June 2025: AS Paper 2, Depth in chemistry

Mark scheme

Examiner's comment

Most correctly related an alkali to the “release of OH- ions”. Responses which did not score a mark usu- ally stated that the ”pH is greater than 7” or an alkali is “a proton acceptor” or gave simple explanations such as a “soluble base” or “neutralises an acid”.

Notes from the mark scheme and report

• releases OH– ions in aqueous solution
• IGNORE mention of pH

Mark scheme

Examiner's comment

Most candidates were able to write the ionic equation. Common incorrect responses included giving the full equation, word equation or using the spectator ions i.e. Na+ + Cl- → NaCl

Notes from the mark scheme and report

• H+ + OH– → H2O
• IGNORE state symbols

Mark scheme

Examiner's comment

Many well-structured calculations were seen, even if all the marks weren’t achieved. Almost all candi- dates calculated the number of moles involved. For those candidates’ hat didn’t get full marks, it was often caused by using an incorrect mass in Q = mcΔT. Some tried to calculate the mass of NaOH and HCl (from moles and Mr values) while others didn’t add the total volumes together i.e. used 40 g rather than 80 g. Some used moles × 2 i.e. 0.112 rather than per mole of water formed. One of the most common errors was the omission of the negative sign for the final enthalpy change. Some put a positive charge instead. Some lost a mark for the incorrect significant figures.

Notes from the mark scheme and report

• common errors (3 marks): –28.1 from using 40 g instead of 80 g or 2 × 0.056 mol, –1690 from adding 273 to the temperature change, +56.1 (no – sign)
• 3 SF AND – sign required
• ALLOW ECF throughout

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

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

More than half of the candidates gave the correct equation. However, many lost the mark for missing or incorrect state symbols. In some responses it was hard to differentiate between (g) and (s) – candidates need to make sure their handwriting is clear so credit can be given. Some other errors that were seen included: losing 2 electrons, incorrect charges, gaining electrons or using Mg rather than Ca.

Notes from the mark scheme and report

• Ca+(g) → Ca2+(g) + e–
• starts from Ca+ not Ca
• IGNORE states on the electron

Mark scheme, with the levels and indicative points

Examiner's comment

Many candidates scored highly on this level of response question, answering all three parts. Generally, candidates found it easier to explain the difference in reactivity between Mg and Ca than the other parts of the question. Some lacked clarity in their responses, describing a decreased “electrostatic attraction” without reference to the nucleus and/or outer electrons. Some muddled the order of reactivity describing the gain of electrons. A few candidates described a general trend down the group without linking to the position of Mg and Ca in the group. A number of candidates had an incorrect equation forming CaO rather than hydroxide and occasional errors in the formula, e.g. CaOH or balancing of the equation were seen. Most gave at least one correct observation. Some candidates suggested that bubbles would be seen but did not have hydrogen pro- duced in their equation. Most candidates were able to recognise that redox meant that oxidation and reduction took place. How- ever, some were not clear here, e.g. “it is redox as calcium is oxidised”. Most understood that to explain why this was a redox reaction they needed to give oxidation number changes and/or number of electrons lost or gained. Some errors were seen in assigning oxidation numbers such as H in water, which was given as +2, rather than +1 for each atom. Lower-attaining candidates often suggested that oxygen was reduced. Candidates should be aware that they do not need to fill the answer space provided. Often longer re- sponses introduce contradictions in responses. An excellent response which achieved Level 3 and gained all 6 marks.

Notes from the mark scheme and report

• Ca + 2H2O → Ca(OH)2 + H2, fizzing and Ca dissolves/white ppt
• Ca 0 to +2 oxidised, H +1 to 0 reduced (error: +2 as initial oxidation state of H)
• compare Mg and Ca directly - larger radius, more shielding, less attraction, lower ionisation energies

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

Around half of the candidates gained a mark for this question. Many suggested the use of “gas syringe” but from their response in (i) it was clear that a significant number didn’t understand how this would im- prove the experiment. Lower-attaining candidates sometimes couldn’t give the correct name for a gas syringe suggesting “gas volume measurer” or “gas tube” instead. Some other incorrect responses included: • Improve resolution on measuring cylinder • Use a larger mass of BaCO3 • Leave reaction for longer • Heat to a constant mass • Make sure delivery tube is further in the measuring cylinder • Make sure connections are airtight

Notes from the mark scheme and report

• gas syringe in place of upturned measuring cylinder, or carbonate in a small tube released into the acid without removing the stopper
• IGNORE gas cylinder

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

Most scored here. A minority used all three titres for the mean and even fewer used 23.30 in the calcula- tion with either of the other two values.

Notes from the mark scheme and report

• mean of concordant titres only (23.05 and 23.15) = 23.10
• ECF titres must be within 0.1 cm3

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Many responses were well constructed and showed most of the stages. This made it much easier to award error carried forward (ECF) marks if a correct response was not obtained. Around a quarter of candidates were able to score all 5 marks. Almost all candidates scored the first 2 marks. However, it was sometimes hard to follow the steps a candidate had taken to obtaining a final response, due to: • Steps not being labelled • Steps merging into one another (all equalling each other despite different processes) • Competing alternative methods described without crossing through incorrect working. Many candidates did not gain marks for unit conversions i.e. not converting volumes from cm3 to dm3 and incorrectly converting g to mg. Some did not gain marks for missing the × 10 step. Lower-attaining candidates often didn’t use the titre value obtained in (ii) but used 25 cm3 instead, muddling which vol- ume value should be attributed to which reactant. Maths Skills for AS Chemistry It is important for candidates to have the opportunity to develop their maths skills as part of the course. Particular attention should be paid as to how to: • Setting out the working for calculations • Keeping the full value of any step in their calculator to use in the next step • Common unit conversions, e.g. dm3 to cm3 • Giving the final answer to an appropriate number of significant figures or that asked for in the question • Checking their working for any errors Some support can be found in our Maths Skills Handbook: https://www.ocr.org.uk/Images/295468-chemistry-mathematical-skills-handbook.pdf This article from the RSC contains some useful resources: https://edu.rsc.org/resources/maths-skills-for-chemistry/4013899.article

Notes from the mark scheme and report

• common errors (4 marks): 0.413/4.13/41.3 incorrect conversion to mg, 1240 not divided by 3 tablets
• 1:2 acid:NaOH
• ×10 for 250 cm3
• 3 SF AND mg

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; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Around half of the candidates scored both marks, understanding the impact on the titre of rinsing the bu- rette with water. It was essential for candidates to refer to the “titre” and not just the volume in their re- sponse. Some understood that the NaOH would be more dilute but then suggested it would give a smaller titre or didn’t give impact on the titre value at all. Some suggested that the water provided OH- ions thus de- creasing the amount of NaOH required to neutralise. A few gave non-committal responses such as “Water would alter the concentration of NaOH, so you wouldn’t get an accurate titre” so scored 0 marks.

Notes from the mark scheme and report

• titre would be greater
• NaOH in burette more dilute so larger volume needed

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Approximately half of the candidates did not score any marks for this question. Many left the OH group on the alkenes formed i.e. removed H2 rather than H2O. Some attempted to draw cis/trans isomers with structures with no C=C in them. Some drew any alkene, e.g. but-2-ene, to show cis/trans, but this was not given here. Common reasons for scoring 1 mark included the isomers being the wrong way round or poor connectivity to -COOH groups.

Notes from the mark scheme and report

• dehydration gives HOOCCH=CHCOOH
• one mark if both stereoisomers shown in wrong columns
• DO NOT ALLOW incorrect connectivity to COOH (first occasion only)

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

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

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 found this unfamiliar calculation quite tricky. Some tried to use the equation q = mcT to try to find a mass. Others tried a Hess cycle. Some multiplied by the Mr for pentane rather than carbon dioxide. A common error was to use 1000 J in the calculation, despite the enthalpy change being given in kJ mol-1, giving a final answer of 62.7g.

Notes from the mark scheme and report

• 5 × 44.0 = 220 g CO2 per 3509 kJ so 0.0627 g
• ALLOW ECF from mole ratio in (b)(i)
• IGNORE significant figures

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 were able to score 2 marks here for this familiar question. However, marks were lost for the following errors: • Double headed arrows or arrow wrong way round for H or missing arrow heads • Arrow for Ea not extending to the maximum • Missing formula for reactants and products • −H label • Ea arrow drawn from products up to maximum A few drew an endothermic profile diagram limiting their mark to 1.

Notes from the mark scheme and report

• products CO2 + H2O below reactants C5H12 + O2 with ∆H arrow downwards (DO NOT ALLOW –∆H or double-headed arrow)
• Ea from reactants to peak (DO NOT ALLOW arrowhead down)
• 1 mark for correctly labelled endothermic diagram

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

Most candidates scored both marks here. Some lost a mark due to slips in naming, e.g. missing ‘di’ from dimethylpropane or incorrect numbering, e.g. 2-dimethylpropane, 2,3-dimethylpropane. Some used structural formula rather than skeletal. Common incorrect structures included cyclopentane, 1- or 2- ethylpropane or 1,2-dimethylpropane (not realising that these are 2-methylbutane) or 6 C structures with addition of a methyl group to pentane.

Notes from the mark scheme and report

• (2-)methylbutane and (2,2-)dimethylpropane
• structures must be skeletal
• all 4 points for 2 marks
• numbers if given must be correct
• DO NOT ALLOW methy/meth/methly

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

Generally well known, with most recognising that HBr is formed as a product. Incorrect responses sometimes had 2 Br instead of Br2 or included other radicals in their response. Some candidates confused propane and pentane, despite being given them in the table. Some started with a longer C chain, breaking the chain and substituting with Br to give bromopropane as state in the question, e.g. C4H10 + Br2 → C3H7Br + CH3Br. Some used structural formulae rather than the molecular formula which was asked for.

Notes from the mark scheme and report

• C3H8 + Br2 → C3H7Br + HBr

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

This question was a slightly different way of asking why a number of different products are formed in rad- ical reactions. However here candidates needed to recognise that it would be due to substitution at dif- ferent positions along the chain. Common incorrect response included ”multiple termination products” or “further substitution”. It was also evident in some responses that candidates had muddled this with nucle- ophilic substitution and described the “stability of the carbocation intermediate”. Some just stated the two different structural isomers that would be formed but didn't gain the mark as needed to say how they are formed.

Notes from the mark scheme and report

• Br can substitute at different positions on the chain (end or middle C) / two different radicals form
• DO NOT ALLOW stability of carbocations
• IGNORE major/minor product

Mark scheme

Examiner's comment

Many candidates lost the mark here for omitting to add lone pairs on chlorine. Some lost the mark as they didn’t use three different symbols for electrons on each of the elements, making it unclear which electrons were associated with each element.

Notes from the mark scheme and report

• three lone pairs on Cl must be shown
• three different symbols for C, H and Cl electrons
• IGNORE inner shells
• circles not required

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, with the levels and indicative points

Examiner's comment

Many understood the necessary steps to purify a liquid and were confident with the calculation. Most knew at least one or two purification steps and could explain their purpose. Reasons marks were lost included: • Not remembering the correct name for equipment i.e. separating funnel • Getting the layers the wrong way round despite being given density information • Unclear which layer an anhydrous salt was added to for drying • Using an inappropriate salt for dry step, e.g. Na2CO3 or MgCO3 • Slips in calculations, e.g. incorrect Mr values, yield inverted or not accounted for • Thinking a solid would be formed • Describing reflux (including drawing apparatus) • Saying to distil at a specific boiling point but not stating the actual value (given in the question) As with all calculations, it is important to set out working clearly as it helps to avoid slips. Remind candi- dates of the importance of checking their Mr values – many made slips here. Candidates should also avoid rounding intermediate values and consider the number of significant figures used for the final re- sponse. Some rounded intermediate values to less than 3 significant figures but gave a final response to 4 or more. The response scored Level 3 and gained all 6 marks. All three purification steps are given in the correct order and explained. The calculation for mass is set out clearly and the final mass is given to 3 signifi- cant figures.

Notes from the mark scheme and report

• separating funnel - organic layer is the top (less dense) layer
• dry with anhydrous MgSO4/CaCl2
• distil collecting fraction at 70 °C
• 0.125 mol × 0.65 × 92.5 = 7.52 g
• IGNORE washing with carbonate/water
• ALLOW one small slip e.g. Mr 73

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; PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Candidates need to be reminded to check structures carefully – common errors included additional H atoms around C=C or missing the -CH3 group. Some used -C4H9 rather than writing out the complete structure; unfortunately, this is ambiguous so didn’t score the mark. Lower-attaining candidates some- times gave an alkane rather than an alkene, reacting with Br2 in presence of UV. Many had the correct reagents but added additional incorrect reagents resulting in loss of the mark, e.g. the addition of H2SO4 as a catalyst alongside NaOH for stage 2.

Notes from the mark scheme and report

• alkene X is 4-methylpent-1-ene (check number of H and CH3 branch)
• Stage 1 Br2
• Stage 2 NaOH/KOH(aq) (ALLOW H2O but DO NOT ALLOW steam)
• DO NOT ALLOW additional reagents
• IGNORE conditions and solvent

Mark scheme

Examiner's comment

Most candidates were able to correctly state the mechanism. Most common incorrect responses in- cluded hydrolysis, hydration or electrophilic substitution.

Notes from the mark scheme and report

• nucleophilic substitution
• DO NOT ALLOW hydrolysis

Mark scheme

Examiner's comment

Most candidates only scored 1 mark as they omitted the charges, despite being asked in the question for the structures of ions. Some gave charge on the fragment P but not on the molecular ion Q. Some also gave ions that would have the correct mass but don’t exist in this molecule, e.g. P CH2CH2CH3+. A few gave molecular formulas rather than structures.

Notes from the mark scheme and report

• P is CH3CO+ (m/z 43), Q is molecular ion CH3COCH2CH3+ (m/z 72)
• + charge required on each
• one mark if both correct with no charge or P/Q swapped

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

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

Mark scheme

Examiner's comment

The majority of candidates said infrared radiation caused the bonds to vibrate. This was not sufficient for the mark as the bonds would already be vibrating – but IR causes them to vibrate more. A significant number thought that IR would cause the bonds to break.

Notes from the mark scheme and report

• bonds vibrate MORE (and absorb energy)
• DO NOT ALLOW bond breaking

Mark scheme

Examiner's comment

Candidates must take care to name functional groups rather than just bonds as O-H bonds are found in both carboxylic acids and alcohols, C=C are found in alkenes and arenes. Many candidates thought that there was a carboxylic acid present in B, not realising this would then have too many O atoms to be a structural isomer. Similarly, some identified a halogen. Candidates must be clear about the IR peaks identified ensuring they give precise values within the range identified on the data sheet. Many candidates still confuse O-H in a carboxylic acid, which gives a much broader peak that merges into the C-H stretch, and O-H in alcohol, still broad but distinct from C- H.

Notes from the mark scheme and report

• alcohol with O–H broad peak at 3200–3600
• alkene with C=C peak at 1620–1680
• name of functional group required not just the bond
• explanation needs bond AND wavenumber range/value
• IGNORE alkane
• DO NOT ALLOW any other functional group
• Candidates are often confused about how IR works. Our delivery guide for identifying un- knowns contains information on IR including how it works and the use of a simple spring model to help explain. There is also a link to a TES resource with a bank of real IR spectra which clearly shows difference between O-H in an alcohol and O-H in a carboxylic acid.
• Delivery Guide (H032): Identifying Unknowns https://teachcambridge.org/item/9698f11f-a6d4-45a6-9f3d-de025f7bd434

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