Graph and data questions

Mark scheme

Examiner's comment

This question required candidates to appreciate that initially concentration of HCl is at its highest, leading to more frequent collisions. As the reaction proceeds, concentration and collision frequency decrease until HCl is used up and the reaction stops (shown by the graph plateauing). The first marking point ignored candidates incorrectly discussing concentration of solid magnesium, but candidates need to be clear that only the (limiting) reactant, HCl, was used up when the reaction stopped for the third marking point. This last point was often missed, and this would be a good question to use with high-attaining candidates. Apart from the last point, there was an even spread of marks given; the second point was often missed through lack of ‘frequency’ or other link to collision rate. Analysing data in graphs Lower scoring candidates often stated that ‘the line increases because rate/concentration increases’. Students should be encouraged to carefully read axes to make sure they are identifying the correct trend from a graph.

Notes from the mark scheme and report

• less frequent collisions/fewer per second (IGNORE 'fewer collisions' or 'fewer successful collisions' with no link to time)
• acid used up (DO NOT ALLOW magnesium/reactants used up, Mg is in excess)
• IGNORE 'magnesium concentration decreases'
• DO NOT ALLOW rate increases

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

Mark scheme

Examiner's comment

Many candidates were able to score all 4 marks. More successful responses were clear and concise, covering all the detail required in much less space that provided here (see exemplar). Less successful responses often scored a mark for the correct trend but stated that this was due to more (covalent) bonds needing to be broken. Some were able to give well-rehearsed explanations for the dif- ference in boiling point but lost the trend mark as state change by the position in the table only, e.g. “as you go down the table, the boiling point increases”. A few muddled different types of intermolecular forces, suggesting that H-bonds were present. There was evidence that some held the misconception that a C-H-bond constitutes a H-bond. Explanations were sometimes missing the ‘contact’ between molecules or alternatively an increase in the number of electrons. Some did not gain marks as their responses were not comparative, e.g. “has sur- face contact so can form London forces” or” requires a lot of energy to break London forces”. It was fairly common to see “more energy to break the bonds” which was not given as it needed to be clear that it was the intermolecular/London forces which are broken. Some referred to enthalpy of combustion data which was not asked about in the question. Some re- sponses had lots of repetition i.e. explained why longer chains had a high boiling point, then repeated the same but for the reverse argument for smaller chains. A clear and concise response which scores all 4 marks. The diagrams at the bottom confirm that this candidate has understood the concepts completely.

Notes from the mark scheme and report

• comparisons needed throughout
• boiling point increases as chain length increases
• more surface CONTACT (surface area alone insufficient)
• more/stronger London forces
• more ENERGY to break them
• DO NOT ALLOW branching, permanent dipoles, hydrogen bonding or covalent bonds breaking
• IGNORE van der Waals

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

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

This was generally a well answered question from most candidates. The definition requires the use of ‘element’ rather than the more general term ‘species’. Candidates often went on to identify N as the element. Less successful candidates identified H or O from the redox change. A misconception for a few was that an ‘atom’ was disproportionated without realising this would be impossible. Oxidation numbers were mostly correct and correct oxidation and reduction identified, either by naming compounds in the answer space or labelling them in the equation. Candidates would be advised to make sure that there is a clear link between the oxidation number and the element that it refers to.

Notes from the mark scheme and report

• oxidation and reduction of the same ELEMENT (IGNORE species)
• N +3 to +5 (in HNO3) AND +3 to +2 (in NO)
• oxidised to HNO3 and reduced to NO
• + signs required
• DO NOT ALLOW N3+, N5+, N2+

Mark scheme

Examiner's comment

Most candidates were given full marks. They successfully calculated the empirical formula but a few then grouped the ligands together giving a molecular formula, e.g. Fe(C5H5N4)Cl2. Most candidates then continued and found the number of ligands and coordination number. Occasionally coordination number was mixed up with the charge on the ion.

Notes from the mark scheme and report

• ratio 1 : 20 : 20 : 4 : 2 gives FeC20H20N4Cl2 (DO NOT ALLOW Fe(C5H5N)4Cl2 as the empirical formula)
• four pyridine ligands
• coordination number 6

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

Most candidates found this familiar question relatively straightforward, achieving all 3 marks. Some did not gain marks as the electron pair came from -OH rather than from the O or it was suggested that multiple lone pairs on the O were donated to the ring. Some suggested that the presence of the -OH group on the benzene ring disrupted the delocalised electron ring thereby making it less stable and so more reactive. Some also attributed the reactivity solely to the -OH group, suggesting that bromine would react to replace the OH. Some did not gain marks for not giving comparative answers. It was necessary to compare the reactivity of both benzene and phenol here. Some lost the mark for ‘increasing electron density’ as they referred to charge density or electronegativity instead. Lower attaining candidates often just stated that ‘phenol is more reactive than benzene’ without considering why this is the case. However, it was clear that many candidates had practised this type of question and knew what to include.

Notes from the mark scheme and report

• lone pair on O (one pair only) delocalised into ring
• electron density higher than in benzene
• ring more susceptible to electrophilic attack/polarises Br2
• responses must be comparative
• IGNORE activating, charge density, electronegativity

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

Although high attaining candidates responded with the formulae of chromium-containing species, it was common to see organic compounds being suggested. Consequently, a large proportion of candidates did not score either of the 2 marks. Many candidates seem to expect to only give organic species in their responses on this paper and would benefit from understanding that inorganic species may also need to be provided.

Notes from the mark scheme and report

• green Cr3+ (or [Cr(H2O)6]3+)
• orange Cr2O7 2–
• formulae AND charges must be correct
• DO NOT ALLOW Cr6+
• 1 mark if the wrong way round

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Almost all candidates knew that a catalyst lowered activation energy and most were aware that an alternative pathway was made possible by a catalyst.

Notes from the mark scheme and report

• lowers activation energy AND alternative route/more molecules exceed Ea
• IGNORE 'shorter route'
• IGNORE more successful collisions

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

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

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

Most candidates were able to successfully explain Step 1, scoring the first 2 marks. When comparing electrode potentials, candidates should avoid the use of higher/lower as these phrases are ambiguous due to the negative signs involved. Describing them as ‘more negative’ or ‘more positive’ is clearer. Candidates are advised to read the instructions contained within the question and to use or comment on all the data presented. Very few candidates linked Step 2 to the reducing effect of zinc to manganate(VII) ions hence the need for filtration. A few candidates explained Step 2 in terms of the warming/cooling and zinc crystalising rather than explaining the redox chemistry given in the table.

Notes from the mark scheme and report

• Step 1: E(Zn) more negative than E(Fe3+/Fe2+) so Zn reduces Fe3+ to Fe2+
• Step 2: E(Zn) more negative than E(MnO4–) so unfiltered Zn would react with MnO4–
• IGNORE larger/smaller

Skills: 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

This question proved difficult and although most candidates score some marks, only the very best responses secured all 4 marks. Candidates often did not link the doublet to two CH3 groups and many only scored 1 mark for their suggested structure as the carbonyl groups were not side by side within the molecule.

Notes from the mark scheme and report

• doublet at 1.1 (6H) = 2 × CH3
• singlet at 2.2 (3H) = CH3–C=O
• multiplet at 2.9 (1H) = CH(CH3)2 next to C=O
• structure must be C6H10O2 with two carbonyl groups
• ALLOW δ ± 0.2 ppm

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

Mark scheme

Notes from the mark scheme and report

• MAX 1 if no + sign for oxidation number
• 1 mark if all oxidation numbers correct but oxidised and reduced wrong way round

Mark scheme

Notes from the mark scheme and report

• K2S strong ionic bonds/giant ionic lattice
• SF2 weak London/dipole–dipole forces between molecules
• IGNORE van der Waals forces

Mark scheme

Notes from the mark scheme and report

• steeper initial gradient and levels off earlier
• same final volume of gas (same moles)

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

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

Mark scheme

Notes from the mark scheme and report

• non-superimposable mirror images

Mark scheme

Examiner's comment

Q1(a) Partial pressure calculation Q1(d) Formula determination from percentage composition by mass Q2(b) Converting mass and volume into moles Q3(b) Unstructured titration calculation Q5(a)(ii) Ideal gas calculation Q5(b)(i) Calculation of the pH of a weak acid • Produced clear and concise responses for a Level of Response question based on a familiar context i.e., Q4(b) rates by continuous monitoring and graphical analysis. • Had difficulty when answering questions requiring application of knowledge and understanding on questions set in novel and unfamiliar contexts, e.g. Q3(b): As with Question 2, candidates had more success with the mole calculation in Q3(b) than the descriptive response in Q3(a). See also comments made for Q1(e) and Q3(b).

Notes from the mark scheme and report

• n(D) in 25 cm3 = 2 × 3.525 × 10–3
• ×4 for 100 cm3
• M = 114 so C5H9COOH
• any 2 cis isomers with different groups on each C of C=C
• common errors: scaling omitted (M = 456), no × 2 (M = 228), 25.00 used as titre (M = 100.8)
• 1 mark for 2 trans isomers

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects; PAG2 Acid–base titration

Mark scheme

Examiner's comment

3(a) More able candidates scored well in this question, setting out their answers in a logical order. They often first discussed Ca and its bonding and structure, linking this to the physical properties and then doing the same for Br. A number of candidates discussed the chemical properties of Ca and Br, such as their ability to bond with other elements, ionisation energy and their reactivity based on their position in the periodic table. Some candidates gave a good description of metallic bonding but then went on to discuss melting point in terms of intermolecular forces. 3(b)(i) Most candidates were able to give the correct diagrams for ionic bonding, although care needs to be taken that diagrams are well drawn with both charges given. Some gave diagrams for covalent bonding. 3(b)(ii) It was important to answer the question asked. A number of responses lost marks for describing the general trend down group 2 without making reference at all to calcium and barium. Most candidates managed to score at least one mark here but a considerable proportion missed the second marking point explaining that nuclear attraction was less in Ba. 3(c)(i) This was dependent on candidates’ ability to give correct formula of aluminium hydroxide, Al(OH)3. Writing correct formulae is an important skill in chemistry, which should be practised regularly in different topics to help candidates master the skill. 3(c)(ii) Most candidates were able to recognise that water had the higher boiling point due to hydrogen bonding. However, it was evident in a number of responses that they were unclear that this is an example of an intermolecular force and doesn’t just refer to the covalent bond formed between O and H. Most avoided discussing the intermolecular forces present in H2Se even although both London forces and permanent dipole-dipole interactions were accepted as they may not have come across this specific example and had not been given electronegativity data. 3(d)(i) Very few candidates scored this mark. Although a number of candidates did give sodium bromate as the answer (with the omission of the oxidation state), many other answers were seen suggesting candidates are not aware of naming conventions for inorganic compounds. 3(d)(ii) This is the first time in a reformed chemistry AS paper that the question space has been left unstructured for oxidation number changes. The highest-attaining candidates set out their responses clearly, dealing with changes for oxidation and reduction separately, and giving the correct oxidation numbers. Some struggled to obtain an oxidation state of Br in NaBrO3 as +5, suggesting +1 instead.

Notes from the mark scheme and report

• same element both oxidised (0 to +5) and reduced (0 to –1)
• 1 mark if all three oxidation numbers correct but oxidation/reduction mis-assigned

Mark scheme

Notes from the mark scheme and report

• mark is for working

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.2 Change the subject of an equation; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

This question asked the candidate to determine the amount of sodium sulfite in food. Many candidates gained full marks. Most candidates calculated the number of moles of MnO4- and SO32-. Some candidates calculated the mass in 525 g of meat, although some used the wrong Mr, e.g. 80, for the sulfite ion. The lower ability candidates did not process the scaling to 1 Kg. Rates of reaction

Notes from the mark scheme and report

• n(MnO4–) × 2.5 for SO3 2–
• × 10 for 250 cm3
• × 126.1 = 0.397 g in 525 g
• scale to 1 kg = 757 mg which is below 850 mg so complies
• ALLOW ECF throughout

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

Nearly all the candidates obtained this mark

Notes from the mark scheme and report

• point at 80 s circled

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

Mark scheme

Examiner's comment

Many students did not sketch this curve or sketched a curved that was less steep and did not finish at 58cm3.

Notes from the mark scheme and report

• steeper initial slope AND levels off earlier
• same final volume (58 cm3) because same moles of HCl

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

Mark scheme

Examiner's comment

Most candidates did not state that the direction of reaction of redox equilibria is dependent on the relative negativity/positivity of the standard electrode potentials. Higher ability candidates described two reductions of the relevant vanadium ions to end up with V2+ ions.

Notes from the mark scheme and report

• VO2+ → V3+ (green) then V3+ → V2+ (violet)
• E of systems 4 and 3 more positive than system 2 (Fe2+/Fe) so they shift right/are reduced
• IGNORE 'lower/higher'
• IGNORE Fe oxidised

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

Some candidates realised that the most suitable indicator for a weak acid / strong alkali titration would be cresol purple. For incorrect responses, other indicators appeared to be selected at random, suggesting that candidates were unclear on the criteria for selected a suitable indicator.

Notes from the mark scheme and report

• cresol purple AND pH range matches the vertical section/equivalence point
• IGNORE 'colour change matches end point'

Mark scheme

Examiner's comment

When identifying a similarity, many candidates confused the term ‘equivalence point’ with the term ‘end point’. For differences, many candidates realised that HCN had a different Ka to C2H5COOH but often did not appreciate that this would lead to a starting point with a higher pH.

Notes from the mark scheme and report

• similarity: same volume of NaOH (20 cm3) to neutralise or same final pH
• difference: HCN higher starting pH or shorter vertical section
• end point must not refer to same pH

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

Mark scheme

Examiner's comment

Candidates are confident in tackling titration analysis and the majority of responses were well structured. Most candidates used the results to determine the Mr of the unknown amino acid and hence identify it as leucine. However, other approaches were also seen and these were given full credit if correct. A common error was incorrect scaling to determine the amount of amino acid used to make the 250 cm3 solution. A significant number of responses simply multiplied the number of moles of acid by ten rather than using a factor of 250/21.30. Error carried forward marks were available and so the majority of candidates scored 3 or 4 marks.

Notes from the mark scheme and report

• amino acid solution is in the burette: n in 250 cm3 = 3.75 × 10^-3 × 250.0/21.30
• M = 131 so leucine, with working (R = 57 or Mr) to justify
• ALLOW ECF

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

A LEVEL CHEMISTRY A Examiners’ report H432/02 Summer 2019 series Version 1 H432 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 2 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 3 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 6 Question 5 ............................................................................................................................................ 7 Question 10 .......................................................................................................................................... 8 Question 12 .......................................................................................................................................... 9 Question 13 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 10 Question 15 ........................................................................................................................................ 11 Section B overview ................................................................................................................................. 12 Question 16 (a) (i) ............................................................................................................................... 12 Question 16 (a) (ii) .............................................................................................................................. 12 Question 16 (b) (i) ............................................................................................................................... 13 Question 16 (b) (ii) .............................................................................................................................. 14 Question 16 (b) (iv) ............................................................................................................................. 14 Question 16 (b) (v) .............................................................................................................................. 15 Question 17 (a) (i) ............................................................................................................................... 17 Question 17 (a) (ii) .............................................................................................................................. 19 Question 17 (b) (i) ............................................................................................................................... 20 Question 18 (a) (i) ............................................................................................................................... 21 Question 18 (a) (ii) .............................................................................................................................. 22 Question 18 (b) ................................................................................................................................... 23 Question 18 (c) ................................................................................................................................... 23 Question 18 (d) ................................................................................................................................... 24 Question 18 (e) (i) ............................................................................................................................... 24 Question 18 (e) (ii) .............................................................................................................................. 26 Question 19 (a) (i) ............................................................................................................................... 27 Question 19 (a) (ii) .............................................................................................................................. 29 Question 19 (b) (i) ............................................................................................................................... 30 Question 19 (b) (iii) ............................................................................................................................. 32 Question 19 (b) (iv) ............................................................................................................................. 34 Question 20 (a) (i) ............................................................................................................................... 36 Question 20 (a) (ii) .............................................................................................................................. 37 Question 20 (a) (iii) ............................................................................................................................. 37 Question 20 (b) (i) ............................................................................................................................... 38 Question 21 ........................................................................................................................................ 39 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 17 (b) (i) Candidates are familiar with the calculation of Rf values and this question was answered very well.

Notes from the mark scheme and report

• glycine: matches leu and gly in solvent W AND ala and gly in solvent X, so same Rf in both solvents

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

Many candidates were able to describe the meaning of ‘heterolytic’, referring to both electrons going to the O atom. However, only some candidates described ‘fission’ and a significant proportion of these referred only to ‘bond breaking’. The best responses correctly stated that fission is the breaking of a covalent bond. Candidate are encouraged to read questions carefully and take notice of words that appear in bold type as these often communicate what is expected in a question.

Notes from the mark scheme and report

• one bonded atom (O) receives both electrons
• breaking of a covalent bond ('bond breaking' alone not sufficient)
• IGNORE formation of ions
• DO NOT ALLOW element or molecule for atom

Mark scheme

Examiner's comment

This part discriminated very well with able candidates identifying that the absorption for the C–Br bond would disappear, with a new peak appearing for the alcohol O–H bond. A significant number of candidates did not seem to understand what was required, with many interpreting the spectrum as that of the alcohol, rather than predicting how the spectrum would change during the reaction. A common error was to interpret the absorption for a C–H bond at ~3000 cm–1 as that of an O–H bond. Copyright acknowledgements Q25(b)(ii) spectrum © SDBSWeb : https://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi (National Institute of Advanced Industrial Science and Technology, accessed 31 January 2017).

Notes from the mark scheme and report

• disappearance of C–Br peak (500–800 cm–1) and appearance of O–H peak (3200–3600 cm–1)
• DO NOT ALLOW responses that only describe the spectrum shown

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

Mark scheme

Examiner's comment

Most candidates were able to state that one mole of gaseous ions was dissolved into water. Common errors seen included candidates referring to the dissolving of one mole of substance (i.e. the enthalpy change of solution); the use of the generic term ‘solvent’ rather than water; and dissolving of gaseous ions into one mole of water.

Notes from the mark scheme and report

• 1 mole of gaseous ions
• dissolve in water/form aqueous ions
• IGNORE 'energy released' or 'energy required'

Mark scheme

Examiner's comment

Examiners were encouraged by the number of good responses to this problem solving question. Most candidates achieved at least one mark in this part, often from a correct structure of J. Although many candidates deduced that the R group for both L and M consisted of 3 C atoms and 7 H atoms, only the highest ability candidates were able to join these correctly. A small but significant number of responses showed R groups that involved O atoms, despite the prompt that the R represented an alkyl group. Candidates are advised to read questions carefully.

Notes from the mark scheme and report

• J has R = CH3
• L and M are the propyl and methylethyl esters (either way round)
• IGNORE 'C3H7' as ambiguous

Mark scheme

Examiner's comment

This question was answered well with most candidates scoring three or four marks. Examiners were impressed by the number of responses that accurately referred to induced dipole-dipole interactions or London forces rather than van der Waals’ forces, which is ambiguous. Some responses lacked detail, as demonstrated in Exemplar 10. This response attributes the higher boiling point of pentan-1-ol to the amount energy required to break hydrogen bonds. However, it does not refer the relative strength of this type of interaction. Consequently, the first paragraph only scores marking point four and not marking point three. The higher boiling point of heptane compared to hexane is explained by a correct comparison of the induced dipole-dipole interactions present in these compounds, so marking point two was achieved. However, the justification for the difference in intermolecular forces lacks precision. Candidates should be encouraged to focus on surface contact or surface interaction between molecules rather than referring to surface area alone.

Notes from the mark scheme and report

• heptane: more surface contact/more electrons so stronger London forces
• pentan-1-ol: hydrogen bonds
• more energy to break the forces
• IGNORE van der Waals/VDW/IDID
• IGNORE 'harder to break' without energy
• IGNORE 'more energy to break bonds'

Mark scheme

Notes from the mark scheme and report

• comparison needed for each mark
• IGNORE atomic number increases, effective nuclear charge, same sub-shell
• IGNORE just 'greater attraction' (must be nuclear attraction on outer electrons)

Mark scheme

Notes from the mark scheme and report

• + sign required in +3 and +1
• 1 mark if elements and oxidation numbers correct but on the wrong lines

Mark scheme

Notes from the mark scheme and report

• same element oxidised and reduced
• Cl +5 to +7 and +5 to –1
• signs required
• IGNORE ionic charges such as Cl5+
• 1 redox mark if numbers correct but oxidation/reduction wrongly assigned

Mark scheme

Notes from the mark scheme and report

• link ∆G = ∆H – T∆S to y = mx + c
• gradient = –∆S (– sign required)
• P = ∆H
• Q = temperature at which feasibility changes
• IGNORE 'minimum/maximum temperature'

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; M3.3 Determine the slope and intercept of a linear graph

Mark scheme

Notes from the mark scheme and report

• reducing agent Zn
• oxidising agent MnO4–

Mark scheme

Notes from the mark scheme and report

• must give rank order: phenol most easily nitrated AND benzoic acid least

Mark scheme

Notes from the mark scheme and report

• comparative: diol has more OH groups AND forms more hydrogen bonds with water
• DO NOT ALLOW hydroxide/OH–

Mark scheme

Notes from the mark scheme and report

• four aromatic carbon environments at 110–160 ppm (all four peaks if labelled)
• C=O at 190–200
• C–C at 20–30
• 4-hydroxyphenyl ethanone (1,4-substituted)

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