer

B

Mark scheme

Examiner's comment

Overall, candidates had little difficulty with this question and most selected B. Options C and D were the main distractors, with few choosing option A. The question did not discriminate well across the ability range, suggesting that many candidates guessed.

Answer

C

Mark scheme

Examiner's comment

About half the candidates correctly chose C. Options A and B were incorrectly chosen by many candidates, presumably by counting from the left or not first identifying the longest carbon chain. This question proved to be an excellent discriminator.

Answer

B

Mark scheme

Examiner's comment

This question assessed understanding of important terms used in organic chemistry, with some candidates correctly choosing B as the correct option. The question discriminated well with less successful responses choosing D, presumably thinking that the two double bonds in a cyclic structure makes the compound aromatic.

Notes from the mark scheme and report

• A cyclic organic molecule containing single and/or double bonds is aliphatic. The molecule is described as aromatic if it contains a benzene ring.

Answer

C

Mark scheme

Examiner's comment

This question proved to be an excellent discriminator with most above average candidates choosing monomer (option C).

Answer

D

Mark scheme

Examiner's comment

Many candidates wrote the functional groups on the structure shown on their scripts, which reflects good exam technique. Most correctly identified that the compound contains ketone and carboxylic acid functional groups (D). This was an excellent discriminator.

Answer

C

Mark scheme

Examiner's comment

Most candidates first worked out the molecular mass of each fragment and annotated their scripts with this information, eliminating options A and C in the process. More successful responses than drew out the structure of the alcohol, to work out that only option C could have produced the fragment. This is evidence of good exam technique. Predictably, option B was the main distractor. This question discriminated well. Section B overview Section B includes short response question styles (structured questions, problem solving, calculations, practical) and extended response questions. This section of the paper is worth 50 marks.

Mark scheme

Examiner's comment

This bond energy calculation contained more values than in recent papers and many candidates found analysing the data difficult. Despite this, the question discriminated extremely well with about half the candidates obtaining the correct enthalpy change of 391 kJ mol–1 for all 3 marks, despite an incorrect final response. Successful responses showed that some candidates had clearly practised this type of problem and they recognised that there were going to be three stages to the calculation: • use of 945 and 4 ´ –464 to obtain 2801 • correct incorporation of the other ∆H values to obtain 1564 for 4 N–H • division by 4 for 1 N–H bond to obtain 391 kJ mol–1 As with all multi-stage calculations, error carried forward was applied, allowing credit to be given for a correct method. The commonest errors were: • use of 2 O–H instead of 4 O–H in the 1st step. • use of 158 for N–N in the 1st step instead of 945 for NN (Did these candidates not recall that N2 has a triple bond?) • omission of some data for the second step.

Notes from the mark scheme and report

• common errors (2 marks): –391 (wrong sign), 159 (2 × O–H instead of 4), 681.5 (wrong sign for –581), 536.25 (∆H omitted)
• ALLOW ECF throughout

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Most candidates were given 3 or 4 marks. The most common omission was the idea of surface contact. Most candidates identified London forces or induced dipole interactions as the relevant intermolecular force. A few candidates gave a general comment in terms of ‘intermolecular’ forces without specifying the type of intermolecular forces. There has been a general improvement in candidate responses to this type of question with fewer candidates than in previous exams suggesting the breaking of hydrogen bonds or covalent bonds. main factors responsible for the trend in boiling points. This response was given the full 4 marks.

Notes from the mark scheme and report

• boiling point decreases with more branching
• less surface contact between molecules
• weaker London forces
• less energy to break intermolecular forces
• DO NOT ALLOW different numbers of electrons
• IGNORE just 'bonds'
• surface area alone not sufficient

Mark scheme

Examiner's comment

The presentation of many candidates’ diagrams in questions such as this one requires improvement. The mark scheme was generous, requiring to first see that a candidate knew what distillation is. A first mark was given for a set up comprising a flask connected to a roughly horizontal condenser. There had to be no gap above the condenser and the overall set up could not be a closed system. A second mark was given for minimal labels: condenser and correct direction of water flow. Nearly half the candidates received no marks for their diagram with only just over a quarter being given both marks. Typical errors included: • a reflux set up • open above the flask • a closed system • water flowing the wrong way • the condenser labelled as condensation tube, cooling tube, cooling funnel, distiller, etc. Less successful responses showed a conical flask, beaker or test tube being heated. The best advice is to spend some time instructing students how to draw diagrams and to label the apparatus using the accepted scientific names. Students increasingly appear to be finding these questions challenging.

Notes from the mark scheme and report

• DO NOT ALLOW any marks for a reflux set up
• condenser horizontal or angled down
• not a sealed system
• water in at bottom and out at top
• IGNORE no heat or thermometer
• Practise the drawing of common organic apparatus as part of the practical work carried out for the practical endorsement.

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Candidates found this question difficult. Many fell into a trap of their own making by comparing the relative reactivities of the halogens chlorine, bromine and iodine and it was common to see responses claiming that a chloroalkane has the fastest rate and that an iodoalkane has the slowest rate. Only the most successful candidates related a correct order of reactivity to the different strengths of the carbon– halogen bond.

Notes from the mark scheme and report

• RI fastest, RCl slowest
• C–I weakest bond / C–Cl strongest
• MUST refer to C–X bond
• IGNORE reactivity of halogens

Answer

B

Mark scheme

Examiner's comment

Overall, candidates had little difficulty with this question and most selected B.

Answer

C

Mark scheme

Examiner's comment

Just under half of candidates did not gain this mark, with A being a common wrong response. This reflects the general difficulty that students have interpreting skeletal formula and would be a good area on which to focus practice.

Answer

C

Mark scheme

Examiner's comment

Candidates found this question challenging. A was the most common wrong response given, suggesting students have difficulty applying Cahn-Ingold Prelog rules.

Notes from the mark scheme and report

• Many students incorrectly assigned CH3CH2- as higher priority than F (presumably by using the Mr of the entire side chain). To correctly assign priority, use the atomic number of the atoms directly attached to each carbon in the double bond.

Skills: M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects; M4.3 Understand the symmetry of 2-D and 3-D shapes

Mark scheme

Examiner's comment

This question discriminated well. It requires the use of the graph to determine that 60 cm3 was the volume of gas evolved and hence calculate moles of H2. Many low scoring candidates did not read the volume of gas from the graph, using 24.5 cm3 instead. ‘Error carried forward’ marks could be given to candidates who subsequently multiplied an incorrectly calculated value by two to reflect stoichiometry and hence calculate moles of HCl reacted. The final mark needed use of moles = concentration x volume to calculate HCl concentration (as well converting to standard form and giving to 2 significant figures). Candidates who scored low on the whole paper tended to struggle to make an attempt on this question.

Notes from the mark scheme and report

• 2 sig figs AND standard form required
• final volume 60 cm3 read from graph
• common errors: using 24.5 cm3 as H2 volume, 60/24 (units), 0.204 not in standard form (2 marks each)
• 2:1 HCl:H2 ratio

Skills: M0.1 Recognise and use expressions in decimal and ordinary form; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question was a very good discriminator. Most candidates knew that silver nitrate should be added to the solutions and knew the colour of precipitates produced. Some candidates did not score the second mark by only giving colours and not stating they were solids. Many candidates were also able to give a correct ionic equation, with only the highest scoring candidates explicitly stating it was the AgX precipitates which had different solubilities in ammonia. A small number of candidates mistook this for a question on halogen displacement, scoring no marks. naming the solution and its result. Ionic equations are given (although only one is needed) and relative solubilities of the precipitates in ammonia are stated. This response was given the full 4 marks.

Notes from the mark scheme and report

• AgNO3
• any two precipitate colours
• ionic equation
• AgCl dissolves in dilute NH3, AgBr in concentrated NH3, AgI insoluble (must be clear the precipitate dissolves)
• HCl/H2SO4 or BaCl2 is a CON (IGNORE HNO3)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

This bond energy calculation proved difficult for some. Roughly a third of candidates obtained the correct enthalpy change of 463 kJ mol–1 for all 3 marks, but 0 marks was the next common score. Despite evidence that candidates had clearly practised this type of problem and they recognised that there were going to be three stages to the calculation, candidates frequently did not use the stoichiometry shown in the equation and therefore divided by 2 and 3 respectively for marking points 1 and 3 (rather than 4 and 6). Another common error was incorrect treatment of the enthalpy of reaction value (subtracting rather than adding 196 for the second marking point or omitting it altogether). Drawing out the total number of molecules helped students understand the numbers of bonds involved and should be encouraged. It is also worth reiterating the sign for bond enthalpies. As with all multi-stage calculations, error carried forward was applied, allowing credit to be given for a subsequent correct method.

Notes from the mark scheme and report

• 4 S=O broken in 2SO2 and 6 S=O made in 2SO3
• common errors: +578 (2 and 3 S=O bonds), +398 (wrong sign for 196), –463
• IGNORE sign of final answer

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Over half of candidates gained this mark, most often by stating chlorine, HCl or, more rarely, CO was produced. Common responses which didn’t score included ‘takes lots of energy and ‘produces greenhouse gases’ (which is a repetition of the question).

Notes from the mark scheme and report

• HCl or chlorine formed (ALLOW CO from incomplete combustion)
• IGNORE 'toxic waste products'

Mark scheme

Examiner's comment

Many candidates scored well on this question, drawing a clear mechanism and correctly giving the major product. However, mistakes were often seen with the direction of arrows and a minority of students gave the minor product for Compound A and/or named the mechanism incorrectly. Partial charges on the carbons of the double bond and lack of charge on the chloride ion were also common errors. This question proved to be an excellent discriminator.

Notes from the mark scheme and report

• curly arrow from C=C to H of H–Cl
• dipole on H–Cl AND arrow breaking H–Cl
• carbocation with full + (tertiary for major product) and arrow from lone pair/– charge of Cl–
• DO NOT ALLOW partial charge on C=C
• DO NOT ALLOW half-headed arrows
• name electrophilic addition

Answer

B

Mark scheme

Examiner's comment

Candidates found this question difficult with less than half choosing B, FeO42–, as the correct response. There was no obvious key distractor, with A, C and D all being chosen by candidates.

Answer

C

Mark scheme

Examiner's comment

Candidates found this question difficult, with the nitric acid N atoms being split between two products, Cu(NO3)2 and NO2. B proved to be the biggest distractor from the correct response of C, probably the result of miscounting the N atoms.

Answer

A

Mark scheme

Examiner's comment

Few candidates selected the correct response of A. There was no clear distractor.

Mark scheme

Examiner's comment

Unlike Q23(a), all intermediate calculations gave values to 3 significant figures. As with Q23(a), many candidates produced largely unreferenced numbers. 23(d) Candidates often find questions linked to practical work challenging.

Notes from the mark scheme and report

• common error 0.227 (no ×2 for OH–) scores 2
• 3 SF required
• ALLOW use of 171

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures

Mark scheme

Examiner's comment

23(a) This question required candidates to calculate the moles of Ba(OH)2 in 100 cm3, to scale this value up to 1 dm3 for the concentration of Ba(OH)2 and finally to multiply this value by 2 to obtain the concentration of OH– ions of 0.454 mol dm–3. This part discriminated extremely well. Lower-attaining candidates would often calculate an incorrect molar mass of Ba(OH)2 and scale up by either 10 or 2. The commonest incorrect answer seen was 0.227, half of the correct answer by omitting the ×2 stage. Most candidates did supply their final answer to the required 3 significant figures. As with all calculations, candidates should round once, at the end of the calculation. Rounding of intermediate values can lead to significant rounding errors. Sometimes, intermediate values were rounded to 2 significant figures when the final value required 3. This use of maths will almost certainly lead to a rounding error in the final answer. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. Many candidates produced largely unreferenced numbers. 23(b) Many candidates seemed to be unfamiliar with the term ‘concordant titres’. Acceptable responses were ‘within 0.1 cm3 and within 0.05 cm3. Common incorrect answers for this important term included ‘the closest titres’, ‘similar titres’, ‘repeated titres’ and ‘within 0.5 cm3’. 23(c) Many candidates were able to calculate the amount of HNO3 in the titration as 4.28 × 10–3 mol. Most candidates were credited for the amount of Ba(OH)2 as 2.14 × 10–3 mol, half the calculated amount of HNO3. Candidates then need to scale up this value by 1000/25 to obtain the concentration as 0.0856 mol dm–3. Unlike Q23(a), all intermediate calculations gave values to 3 significant figures. Candidates found this stock titration calculation easier than the more unfamiliar 23(a), with many scoring all 3 marks. Discrimination was extremely good, but about a third of candidates did not receive any marks. Candidates should be encouraged to practise stock titration calculations as part of their preparation for the examinations. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. As with Q23(a), many candidates produced largely unreferenced numbers. 23(d) Candidates often find questions linked to practical work challenging. The two reaction routes involving Group 2 reactions are drawn directly from the specification: the reaction of barium with water, and the reaction of barium with oxygen, followed by water. Most candidates found this difficult, and the one-step route using barium and water was seen far more often than the two-step oxygen then water approach. Only the highest-attaining candidates were able to suggest two routes, with scores of 3/4 or 4/4 marks being comparatively rare. As with 21(c)(ii) and 25c(iii), candidates had difficulty in interpreting provided information. OCR support Our new practical question packs, split up by PAG, provide additional opportunities for students to practise questions relating to practical work. They are available on Interchange: https://interchange.ocr.org.uk/Downloads/H432_H433_Chemistry_PAG_ Practice_Question_Sets.zip

Notes from the mark scheme and report

• within 0.1 cm3
• DO NOT ALLOW incorrect units e.g. dm3

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

23(a) This question required candidates to calculate the moles of Ba(OH)2 in 100 cm3, to scale this value up to 1 dm3 for the concentration of Ba(OH)2 and finally to multiply this value by 2 to obtain the concentration of OH– ions of 0.454 mol dm–3. This part discriminated extremely well. Lower-attaining candidates would often calculate an incorrect molar mass of Ba(OH)2 and scale up by either 10 or 2. The commonest incorrect answer seen was 0.227, half of the correct answer by omitting the ×2 stage. Most candidates did supply their final answer to the required 3 significant figures. As with all calculations, candidates should round once, at the end of the calculation. Rounding of intermediate values can lead to significant rounding errors. Sometimes, intermediate values were rounded to 2 significant figures when the final value required 3. This use of maths will almost certainly lead to a rounding error in the final answer. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. Many candidates produced largely unreferenced numbers. 23(b) Many candidates seemed to be unfamiliar with the term ‘concordant titres’. Acceptable responses were ‘within 0.1 cm3 and within 0.05 cm3. Common incorrect answers for this important term included ‘the closest titres’, ‘similar titres’, ‘repeated titres’ and ‘within 0.5 cm3’. 23(c) Many candidates were able to calculate the amount of HNO3 in the titration as 4.28 × 10–3 mol. Most candidates were credited for the amount of Ba(OH)2 as 2.14 × 10–3 mol, half the calculated amount of HNO3. Candidates then need to scale up this value by 1000/25 to obtain the concentration as 0.0856 mol dm–3. Unlike Q23(a), all intermediate calculations gave values to 3 significant figures. Candidates found this stock titration calculation easier than the more unfamiliar 23(a), with many scoring all 3 marks. Discrimination was extremely good, but about a third of candidates did not receive any marks. Candidates should be encouraged to practise stock titration calculations as part of their preparation for the examinations. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. As with Q23(a), many candidates produced largely unreferenced numbers. 23(d) Candidates often find questions linked to practical work challenging. The two reaction routes involving Group 2 reactions are drawn directly from the specification: the reaction of barium with water, and the reaction of barium with oxygen, followed by water. Most candidates found this difficult, and the one-step route using barium and water was seen far more often than the two-step oxygen then water approach. Only the highest-attaining candidates were able to suggest two routes, with scores of 3/4 or 4/4 marks being comparatively rare. As with 21(c)(ii) and 25c(iii), candidates had difficulty in interpreting provided information. OCR support Our new practical question packs, split up by PAG, provide additional opportunities for students to practise questions relating to practical work. They are available on Interchange: https://interchange.ocr.org.uk/Downloads/H432_H433_Chemistry_PAG_ Practice_Question_Sets.zip

Notes from the mark scheme and report

• DO NOT ALLOW 4.3 × 10–3 for n(HNO3) but ECF for remaining marks
• 1:2 ratio Ba(OH)2:HNO3

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

23(a) This question required candidates to calculate the moles of Ba(OH)2 in 100 cm3, to scale this value up to 1 dm3 for the concentration of Ba(OH)2 and finally to multiply this value by 2 to obtain the concentration of OH– ions of 0.454 mol dm–3. This part discriminated extremely well. Lower-attaining candidates would often calculate an incorrect molar mass of Ba(OH)2 and scale up by either 10 or 2. The commonest incorrect answer seen was 0.227, half of the correct answer by omitting the ×2 stage. Most candidates did supply their final answer to the required 3 significant figures. As with all calculations, candidates should round once, at the end of the calculation. Rounding of intermediate values can lead to significant rounding errors. Sometimes, intermediate values were rounded to 2 significant figures when the final value required 3. This use of maths will almost certainly lead to a rounding error in the final answer. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. Many candidates produced largely unreferenced numbers. 23(b) Many candidates seemed to be unfamiliar with the term ‘concordant titres’. Acceptable responses were ‘within 0.1 cm3 and within 0.05 cm3. Common incorrect answers for this important term included ‘the closest titres’, ‘similar titres’, ‘repeated titres’ and ‘within 0.5 cm3’. 23(c) Many candidates were able to calculate the amount of HNO3 in the titration as 4.28 × 10–3 mol. Most candidates were credited for the amount of Ba(OH)2 as 2.14 × 10–3 mol, half the calculated amount of HNO3. Candidates then need to scale up this value by 1000/25 to obtain the concentration as 0.0856 mol dm–3. Unlike Q23(a), all intermediate calculations gave values to 3 significant figures. Candidates found this stock titration calculation easier than the more unfamiliar 23(a), with many scoring all 3 marks. Discrimination was extremely good, but about a third of candidates did not receive any marks. Candidates should be encouraged to practise stock titration calculations as part of their preparation for the examinations. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. As with Q23(a), many candidates produced largely unreferenced numbers. 23(d) Candidates often find questions linked to practical work challenging. The two reaction routes involving Group 2 reactions are drawn directly from the specification: the reaction of barium with water, and the reaction of barium with oxygen, followed by water. Most candidates found this difficult, and the one-step route using barium and water was seen far more often than the two-step oxygen then water approach. Only the highest-attaining candidates were able to suggest two routes, with scores of 3/4 or 4/4 marks being comparatively rare. As with 21(c)(ii) and 25c(iii), candidates had difficulty in interpreting provided information. OCR support Our new practical question packs, split up by PAG, provide additional opportunities for students to practise questions relating to practical work. They are available on Interchange: https://interchange.ocr.org.uk/Downloads/H432_H433_Chemistry_PAG_ Practice_Question_Sets.zip

Notes from the mark scheme and report

• Route 1 Ba + 2H2O
• Route 2 via BaO then water
• ALLOW multiples
• 1 mark for both reactants O2 then H2O in route 2 without equations

Mark scheme

Examiner's comment

This was a good discriminator. Many candidates were credited the first two marks, and higher ability candidates went on to obtain 3 or 4 marks. Common errors were not rounding to a whole number at the end and not scaling up by 250/27.30. Many candidates did not scale up at all, obtaining an answer of 1081, or merely multiplied by 250/25 and obtained a molar mass of 108.

Notes from the mark scheme and report

• acid A is in the burette so scale by 250.0/27.30
• 1:2 ratio A:NaOH
• common ECF answers 108, 1081, 59, 114, 99 (3 marks)

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme, with the levels and indicative points

Examiner's comment

Candidates were not prepared to answer this type of question and the diagrams were hard to give credit to. Many had significant safety implications such as open beakers of butan-1-ol being heated by a Bunsen burner. Most mis-read the question and just outlined the method for purification and struggled to recall the practical details. Very few candidates mentioned the use of anhydrous salts, referring instead to ‘boiling off’ the water. This candidate was credited 4 marks for this level 2 answer. Although they have drawn distillation apparatus instead of reflux, they have considered the boiling point of the product, detailed using a separating funnel, a drying agent and that the lower organic layer would be drawn off first.

Notes from the mark scheme and report

• reflux with water in at bottom of condenser and open system
• separating funnel – organic layer is the lower (denser) layer
• dry with anhydrous salt
• redistil collecting at 102 °C

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

In general, the diagrams were poorly drawn or inadequately labelled. Many candidates lost marks as they did not specify the movement of electrons as the reason why metals conduct electricity. Too many candidates wrote “delocalised electrons carry the charge” or negated the metallic bonding description with “intermolecular forces” in their responses. This candidate scored 3 marks for the description but lost 2 marks as the electrons were not throughout the structure, merely drawn around the ions, and the positive ions were not labelled. This candidate scored 5 marks for the excellent description with a well drawn and labelled diagram, clearly showing the electrons throughout the structure in-between the ions.

Notes from the mark scheme and report

• diagram needs at least two rows of labelled + ions with labelled electrons between
• strong attraction between cations and delocalised electrons
• delocalised electrons MOVE for conductivity
• DO NOT ALLOW intermolecular forces

Mark scheme

Examiner's comment

Most candidates answered the similarity part correctly, many wrote vague answers to the difference and were not specific. A common error focused on the peak at m/z = 71, very few recognised that the Z peak at 43 would not be present for 2-iodo-2-methylbutane

Notes from the mark scheme and report

• similarity: both have peak at m/z 198 (or 71 or 29)
• difference: 2-iodo-2-methylbutane has no peak at 43
• IGNORE 'different ion peaks' with no m/z

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Almost half of the candidates answered this question correctly, the remainder did not realise that the question gave them the answer to the state that the copper sulphate solution was in. Many answers stated that it could be solid or aqueous, so difficult to classify.

Notes from the mark scheme and report

• reactants are in different physical states - CuSO4(aq) same phase as acid but different from zinc

Mark scheme, with the levels and indicative points

Examiner's comment

This question discriminated well and most were able to attempt to calculate a mass and explain the purification steps with some fine detail. It was evident that most candidates were aware of the apparatus required but the logic in the order was sometimes out of sequence. Drying agents were mentioned, some of the examples used were incorrect and errors were made with the boiling points if candidates mentioned distillation. This in a response of an excellent approach to this question, it was given 6 marks. The correct mass has been calculated and all of the fine detail, except distilling at 63°C

Notes from the mark scheme and report

• 0.0500 mol hex-1-ene needs 0.0800 mol hexan-1-ol = 8.16 g
• partly correct: 5.1 g (% yield omitted), 3.19 g (% yield inverted)
• separating funnel (upper organic layer), dry with anhydrous MgSO4/CaCl2, distil collecting fraction at 63 °C
• incorrect purification method not creditworthy

Skills: M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Most candidates obtained the first mark, but very few stated that hex-2-ene forms which is why the yield is less. Many candidates stated this was due to secondary alcohols being more stable than primary ones and a significant number thought the yield would be the same as they had the same Mr.

Notes from the mark scheme and report

• yield of hex-1-ene is less because a mixture with hex-2-ene forms

Mark scheme

Examiner's comment

Over half of the candidates answered this question incorrectly. Many drew the wrong repeat units or did not show two repeat units. The structural diagrams were very hard to decipher and not well drawn. Although the connectivity is not exact, this was allowed. Many students drew full displayed structures which were very difficult to decipher as they often encroached over the writing in the question.

Notes from the mark scheme and report

• two repeat units with C4H9 side chains and side links (end bonds)
• IGNORE brackets and n

Mark scheme

Examiner's comment

A very challenging question with very few candidates scoring both marks. The response needed a clear indication of how the method would be altered and a justification for why this would work. Lots of candidates recognised the need to dilute the citric acid to obtain the correct titre but were not able to give a method of how to do this or any indication of quantities needed. Some candidates said to use a larger volume of NaOH – not recognising that this would be the titre value, e.g. “in order to keep the same titre but lower concentration of NaOH the student should titre more NaOH”. Some gave the method of how to dilute the NaOH or even just said to add water. A few suggested using a higher concentration of lime juice. Candidates need to be given opportunities to plan practical work to fully appreciate the impact that any changes will have (specification 1.1.1).

Notes from the mark scheme and report

• use half a lime or make up juice in 1 dm3 volumetric flask
• justified by factor of 4 (NaOH 4 times more dilute)
• ALLOW any feasible method giving dilution factor of 4
• Further information about practical skills assessed on written exams can be found in section 3 of the practical skills handbook - https://www.ocr.org.uk/Images/208932-chemistry-practical- skills-handbook.pdf. If using our suggested practicals, then encourage candidates to answer the extension opportunity questions to help develop a deeper understanding in preparation for written assessments.

Skills: M0.2 Use ratios, fractions and percentages; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Only a small number of candidates managed to score all 3 marks for this question. Lots began their response describing why the rate is high at the start. This often filled the space available with marks being gained only in the last sentence. Very few gave descriptions with this specific reaction in mind, such as recognising that the HCl was the limiting reagent so gets used up, leaving some MgCO3 remaining. Common errors included referring to “fewer collisions” but without any indication of time, or “less reactant” without considering the volume of the reaction. We have produced a delivery guide on rates with some useful resources to help consolidate ideas and avoid misconceptions such as these: https://www.ocr.org.uk/Images/231742-rates-delivery-guide.pdf

Notes from the mark scheme and report

• acid CONCENTRATION decreases (IGNORE amount)
• fewer collisions per second/less frequent ('fewer collisions' alone not linked to rate)
• stops when acid/limiting reagent used up
• Many candidates described a decrease in energy in the reaction or gave responses related to activation energy. For example: “lose energy and start to collide less often”, “reactants have less kinetic energy” and “only a certain number of particles which have enough energy above the activation energy…over time energy will decrease and less particles have enough energy”.

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

About a third of candidates didn’t score any marks here. Some did not follow the instructions to add their curve to the previous graph. The sketches produced in the blank space then usually lacked any scales on the axes so could not be given marks. Lines added were often inaccurately drawn with lines often extending below 91.55g before returning to this mass at the end. It was often difficult to interpret if a graph levelled off before the original with lots tracking the same curve with a very slightly steeper slope. Some gave a slower rate but did manage to gain marks for finishing at 91.55g.

Notes from the mark scheme and report

• steeper slope AND levels off earlier
• same loss in mass - levels off at about 91.55 g

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Candidates found this question very challenging. Most recognised that the addition of the catalyst would speed up the reaction but could not translate this into observations for this specific practical. For example, “reacts faster”, “releases more hydrogen” or “stops reacting quicker”. Giving candidates the opportunity to observe a range of both familiar and unfamiliar reactions, noting qualitative observations can help to improve their skills here. Many just noted that one tube would be blue and the other colourless rather than considering the reaction taking place. Some indicated that only 1 tube would react and the other would not. A few candidates recognised that a competing displacement reaction would take place between Zn and CuSO4 so were given marks for this.

Notes from the mark scheme and report

• more vigorous bubbling
• zinc dissolves more quickly
• one alternative mark for Cu displacement (red/brown solid or blue solution turns colourless)

Mark scheme, with the levels and indicative points

Examiner's comment

Only a small number of candidates were given all 6 marks for their response. Many responses made little or no reference to equilibrium, considering only the rate of reaction. A clear understanding of the difference between rate and equilibrium was essential to scoring highly on this question. Excellent descriptions were given regarding the role of the catalyst on the rate of reaction, possibly influenced by the previous question. Some gave detailed responses in terms of the impact of both pressure and temperature on the equilibrium position but then did not give the changes made by industry such as reducing temperature due to energy demand or increasing pressure to increase rate. Some candidates indicated that 15atm was a low pressure including reference to it being “lower than normal atmospheric pressure” which suggested that they did not understand either the measurement in atmospheres and/or scale of pressure. A common incorrect response was to suggest that a higher pressure gives increased yield as more moles on right/products. Some stated that the reaction was exothermic, clearly confusing the enthalpy change signs. Some also described the “endothermic side” demonstrating a lack of understanding that enthalpy changes require a difference between products and reactants. Many did not correctly consider the use of excess of steam to increase concentration of reactant and shift equilibrium to products/right but talked in terms of limiting reagents, e.g. “This is used so all of the methane is reacted” or increased collisions. Many were prone generalisations, e.g. “These conditions are used industrially to achieve the most hydrogen efficiently with the least amount of undesired (waste) products”. Candidates need to be encouraged to plan their answers, even using subheadings to organise information and follow the key approach: point, evidence, explain. Candidates should also be encouraged to re-read their answers to check communication is clear and accurate without any contradictory statements. Many included irrelevant or inaccurate information such as describing the toxicity of carbon monoxide, so reducing yield of that being beneficial, or “using a catalyst improves atom economy”. This exemplar shows a candidate who achieved L3 with all 6 marks given. They have considered both the effect of temperature and pressure on equilibrium and potential compromises used industrially. They have also considered the impact of using a catalyst. Their response is logically structured and has a clear line of reasoning.

Notes from the mark scheme and report

• endothermic so high temperature shifts right
• 2 mol gas to 4 mol so low pressure favours products but reduces rate
• compromise pressure
• high temperature costs energy
• catalyst increases rate without changing equilibrium position
• excess steam shifts right
• This question was good at highlighting candidates understanding of the impact of different conditions on equilibrium reactions. We have produced a delivery guide to help with teaching about equilibrium: https://www.ocr.org.uk/Images/261544-equilibrium.pdf

Mark scheme

Examiner's comment

Candidates continue to find understanding of shapes of molecules a very challenging topic to grasp. Very few were able to gain all 4 marks, with most gaining little or no marks. The most common mark given was for lone pairs repelling more than bonding pairs. Many only stated a lone pair decreases bond angle by 2.5o but did not explain that this was caused by increased repulsion. The most common error was stating that CO2 has 2 bonding pairs – it doesn’t, as it has 4 bonding pairs which are arranged as 2 double bonds. They could also state that it has 2 bonding regions. Lots stated CO2 has no lone pairs and H2O has 2 lone pairs but without reference to bonding pairs of electrons so lost out on 2 marks. In fact, it was fairly common that candidates indicated that only lone pairs cause repulsion, e.g. stating no repulsion in CO2 “no lone pairs so there will be no repulsion”. Though these two molecules are ones candidates will have prior knowledge of before A Level, a range of bond angles were suggested. The most successful candidates drew out dot cross diagrams and/or displayed formula to be able to count pairs of electrons around the central atom.

Notes from the mark scheme and report

• 180° and 104.5° (ALLOW 104–105)
• CO2 two bonding regions/double bonds
• H2O two bonded pairs AND two lone pairs
• lone pairs repel more
• IGNORE names of shapes
• DO NOT ALLOW atoms repel

Skills: M4.1 Use angles and shapes in regular 2-D and 3-D structures

Mark scheme

Examiner's comment

Candidates found this question very challenging and few scored both marks. Many responses considered only the formation of HBr (other product) and/or general statements about other products with no indication of how they were formed. Some described losses due to the purification method or incomplete reaction (due to conditions such as T and P) or low atom economy. Some referred to the stability of the radical intermediate, showing possible confusion with electrophilic addition. Candidates who understood the mechanism were more confident in answering this question, at least recognising that further substitution was possible.

Notes from the mark scheme and report

• further substitution/di- and tribromobutanes or other termination products
• substitution at a different position (1-bromobutane)

Mark scheme

Examiner's comment

Very few candidates gave the correct answer for this question. The most common errors included: titanium oxide, titanium(IV) dioxide, titanium oxide(IV), titanium(II) oxide. A few also attempted to give names like those for organic compounds: 1,1-titanium dioxide or the reverse 1,1-dioxytitanium. How Science Works It is important in Chemistry to have clear communication by use of systematic and unambiguous nomenclature. This includes the use of Roman numerals to indicate the magnitude of the oxidation number when an element, such as Ti, may have different oxidation numbers in different compounds. See specification statement 2.1.5(c) and HSW8.

Notes from the mark scheme and report

• titanium(IV) oxide
• DO NOT ALLOW titanium dioxide

Mark scheme

Examiner's comment

Candidates found this calculation quite challenging, with less than a quarter achieving full marks. The most common errors are highlighted on the mark scheme. Many that struggled were often given credit for the x4 ratio mark but only if it was possible to see this in the working. Many gave multiple, often contradictory attempts at the calculation. It was not always clear how the final answer had been obtained. Clear working enables us to follow the logic and give ECF where appropriate. Many divided 1000 g by the molar mass for TiCl4 and then found 72% of this. It was important here to read the question carefully to ensure complete understanding. This candidate achieved 3 out of the 4 possible marks. The steps in their calculation are logical and it is easy to follow their working and therefore spot the error in their calculation. They have divided by 4 rather than multiplying. It also shows the calculation can be performed in a different order to that on the mark scheme. All intermediate values are used in calculations as calculator values without rounding to ensure an accurate answer.

Notes from the mark scheme and report

• 3 SF AND kg required (2.67)
• common errors: 1.92 (yield ignored), 1.38 (yield inverted), 0.673 (Mr of TiCl4 used for Ti)
• ALLOW ECF throughout

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Less than half the candidates gained credit for this challenging question. There was lots of information to process. Many struggled to give the correct formula for the products, e.g. NaNO3, Ba2SO4, or had issues with balancing. Some tried to involve the NaCl in the reaction, either recognising that it didn’t react (acceptable on the mark scheme) or forming barium chloride or even Cl2. Lots of candidates lost the mark for state symbols as they left Ba(NO3)2 as (s), not recognising that in step 1 the mixture was dissolved in water so should now be (aq).

Notes from the mark scheme and report

• BaSO4(s) with other species (aq)
• ALLOW ionic equation
• state symbol mark dependent on balanced equation

Mark scheme

Examiner's comment

This was a tricky calculation, made more challenging if candidates hadn’t been able to successfully complete (i). Many were able to calculate the moles of BaSO4 but often rounded their answer to only 2 significant figures at this stage i.e. 0.014. Many assumed a direct ratio between BaSO4 and NaCl so mass was found by multiplying moles by 58.5 (molar mass for NaCl) - if this was done then credit was given for ECF for the final marking point.

Notes from the mark scheme and report

• n(BaSO4) → mass Ba(NO3)2 (× 261.3) → mass NaCl by difference → 26.6% to 3 SF
• using n × 58.5 gives 16.4 (ECF only)
• ALLOW ECF from equation in (b)(i)
• The M1 section of the Mathematical Skills handbook contains useful information on handling data, including M1.1 use of significant figures.

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG1 Moles determination

Mark scheme, with the levels and indicative points

Examiner's comment

Very few candidates managed to access the higher level on this question. They found the calculation very challenging. In addition to a typical mole calculation, they needed to use the tonnes conversion (given on the data sheet), look at the ratio of I2 to KI and use a mass concentration. Many used the mass concentration as a molar concentration. Even if a balanced equation was given, it didn’t guarantee the correct ratio in calculations and a common error was to find moles of I2 using the relative atomic mass of iodine (126.9). As with previous calculations on this paper, it was often challenging to follow the logic in the steps of the calculation. It is important to encourage candidates to label each step of their calculation, showing them as independent steps and only using = when terms are equal. Explanations of relative reactivities of chlorine and iodine often lacked detail or had extra irrelevant information, such as reference to electronegativity, ionisation energy, bond enthalpy or intermolecular forces. Some candidates gave responses for the general trend in reactivity in the group rather than comparing Cl and I. Confusion around nuclear charge and nuclear attraction was also often seen. A key skill for longer answer questions is to plan out answers to avoid contradictions. Candidates should also be encouraged to re-read their answers to check that what they have written makes sense. This exemplar demonstrates a L3 6 mark response. A comprehensive explanation of relative reactivities of chlorine and iodine is given. The steps in the calculation are logical with each step labelled, making it easy to follow. A balanced equation is also given.

Notes from the mark scheme and report

• comparison required throughout
• IGNORE 'nuclear charge' for nuclear attraction
• 2:1 ratio I–:I2
• n(I2) = 1 × 10^6/253.8
• acceptable errors: 126.9 used for I2 (1.74 × 10^7) or missing × 2 (4.36 × 10^6), not both for L3

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.1 Recognise and use expressions in decimal and ordinary form; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Most were able to explain the term disproportionation. Some missed the mark by not stating an element or chlorine. A very common error was giving final oxidation numbers of Cl as +2 and/or -2, rather than per atom. The link between oxidation number and species was not always clearly indicated or changes not specified as oxidation/reduction (or given as the wrong way round). It is vital to set out answer s clearly showing oxidation numbers, species and stating if oxidised or reduced. It is not enough to write on the equation given in the question as it often challenging to read these numbers, or they contradict the main answer. Some attempted to show that Ca has been disproportionated.

Notes from the mark scheme and report

• oxidation AND reduction of the same element
• 0 to +1 in Ca(OCl)2 and 0 to –1 in CaCl2
• IGNORE 'species'/'reactant' for element
• 1 mark for three correct oxidation numbers without oxidation/reduction stated

Mark scheme

Examiner's comment

Candidates’ working was often unclear and very difficult to follow for this question, making it challenging to find errors and award ECF where appropriate. Despite being given the displayed formula, many were unable to correctly count the bonds in the molecules. Missing some or all of C-C, C-O and 3 x O=O from the bonds broken was often seen. Some only counted 2 x C=O bonds in the products. Some subtracted the bonds broken from bonds made or omitted the enthalpy change from their calculation. Very few candidates gave a negative value for their final bond enthalpy.

Notes from the mark scheme and report

• +805
• +3220 (not divided by 4) scores 3
• common errors: –805 (sign), 718 (C–C missed), 431.5 (3 × O=O missed), 167 (rearrangement)
• 6 O–H made, 1 O–H broken

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Very few candidates scored both marks here. Many just gave a generic response describing an exothermic reaction i.e. ‘more energy is released in bond forming than needed for bond breaking’ rather than answering the question asked. It was evident from many responses that candidates are unclear that energy is needed for bond breaking and released in bond forming. Some tried to compare specific bonds but often failed to account for them all, comparing presence of C-O and extra C-H in methoxymethane to O-H in ethanol but forgetting the C-C bond, resulting in methoxymethane having stronger bonds/more energy to break bonds. Lots didn't recognise that the same products were formed in both reactions.

Notes from the mark scheme and report

• less energy needed to break bonds in methoxymethane (bonds weaker)
• energy released forming bonds is the same (same bonds formed)
• IGNORE calculations

Mark scheme

Examiner's comment

Many responses made no reference to ‘priority’ and/or discussed alkene C, suggesting that they didn’t read the question fully. Candidates often struggled to find the right language to express themselves, such as reference to ‘functional groups’ or ‘molecules’ rather than priority groups. Lots discussed using Mr to assign priority with only a few stating correctly that it is atomic number that is used for CIP rules. Many, despite stating that priority groups are on the same side, didn't identify these groups so didn't get the second mark.

Notes from the mark scheme and report

• priority groups (CH3CH2 and CH3) on the same side
• IGNORE relative mass/Mr of groups: priority is by atomic number

Skills: M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects

Mark scheme

Examiner's comment

About a quarter of candidates didn't gain any credit here as they struggled to identify useful processes. Lots referred to cracking or breaking down into smaller chains, possibly thinking about fractional distillation of crude oil and how we make better use of larger fractions. Some identified possible use as a fuel but didn’t say to generate energy/electricity. We also saw reference made to photodegradable/biodegradable polymers which isn't relevant to hydrocarbon polymers. There was evidence of the misunderstanding of the use as ‘feedstock’ with reference being made to animals (livestock or animal feed) or as food to eat.

Notes from the mark scheme and report

• recycling / combustion for energy / organic feedstock
• IGNORE reused

Mark scheme

Examiner's comment

Many managed to score at least one mark here but most struggled to give a comprehensive definition. A mark was often lost mark for omission of ‘atom’, i.e. not saying what was attracting electrons, or for saying ‘element’ instead. The ability to attract an electron was not accepted as this is describing electron affinity rather than electronegativity. Many described the formation of dipoles or partial charges across the bond due to the difference in electronegativity but didn’t demonstrate a clear understanding of what electronegativity is. Lower-scoring candidates often referred to the formation of ions and/or loss of electrons.

Notes from the mark scheme and report

• ability of an atom to attract electrons
• in a covalent bond
• DO NOT ALLOW 'element' for 'atom'
• DO NOT ALLOW attract 'an electron' (single electron)
• Learning key words and terminology is an important part of understanding Chemistry.
• Encourage students to make their own flash cards for each topic – this can be done on card or electronically using websites such as Quizlet.

Mark scheme

Examiner's comment

Around half of candidates scored both marks with a well-drawn diagram (such as that shown in the exemplar below). Practising drawing hydrogen bonds between different molecules is a really good way of exploring students’ understanding of what they are. The most common errors were missing the lone pair or showing a hydrogen bond that didn’t originate from those drawn. Lower-scoring candidates often didn’t draw a second water molecule, labelling the covalent bond between O and H as a hydrogen bond. Quite a few diagrams were seen with δ+ and δ- only on one molecule or with δ+ on one structure and δ- on the other. It was essential here to show that the O-H bond in both molecules is polar. Diagrams were often unclear so sometimes difficult to interpret. This is an example of a good diagram for hydrogen bonding.

Notes from the mark scheme and report

• δ+ on H and δ– on O on each molecule
• hydrogen bond must go from H to a lone pair on O of the other molecule
• DO NOT ALLOW more than 2 lone pairs on O

Mark scheme

Examiner's comment

Some good answers were seen referring to hydrogen bonds holding/keeping water molecules further apart or creating an open lattice. However, a number did not score for not mentioning hydrogen bonds or for referring to alternative intermolecular forces. For example, ‘In solid state, the London forces are weaker’ and ‘Ice has less induced dipole-dipole forces’. Some also indicated that ‘air’ was in the structure and this wasn’t accepted as air molecules are too large to fit in the gaps between water molecules in the ice crystal structure. Responses often demonstrated some significant misconceptions in candidate understanding. Many felt that the length of the hydrogen bond changed, for example ‘when ice freezes hydrogen bonds expand’ ‘Atoms are further apart so longer hydrogen bonds’. Some suggested that the strength of the hydrogen bond changed (for example, ‘ice has weaker H-bonds than water’). Some recognised that ice has a crystalline structure but didn’t explain how this is different from other solid structures, which would be denser than their liquid counterparts. Lower-scoring candidates struggled to understand density (for example, ‘ice is more compact’ or ‘molecules closer together in ice’). Terminology was also a problem with candidates often referring to ‘ice’ or ‘particles’ rather than ‘molecules’.

Notes from the mark scheme and report

• must refer to hydrogen bonds holding molecules apart/open lattice
• DO NOT ALLOW 'atoms' for 'molecules'
• DO NOT ALLOW gaps containing air

Mark scheme

Examiner's comment

Despite being told in the question that ammonia contains hydrogen bonds, many gave responses in terms of ammonia having either London forces and permanent dipole-dipole forces which are weaker than hydrogen bonds. For example, ‘ammonia consists of permanent dipole-dipole interactions which are weaker than hydrogen bonding in ice’ and ‘NH3 has 17 electrons and H2O has 18 electrons. Due to NH3 having fewer electrons, there are fewer London forces’. Lower-scoring candidates often confused hydrogen bonds and covalent bonds, consistent with what was seen in 1(b)(i). For example, ‘O-H bond is stronger than N-H bond’ and ‘more energy needed to break O- H bonds rather than N-H bonds’. Some of these candidates did score a mark for recognising that N is less electronegative than O. For others they understood that ammonia has weaker hydrogen bonds but then struggled to give a reason either in terms of lone pairs or electronegativity.

Notes from the mark scheme and report

• comparative: weaker/fewer hydrogen bonds in ammonia
• N has one lone pair, O has two OR N less electronegative
• DO NOT ALLOW breaking N–H/O–H covalent bonds
• IGNORE other intermolecular forces

Mark scheme

Examiner's comment

Just under half of the candidates scored all 3 marks. A range of responses were seen for this question but the most common error was to use the incorrect Mr for the hydrated which included either using the Mr for the anhydrous salt or the Mr for ZnSO4.H2O (179.5). It was evident that candidates struggle to understand what is meant by water of crystallisation. Some scored 2 marks for the correct Mr and finding moles but then were unsure how to proceed with the calculation. For example, some divided moles by 7 before multiplying by 161.5. Many candidates took the more complicated alternative route to find the mass of water before subtracting from mass of hydrated compound. Low-scoring candidates often struggled to correctly work out Mr values. Their working was set out so that individual steps were unclear, often with contradictory calculations.

Notes from the mark scheme and report

• M(ZnSO4•7H2O) = 287.5
• n = 0.04
• × 161.5 = 6.46 g
• no ECF if 161.5 used as molar mass of hydrate
• alternative via mass of water lost

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Many found this question challenging despite doing well in Question 2(a)(i). The question states ‘use your answers from (a)(i)’ but not many candidates wrote about the electron configurations they had given. Many gave very simplistic responses in terms of the number of electrons increasing but made no reference to how those electrons are arranged (e.g. ‘number of electrons increases across a period as the electron configuration gets higher’ or ‘atomic number increases’). Some candidates struggled with terminology, often referring to ‘block’ or ‘orbital’ instead of subshell (e.g. ‘outer electrons are in same block’, ‘going across a period the number of orbitals increases’, ‘elements in same group have their highest energy electron in same block’ or ‘orbital’). Candidates need clarity on the terminology used for electron configurations and periodic table i.e. blocks, shells, sub-shells and orbitals. It was rare for candidates to score both marks as this was a question that they were unfamiliar with. However, some did gain a mark for linking the number of outer shell electrons to the group number or stating that elements in the same period have the same number of shells. It was not enough to refer to the highest energy electron being in the s-subshell or p-subshell as this is the link to the block, but all groups in same block will be the same. Some described the trend in other physical or chemical properties. Some examples included: ‘Elements have same chemical and physical properties due to similar electronic configuration’; ‘as you go across period, number of electrons increase and their boiling and melting points increase’; and ‘electrons are more easily lost in a paired orbital, due to greater repulsion and so have lower ionisation energies’.

Notes from the mark scheme and report

• s sub-shell fills then p across Period 2
• same pattern repeated across other periods
• ALLOW same outer-shell configuration within a group
• We have produced a transition guide on the topic of atomic structure. It covers content from KS4 and how this is developed at KS5 with a wide range of suggested resources to support teaching. At KS4, candidates are expected to be able to explain how the position of an element in the Periodic Table is related to the arrangement of electrons in its atoms, with development at KS5 to arrangement in to s, p and d orbitals.
• https://ocr.org.uk/Images/170375-atomic-structure-ks4-ks5.pdf

Mark scheme

Examiner's comment

The term was not well known, with only a small proportion of candidates scoring this mark. Many gave the definition for enthalpy change of combustion or formation or bond enthalpy. Many referred to one mole of either reactants or products or bonds, or sometimes a combination of these. Lower-attaining candidates often said it was the temperature change.

Notes from the mark scheme and report

• enthalpy change for the molar quantities in the stated equation
• DO NOT ALLOW 'energy released'
• IGNORE standard conditions

Mark scheme, with the levels and indicative points

Examiner's comment

The calculation of enthalpy change was generally well-answered and the majority of candidates were able to recall the equation q = mcT. Many candidates forgot the minus sign or gave a positive sign for final enthalpy change. Errors in calculation were most commonly for using an incorrect mass, usually by finding the mass of copper(II) nitrate (from moles and Mr). Some also used the wrong value for the heat capacity, selecting the value for R from the data sheet instead. Many gave the final answer to an inappropriate six significant figures. Candidates often found it challenging to give appropriate assumptions and improvements, limiting the level achieved to Level 2. However, many candidates did correctly give the assumptions that the specific heat capacity and density of the solution was the same as water. This is usually stated for the candidates with these types of questions. The most common improvements suggested were use of a polystyrene cup, adding a lid or using a thermometer with a higher resolution. Quite a few candidates suggested using a larger volume of solution which would indeed reduce the % uncertainty in the volume measurement. However, it would lead to a smaller temperature change, increasing the % uncertainty in the temperature measurement. Some confused the question with a calculation of enthalpy change of combustion and gave improvements accordingly, e.g. ‘use a copper or bomb calorimeter’, ‘draft shields’, ‘heat for longer’, ‘position of flame and supplies of oxygen’. This response achieved Level 3 - 6 marks. There is a correct calculation for H, the final value has a correct negative sign and is given to 4 significant figures. Lots of valid assumptions and improvements are given.

Notes from the mark scheme and report

• q = 100 × 4.18 × 18.6 = 7.77 kJ
• n(Cu(NO3)2) = 0.05
• ∆H = –155 kJ mol–1 with negative sign
• assumptions (density/c same as water, no heat loss, ignore zinc)
• improvements (polystyrene cup, lid, cooling curve, more precise thermometer)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

This is a familiar question with around half of candidates scoring all 4 marks. Common errors included drawing 2 lines, as you would have with different temperatures, but labelling one with catalyst and other without. Some had incorrect or missing labels on the axes. The most frequently gained mark was for knowing that the activation energy was lowered by a catalyst; this could be given by correct lines and labelling shown on the distribution, although care needed to be taken as some contradicted their answers with their diagram. Some labelled Ea and Ec lines but didn’t have energy on x-axis scale. Some struggled with the final marking point, not recognising that more molecules have the required activation energy. For example, ‘more frequent successful collisions’ with no reference to the activation energy, i.e. why they are successful.

Notes from the mark scheme and report

• one curve only, starting at origin and not touching x-axis (DO NOT ALLOW two curves – confusion with temperature)
• axes: number of molecules vs energy (DO NOT ALLOW atoms/enthalpy)
• Ec below Ea
• more molecules with energy above activation energy
• IGNORE 'more collisions'

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Most scored both marks here. Some didn’t circle the anomalous result and some lost a mark for a poorly drawn curve. Candidates must ensure they have a sharp pencil and draw a single line through all the points (except the anomalous point). Some didn’t start at the origin or didn’t level off at around 90 cm3.

Notes from the mark scheme and report

• smooth curve through all points except 100 s
• anomaly at 100 s circled
• curve levels off at about 90 cm3

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Over half of candidates scored all 3 marks here. However, around a quarter did not gain any credit at all. Some confused the volume of H2O2 for a volume for a gas and attempted to find moles using molar gas volume (i.e. used 50cm3 rather than 90cm3). This often lost all 3 marks as they then divided by 2 and to find concentration divided by 90/1000 instead. A few attempted to use the ideal gas equation but this rarely yielded a correct value. Candidates must be encouraged to set out working clearly, showing logical steps and preferably labelling each step with what is being calculated and giving units. Many wrote the calculation as series of steps which all equalled the previous. e.g. 90  24000 = 3.75 x 10-3 x 2 = 7.5 x 10-3  0.05 = 0.15

Notes from the mark scheme and report

• n(O2) = 90/24000
• × 2 for H2O2
• ÷ 0.0500 dm3 = 0.15
• common errors 0.075 (no × 2) and 150 (no cm3 to dm3)
• ALLOW ideal gas equation with sensible p and T
• Encourage students to assign information in the question to the correct chemical. One way to do this is to write out the equation and then underneath each species put correct volumes as given. It also helps to highlight ratios shown in the equation. It is important here to pay close attention to state symbols as it helps identify correct calculations to use.
• For example; 2H2O2 (aq) → 2H2O (l) + O2(g) 50.0cm3 90 cm3 Solution Gas at RTP Conc?

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Approximately half of candidates scored all 4 marks. Very few scored no marks as they were able to give the correct structure of the named product (2-bromobutane). The most common reasons for losing marks included: • Adding dipoles to C=C • Missing dipoles on H-Br or reversed dipole on H-Br (i.e. + Br) • Missing charge on bromide ion or adding - • Arrows the wrong way round or not coming from a bond or lone pair (or negative charge for bromide ion) Some attempted a radical mechanism.

Notes from the mark scheme and report

• arrow from C=C to H of H–Br
• dipole on H–Br and arrow from bond to Br
• arrow from Br– (lone pair or charge) to C+ of secondary carbocation
• DO NOT ALLOW δ+ on carbocation or partial charge on C=C
• product mark independent
• ALLOW ECF for product from incorrect carbocation
• For ideas on teaching this topic please look at our Topic Exploration Pack: Electrophilic Addition and Markownikoff’s rule: https://teachcambridge.org/item/b4220e86-bc04-492c-b354-8103687ce594

Mark scheme, with the levels and indicative points

Examiner's comment

Over a third of candidates achieved Level 3, gaining 5 or 6 marks. A correct structure (either aldehyde or ketone) alone was not enough to award Level 3 and candidates were expected to give a comprehensive description of how the evidence helped them determine the structure. The biggest challenge for many candidates was finding the correct empirical formula. The ratio worked out to 1:1.33:1 so many incorrectly rounded this to either 1:1:1 or 1:2:1, which meant they struggled to find a molecular formula that worked and added up to 88. Incorrect molecular formulas seen included C3H3O3, which adds to 87 (often the extra H was just added to make it fit), or C4H8O2, which does add to 88. Most candidates could analyse the IR spectrum, identifying peaks corresponding to C=O or O-H. Candidates should identify bonds present before making conclusions about the functional groups. Many were able to use mass spectra to determine the Mr value from the M+ peak. Some did go on to make use of other peaks, identifying fragments and confirming whether the structure was an aldehyde or ketone depending on analysis. For example, CHO+ at m/z = 29 suggests an aldehyde, or conversely CH3+ at m/z = 15 suggests a ketone. Candidates should always be encouraged to comment on all the data provided. This can be through good annotation of the spectra and notes added to the first page of the question. Many candidates didn’t mention the evidence from the bromine test. If candidates pieced together information to give a structure that is chemically feasible containing either a C=O or COOH group then they could achieve Level 2. Without a structure they were limited to Level 1. The most common incorrect structures seen included butanoic acid, 2-hydroxypropenoic acid or structures with 2 × C=O and an alcohol OH. This response achieved Level 2 - 4 marks. Despite a correct calculation for empirical formula, they rounded incorrectly to CHO. They then state that molecular formula is C4H8O2 which matches the Mr value determined from the mass spectrum but not their empirical formula. They demonstrated good analysis of the IR spectrum and have even looked at potential fragments from the mass spectrum. The response meets the Level 2 descriptor as they have a feasible structure with a COOH group and have some analysis of all three scientific points. The communication is good with a clearly laid out calculation and bonds identified from IR and MS fragments have a positive charge.

Notes from the mark scheme and report

• empirical formula C3H4O3
• M+ at m/z 88 so molecular formula C3H4O3
• IR broad O–H 2500–3300 and C=O 1630–1820 so carboxylic acid
• no C=C (bromine water)
• fragments m/z 43 CH3CO+, 29 CHO+, 15 CH3+ with positive charge
• CH3COCOOH or OHCCH2COOH

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Many candidates found this question challenging. Some managed to recognise the idea that the moles remained the same. Fewer recognised that the en- thalpy change stayed the same while the mass/volume decreased. A minority combined both ideas to- gether. Many gave responses in terms of increasing concentration increases rate, attributing a higher tempera- ture to more collisions.

Notes from the mark scheme and report

• same number of moles so same energy released AND smaller total volume/mass heated
• IGNORE responses about rate of reaction
• Candidates struggled to understand the difference between the rate of reaction and the en- thalpy change, often indicating that more collisions would result in more energy being re- leased. Make sure that candidates know that overall energy change depends on the net dif- ference in bond energies between reactants and products per mole. While an increase in con- centration can lead to a faster rate of reaction, it will not change the enthalpy change of the reaction.
• We have some useful resources to support this: Topic Exploration Pack: Kinetics and Catalysis https://teachcambridge.org/item/9fbefaed-4ade-4f22-95ec-3defcfef5a46 Delivery guide (H032) Enthalpy changes https://teachcambridge.org/item/6f16de69-f654-407c-b513-3ed7902ef9ce

Skills: M0.2 Use ratios, fractions and percentages; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Most were able to correctly calculate the number of moles with the molar mass given in the question. Many were then able to use the molar gas volume at RTP, given on the data sheet, to find the volume of gas. The most common errors seen here were an incorrect number of significant figures, not converting to cm3 but leaving as dm3, and some multiplied by 2. A few candidates attempted to use the ideal gas equation here and if this was done correctly marks were credited.

Notes from the mark scheme and report

• answer must be 3 SF and in cm3 (184)
• ALLOW ECF from incorrect n(BaCO3)
• ALLOW ideal gas equation with appropriate T and p

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG1 Moles determination

Mark scheme

Examiner's comment

A very challenging question with most candidates failing to score any marks. Only a few correctly sug- gested the gas dissolving in water or the use of non-standard conditions. Most correct responses were for gas lost while replacing the stopper. Most mentioned loss of gas but didn’t link to the procedure to say how it was lost. Lots of candidates suggested that the reaction was incomplete despite being told the acid is in excess in the question. Candidates should consider that if a reagent is in excess, the reaction is expected to go to completion. Many suggested faults with setting up apparatus – possibly from personal experience. We wanted them to consider the limitations should this be set up without ‘human’ errors. A lot of candidates discussed the various leaks in the apparatus as the sources of gas loss. Some made reference to gas present in the tubing already not understanding that it would push through and displace gas to fill the measuring cylin- der, with equivalent amount being left in tubing at the end.

Notes from the mark scheme and report

• gas escapes while bung is replaced
• some CO2 dissolves in the water
• conditions not at RTP
• IGNORE incomplete reaction and human errors such as unsealed apparatus
• We provide support to help students prepare for questions based on the practical work they have carried out. Each of our suggested practicals has extension questions which can be used to explore practical work in more detail and evaluate different procedures.
• This question links well with PAG 1.1: https://teachcambridge.org/item/b9be7815-8c49-4fd6- b0eb-8f476393c653

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Very few lost this mark and when they did it was usually for the giving the first titre to only 1 decimal place. Occasionally arithmetic errors were seen.

Notes from the mark scheme and report

• 23.30, 23.05, 23.15
• DO NOT ALLOW 23.3 (must be 2 d.p.)

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

The majority gave a correctly balanced equation here. The most common error was an incorrect number of oxygen molecules. A few candidates gave the equation for propane rather than pentane.

Notes from the mark scheme and report

• C5H12 + 8O2 → 5CO2 + 6H2O
• DO NOT ALLOW multiples (one mole of pentane)

Mark scheme

Examiner's comment

Many struggled to give the correct response here. They were expected to find the mid-point between the values given for propane and pentane. (The difference between pentane and hexane could also be used.) They then needed to give the value to 2 significant figures. Candidates were generally confident at giving final answers to 3 significant figures but struggled here when asked for only 2. A significant number also forgot to include the – sign.

Notes from the mark scheme and report

• –2900
• must be 2 SF with negative sign

Skills: M0.3 Estimate results; M1.1 Use an appropriate number of significant figures

Mark scheme

Examiner's comment

Candidates struggled to give a full explanation here. They needed to say what fission is and explain ho- molytic. Many said “bond breaking to form radicals” without explaining what radicals are or what happens to electrons from the bond. Many referred to elements rather than atoms in their response, e.g. “covalent bond is broken so that each element gets 1 electron”. It was evident that some had attempted to learn a definition but didn’t really understand what it meant, e.g. “each of the bonding atoms takes one of the electron pairs.” The description of what happened to electrons from the bond was often too vague, e.g.” electrons are shared evenly between atoms.”

Notes from the mark scheme and report

• bond breaks AND each bonded atom receives one electron from the shared pair
• DO NOT ALLOW molecule/compound/particle/element for atom
• IGNORE equations

Mark scheme

Examiner's comment

Most scored both marks here. The most common error involved substituting the Br in the first step – this lost both marks. Very few candidates scored 1 mark; this was usually for a correct first step but then re- acting the C3H7 radical with either 2 Br or HBr in second step. Candidates must check that equations balance in terms of atoms. Candidates should also understand that hydrogen radicals are highly reactive and are therefore unlikely to exist under standard conditions.

Notes from the mark scheme and report

• C3H8 + Br• → •C3H7 + HBr then •C3H7 + Br2 → C3H7Br + Br•
• 1 mark if both correct but dots omitted

Mark scheme

Examiner's comment

Most scored both marks here, as they were able to use the ‘dot-and-cross’ diagram drawn in (i). Some struggled with recalling the bond angle. A few “trigonal planar” and “120” responses were seen perhaps confusing themselves with the next part of the question.

Notes from the mark scheme and report

• tetrahedral
• 109.5° (ALLOW 109–110)

Skills: M4.1 Use angles and shapes in regular 2-D and 3-D structures

Mark scheme

Examiner's comment

A range of marks is scored on this question with most scoring at least 1 mark for the correct bond angle. A larger range of bond angles were allowed here to account for the impact of the double bond. Candi- dates were not expected to account for the additional repulsion from the double bond but should be able to use electron pair repulsion theory to predict and explain. Many candidates found the explanation challenging, with a significant number not making any compari- son between chloroethene with chloromethane as required in the question. As seen in previous years, a mark was available for stating that “electron pairs repel” but this was often omitted despite being the main principle that determines molecular shapes. Many candidates felt like they needed to discuss lone pairs – in some cases saying they were present and therefore caused more repulsion. Candidates need to be reminded to focus only on the electron pairs around the central atom. Discussion of regions or areas of electron density or the number of bonded atoms was not enough to gain credit here.

Notes from the mark scheme and report

• 120° (ALLOW 116–124)
• CH3Cl has 4 bonded pairs AND H2C=CHCl has 3 bonded regions
• electron pairs repel (DO NOT ALLOW atoms repel
• IGNORE electrons/bonds repel)
• IGNORE names of shapes
• Many students think that molecular shapes are determined solely by lone pairs or by repulsion between bonded atoms. The principle behind molecular shapes is called electron pair repulsion theory because it is based on repulsion between electron pairs, which may be bonded pairs or lone pairs, but not atoms.

Skills: M4.1 Use angles and shapes in regular 2-D and 3-D structures

Mark scheme

Examiner's comment

Many recognised it as the M+1 peak but did not explain why it is there. Some suggested the addition of a H to the molecular ion. Some suggested it was due to different isotopes but did not specify for C-13. Other isomers, e.g. H-2, exist in such small natural abundance they would not be detectable. Lower-attaining candidates that struggled with (i) often thought that it was the M+ peak and so gave the Mr of the compound.

Notes from the mark scheme and report

• M+1 peak from small proportion of carbon-13

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Answer

C

Mark scheme

Examiner's comment

This relatively difficult pH calculation was readily done successfully by higher ability candidates, but lower ability candidates found it difficult, with answer B proving a popular choice.

Answer

A

Mark scheme

Examiner's comment

The correct answer to this was known by the more able candidates. Lower ability candidates struggled.

Notes from the mark scheme and report

• 2 mol H2

Skills: M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Candidates almost universally secured the first mark for equilibrium shifting to the right. Many scored this by simple application of Le Chatelier’s principle, and then went on to incorrectly explain Kp increased because of this shift. Very few realised that (a constant) Kp drives Le Chatelier’s principle (and not the other way around). An increase of pressure will increase the value of the partial pressures in the bottom half of the Kp expression more than the top half, thus (initially) decreasing the Kp ratio. Therefore, to restore Kp, the amount of NO2 present must increase; consequently, the equilibrium shifts to the right.

Notes from the mark scheme and report

• shifts right
• denominator of Kp expression increases more than numerator so ratio falls below Kp
• more NO2 formed to restore Kp
• ALLOW 'Kp initially decreases' only if Kp is then restored

Skills: M0.3 Estimate results

Mark scheme

Examiner's comment

This proved a more difficult calculation than expected, but higher ability candidates realised that [H+] (determined from the pH) divided by the given concentration of CH3COOH was required. Answers had to be expressed to three significant figures in order to receive credit.

Notes from the mark scheme and report

• 3 sig figs required

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures

Mark scheme

Examiner's comment

Candidates found the unit conversion into the pV = nRT equation difficult but were able to rearrange the equation to arrive at a value of n. They were then able to determine a molar mass by dividing n into 0.4485 g. Having arrived at 183.0 g mol–1, it was expected that candidates would realise that there must be an even number of Cl atoms. The next step would be to subtract 71.0 from 183.0. This gives 112.0, which divides by 16.0 to give 7, thus leading to Cl2O7. The most common errors were seen in the unit conversions. Pressure is measured in Pa (not kPa), volume is measured in m3 (not dm3) and temperature is measured in K (in this case 373 K). Another source of error was to attempt to determine n by dividing the volume by 24,000 cm3.

Notes from the mark scheme and report

• T = 373 K and V = 76.0 × 10^–6 m3
• n = 2.45 × 10^–3
• M = 183
• Cl2O7
• use of 24 000 cm3 scores max 2 by ECF (M = 141.5)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

This unstructured calculation was done well by the higher ability candidates. Lower ability candidates struggled to show what they were attempting to calculate and in particular did not appreciate the 1 : 6 ratio of S2O3 2–(aq) to IO3 –(aq). Candidates might be advised to start n(formula) = … at the start of each line of calculation eg n(S2O3 2–) = … mol No credit was given to candidates who grasped the identity of the Group 1 iodate from nowhere and calculated the theoretical relative formula mass.

Notes from the mark scheme and report

• n(S2O3 2–) ÷ 6 for IO3–
• × 10 scaling to 250 cm3
• relative formula mass 260.5 so RbIO3
• formula must be derived from the calculated mass (DO NOT ALLOW RbIO3 without it)
• ALLOW ECF for another Group 1 metal

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme, with the levels and indicative points

Examiner's comment

Many candidates had a stab at identifying C–F but neglected to include equations for the three reactions described or to show relevant working. Most candidates recognised C as the ammoniacal copper(II) ion but the formula was frequently incorrect and correct attempts at a ligand substitution equation from [Cu(H2O)6]2+ was rarely seen. Diagrams showing the trans isomer were attempted but often of poor quality due to incorrect linking. Candidates recognised D as being CuSO4 but often did not identify E as Cu due to a lack of familiarity with this common disproportionation reaction. Cu(OH)2(s) was a common incorrect identification of E. Only the best responses described the oxidation number changes which made this a disproportionation reaction. F was identified by a percentage by mass calculation to determine an empirical formula and then by deduction to produce Cu(NO3)2.3H2O. Having done this, many candidates did not give the relatively simple equation for reaction 3 between copper(II) oxide and dilute nitric acid. This exemplifies how considered structuring of candidate responses can enhance their clarity. The clear labelling of C-F in the response, along with numbering which corresponds to the reactions in the question stem, make the candidate’s line of reasoning easy to follow. The underlining and annotations in the question stem show good practice in picking out and interpreting key information.

Notes from the mark scheme and report

• C = [Cu(NH3)4(H2O)2]2+ drawn as trans isomer with correct wedges and connectivity
• D = CuSO4, E = Cu with Cu(+1) → Cu(+2) + Cu(0)
• F = Cu(NO3)2•3H2O from mole ratio CuH6N2O9
• formulae with missing brackets or numbers limit the level

Skills: M0.2 Use ratios, fractions and percentages; M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects

Answer

C

Mark scheme

Examiner's comment

This question proved to be difficult, with only the most able candidates selecting the correct answer of C. Many candidates confused the trends in oxidising power of the halogens with trends in the reducing power of the halide ions. Down the group, the oxidising power of the halogens decreases but the reducing power of the halide ions increases. Option A is incorrect because iodine is a weaker oxidising agent than bromine, option B (the most common incorrect answer) is incorrect because chlorine is not a reducing agent (it’s an oxidising agent) and option D is incorrect because the chloride ion is not an oxidising agent (it’s a reducing agent).

Answer

B

Mark scheme

Examiner's comment

This was a challenging question, the correct answer being B. Candidates had to know the correct formula for sodium carbonate in order to deduce the 2:1 molar ratio of HCl to Na2CO3 and to calculate its Mr (106). Option D was arrived at by assuming the molar ratio was 1:1, option A by incorrectly thinking the formula was NaCO3 , and option C by making both of these mistakes.

Answer

A

Mark scheme

Examiner's comment

This was another challenging question, with only the most able candidates choosing the correct answer of A. Candidates had to calculate the number of moles of O3 (by dividing 1.0 by 2.5) and then had to use the information in the equation (that 2 moles of O3 releases 284 kJ). The most common error was option B, presumably because candidates thought that 1 mole of O3 releases 284 kJ (rather than 2 moles). Candidates should be aware that a given enthalpy change relates to the molar quantities as shown in the equation.

Answer

B

Mark scheme

Examiner's comment

This was a challenging question, with only the most able candidates giving the correct response of B (one trans isomer and two optical cis isomers). Many candidates drew out the shapes of the isomers but some missed the two optical cis isomers (giving option A) while others thought there were two cis and two trans optical isomers, giving option C.

Mark scheme

Examiner's comment

Many candidates knew the block magnesium belonged but only very few could explain this was because magnesium’s highest energy electron was in a s sub-shell or s orbital.

Notes from the mark scheme and report

• s-block AND highest energy/outer electron in an s orbital or s sub-shell
• DO NOT ALLOW s shell/energy level
• IGNORE electron configurations

Mark scheme

Examiner's comment

This was mathematically challenging, but candidates who did not arrive at the correct answer were able to get partial credit for showing working.

Notes from the mark scheme and report

• third isotope abundance 11.01%
• answer must be to 5 SF (25.982)
• ECF for transcription errors in first sum

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.2 Change the subject of an equation; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Very few candidates correctly identified the CrO4 2– ion here. Candidates should be aware that oxidation of Cr(OH)3 produces CrO4 2– (which can then be acidified to produce Cr2O7 2-).

Notes from the mark scheme and report

• CrO4 2–
• IGNORE compounds e.g. Na2CrO4

Mark scheme

Examiner's comment

Only a very small minority of candidates appreciated that a larger titre reduces percentage error in titre values. Most erroneously described a reading of 1.35 cm3 as being less accurate than a reading of 13.5(0) cm3. The accuracy of these is equal in the same scaled apparatus.

Notes from the mark scheme and report

• dilute solution gives a larger titre and so a smaller percentage error/uncertainty

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Examiner's comment

A LEVEL CHEMISTRY A Examiners’ report H432/01 Summer 2019 series Version 1 H432 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 1 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 2 ............................................................................................................................................ 6 Question 3 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 7 Question 5 ............................................................................................................................................ 7 Question 6 ............................................................................................................................................ 8 Question 8 ............................................................................................................................................ 8 Question 9 ............................................................................................................................................ 9 Question 10 .......................................................................................................................................... 9 Question 11 ........................................................................................................................................ 10 Question 13 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 11 Question 15 ........................................................................................................................................ 11 Section B overview ................................................................................................................................. 12 Question 16 (a) ................................................................................................................................... 12 Question 16 (b) ................................................................................................................................... 12 Question 16 (c) (i) ............................................................................................................................... 12 Question 16 (c) (ii) .............................................................................................................................. 13 Question 16 (d) (i) ............................................................................................................................... 14 Question 16 (d) (ii) .............................................................................................................................. 15 Question 16 (e) (i) ............................................................................................................................... 16 Question 16 (e) (ii) .............................................................................................................................. 16 Question 17 (a) ................................................................................................................................... 17 Question 17 (b) ................................................................................................................................... 19 Question 18 (a) (i) ............................................................................................................................... 20 Question 18 (a) (ii) .............................................................................................................................. 20 Question 18 (a) (iii) ............................................................................................................................. 21 Question 18 (a) (iv) ............................................................................................................................. 22 Question 18 (a) (v) .............................................................................................................................. 22 Question 18 (b) (i) ............................................................................................................................... 22 Question 18 (b) (ii) .............................................................................................................................. 23 Question 18 (c) (i) ............................................................................................................................... 24 Question 18 (c) (ii) .............................................................................................................................. 24 Question 19 (a) (i) ............................................................................................................................... 25 Question 19 (a) (ii) .............................................................................................................................. 25 Question 19 (a) (iii) ............................................................................................................................. 26 Question 19 (b) (i) ............................................................................................................................... 26 Question 19 (b) (ii) .............................................................................................................................. 27 Question 19 (b) (iii) ............................................................................................................................. 27 Question 19 (b) (iv) ............................................................................................................................. 28 Question 19 (c) (i) ............................................................................................................................... 28 Question 19 (c) (ii) .............................................................................................................................. 29 Question 20 (a) ................................................................................................................................... 29 Question 20 (b) (i) ............................................................................................................................... 30 Question 20 (b) (ii) .............................................................................................................................. 30 Question 20 (b) (iii) ............................................................................................................................. 31 Question 20 (b) (iv) ............................................................................................................................. 32 Question 20 (b) (v) .............................................................................................................................. 32 Question 20 (c) ................................................................................................................................... 33 Question 21 ........................................................................................................................................ 34 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 18 (c) (i) Only a very small minority of candidates appreciated that a larger titre reduces percentage error in titre values. Question 18 (c) (ii) Many candidates coped well with this multi-step calculation.

Notes from the mark scheme and report

• A gives 66 mg Fe and B gives 69 mg Fe so B
• working needed

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

The calculation was done well. Some candidates found it difficult to convert ΔS values from J to kJ or temperature values from °C to K, and this was frequently seen in responses. The final mark was often lost through vague phrasing. ‘Yes’ was an insufficient answer, as it does not identify the reaction as feasible or non-feasible. ‘Gibbs free energy is negative’ was also insufficient, as it should be identified that ΔG is negative.

Notes from the mark scheme and report

• ∆S = –391 J K–1 mol–1 converted to kJ
• T = 293 K
• common errors –1117 (T = 20), +113438 (J/kJ mix), –1008 (298 K) score 2 calculation marks
• feasible as ∆G < 0 (ECF)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

This calculation proved very difficult. Many candidates did not deduce the correct cycle. Of those who did, a significant majority did not appreciate the stoichiometry of the equation and did not double the ΔcH values or half the initially calculated value of ΔfH(H2S).

Notes from the mark scheme and report

• both ∆cH values ×2
• subtract ∆rH
• divide by 2
• +20(.1) scores 2
• ALLOW –20.1

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Candidates tend to find Kp calculations difficult and so may need guidance in developing a strategy to work their way through them. A typical approach is: • Calculation of initial moles present • Calculation of the change in moles present • Deduction of the number of moles present at equilibrium • Determination of total moles present at equilibrium • Calculation of mole fractions at equilibrium • Calculation of partial pressures at equilibrium • Inserting partial pressure vales into the Kp expression • Writing an answer to the required number of significant figures.

Notes from the mark scheme and report

• initial SO2 from 10.2/24.0 and O2 from 12.0/32.0
• O2 used = 0.350/2
• total moles 0.625
• common errors 1.45 (n(SO2) = 2 × n(O2)) and 15.1 (12/16 for O2) score 4
• 3 SF

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Showing by calculation something already known is a skill that some candidates find challenging. Many responses included rows of figures with ‘20 cm3’ appearing at the end. Clarity of working is essential and in questions such as this, candidates are advised to include word descriptions of what they are calculating, even if it is abbreviations such as ‘n’ for number of moles.

Notes from the mark scheme and report

• mark is for working: n(acid) = 0.002 mol and V(NaOH) = 0.002/0.100

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

This calculation proved difficult with once again, many figures and sums appearing with little indication as to their relevance. Responses to 20bi/ii often featured rows of figures and random sums without a single word about what the figures were or sums were set to calculate. Candidates should remember to provide written indications of what it is they’re working out – presenting the calculations without any annotations can make it harder for error carried forward marks to be given if there is an error in their calculation.

Notes from the mark scheme and report

• excess OH– = 0.0025 mol in total volume 70.0 cm3
• common errors 12.81 (45 cm3 volume) and 11.40 (moles not concentration) score 3
• ALLOW pOH method

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

This weak acid / strong alkali titration curve required candidates to apply their knowledge. Some candidates found it difficult to draw an adequate titration curve. The key points to titration curves are: • A ‘vertical’ section at the end point • The vertical section with pH range correct to relative to the strength of acid/alkali • A correct equivalence point relative to pH 7 with respect to the strength of acid/alkali • A ‘shallow curve’ leading from 0 cm3 to vertical section • A ‘shallow curve’ leading from the vertical section to the total volume added For this reaction: • The vertical section was at 20 cm3 (given in 20bi) • As it was a weak acid and strong alkali reacting, the vertical section should start above pH 5 and finish around pH 11 • The equivalence point (half-way up the vertical section) for a weak acid / strong alkali titration should be above pH 7 • The starting pH should be that of the weak acid, C2H5COOH (answer to 20(a)) • The final pH should be that of the final solution (answer to 20(b)(ii))

Notes from the mark scheme and report

• vertical section at about 20 cm3, within pH 5–12, at least 3 pH units, midpoint above 7
• if curve drawn the wrong way round only the end-point mark is available

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Very few candidates scored the mark for this question. The most common error was to write ‘HIO3 dissociates’. While correct, this is true of all weak acids. The subtlety in this question was to realise that because Ka was relatively high, the resultant acid strength would mean that the degree of dissociating was significant enough so initial [HIO3] was significantly less than [HIO3] at equilibrium. In other words, the assumption [HIO3]initial = [HIO3]equilibrium is not valid in this case.

Notes from the mark scheme and report

• HIO3 dissociation not negligible (large Ka, 'stronger' weak acid) so [HIO3]eqm significantly lower than [HIO3]initial
• IGNORE [HIO3]eqm < [HIO3]initial alone

Answer

D

Mark scheme

Examiner's comment

This question proved to be difficult, with only the most able candidates selecting the correct answer of D. A was often given as an incorrect answer as candidates recognised that AgCl would be the only halide precipitate to show a change with dilute ammonia but did not realise that as it would redissolve, it would be the only one not in the filtrate.

Notes from the mark scheme and report

• AgCl dissolves in dilute NH3
• AgBr and AgI do not

Skills: PAG4 Qualitative analysis of ions

Answer

B

Mark scheme

Examiner's comment

This was a challenging question, the correct answer being B. The candidates would be advised to use the space around the question to perform the calculation. A was a common error as the Al ion was seen as the obvious choice without doing any calculations due to being Al3+.

Answer

A

Mark scheme

Examiner's comment

This was another challenging question, with fewer candidates choosing the correct answer of A. The distractor, D, was obtained by swapping products and reactants in the calculation or in an incorrect enthalpy cycle. Candidates should have ruled out C and D straight away as enthalpy of combustion is always exothermic. Where working was shown, candidates omitted one or more of the multiples in the bonds formed or broken.

Answer

A

Mark scheme

Examiner's comment

This question proved to be difficult, with fewer candidates selecting the correct answer of A. Option C was the most common distractor as many candidates did not know the standard state of bromine. Option B was selected by those candidates who confused the definitions of average bond enthalpy with the enthalpy change of atomisation.

Notes from the mark scheme and report

• 1 mol of gaseous atoms from the element in its standard state (liquid Br2)

Answer

B

Mark scheme

Examiner's comment

This was another challenging question, with fewer candidates choosing the correct answer of B. Successful candidates often had the electronic configuration written down and focused on the higher energy electrons as to whether they were paired or not. Missing the 4s1 in Cu was the main omission to identify the correct answer. Section B overview

Notes from the mark scheme and report

• Cu+ is 3d10

Mark scheme

Examiner's comment

Many candidates were able to explain clearly the second order dependence on [NO] but many found it more challenging to identify and explain the first order relationship for [CO], where the candidate had to consider the second order dependence on [NO] to work out the effect on rate for the change in [CO]. Most gave a rate equation of the correct format based on the orders they had stated, and worked out a value for k. Although some did not determine the correct units, there was an encouraging number who managed this either from memory or by cancelling. The most common error was not including s-1

Notes from the mark scheme and report

• NO second order from expts 1 and 2
• CO first order needs both concentrations changing (expts 2 and 3: rate × 16)
• k = 3.37 × 10^6 dm6 mol–2 s–1
• common errors 4.65 × 10^9 mol–3 dm9 s–1 (CO second order) and 2446 mol–1 dm3 s–1 (CO zero order) score 4 with ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Many candidates made some attempt at this unusual twist on a Q = mcΔT calculation, but only a minority of candidates produced a fully correct answer. A temperature of 84.9 oC was seen much more often than 52.4 oC. The calculation of n(AgNO3) was performed well. Common errors were not dividing the moles by 2, not recognising the need to calculate the energy released per mole of AgNO3, confusing the mass of water with the mass of the silver nitrate solid or of using an energy in kJ alongside a value of c based on J.

Notes from the mark scheme and report

• n(AgNO3) = 0.0400
• energy = 678 × 0.0400/2 = 13.56 kJ (2 mol AgNO3 per ∆H)
• ∆T = 32.4
• add 20.0 gives 52.4 °C to 3 sig figs
• common errors 32.4 (∆T only) and 84.9 (no ÷2) score 3

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Although there were a few well explained, correct answers, this question proved challenging for most candidates. Candidates needed to link the changes as a proportion (e.g. half) to be given the mark. Candidates often only considered the change to energy or mass, but not both, e.g. dealt only with the decrease in moles reacting leading to a smaller temperature rise.

Notes from the mark scheme and report

• same maximum temperature AND half the energy/moles AND half the mass/volume

Skills: M0.2 Use ratios, fractions and percentages; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

This calculation proved difficult. Many candidates did not deduce the correct cycle. Of those who did, a significant majority did not appreciate the stoichiometry and/or use the correct signs from the cycle. Many students scored 1 mark for this question which was gained as an error carried forward, providing all the values had been used in the enthalpy cycle. -90 was a common error.

Notes from the mark scheme and report

• 4∆fH(NO) = –1172 + 1716 – 184 = +360
• ÷4 = +90
• –90 scores 1

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Few candidates scored full marks here, with some candidates not increasing the 1st IE across the period, and many getting the dip for the wrong element (not O) and finishing too high with Ne. Less successful candidates had clearly confused it with MP trend across the period.

Notes from the mark scheme and report

• general increase from B to Ne with Ne lower than He
• O lower than N but higher than C, F higher than O

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Many candidates did not achieve full marks as they did not discriminate between the three different comparisons that were being tested in Question 18 (c) and 18 (d). Few candidates did identify the electron being in the s or p but then explained the ionisation energy in terms of full subshells, electron pair repulsion and described the 2s sub-shell as closer to the nucleus rather than as lower energy.

Notes from the mark scheme and report

• Be electron removed from 2s, B from 2p
• 2p is at higher energy
• DO NOT ALLOW 'shell'
• DO NOT ALLOW unpaired electron argument
• IGNORE distance and shielding

Mark scheme

Examiner's comment

Many candidates did not answer the question and instead described what a buffer was. Very few candidates correctly explained that a weak acid was being added to a base, sometimes mentioning the formation of the salt or conjugate base. The majority also did not include the importance of there being excess acid, or some acid remaining, after the partial neutralisation.

Notes from the mark scheme and report

• acid in excess/partially neutralised AND glycolate ions (conjugate base/salt) formed

Mark scheme

Examiner's comment

Some candidates coped well with this question which was based on the AS part of the specification and gained both marks. More candidates gained 1 mark through writing one half equation, usually the oxidation of magnesium. Common errors were for chlorine to featuring in the reduction half equation and the lack of electrons in their answers. Very few candidates mixed up the oxidation and reduction equations.

Notes from the mark scheme and report

• Mg → Mg2+ + 2e–
• 2H+ + 2e– → H2
• only species that change
• 1 mark if both correct but labelled the wrong way round

Mark scheme

Examiner's comment

This proved a difficult question for most candidates. Most were able to correctly calculate the moles of fertiliser by converting kg to g. The next step was to deduce that 5 moles of calcium carbonate would be required for each mole of Z and multiply by 5, rather than the common error of dividing by 5. Few candidates were able to multiply by 100/95, to account for the impurities in limestone, with many multiplying by 95/100.

Notes from the mark scheme and report

• n(Z) = 1500/1080.5
• × 5 for CaCO3
• × 100.1 (DO NOT ALLOW 100)
• ÷ 0.950 = 731 g
• common errors 146 (no × 5), 660 (× 0.95), 29.3 (÷5) score 2

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures

Mark scheme

Examiner's comment

This was another very challenging question using an unfamiliar mineral. Most candidates identified a formula of salts containing both magnesium and calcium, or carbonates of the separate elements. Only the most successful candidates were able to give the correct formula. Common errors, for those who solved the formulae, were the use of “4”HCl in balancing and the absence of state symbols.

Notes from the mark scheme and report

• Mg3Ca(CO3)4(s) + 8HCl(aq) → 3MgCl2(aq) + CaCl2(aq) + 4H2O(l) + 4CO2(g)
• balancing/state symbols mark dependent on correct formulae

Mark scheme, with the levels and indicative points

Examiner's comment

Answers were distributed across all 3 levels of achievement. Most of the candidates managed to identify at least some of the substances. Of the equations, the reaction of copper (II) oxide with nitric acid was most regularly seen correct, although many students could also represent the ligand replacement in Experiment 1. Many candidates were able to calculate Mr for gas D but some of those suggesting SO2 as a possible formula preferred to have an equation in experiment 2 producing hydrogen. A few candidates used the Mr to suggest that the gas was 2O2 and as such candidates found the equation between copper and sulphuric acid challenging. A good number of candidates identified F and G, recognising what they had learned from their work on redox titrations, and some were able to reproduce the equation. Incorrect formula of copper (I) iodide (CuI2) was a common error. Many candidates made no attempt at identifying changes in oxidation states. Candidates are advised to address all parts of the question in order to access the higher levels and to allow sufficient time to attempt the LoR questions. The candidate has clearly set out the three experiments, written the equation and followed with a clear identification of each of the formulae B-G. The calculation is well laid out and the response clearly indicates how the final value has been calculated. Oxidation numbers have then been commented on where appropriate. The colours of the transition metal ions and complexes can be reviewed at: A Level Chemistry A - Colours of inorganic ions and complexes (ocr.org.uk) If any of your students’ results are not as expected, you may wish to consider one of our post-results services. For full information about the options available visit the OCR website. We send a weekly roundup to tell you about important updates. You can also sign up for your subject specific updates. If you haven’t already, sign up here. Attend one of our popular CPD courses to hear directly from a senior assessor or drop in to a Q&A session. Most of our courses are delivered live via an online platform, so you can attend from any location. Please find details for all our courses on the relevant subject page on our website or visit OCR professional development. ExamBuilder is the question builder platform for a range of our GCSE, A Level, Cambridge Nationals and Cambridge Technicals qualifications. Find out more. ExamBuilder is free for all OCR centres with an Interchange account and gives you unlimited users per centre. We need an Interchange username to validate the identity of your centre's first user account for ExamBuilder. If you do not have an Interchange account please contact your centre administrator (usually the Exams Officer) to request a username, or nominate an existing Interchange user in your department. Review students' exam performance with our free online results analysis tool. It is available for all GCSEs, AS and A Levels and Cambridge Nationals. It allows you to: • review and run analysis reports on exam performance • analyse results at question and/or topic level • compare your centre with OCR national averages • identify trends across the centre • facilitate effective planning and delivery of courses • identify areas of the curriculum where students excel or struggle • help pinpoint strengths and weaknesses of students and teaching departments. Find out more. Post-results services Keep up-to-date OCR Professional Development Signed up for ExamBuilder?

Notes from the mark scheme and report

• B = [CuCl4]2–
• D: n = 45/24000, M = 64 so SO2, C = CuSO4 with Cu 0 to +2 and S +6 to +4
• E = Cu(NO3)2
• F = CuI (white) and G = I2 with Cu +2 to +1 and I –1 to 0
• Level 3 needs Mr calculation, most equations and some oxidation number changes
• The AS delivery guide ‘Theme: Identifying Unknowns’ https://www.ocr.org.uk/Images/208563-identifying-unknowns.pdf ‘Transition elements’: Delivery Guide for OCR AS/A Level Chemistry A

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Answer

C

Mark scheme

Examiner's comment

Candidates found this question challenging with the correct answer being C. A or B were the common errors selected by most candidates.

Answer

A

Mark scheme

Examiner's comment

The correct answer was A. This question proved to be challenging, with the common incorrect answer being C.

Answer

D

Mark scheme

Examiner's comment

This question was another challenging idea. The correct answer was D. Many candidates chose A or C, possibly due to fluorine’s high electronegativity.

Mark scheme

Examiner's comment

Many candidates calculated the correct value of q and scored all three marks. Common errors saw some candidates using the incorrect mass, of either 0.133 or that added to or subtracted from the 150. The temperature change was given but a few candidates added the 10.5 to 273 in order to, incorrectly, convert to K. The final mark required the candidate to link the temperature increase to an exothermic value and include a minus sign.

Notes from the mark scheme and report

• q = 150 × 4.18 × 10.5
• n = 0.133/100
• – sign required
• common errors for 2 marks: +4950, –5077/–5064 (use of 2 SF)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

This calculation proved difficult. Many candidates did not deduce the correct cycle. Of those who did, a significant majority did not appreciate the stoichiometry and/or use the correct signs from the cycle. Many candidates scored 1 mark for this question, which was gained as an error carried forward, in the enthalpy cycle. -3163 was a common error. This candidate showed clarity in their working by drawing a correct Hess cycle and then applying correct multiples and directions for the enthalpy changes.

Notes from the mark scheme and report

• 186 = ∆cH(C9H20) – ∆cH(C5H12) – 2∆cH(C2H4)
• common errors for 1 mark: +3535, –4946 (1 × –1411), –3163 (+186)

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Candidates tend to find Kp calculations difficult and so a strategy to work their way through them could include: • write the Kp expression, with units, ensuring square brackets are not used. Common mistakes with units included atm-1 mol-1, mol-1 dm3, kPa-1 • calculation of initial moles present, with careful consideration of the use of appropriate significant figures • calculation of the change in moles present • deduction of the number of moles present at equilibrium • determination of total moles present at equilibrium. These steps are often best completed as RICE tables (Ratio, Initial, Change, Equilibrium) and should look to use the appropriate amount of significant figures: • calculation of mole fractions at equilibrium • calculation of partial pressures at equilibrium • inserting partial pressure vales into the Kp expression and avoiding any unnecessary unit conversions • writing an answer to the required number of significant figures.

Notes from the mark scheme and report

• DO NOT ALLOW square brackets in Kp
• n(N2O4) = 0.300 and total 5.70
• units atm–1
• 3 SF (1.17 × 10–2)
• common error 2.47 × 10–2 (0.6 mol N2O4) scores 2 calculation marks

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

ocr.org.uk/science Oxford Cambridge and RSA CHEMISTRY A H432 For first teaching in 2015 A LEVEL Version 1 H432/01 Summer 2023 series Examiners’ report Contents Introduction .......................................................................................................................................... 4 Paper 1 series overview........................................................................................................................ 5 Section A overview ............................................................................................................................... 6 Question 1 .................................................................................................................................... 6 Question 2 ....................................................................................................................................... 6 Question 3 ....................................................................................................................................... 7 Question 4 ....................................................................................................................................... 7 Question 5 ....................................................................................................................................... 8 Question 6 ....................................................................................................................................... 8 Question 7 ....................................................................................................................................... 9 Question 8 ....................................................................................................................................... 9 Question 9 ..................................................................................................................................... 10 Question 10 ................................................................................................................................... 10 Question 11 ................................................................................................................................... 11 Question 12 ................................................................................................................................... 11 Question 13 ................................................................................................................................... 12 Question 14 ................................................................................................................................... 12 Question 15 ................................................................................................................................... 13 Section B overview ............................................................................................................................. 14 Question 16 (a) (i) ........................................................................................................................... 14 Question 16 (a) (ii) ......................................................................................................................... 15 Question 16 (b) .............................................................................................................................. 16 Question 17* .................................................................................................................................. 17 Question 18 (a) ............................................................................................................................... 18 Question 18 (b) .............................................................................................................................. 19 Question 18 (c) (i) .......................................................................................................................... 20 Question 18 (c) (ii) .......................................................................................................................... 21 Question 19 (a) (i) .......................................................................................................................... 23 Question 19 (a) (ii) ......................................................................................................................... 24 Question 19 (b) .............................................................................................................................. 24 Question 20 .................................................................................................................................... 25 Question 20 (a) (ii) ......................................................................................................................... 26 Question 20 (b) .............................................................................................................................. 26 Question 20 (c) (i) .......................................................................................................................... 27 Question 20 (c) (ii) .......................................................................................................................... 27 Question 20 (d) (i) .......................................................................................................................... 28 Question 20 (d) (ii) ......................................................................................................................... 28 Question 20 (e) (i) .......................................................................................................................... 29 Question 20 (e) (ii) ......................................................................................................................... 29 Question 20 (f) ............................................................................................................................... 30 Question 21 (a) (i) ........................................................................................................................... 31 Question 21 (a) (ii) ......................................................................................................................... 31 Question 21 (iii) .............................................................................................................................. 32 Question 21 (b) .............................................................................................................................. 33 Question 22 (a)* .............................................................................................................................. 34 Question 22 (b) (i) .......................................................................................................................... 36 Question 22 (b) (ii) ......................................................................................................................... 36 Question 22 (c) .............................................................................................................................. 37 Question 22 (d) .............................................................................................................................. 37 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 20 (a) (ii) This question proved difficult with only a few candidates able to state that in neutral water, [H+] = [OH-].

Notes from the mark scheme and report

• pH = 6.77 (2 DP)
• DO NOT ALLOW use of A– or X–

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

This question proved difficult with only a few candidates able to state that in neutral water, [H+] = [OH-]. Many candidates said that as the pH is close to 7, water is therefore neutral.

Notes from the mark scheme and report

• DO NOT ALLOW 'ratio [H+]:[OH–] doesn't change'

Mark scheme

Examiner's comment

Only a few candidates used the mass value given in the question to link the number of moles of the group 2 metal carbonate and the number of moles, and hence volume, of gas produced.

Notes from the mark scheme and report

• Mr of SrCO3 greater so fewer moles in 1.00 g
• more moles of CaCO3 gives more gas
• Many candidates answered this question in terms of the relative reactivity, or solubility of Ca and Sr and then continuing by explaining their respective ionisation energies.

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Ionic equations still present candidates with a challenge. A few candidates scored the mark but many candidates gave a full equation or one that contained a mismatch of spectator ions as well as the correct ions.

Notes from the mark scheme and report

• Mg + 2H+ → Mg2+ + H2
• ALLOW multiples

Mark scheme

Examiner's comment

This question proved challenging for the candidates to identify the three ideas: Those of comparing acids, comparing moles and comparing rates. Very few candidates were able to score the 3 marks. Most candidates recognised the different strength of the two acids, but some only used comparative language. Some linked the moles of acid used to the volume of gas produced but many simply restated the same volume and concentration which is given within the question. Only a few candidates linked the higher initial [H+] in HCl to the increased rate through more frequent collisions. A common issue was describing the rate of dissociation rather than the [H+] present in determining the rate of the reactions or mentioning that it dissociates more but not linking this to the H+ concentration.

Notes from the mark scheme and report

• HCl completely dissociates AND CH3COOH partially
• greater [H+] in HCl so more frequent collisions
• same total moles of H+ released as CH3COOH continues to dissociate
• IGNORE 'HCl is a stronger acid'

Mark scheme

Examiner's comment

Very few candidates wrote the complete definition of a monobasic acid. Most wrote “donates one proton” only, omitting mole or molecule. Some candidates described donating electrons or OH-.

Notes from the mark scheme and report

• one mole of acid donates one mole of H+ (or one H+ per molecule)

Mark scheme

Examiner's comment

This calculation proved difficult with many figures and sums appearing with little indication as to their relevance. Candidates should remember to provide written indications of what it is they’re working out – presenting the calculations without any annotations can make it harder for error carried forward marks to be given if there is an error in their calculation. Few candidates scored all 4 marks. Most found the concentration of H+ from the pH and the moles of KOH correctly but did not recognise they had to take away the moles of KOH from those of HA to find the remaining concentration of HA. Some candidates then used the [H+] as the [HA]. A few candidates tried a [H+] squared expression of a weak acid. The candidate has clearly set out the calculation so each numerical value can be linked and the steps understood. The only error is not calculating the excess acid (i.e. not subtracting moles OH- from initial moles acid) so 3 marks were given.

Notes from the mark scheme and report

• n(A–) = n(KOH) = 0.0604
• n(HA) remaining = 0.094 – 0.0604
• Ka = [H+][A–]/[HA] = 1.53 × 10–5
• common error 5.47 × 10–6 (KOH not subtracted from HA) scores 3

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

ocr.org.uk/science Oxford Cambridge and RSA CHEMISTRY A H432 For first teaching in 2015 A LEVEL Version 1 H432/01 Summer 2023 series Examiners’ report Contents Introduction .......................................................................................................................................... 4 Paper 1 series overview........................................................................................................................ 5 Section A overview ............................................................................................................................... 6 Question 1 .................................................................................................................................... 6 Question 2 ....................................................................................................................................... 6 Question 3 ....................................................................................................................................... 7 Question 4 ....................................................................................................................................... 7 Question 5 ....................................................................................................................................... 8 Question 6 ....................................................................................................................................... 8 Question 7 ....................................................................................................................................... 9 Question 8 ....................................................................................................................................... 9 Question 9 ..................................................................................................................................... 10 Question 10 ................................................................................................................................... 10 Question 11 ................................................................................................................................... 11 Question 12 ................................................................................................................................... 11 Question 13 ................................................................................................................................... 12 Question 14 ................................................................................................................................... 12 Question 15 ................................................................................................................................... 13 Section B overview ............................................................................................................................. 14 Question 16 (a) (i) ........................................................................................................................... 14 Question 16 (a) (ii) ......................................................................................................................... 15 Question 16 (b) .............................................................................................................................. 16 Question 17* .................................................................................................................................. 17 Question 18 (a) ............................................................................................................................... 18 Question 18 (b) .............................................................................................................................. 19 Question 18 (c) (i) .......................................................................................................................... 20 Question 18 (c) (ii) .......................................................................................................................... 21 Question 19 (a) (i) .......................................................................................................................... 23 Question 19 (a) (ii) ......................................................................................................................... 24 Question 19 (b) .............................................................................................................................. 24 Question 20 .................................................................................................................................... 25 Question 20 (a) (ii) ......................................................................................................................... 26 Question 20 (b) .............................................................................................................................. 26 Question 20 (c) (i) .......................................................................................................................... 27 Question 20 (c) (ii) .......................................................................................................................... 27 Question 20 (d) (i) .......................................................................................................................... 28 Question 20 (d) (ii) ......................................................................................................................... 28 Question 20 (e) (i) .......................................................................................................................... 29 Question 20 (e) (ii) ......................................................................................................................... 29 Question 20 (f) ............................................................................................................................... 30 Question 21 (a) (i) ........................................................................................................................... 31 Question 21 (a) (ii) ......................................................................................................................... 31 Question 21 (iii) .............................................................................................................................. 32 Question 21 (b) .............................................................................................................................. 33 Question 22 (a)* .............................................................................................................................. 34 Question 22 (b) (i) .......................................................................................................................... 36 Question 22 (b) (ii) ......................................................................................................................... 36 Question 22 (c) .............................................................................................................................. 37 Question 22 (d) .............................................................................................................................. 37 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Candidates who did well on this paper generally: Candidates who did less well on this paper generally: • produced clearly structured working for calculations – Questions 18 (a),18 (b), 19 (a) (i),19 (b), 20 (b), 20 (e) (ii) and 21 (c) • applied equations to a graphical representation Question 18 (c) (ii)) • produced clear and concise responses for the two Level of Response Questions 17 (a) and 22 (a) • produced clear and concise responses to descriptions in trends of ionisation energy Question 16 (b) • were able to make and justify predictions based on their knowledge Questions 20 (c), (d) • explained equilibrium changes Question 19 (a) (ii) • gave answers to the correct number of significant figures Questions 19 (a) (i), 20 (a) (i) and 20 (b) • converted between units when required Questions 18 (c) (ii) and 19 (b) • correctly explained experimental procedures using electrode potentials Question 21 (b) • wrote a balanced equation for unfamiliar compound Question 22 (d) • understood the effect of addition of chemical substances on the pH of a buffer solution Question 20 (f). • found it difficult to apply what they had learned to unfamiliar situations • produced unstructured responses to Level of Response questions which were lacking in depth or explanation or contained inaccurate definitions Questions 17 (a) and 22 (a) • did not clearly set out calculations, making it difficult for marks to be given for working Questions 18 (a),18 (b), 19 (a) (i), 19 (b), 20 (b), 20 (e) (ii) and 21 (c) • did not give answers to calculations to the specified number of significant figures Questions 19 (a) (i), 20 (a) (i) and 20 (b) • either did not realise the need to convert between units for some calculations or found this difficult Questions 18 (c) (ii) and 19 (b) • did not produced clear and concise responses to descriptions in trends of ionisation energy Question 16 (b) • did not write a balanced equation, and ionic equations, for reactions with group 2 metal and metal compounds Questions 20 (b), 20 (c) (i) and 20 (d). Question 20 (f) Misconception Only the most successful candidates linked the pH not changing to the ratio of [HA] and [A-] not changing on addition of water.

Notes from the mark scheme and report

• ratio [HA]/[A–] stays the same on dilution
• Only the most successful candidates linked the pH not changing to the ratio of [HA] and [A-] not changing on addition of water. The majority of candidates described the buffer being able to minimise pH change on addition of small amounts of acid or base or that water being neutral would not affect the pH.

Mark scheme

Examiner's comment

Only a few candidates were given this mark. Some had the colour change inverted and most stated a variety of different colours.

Notes from the mark scheme and report

• colourless to (pale) pink
• DO NOT ALLOW purple

Skills: PAG2 Acid–base titration

Mark scheme, with the levels and indicative points

Examiner's comment

Only the higher-attaining candidates scored full marks. Very few candidates were able to define d-block element correctly without the minor slip of saying outermost electron instead of highest energy or valence electron. Many candidates often did not mention ions for the transition metal definition. Many did not include any of the electron arrangements for d-block elements and transition elements. The majority of candidates who were able to recognise zinc and scandium as d-block elements, but not transition elements, gave their electronic configurations correctly. Common errors included the presence of the 4s electrons in the electron configurations of the ions and incorrect electron configurations of copper and chromium atoms. A few candidates thought chromium and copper were not transition elements due to the 4s1 electron configuration. This candidate has mentioned outer electrons rather than highest energy but was this is a minor slip and they were still given a Level 3 response as everything else is correct. A holistic approach for LoRs is used and not a point-based marking system.

Notes from the mark scheme and report

• d-block: highest energy electron in d sub-shell (DO NOT ALLOW d shell/d block)
• transition element forms an ion with incomplete d sub-shell
• Sc3+ (empty 3d) and Zn2+ (full 3d10) with configurations

Mark scheme

Examiner's comment

Most candidates identified the charge as -1 and a coordination number of 6. A few candidates identified the coordination number as 4 as they did not spot that ethanedioate is bidentate and charge of +3 stated.

Notes from the mark scheme and report

• charge 1–
• coordination number 6 (bidentate ligands count twice)
• DO NOT ALLOW Co–

Mark scheme

Examiner's comment

Very few candidates were able to produce the balanced overall equation; a few had the correct reactants and products but not balanced. Candidates are advised to look for the information contained within the question. The formulas were given, and it was stated that the solution was acidified, leaving only water to be identified. Some candidates approached this through two half equations whereas others used oxidation numbers to balance their equations.

Notes from the mark scheme and report

• 3V3+ + Cr2O7 2– + 2H+ → 3VO2+ + 2Cr3+ + H2O
• first mark for all species
• DO NOT ALLOW electrons in final answer

Skills: M0.2 Use ratios, fractions and percentages

Answer

D

Mark scheme

Examiner's comment

The correct answer was D. Wrong answers were very rarely seen as candidates had secure knowledge of filling orbitals individually before pairing.

Answer

D

Mark scheme

Examiner's comment

The correct answer was D. This proved a more challenging question. Successful candidates often presented oxidation numbers above the equations to identify the element that was simultaneously oxidised and reduced. Most candidates recognised that A and B could be ruled out, with C being the most common error.

Answer

B

Mark scheme

Examiner's comment

The correct answer was B. Candidates generally find the use and manipulation of the Arrhenius equation challenging. Focus was need on the unit conversion when using R (Jmol-1K-1) for the gradient and activation energy (kJ mol-1). This led to a few candidates selecting D. It is worth reminding candidates that the two forms of this equation are given on the data sheet.

Answer

D

Mark scheme

Examiner's comment

The correct answer was D. Candidates performed well on this question with incorrect answers rarely seen. A minority of candidates confused rate-concentration graphs with concentration-time graphs.

Answer

A

Mark scheme

Examiner's comment

The correct answer was A. Formation of a buffer solution can either be from either: • a weak acid and a salt of the weak acid, e.g. CH3COOH/CH3COONa • an excess of a weak acid and a strong alkali, e.g. excess CH3COOH/NaOH A few candidates suggested the other alternatives in equal measure.

Answer

D

Mark scheme

Examiner's comment

The correct answer was D. Many candidates were able to identify the green precipitate as Fe(OH)2 and the white precipitate as BaSO4. A few candidates suggested C, identifying BaCl2 as the white precipitate, or B, identifying Cu(OH)2 as the green precipitate.

Mark scheme

Examiner's comment

This question proved more difficult for candidates with lots of inaccuracies. The profile was dependent on the calculation for ΔH in Question 16 (a) (i). The arrowhead for ΔH needs to be pointing from the reactants to the products. The activation energies, again, need to start at the reactant line and go to the maximum level of the curve. Those that needed to draw an endothermic profile were far more likely to make an error with the Ea and Ec arrows, often starting from the product line or even from the base line of the graph. A significant number of candidates did not add arrows and instead labelled the curves Ea and Ec. Some candidates drew a Boltzmann distribution curve scoring 0 marks. The candidate has the correct exothermic profile but has the incorrect starting point for the activation energy going from the product line.

Notes from the mark scheme and report

• profile must match the sign of ∆H from (a)(i) (ECF allowed)
• Ea and Ec lines both start at reactant level

Mark scheme

Examiner's comment

This question proved more challenging for candidates. Candidates stating +4 was the most common error; this is the oxidation state in MnO2. Some candidates stated fractions, negative values and gave the state symbol instead i.e. solid and liquid.

Notes from the mark scheme and report

• + sign required
• DO NOT ALLOW Mn2+/Mn3+ (ions, not oxidation states)
• DO NOT ALLOW +4

Mark scheme

Examiner's comment

This question proved difficult for candidates to explain well. Collision theory linked to rate requires a quantitative approach, e.g. less particles per unit volume and less collisions per unit time. A lot of candidates wrote vague responses about fewer particles so less collisions and did not gain credit.

Notes from the mark scheme and report

• 'fewer collisions' needs 'per unit time' / less frequent
• IGNORE 'fewer reacting particles'
• Some candidates described a decrease in energy in the reaction or gave responses related to activation energy for the concentration effect. For example, ‘reactants have less kinetic energy’ and ‘only a certain number of particles which have enough energy above the activation energy…over time energy will decrease and less particles have enough energy’.
• We have produced a delivery guide on rates with some useful resources to help consolidate ideas and avoid misconceptions such as these: Teach Cambridge (ocr.org.uk)

Mark scheme

Examiner's comment

Candidates tend to find Kp calculations difficult and so a strategy to work their way through them could include: • Write the Kp expression using the molar ratio given in the question. Care should be taken not to change the molar ratio to help an easier calculation. Square brackets should not be used as these represent concentration. • Calculation of initial moles present, with careful consideration of the use of appropriate significant figures • Calculation of the change in moles present • Deduction of the number of moles present at equilibrium • Determination of total moles present at equilibrium These steps are often best completed as RICE tables (Ratio, Initial, Change, Equilibrium) and should look to use the appropriate amount of significant figures to avoid having a rounding error in the final answer.

Notes from the mark scheme and report

• equilibrium moles via ½ stoichiometry for O2
• mole fractions × total pressure
• units atm½
• Kp values are for the equation as stated. Candidates should recognise that changing the stoichiometry of the equation changes the Kp value.

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Candidates performed well with this question and many stated that Kp would increase. Some identified the forward reaction as endothermic but did not link this to equilibrium being shifted to the right, thus increasing the ratio within the Kp expression. A few candidates sought to incorrectly explain the effect by using Le Chatelier effect on pressure. Candidates found this question demanding. Many candidates explained the formation of a polar bond by describing the difference in electronegativity between S and O. Many answers had descriptions about symmetry but explained polarity in terms of polar bonds/charges cancelling instead of dipoles. A symmetric molecule (all bonds identical and no lone pairs) will not be polar even if individual bonds within the molecular are polar. The individual dipoles on the bonds ‘cancel out’ due to the symmetrical shape of the molecule. There is no net dipole moment: the molecule is non-polar

Notes from the mark scheme and report

• ORA
• must link endothermic to Kp increase with T
• DO NOT ALLOW 'T1 has greater Kp'

Mark scheme

Examiner's comment

This proved a challenging question where candidates did not stick to the principle that Kp (or Ka) values only change due to temperature changes. Only a few candidates scored both marks with many having the Kp value changing due to increased pressure.

Notes from the mark scheme and report

• catalyst: no change
• pressure: no change (Kp constant)

Mark scheme

Examiner's comment

Another challenging question, requiring the identification of 3 double bonds (and no lone pair). The most common error was stating that SO2 has 3 bonding pairs – it doesn’t, it has 6 bonding pairs which are arranged as 3 double bonds. They could also state that it has 3 bonding regions.

Notes from the mark scheme and report

• 3 bonding regions/areas of electron density
• DO NOT ALLOW 'bonding pairs'

Mark scheme

Examiner's comment

ocr.org.uk/science Oxford Cambridge and RSA CHEMISTRY A H432 For first teaching in 2015 A LEVEL Version 1 H432/01 Summer 2024 series Examiners’ report Contents Introduction .............................................................................................................................................. 4 Paper 1 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 1 ............................................................................................................................................ 6 Question 2 ............................................................................................................................................ 7 Question 3 ............................................................................................................................................ 7 Question 4 ............................................................................................................................................ 8 Question 5 ............................................................................................................................................ 8 Question 6 ............................................................................................................................................ 9 Question 7 ............................................................................................................................................ 9 Question 8 .......................................................................................................................................... 10 Question 9 .......................................................................................................................................... 10 Question 10 ........................................................................................................................................ 11 Question 11 ........................................................................................................................................ 12 Question 12 ........................................................................................................................................ 13 Question 13 ........................................................................................................................................ 13 Question 14 ........................................................................................................................................ 14 Question 15 ........................................................................................................................................ 15 Section B overview ................................................................................................................................. 16 Question 16 (a) (i) ............................................................................................................................... 16 Question 16 (a) (ii) .............................................................................................................................. 17 Question 16 (b) (i) ............................................................................................................................... 18 Question 16 (b) (ii) .............................................................................................................................. 19 Question 16 (b) (iii) ............................................................................................................................. 20 Question 16 (c) (i) ............................................................................................................................... 20 Question 16 (c) (ii) .............................................................................................................................. 21 Question 16 (c) (iii) .............................................................................................................................. 22 Question 17 (a) ................................................................................................................................... 23 Question 17 (b)* .................................................................................................................................. 24 Question 18 (a) (i) ............................................................................................................................... 26 Question 18 (a) (ii) .............................................................................................................................. 27 Question 18 (a) (iii) ............................................................................................................................. 27 Question 18 (b) (i) ............................................................................................................................... 28 Question 18 (a) (ii) .............................................................................................................................. 28 Question 19 (a) (i) ............................................................................................................................... 29 Question 19 (a) (ii) .............................................................................................................................. 29 Question 19 (a) (iii) ............................................................................................................................. 30 Question 19 (b) (i) ............................................................................................................................... 31 Question 19 (b) (ii) .............................................................................................................................. 34 Question 20 (a) (i) ............................................................................................................................... 35 Question 20 (a) (ii) .............................................................................................................................. 35 Question 20 (b) ................................................................................................................................... 36 Question 20 (c)* .................................................................................................................................. 37 Question 21 (a) ................................................................................................................................... 38 Question 21 (b) ................................................................................................................................... 40 Question 21 (c) ................................................................................................................................... 41 Question 21 (d) (i) ............................................................................................................................... 42 Question 21 (d) (ii) .............................................................................................................................. 43 Question 22 (a) ................................................................................................................................... 44 Question 22 (b) (i) ............................................................................................................................... 45 Question 22 (b) (ii) .............................................................................................................................. 45 Question 22 (c) ................................................................................................................................... 46 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Candidates who did well on this paper generally: Candidates who did less well on this paper generally: • produced clearly structured working for calculations – Questions 16 (a),16 (c) (ii), 18 (a), 19 (a) (iii), 19 (b) (i), 21 (b), 22 (c) • produced clear and concise responses for the two Level of Response questions – Questions 17 (b) and 20 (c) • produced clear and concise responses to describe a polar molecule – Question 18 (b) (ii) • deduced the rate determining step and rate equation – Question 20 (b) • correctly used electrode potentials – Question 21 (d) (ii) • wrote a formula for an unfamiliar complex ion – Question 22 (b) (ii) • deduced the oxidation numbers of an unfamiliar compound • drew an enthalpy profile for an exothermic or endothermic reaction – Question 16 (b) (i) • drew a labelled diagram to measure a standard electrode potential. • found it difficult to apply what they had learned to unfamiliar situations • produced unstructured responses to Level of Response questions which were lacking in depth or used incorrect terminology – Questions 17 (b) and 20 (c) • did not clearly set out calculations, making it difficult for marks to be given for working – Questions 16 (a), 16 (c) (ii), 18 (a), 19 (a) (iii), 19 (b) (i), 21 (b), 22 (c) • did not give answers to calculations to the specified number of significant figures – Questions 18 (a) (i), 19 (a) (iii), 21 (b) • did not appropriately convert between units for calculations – Question 21 (b) • found it difficult to write a balanced equation, or ionic equations, for reactions with group 2 metal and metal compounds or group 7 redox reactions – Questions 20 (a) (ii), 21 (a) • drew an inaccurate enthalpy profile for an exothermic or endothermic reaction – Question 16 (b) (i) • drew an inaccurate or incomplete labelled diagram to measure a standard electrode potential. Question 18 (b) (i) Another challenging question, requiring the identification of 3 double bonds (and no lone pair).

Notes from the mark scheme and report

• electronegativity difference S/O
• SO2 non-linear so dipoles do not cancel, SO3 symmetrical

Mark scheme

Examiner's comment

Candidates found this question more challenging. Many candidates deduced Step 1 and then went on to state the rate equation. A few candidates incorrectly deduced F rather than F2 and/or obtained a second order for NO2

Notes from the mark scheme and report

• Step 1 = overall minus Step 2
• rate equation from slow step must include k

Mark scheme, with the levels and indicative points

Examiner's comment

Structure and bonding continue to be a difficult concept for many candidates. High-attaining candidates were able to identify why the element had a certain magnitude of melting point. They clearly linked the structure type with the type of bonding. They then described, in detail, the nature of the bond. The strength of force required to break/overcome the bond/London Force was linked to the melting point. It was very common for ‘giant’ to be omitted in the name of the lattice, especially in Al. Candidates find it particularly challenging to associate the correct terminology with the correct structure, often describing intermolecular forces in giant covalent explanations or use of molecules in giant metallic explanations. London forces were mentioned widely but sometimes not described as being forces between molecules and not linked to the increased number of electrons. A holistic, rather than a point based, approach is used in marking these responses. This allowed Level 2 to be given when the candidate did not use all of the correct terminology throughout the three structure types. Several candidates described the varying melting point going across the period as being due to atoms having more electrons in the outer shell and a greater nuclear charge.

Notes from the mark scheme and report

• giant metallic (Al), giant covalent (Si), simple molecular P4/S8 with London forces
• S8 > P4 because more electrons
• Our bonding delivery guide provides details of common misconceptions students hold relating to this topic, and also includes resources and guidance that can help overcome them: Teach Cambridge (ocr.org.uk)

Mark scheme

Examiner's comment

Successful candidates drew a complete circuit and voltmeter with the labelled salt bridge dipped into the two solutions. Both cells had Pt as the electrodes. One cell contained 1 moldm-3 Fe2+ and Fe3+ where the other had a delivery mechanism for H2 (at 1 atm) and H+ (at 1 mol dm-3). Standard temperature of 298K (or 25oC) was stated. Common errors included: not have solutions in the beakers, used Fe or Fe2+ electrodes, the hydrogen cell was missing and/or the H2 with a device for adding it. A few candidates suggested two cells with only Fe2+ one side and only Fe3+ on the other.

Notes from the mark scheme and report

• Pt electrode in Fe2+/Fe3+ half-cell
• standard hydrogen electrode with H2 delivery and H+
• salt bridge and voltmeter
• 1 mol dm–3, 298 K, 1 atm/100 kPa

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

Most candidates identified the formula and the charge as 1+. A few candidates stated no charge or 3+. Candidates should consider the use of brackets in the formula, e.g. square brackets to show the complex and curly brackets to show the number of ligands attached. A few candidates used NH4 rather than NH3 for the ammonia ligand.

Notes from the mark scheme and report

• molar mass 197.9 fits [Co(NH3)4Cl2]
• overall charge +1

Skills: M0.2 Use ratios, fractions and percentages; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

The key chemistry that candidates needed to discuss in their response was as follows: • O2 molecules forming coordinate bonds with and Fe2+ ions in haemoglobin. Often candidates omitted the Fe2+ and just stated it was to haemoglobin • O2 molecules being replaced by another ligand (e.g. H2O or CO2) The calculation using the [HCO3–] : [H2CO3] ratio of 8.5 : 1 was well described, although sometimes the final expression of the ratio left ambiguity as it was hard to tell whether the ratio given referred to the [HCO3–] : [H2CO3] ratio or the [H2CO3] : [HCO3–] ratio. ECF was given for the [H+] and then the pH linked to whether the blood was healthy. A smaller number of candidates approached the question by calculating the ratio of [HCO3–] : [H2CO3] for both pH 7.35 and pH 7.45 and then compared both ratios to the ratio of 8.5 : 1 for healthy blood. A few candidates attempted the calculation by the weak acid approach using [H+]2. In this case only the [H+] was given.

Notes from the mark scheme and report

• O2 forms dative bond to Fe(II) reversibly, replaced by H2O/CO2
• pH = pKa + log(8.5) = 7.30, below 7.35 so not healthy

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Answer

C

Mark scheme

Examiner's comment

The correct answer was C. Incorrect answers were rarely seen as candidates had a secure knowledge of isotopes.

Answer

C

Mark scheme

Examiner's comment

The correct answer was C. Candidates generally were able to complete this calculation to obtain the value for x by finding the molar ratio. A few candidates selected A from an incorrect calculation of the number of moles.

Answer

B

Mark scheme

Examiner's comment

The correct answer was B. This question proved slightly more challenging, but a significant number of candidates selected this response. The most common incorrect answer was C due to, presumably, basing their reasoning on the stoichiometric equation rather than the mechanism.

Answer

C

Mark scheme

Examiner's comment

The correct answer was C. This question was slightly more challenging for some candidates who often selected B due to incorrectly observing where the electrons were placed.

Answer

B

Mark scheme

Examiner's comment

The correct answer was B. This was often the most challenging question for candidates. Redox and electrode potentials continue to be a higher-level concept. The terms reducing ‘agent’ and the species being reduced can often be confused. A good number of candidates did not reverse the more negative equilibrium system before they chose their answer.

Mark scheme

Examiner's comment

This question required the candidates to know the trend in melting points across period 3. This question proved difficult for candidates with few being given both marks. Many candidates did not recognise Mg as a metal with a higher melting point than the non-metals and continued the trend upwards above Si. The candidates may be aware that Ar and Cl2 are gases at room temperature, but they didn’t relate this into the concept of melting points. Some candidates plotted points lower than Mg in a straight line on a downward trajectory. Sulfur, with a higher melting point than phosphorus, appears to be not well-known. One misconception seemed to be confusing this with a graph for ionisation energy, maybe by using the first ionisation energy data given on the next page.

Notes from the mark scheme and report

• S higher than P, P higher than Cl, Cl higher than Ar
• all four points below the height of Mg

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

This question required the comparison of structure and bonding. The structure and bonding topic continues to be a difficult one for many candidates. Candidates who did well on this paper were able to identify why each of the elements had a certain magnitude of melting point. They clearly linked the structure type with the type of bonding. They then described the strength of force and gave a detailed description of the bond that was being broken. It was very common for ‘giant’ to be omitted in the name of the lattice for aluminium. Candidates find it particularly challenging to associate the correct terminology with the correct structure, often describing intermolecular forces in giant covalent explanations or use of molecules in giant metallic explanations. London forces were mentioned widely and described when comparing the strength of the bonding and this also appeared after a well written answer causing a CON for the last marking point. Less successful candidates gave an answer that appeared to confuse the melting point with ionisation energy and described number of electrons, shielding and nuclear attraction. The candidate correctly identified silicon as having a giant covalent structure but misidentified aluminium as having a simple molecular structure. This resulted in in incorrect bonding being compared for the final marking point.

Notes from the mark scheme and report

• Al giant metallic AND Si giant covalent
• covalent bonds in Si stronger/need more energy to break than metallic bonds
• CON if intermolecular forces compared
• ALLOW macromolecular for Si
• Our bonding delivery guide provides details of common misconceptions students hold relating to this topic, and also includes resources and guidance that can help overcome them: Teach Cambridge (ocr.org.uk)

Mark scheme

Examiner's comment

This question required the candidates to know the observations of the reactions of group 7 and to recognise a change in colour to show the evidence of a chemical reaction. This question proved difficult for candidates with few being given all three marks. Colours were not well-known with I2 in aqueous solution often described as purple. Other incorrect answers included no colours stated, effervescence and descriptions of the formation of the halide precipitates. Several candidates gave an equation instead of an observation. The PAGs – section 1.2.1 (d) mentions accurate use of terminology to describe change instead of just final colours. A good number of candidates were able to provide the correct ionic equation, although several incorrectly included Na in their equations or wrote half equations. Occasionally candidates wrote more than one incorrect equation, often forming Cl2.

Notes from the mark scheme and report

• NaI: orange to brown (DO NOT ALLOW purple)
• NaCl: stays orange/no colour change (IGNORE 'no reaction'
• DO NOT ALLOW 'turns orange')
• Br2 + 2I– → 2Br– + I2
• IGNORE half-equations
• We have produced a teacher and delivery guide to assist with learning about the reaction of group 7 elements and their compounds: Teach Cambridge (ocr.org.uk) Links to the legacy coursework tasks and PAG practice question sets can be found on Teach Cambridge.
• Exam hints for students can be found on the OCR website.

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Many candidates were given both marks with the correct answer of -25 kJ mol-1. The best responses showed clear working using a Hess’s law cycle. Common errors of +25 and not using the correct multiples were seen from a few candidates.

Notes from the mark scheme and report

• common errors (1 mark): +763 (no 3 × –394), +25 (sign), –247 (no 3 × –111), –2339 (added reactants)
• answer –25

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Many candidates were given both marks with a clear description of the difference in states and a good explanation describing the increase in disorder within a gaseous system. A few candidates linked the standard entropy to the increase in moles (from one mole of Fe2O3 to three moles of CO2), showing a misunderstanding between ‘entropy change of reaction’ and ‘standard entropy’. Occasionally the candidate did not state that Fe was a solid.

Notes from the mark scheme and report

• Fe2O3 is a solid and CO2 is a gas
• gas more disordered/energy more dispersed/more ways of arranging

Mark scheme

Examiner's comment

This question required the calculation of two partial pressures. This question proved to be a challenge for most students. Common errors were to divide the 5476 by 2 instead of using the square root. Many were given a mark for recognising that the partial pressures needed to be the same. A common error was 602 from the incorrect rearrangement of the Kp expression and some candidates used the value for R (8.31) rather than 8.13 in the Kp expression. Less successful candidates attempted, unsuccessfully, to use an ICE table to work out the moles of everything at equilibrium.

Notes from the mark scheme and report

• p(H2O) × p(CO) = 211 × 211 ÷ 8.13 = 5476
• p(H2O) = p(CO) so take square root = 74 kPa for both

Skills: M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme

Examiner's comment

Many candidates gave a good Boltzmann distribution graph, even though a few did not start at the origin, and some were horizontal on the far right-hand side, much above the 0 line. The quality of the Boltzmann distribution could be improved by the candidates, although marks were given. Occasionally candidates drew multiple lines, presumably addressing a change of temperature. This question proved difficult for some candidates to explain well. Collision theory linked to catalyst requires the link between number of particles having energy ≥ the activation energy. A lot of candidates wrote vague responses, often confusing or mixing temperature and concentration effects and did not gain credit. The candidate drew a curve clearly starting at 0,0 but then drew a horizontal line above 0 on the right- hand side. This line would never approach 0 and has a constant number of molecules with the increasing energy and so was not given credit.

Notes from the mark scheme and report

• curve starts at/near origin, does not touch x-axis at high energy and is not level
• DO NOT ALLOW multiple curves
• catalysed Ec marked to the left of Ea (lower activation energy) AND greater proportion of molecules with E ≥ Ea
• DO NOT ALLOW 'more frequent collisions' or 'molecules have more energy'
• We have produced a delivery guide on rates with some useful resources to help consolidate ideas and avoid misconceptions such as these: Teach Cambridge (ocr.org.uk)

Skills: M3.1 Translate information between graphical, numerical and algebraic forms

Mark scheme

Examiner's comment

Lots of candidates were given 5 marks. They were able to use the experimental evidence to deduce the orders, write a rate equation and calculate k. The best candidates listed their reasoning for the orders rather than writing in large paragraphs of prose which sometimes got confused and led to contradictions. A good proportion of candidates recognised that 2.25 was 1.52, although a few thought that the 2nd order with respect to NO was a result of ‘2.25 being approximately 2 times 1.5’. Less successful candidates assumed that if they couldn’t identify what the relationship was between the concentration and the rate, that meant it was zero order. Less successful candidates were also unsuccessful in working out the units. A few candidates omitted k in their rate equations or didn’t write a rate equation. Many candidates were given marks as ECF from incorrect orders.

Notes from the mark scheme and report

• rate = k[NO]2[O2]
• k = 13.3
• units dm6 mol–2 s–1
• ALLOW ECF from incorrect orders and rate equation

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Excellent responses were seen with either 11.35 or 11.4 given. A few candidates suggested 10.4 or 11.40. This could have been from completing Question 18 (c) (ii) first and then choosing the value for part (c) (i) based on their line of best fit (e.g. 10.40).

Notes from the mark scheme and report

• ln(8.51 × 10^4) = 11.4 / 11.35

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

Most candidates were successful in calculating pH. A few candidates gave the pH value to 1 or 3 decimal places, and few quoted 13.18 as if HNO3 were an alkali.

Notes from the mark scheme and report

• pH = –log(0.150) = 0.82
• 2 d.p. required

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

This question required that the candidate understood that pH was a function of both the dissociation and concentration of H+ for a weak acid. This did prove more challenging and often was not explained well. Less successful candidates often quoted a lower pH, explained that it didn’t fully dissociate (rather than partially dissociate) or didn’t link the number of H+ ions to a concentration.

Notes from the mark scheme and report

• higher pH AND nitrous acid partially dissociates
• lower concentration of H+
• DO NOT ALLOW 'nitric acid is a stronger acid' (given in question)
• IGNORE 'does not fully dissociate/dissociates less'

Mark scheme, with the levels and indicative points

Examiner's comment

This was our second Level of Response question which proved more challenging for candidates. Only a few candidates scored full marks. Candidates are reminded to structure their answers and make sure they answer all points in the question. Candidates often find pH of buffer solution calculations difficult to process. The most difficult part in this calculation was the concentration of the salt. The majority that calculated salt concentration were able to calculate the mass. We should encourage the candidates to always write the units when calculating any quantity as concentration and moles were often confused within the calculation. Some candidates were given Level 2 for converting what they thought was the salt concentration into a value for the mass of salt needed, using a correct method. Less successful candidates calculated the correct [H+] but then used a weak acid calculation approach. A few responses attempted to use the Henderson-Hasselbalch approach, but most were unsuccessful due to the mathematical inaccuracies. The buffer action description proved to be a more challenging part to the question. Some candidates wrote a clear and concise description, linked to a written equilibrium equation, of the reaction of an alkali and the resulting equilibrium shift. Level 2 candidates often described an equilibrium shift of a non- existent equilibrium equation. Level 1 candidates were often only able to calculate [H+] and tried to explain the action of a buffer.

Notes from the mark scheme and report

• n = 0.168 × 0.125 = 0.0210 mol
• mass = 0.0210 × 96.0 = 2.02 g (2.00–2.04)
• added OH– reacts with H+/CH3CH2COOH and equilibrium CH3CH2COOH ⇌ H+ + CH3CH2COO– shifts right restoring [H+]
• use full formulae rather than HA/A–

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

The term ‘monodentate ligand’ was well-known by most candidates. A few candidates didn’t describe the electron pair being donated or forming a coordinate bond. Both these parts were required to be given the mark.

Notes from the mark scheme and report

• donates ONE electron pair AND forms ONE coordinate (dative covalent) bond to a metal ion

Mark scheme

Examiner's comment

This question required the candidates to identify the isomerism in an unfamiliar complex. This question was one of the more demanding ideas on the paper. A few candidates successfully drew the cis isomer and identified the type as cis/trans isomerism. Many candidates suggested, incorrectly, that the isomerisation was optical. A few did draw the cis complex ion but then identified the isomerism as optical.

Notes from the mark scheme and report

• draw the cis isomer (like groups on the same side of Cu)
• cis–trans (ALLOW geometric or E/Z)
• IGNORE charge and brackets

Skills: M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects; M4.3 Understand the symmetry of 2-D and 3-D shapes

Mark scheme

Examiner's comment

Candidates found this question quite challenging, with only a few giving a correct explanation. A common incorrect theme among less successful candidates was to consider that since it was a titration, then it must be a neutralisation reaction. Common errors involved the suggesting that evaporation of the water concentrating the copper solution, a bigger RFM, and confusion between SO42– and S2O32–. A fair number of candidates did not attempt this question.

Notes from the mark scheme and report

• Fe3+ oxidises I– to form (extra) iodine
• ALLOW Fe3+ is reduced by/reacts with thiosulfate

Mark scheme

Examiner's comment

Many candidates got the second ionisation energy equation. Very few candidates got the correct state symbol on the lower line for Br2, with solid being the common response.

Notes from the mark scheme and report

• Mg2+(g) + 2Br(g) + 2e– and Mg(s) + Br2(l)
• state symbols required – bromine is a liquid

Mark scheme

Examiner's comment

Most candidates drew a circuit containing a voltmeter and the silver half cell but very few candidates included the H+ in the MnO4-/Mn2+ cell.

Notes from the mark scheme and report

• voltmeter and labelled salt bridge
• Ag in 1 mol dm–3 Ag+
• Pt in H+/MnO4–/Mn2+ (1 mol dm–3 or equimolar)
• IGNORE pressure
• 1 mark for both half-cells if concentrations omitted

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

Very few candidates were given full marks. Higher-attaining students calculated one half life in range but very few could come up with a second half life as the graph did not allow another successive half life to be obtained. Higher-attaining candidates chose alternative half lives from the data given.

Notes from the mark scheme and report

• two half-lives shown on graph (102–110 min) AND statement that half-life is constant
• values on the graph.
• Further guidance on rates of reaction can be found at: https://www.ocr.org.uk/Images/371956-experiments-on-rates-of-reaction.doc

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question required the candidate to draw a line of best fit and then draw a tangent at t=0. Many candidates did not draw a line of best fit, and many did not get a tangent in the acceptable range. Very few candidates processed the gradient by using the correct subtraction on the y axis (scale was from 1 to 5) or by using the 10-3 on the axis label. Electrode potentials

Notes from the mark scheme and report

• tangent at t = 0 (gradient 2.9–4.0 × 10^–5) OR k = ln 2/t½ then rate = k × 5.0 × 10^–3
• note 10^–3 on y-axis
• ALLOW ECF from t½

Skills: M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.1 Translate information between graphical, numerical and algebraic forms; M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Question 19(b)(ii): Only a few candidates were able to link the electrode potentials and equilibrium shifts.

Notes from the mark scheme and report

• E of system 3 less positive than system 5 (must be comparative
• IGNORE lower/higher)
• system 3 shifts left AND system 5 shifts right

Answer

A

Mark scheme

Examiner's comment

Candidates found this question more challenging than anticipated. While many correctly chose A, a significant proportion of candidates selected C.

Answer

B

Mark scheme

Examiner's comment

This question discriminated well, with higher ability candidates correctly identifying B as a non-planar molecule. Common incorrect responses included C and D. Some candidates drew structures alongside each option to aid their choice.

Answer

D

Mark scheme

Examiner's comment

Candidates found this multiple choice question difficult. While some correctly selected D, many candidates chose B.

Answer

B

Mark scheme

Examiner's comment

This question proved difficult. Candidates who drew out the different isomers of chloroheptane were able to identify B as the correct response.

Answer

B

Mark scheme

Examiner's comment

Candidates found this question difficult, presumably as it involved reactions of different functional groups within the same compound. Many candidates identified B as the correct response. The most common incorrect responses were C and D. Section B overview Section B includes a mixture of short answer and extended response questions, including two questions marked using a level of response mark scheme (20d and 22d). This section of the paper is worth 85 marks.

Mark scheme

Examiner's comment

This question proved difficult for candidates. Although many candidates were able to identify the correct organic product, only the higher ability candidates were able to construct an appropriate balanced equation. A common error was to omit the role of the acid; this is shown in Exemplar 2 below. Lower ability candidates appeared not to recognise this reaction and suggested an alkoxide salt, rather than a haloalkane as the organic product. This type of response was seen frequently by examiners. The candidate has drawn the correct structure of the haloalkane formed and scores the first mark. However, the response fails to recognise that the reaction occurs under acidic conditions and omits the sulfuric acid from the equation.

Notes from the mark scheme and report

• DO NOT ALLOW equations that form NaOH
• ALLOW Na2SO4 version, H+ for H2SO4, or equation with HCl

Mark scheme

Examiner's comment

This question required candidates to apply their knowledge of the oxidation of alcohols to complete the equation for the complete oxidation of compound B. This question discriminated well. Many candidates correctly identified the organic product but only the higher ability candidates could complete the equation. A common error was to omit water as a product of the reaction.

Notes from the mark scheme and report

• reflux with excess: primary OH to COOH, secondary OH to C=O, tertiary OH unchanged
• 3[O] and 2H2O

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

In contrast to part (a) this question proved difficult and discriminated well. The higher ability candidates correctly identified the dichloride salt produced by the reaction of lysine with excess acid. Some candidates formed ammonium ions on both nitrogen atoms but omitted the chloride ions. Others only formed a monochloride salt, leaving the R group unchanged. These two approaches scored one mark.

Notes from the mark scheme and report

• BOTH amine groups protonated (excess HCl)
• Cl– required because question asks for salt
• 1 mark if only one NH2 protonated or no Cl–