Short-answer questions

Mark scheme

Examiner's comment

Over half of candidates scored full marks on this question, clearly stating the definition of disproportionation. Where a response did not state ‘element’ in the definition, the candidate usually went on to gain the first mark by clearly stating that bromine is oxidised and reduced. Oxidation numbers were usually assigned correctly, but marks were missed by not linking changes to chemicals, or by not stating which change was oxidation and/or reduction. These marks could sometimes be given from working around the equation, but candidates should be encouraged to include these links in their written response.

Notes from the mark scheme and report

• 'atom' insufficient for element
• oxidation numbers need sign and number and must be linked to NaBrO and NaBr
• 1 of 2 redox marks if oxidation/reduction assigned the wrong way round
• IGNORE electron loss/gain

Mark scheme

Examiner's comment

This long response question was approached well and there were some excellent and concise answers. However, some candidates tried to link reactivity to electronegativity or confused the question with boiling point trends and London forces. Candidates are well-versed with tackling this type of question and most were able to write sensible explanations. However, marks could not be given for insufficiently specific responses, for example, repeating the idea of attracting an electron for the third marking point. Responses could be improved if candidates read through what they write to see if it makes cohesive sense. There was a fairly even spread of marks obtained, with 2 marks given most frequently.

Notes from the mark scheme and report

• comparison required throughout
• IGNORE 'nuclear charge' for nuclear attraction and 'effective nuclear charge'
• IGNORE fewer orbitals/sub-shells
• IGNORE electronegativity

Mark scheme

Examiner's comment

This question was generally well answered, although a few candidates put an advantage down instead such as ‘kills harmful bacteria’.

Notes from the mark scheme and report

• IGNORE 'harmful/dangerous'
• IGNORE chlorine is carcinogenic
• IGNORE forms HCl

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

Most candidates identified at least one feature of a dynamic equilibrium. ‘In a closed system’ was given most often, and other mark-worthy features were ‘the same rate for forward and reverse reactions’ and ‘concentrations do not change’. Less successful responses that were not given marks included ‘forward reaction is the same as reverse’ and ‘concentrations are the same’.

Notes from the mark scheme and report

• DO NOT ALLOW concentration of reactants = concentration of products
• IGNORE 'reversible reaction'

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

This mark required ‘more stable carbocation’, and just under half of candidates gained it. Some responses were excellent, identifying that the carbocation would be tertiary as opposed to primary. Incorrect identification of the carbocation as secondary was ignored, meaning more candidates were able to score. Common responses which didn’t score discussed Markownikoff’s rule or said that the product was more stable.

Notes from the mark scheme and report

• must refer to stability of the (tertiary) carbocation intermediate
• IGNORE references to Markownikoff's rule alone
• IGNORE secondary

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

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

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

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

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

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

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

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

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

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

Most candidates were able to explain the lower first ionisation energy in sulfur. The description of the pairing of electrons for sulfur was better for those who used the orbital diagram with electrons clearly paired. Imprecise language was the most common cause of the mark not being given, e.g. ‘sulfur has four electrons in the subshell’ or incorrectly stating within the orbital. Less successful candidates approached this by explaining in terms of atomic radius, nuclear charge and nuclear attraction which demonstrated that they were not aware of the exceptions to the general trend.

Notes from the mark scheme and report

• paired electrons in a (3)p orbital in S
• repulsion between the paired electrons
• DO NOT ALLOW shell alone or s-orbital
• IGNORE distance from nucleus

Mark scheme

Examiner's comment

Most candidates were able to identify that Fe is already in its elemental state and the enthalpy change of formation would, therefore, be 0 kJ mol-1. Less successful candidates described iron as being in its natural state or an atom and thus didn’t have a formation enthalpy.

Notes from the mark scheme and report

• iron is an element (in its standard state)

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 was well answered by many candidates. There was a clear description of units cancelling, although some did correctly compare the molar ratios. Some candidates showed this in a clearly written equation. Less successful candidates were not given the mark due unclear terminology or explanation. The question was to explain Kp so the units would need to be a pressure unit rather than concentration (as in a Kc expression).

Notes from the mark scheme and report

• equal number of gaseous moles/molecules of reactants and products so (pressure) units cancel
• DO NOT ALLOW references to concentration
• IGNORE 'same number of reactants and products' without moles

Mark scheme

Examiner's comment

This question required the candidate to link the change in Kp value to the change in temperature. The question was answered well by many candidates. Most identified it was lower and exothermic, but not all went on to say that equilibrium shifted. A few candidates mixed up their lefts and rights and could be advised to use the terms products side and reactant side, as identified by the written equation. Misconceptions included that a higher T was because Kp is higher and that ‘Kp shifted to the right’ rather than the equilibrium.

Notes from the mark scheme and report

• lower temperature AND forward reaction exothermic
• Kp has increased so equilibrium position shifted to the right
• ALLOW ORA

Skills: M0.3 Estimate results

Mark scheme

Examiner's comment

This was a well answered question. A few candidates, incorrectly, suggested that it was heterogeneous due to the reactants and products being in different states, and didn’t mention the catalyst or suggested the catalyst was in same state.

Notes from the mark scheme and report

• Pt in a different state/phase from the reactants (solid, reactants gases)
• IGNORE products

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

Most responses were excellent, with many candidates mentioning more than one of the marking criteria. A typical response would be ‘lower temperatures can be used, decreasing use of (fossil) fuels, and reducing CO2 / greenhouse gas emissions.’ Some responses instead mentioned that catalysts can be reused. The most common incorrect response was to say fewer toxic products were released or similar.

Notes from the mark scheme and report

• lower temperature/less energy demand so less fossil fuel burnt and less CO2
• catalyst can be reused

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

Examiner's comment

This question required the candidates to explain the two step reduction of VO2+. The question was only answered well by a few candidates and electrode potentials continues to be a conceptually difficult topic. Choosing the correct systems was difficult for most candidates. Some used the Zn system, perhaps because of the diagram and others joined systems: 4+5, 1 and 7. Writing the two equations proved challenging. Equations were often unbalanced, written the wrong way round or missing the second reduction step completely. There was evidence of confusion between VO2+ and VO2+. When comparing electrode potentials, candidates should avoid the use of higher/lower are these phrases are ambiguous due to the negative signs involved. Phrases such as more negative/less positive removes this ambiguity. Candidates are advised to read the instructions contained within the equation and to use or comment on all the data presented. Equilibrium shifts were often stated as shifting left/right without making it clear to which equilibrium the candidate was referring to. Often only one equilibrium shift was mentioned. Candidates should be reminded to use lower case for the second letter in a chemical symbol. CO was often written for Co. While this was accepted for this question, clearly there is a significant chemical difference between these.

Notes from the mark scheme and report

• Co + 2VO2+ + 4H+ → Co2+ + 2V3+ + 2H2O then Co + 2V3+ → Co2+ + 2V2+
• E of Co system more negative than both systems 5 and 4
• Co system shifts left and V system shifts right
• IGNORE 'higher/lower' electrode potential
• ALLOW E cell +0.62 V and +0.02 V

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates were successful in writing the electron configuration for Fe. Many candidates did not realise that when transition metal ions are formed, the first electrons removed from atoms are the 4s electrons and so wrote 2s2 2p6 3s2 3p6 3d7 4s2. Others, incorrectly suggested, 4s1 or 4d7.

Notes from the mark scheme and report

• Fe …3d6 4s2
• Cu2+ …3d9 (4s electrons lost first)
• DO NOT ALLOW [Ar] shorthand

Mark scheme

Examiner's comment

Most candidates were able to identity the correct colour and followed this by describing a precipitate or solid. A few candidates missed the observation of the precipitate. Incorrect colours suggested were yellow, cream and brown. The latter was perhaps as a confusion with the Fe3+ ion.

Notes from the mark scheme and report

• green precipitate (Fe2+)
• (pale) blue precipitate (Cu2+)
• 1 mark for both colours without 'precipitate'
• DO NOT ALLOW dark blue

Skills: PAG4 Qualitative analysis of ions

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

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

The majority of candidates scored the mark here. The most common incorrect responses included: • Incorrect spelling of ‘methyl’ (although less common than in previous series) • Missing or incorrect locant number for C=C: 2-methylpentene or 2-methyl pent-3-ene • Inclusion of ‘an’ in the name: 2-methylpentan-2-ene • Miscounting the number of carbons: 2-methylhex-2-ene or 2-methylbut-2-ene

Notes from the mark scheme and report

• 2-methylpent-2-ene
• DO NOT ALLOW 2-methylpentan-2-ene or misspellings of methyl
• IGNORE missing hyphens

Mark scheme

Examiner's comment

Very few candidates scored both marks here. Many appeared not to understand what was being asked so explained how to assign priority groups to be able to identify isomers as either E or Z, but they didn’t say why these isomers occur. Some just gave the definition of a stereoisomer. Many referred to the presence of the C=C or double bond but not the reason why it causes E/Z isomers i.e. the double bond doesn’t rotate. Some said that the molecule couldn’t rotate but it wasn’t clear this was due to the presence of the double bond. Some candidates suggested that the isomers arise ‘due to free rotation’. This highlighted a possible misconception which was evident when candidates justified their responses as to why 2 isomers would form – i.e. with priority groups on the same side or opposite side once the C=C had rotated. Many struggled to explain that there are different groups on each carbon of C=C, and common errors included: • no reference to the C=C so could be any C in structure • 4 different groups around the C=C. Here there are only 3 different groups • 2 different groups on each side of C=C (could be above/top or below/bottom) • use of ‘functional groups’, ‘molecules’, ‘compounds’ or ‘species’ instead of groups • has priority groups (still must be clear that there are 2 on each C of C=C) A very small number mentioned optical isomerism.

Notes from the mark scheme and report

• restricted rotation of C=C
• each carbon of the C=C has two different groups
• DO NOT ALLOW 'functional groups' or 'molecules' for groups
• IGNORE 'C=C bond has two different groups' and references to priority
• Many candidates were unclear as to why E/Z isomerism would arise in some alkenes.
• Our Delivery Guide for Organic Chemistry offers some useful suggestions on teaching about E/Z isomerism in alkenes.

Mark scheme

Examiner's comment

Most gained credit here. Many identified chlorine as the toxic gas instead of HCl. However, a mark was not given if candidates stated ‘chlorine is toxic’ – it was necessary to say chlorine or other toxic products are formed (when polymers are incinerated). A common reason for losing the mark here was for saying that incineration formed ‘harmful’ gases which was insufficient. Some referred to the high bond enthalpy of the C-Cl bond, making it more difficult to dispose of as more energy is needed. Some suggested that polymers with chlorine in had high boiling points. Others thought it would produce chlorine radicals and linked their answers to ozone depletion.

Notes from the mark scheme and report

• produces toxic products/HCl
• IGNORE harmful, pollutant, dangerous

Mark scheme

Examiner's comment

The majority of candidates recognised this to be nucleophilic substitution. The most common incorrect response seen was nucleophilic addition.

Notes from the mark scheme and report

• nucleophilic substitution

Mark scheme

Examiner's comment

This question was well answered by most candidates. Incorrect responses often gave just the salt, NaBr, without the acid. Most common errors seen: • KBr or NaBr without acid • NaX or sodium halide without specifying that it is bromide needed here • Br2 • Using H+ without specifying which acid to use (need to be specific as HCl would be inappropriate).

Notes from the mark scheme and report

• NaBr AND H2SO4 (ALLOW HBr, KBr, H3PO4, PBr3)
• NOT HCl
• IGNORE H+/acid (must specify the acid)

Mark scheme

Examiner's comment

Candidates appeared better prepared for this type of question than in previous sessions, with a significant proportion achieving 3 or 4 marks. It differentiated well between candidates at different grades. Most candidates recognised the organic layer would be at the bottom but couldn’t always name the apparatus used to separate these two layers with incorrect responses including burettes, pear-shaped flasks, separating flasks and separating tubes. Some were confused about the density information, thinking that as the organic layer is denser it would be on top, or if adding more water it would then switch around the layers. Many gave the first step but not step 2 and 3. Lots of detail was given about how to use the separating funnel, e.g. invert to mix, vent to release pressure build up etc., to fill up the space provided. Most that went on to add an anhydrous salt were able to explain that it was used to dry the organic layer and were able to give a suitable example. Some candidates often misunderstood how to use them suggesting the anhydrous salt should be added to the separating funnel or the aqueous layer. Some candidates also suggested washing the layer with water or aqueous sodium carbonate to remove the acid once the organic layer had been dried. Many candidates either omitted the temperature to distil at to obtain pure 1-bromopropane or confused it with the boiling point of propan-1-ol (reactant) also given in the question. Some said to collect the distillate at its boiling point but didn’t quote the value that they had been given in the question so did not score the final mark. Many candidates introduced additional, unnecessary steps. Lower attaining candidates often confused the purification with that of an organic solid i.e. recrystallisation and scored 0 marks. A clear and concise response which scored all 4 marks.

Notes from the mark scheme and report

• separating funnel
• keep bottom (denser organic) layer
• dry with anhydrous salt (MgSO4/CaCl2)
• redistil collecting at 71 °C (70–72)
• no further marks if purification of a solid described
• We have produced a range of practice exam questions linked to the purification of an organic liquid (PAG5) which can be found on the Teach Cambridge website.
• This video produced by MaChemGuy is helpful for reviewing PAG 5: PRACTICAL SKILLS - PAG 5 - SYNTHESIS OF AN ORGANIC LIQUID.

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Most candidates scored at least one mark for recognising that the C-C bond lengths in the delocalised model of benzene are the all the same. Many found it difficult to articulate what they meant, e.g. ‘all the C-C and C=C bond lengths are the same’. Candidates were given the mark for stating that there were ‘equal bond lengths’. Although this is technically not true, as the C-H bonds are not the same length as the C-C bonds, we gave the mark as only skeletal structures are shown here without C-H bonds. Many also scored the second mark but slips were often made by reference to ‘hydration’ or ‘bond’ enthalpy. Some just said ‘enthalpy will be less exothermic’. Some described the hydrogenation value as being lower or higher as we are considering negative values it is much clearer to do this in terms of ‘less exothermic’ or ‘less negative’. A few described the enthalpy of hydrogenation as being ‘more exothermic than expected’. Many candidates were able to quote the actual figures in their response, and while it is not necessary for candidates to learn the values, their effort is commendable. A small number of candidates discussed the lack of reactivity in terms of electrophilic addition – not realising this had already been given in the question in terms of reaction with bromine. STEM Contributors We have produced a STEM Contributors spreadsheet linked to the H432 specification. This includes the work by Kathleen Lonsdale and Linus Pauling on our understanding of the structure of benzene. This is a great resource for broadening understanding of the contribution made by individuals to the topics being taught in the specification.

Notes from the mark scheme and report

• all C–C bond lengths the same/between single and double
• enthalpy of hydrogenation less exothermic than expected
• DO NOT ALLOW enthalpy of hydration
• IGNORE resistance to reaction (given)

Mark scheme

Examiner's comment

This mark was achieved by most candidates. Common errors included: • CH3 or CH4 as reagent • Missing the reagent and only giving the halogen carrier • CH3COCl as halogen carrier or reactant • AgCl3 as halogen carrier

Notes from the mark scheme and report

• CH3Cl AND AlCl3 (ALLOW CH3Br, FeCl3, FeBr3, AlBr3)

Mark scheme

Examiner's comment

To gain credit here candidates needed to explain what is meant by both ‘group’ and ‘functional’. Consequently, many didn’t gain credit here. Many responses began ‘A group…’ which didn’t gain the mark, as they needed to explain what a group is i.e. ‘atoms’ or ‘part of a molecule’. We allowed ‘part of an organic compound’ but not just ‘part of a compound’ as this term doesn’t apply to inorganic compounds. Some found it hard to explain what they meant. It was common to see the use of incorrect terminology here such ‘molecule’ or ‘elements’ or ‘species’ or ‘bonds. Most recognised that a functional group is responsible for the reactions of the molecules. Some were not specific enough, e.g. ‘properties’ or ‘characteristics’ but without chemical wasn’t enough. Some related a ‘functional group’ to belonging to a particular homologous series, but this wasn’t sufficient. For example, ‘group which determines the homologous series’ or ‘homologous series shares the same functional group.

Notes from the mark scheme and report

• atom/group of atoms responsible for the characteristic reactions
• IGNORE physical properties

Mark scheme

Examiner's comment

The majority of candidates scored both marks here. A few lost the second mark for just saying ‘turned orange’ without ppt. Some gave tests in (c) here as well but were usually consistent showing all three reacting in the table.

Notes from the mark scheme and report

• 2,4-DNP(H)/Brady's reagent
• orange/yellow/red precipitate (marked independently)

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

A wide range of marks were given for this question depending on candidates’ knowledge of different reactions. It was fairly common to see candidates drawing out synthetic route pathways on the additional blank pages to help them identify different reactions. This question was a good differentiator – those candidates with a wider knowledge of organic reactions were able to recall reagents and work out the structures of products, often gaining all 7 marks. Most candidates knew that SOCl2 is needed to form an acyl chloride with many giving the correct acyl chloride structure. Some attempted to use Cl2 or HCl instead of SOCl2. Some added Cl to the other end of the molecule leaving the -COOH group intact. Some had added a Cl on the structure removing only H on the -COOH group i.e. Many found it trickier to form compound P. Compound P often had -NH3 rather than -NH2. Some candidates reacted NH3 with various positions on the molecule. Many were able to react pyruvic acid with NaOH to form a salt as well as reaction with methanol in acid to for an ester. Care needed to be taken to keep the ketone group in the molecule and not attempt to react this as well. The trickiest step was the last one, which required a C-C bond forming reaction to extend the chain. Many knew that cyanide could be reacted here. Common incorrect reagents were KCN or NaCN with no H+, ammonia or NaBH4. The structure proved challenging for most candidates. Connectivity was sometimes an issue with the product formed with -NC or the omission of the triple bond between C and N if shown fully i.e. shown as C-N only. Some candidates left the C=O in place or missed out the methyl group or added an extra C before attaching the -CN i.e. Lower attaining candidates often attempted to just add NH3 or CH3NH2 here.

Notes from the mark scheme and report

• SOCl2 (ALLOW PCl5/PCl3
• DO NOT ALLOW (aq))
• acyl chloride then primary amide (DO NOT ALLOW –CONH3+)
• sodium carboxylate
• methyl ester
• NaCN/KCN AND acid (acid required) giving hydroxynitrile
• DO NOT ALLOW –HO or –NC connectivity
• Useful synthetic route maps for the whole specification, both with and without reagents, can be found on Teach Cambridge.

Mark scheme

Examiner's comment

The majority were able to correctly calculate Rf values and gain a mark here. A few did not gain marks for calculator errors (they had corrected measurements but gave the wrong answer) or for incorrect measurements, e.g. from the bottom of the TLC rather than the start line.

Notes from the mark scheme and report

• Rf = 0.75 (0.71–0.79) AND 0.25 (0.22–0.28)
• ±1 mm on each measurement
• working not required

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms; PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

A difficult question that few candidates fully understood. Even for those that knew the difference, it was rare to get the mark as they would only describe one of the acids rather than both, e.g. ‘C=O is next to the COOH in an  keto acid’ but with no reference to  keto acids. Teachers should remind candidates that a comparison question needs to have information about both or all items that are being compared. Many struggled to make the comparison to  amino acids (as given to help in the question) to help them explain what  keto acids are. Some suggested that  keto acids were  amino acids and that the difference to  keto acids was due to presence of C=O rather than NH2. Some linked  keto acids to  amino acids saying that the -NH2 group is linked to the next C along. It was also not sufficient to just say ‘in  keto acids the C=O is next to COOH but  keto acids it is found elsewhere on the molecule’ – it had to be clear where the C=O was relative to the COOH in each. However, we allowed for  keto acid that the C=O is on the adjacent C where the C of the C=O is the adjacent C, as candidates used the terminology given for  amino acids in the question. The clearest responses usually drew and labelled structures to show the difference in the positioning of C=O relative to COOH. Candidates should be advised that a well labelled diagram can help if they find it difficult to explain a point.

Notes from the mark scheme and report

• α: C=O on the carbon adjacent to COOH
• β: C=O two carbons away/one carbon in between
• IGNORE 'bonded to a different C' (must be specific)

Mark scheme

Examiner's comment

Very few candidates scored all 4 marks. The mark for identifying the alcohol functional group due to the presence of O-H bond in IR between 3200-3600 cm-1 was often lost for missing information. O-H or OH or hydroxy alone was not accepted as OH groups can be found within other functional groups i.e. phenol and carboxylic acid. Equally, it was important to identify the correct bond from the IR spectrum; in this case, O-H gives the most definitive answer. Other bonds are present, e.g. C-H and C-O but do not tell us that it is an alcohol. A few misinterpreted the ‘reasoning’ question stem and said alcohols would have the correct general formula to fit C4H10O or that alcohols could be dehydrated to make different alkenes, which is correct, but confirmation needed to come from the IR provided in the question. Many candidates gave a primary alcohol, butan-1-ol, as K. It appears many candidates focused on the fact that K was refluxed with H2SO4 and assumed this was oxidation, despite there being no K2Cr2O7, giving L and M as aldehydes, ketones or carboxylic acids. Some just gave alcohols with different structural formula for the alcohol for L and M, attempting to use the 13C NMR data. A significant proportion of candidates did not gain any marks on this question.

Notes from the mark scheme and report

• alcohol AND O–H AND peak at 3200–3600 cm–1 (DO NOT ALLOW phenol/carboxylic acid
• IGNORE C–O)
• K = butan-2-ol, L = but-1-ene, M = but-2-ene (E or Z)
• 1 mark if L and M correct but in wrong boxes

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

Mark scheme

Examiner's comment

Most candidates gave vague responses in terms of the properties of isotopes. Some candidates were able to work out that the 158 and 162 peaks resulted from two 79Br and two 81Br isotopes respectively. The peak at 160 proved to be more difficult with many claiming that it resulted from two 80Br isotopes rather than one 79Br and one 81Br isotope. Predicting the relative abundance of the peaks was difficult, the most common response being that the peaks would have the same heights, presumably because the 79Br and 81Br isotopes have similar abundances.

Notes from the mark scheme and report

• 158 = 79Br79Br, 160 = 79Br81Br, 162 = 81Br81Br: m/z must be linked to isotopes
• 1 : 2 : 1
• IGNORE charges

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates identified the number of peaks, for which one mark was available. The splitting patterns gave a variety of answers, with the triplet and pentet splitting from BrCH2CH2CH2Br proving to be the easier response. Most candidates predicted that BrCH2CH2Br would produce a triplet, rather than a singlet. This part of the question assessed whether candidates could identify protons with the same environment, and that they do not split one another.

Notes from the mark scheme and report

• BrCH2CH2Br: 1 peak AND singlet (equivalent protons do not split)
• BrCH2CH2CH2Br: 2 peaks
• triplet AND quintet
• Many candidates do not consider chemical environments when predicting splitting patterns.
• Protons with the same environment do not split one another.

Mark scheme

Examiner's comment

This question was answered well, with most candidates identifying that ibuprofen contains a phenol group or that it does not contain a carboxylic acid. A named functional group was required in the answer. Some described phenol incorrectly as not being an acid and this was not credited. It was rare for incorrect functional groups to be seen in candidate responses.

Notes from the mark scheme and report

• it is a phenol / has no COOH: a correct functional group must be named
• IGNORE 'too weak an acid'
• DO NOT ALLOW 'not acidic', ketone or amine

Mark scheme

Examiner's comment

Candidates answered this question well. Overall, there has been an improvement of candidates’ ability to write systematic names involving Roman numerals Most incorrect responses either didn’t use a Roman numeral or referred to sulfur oxides or sulfides. Although not a systematic name, the mark scheme did allow sulfite. The old spellings, “sulphate” and “sulphur” are still commonly seen and were allowed.

Notes from the mark scheme and report

• sulfate(IV): IV essential
• ALLOW sulfite
• DO NOT ALLOW 'oxide' in name

Mark scheme

Examiner's comment

Candidates produced a range of answers, and the question discriminated extremely well. The higher attaining candidates often gave 6 or 7 correct formulae. The flowchart contained many clues which most candidates used. Most were able to provide at least 3-4 of the required formulae and there was no consistently wrong answer. Most candidates gave H+ and Cl– as two of the three ions, and AgCl for the white precipitate with Ag+(aq). Both S and S8 were allowed for sulfur but S2 was not allowed. As SO42– ions are present, Ag2SO4 was also allowed for the precipitate with Ag+(aq). Common errors included acid formulae such as H2SO4 in place of water, an incorrect formula for BaSO4, giving BaSO3, writing SO4– for the sulfate ion, and OH– as one of the three ‘acidic’ ions. Lower attaining candidates added formulae without using information in the question. Examples include full formulae such as HCl in the box labelled ‘ions’ and CuCl42– for the yellow solid, despite Cu being absent in the flowchart.

Notes from the mark scheme and report

• all responses must be formulae: AgCl (ALLOW Ag2SO4), BaSO4, three ions from H+, Cl–, SO42– (ALLOW ClO–, HSO4–), S (ALLOW S8
• DO NOT ALLOW S2 or 'sulfur'), H2O

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Candidates produced a range of formulae, mainly from not counting H or C properly or omitting one of the elements, often Ni. A significant number of candidates wrote a structural formula rather than the empirical formula asked for in the question.

Notes from the mark scheme and report

• C6H16N2NiO6: must be empirical formula
• DO NOT ALLOW Ni(C3H6NO2)2(H2O)2

Mark scheme

Examiner's comment

This name caused problems for most candidates. Phenyl methanol was expected, and unambiguous representations were allowed including 1- phenylmethanol, phenylmethan-1-ol and 1-phenylmethan-1-ol Many candidates incorrectly based the name on methylphenol or hydroxymethylbenzene.

Notes from the mark scheme and report

• phenylmethanol
• DO NOT ALLOW hydroxymethylbenzene or methylphenol

Mark scheme

Examiner's comment

This question was answered correctly by most candidates, the most common error being the omission of ‘pair' in the term. Lower attaining candidates often used incorrect terms such as ‘proton donor’ and ‘electron acceptor’.

Notes from the mark scheme and report

• donates an electron pair / lone pair

Mark scheme

Examiner's comment

Most candidates found this question difficult with only higher attaining candidates being able to synthesise the information provided in the method and the mechanism. In 5(a)(iv), the last step in the mechanism for Step 2 gave a clue with the two products shown with a negative charge on both O atoms. The mark scheme required an understanding that an H+ ion was involved in Step 3. With the preparation involving H– ions, responses referring to a ‘hydrogen ion’ were ambiguous and imprecise. A common incorrect response referred to hydrolysis of the ester.

Notes from the mark scheme and report

• H+ protonates the O– / negative ion to form the alcohol: must include idea of H+ being gained
• IGNORE 'to form an OH group/alcohol' alone

Mark scheme

Examiner's comment

This question required candidates to work out a systematic name from a formula. Transition elements can have different oxidation numbers in their compounds and the systematic name needs to contain a Roman numeral. Approximately half the candidates were able to write the correct name as iron(III) oxide. An array of incorrect names were seen, commonly iron(II) oxide, presumably from the number of iron atoms in Fe2O3.

Notes from the mark scheme and report

• must be systematic: iron(III) oxide
• IGNORE iron(3) oxide, iron(III) dioxide
• ALLOW no brackets
• A systematic name may contain the oxidation number, not the number of atoms in the formula. So Fe2O3 is iron(III) oxide and not iron(II) oxide.

Mark scheme

Examiner's comment

Most candidates described a strong and a weak acid in terms of dissociation or ionisation, with few just describing one of the two types of acid.

Notes from the mark scheme and report

• strong acid fully dissociates AND weak acid partially dissociates
• DO NOT ALLOW 'weak acid does not fully dissociate' alone
• DO NOT ALLOW comparison of concentrations
• IGNORE dissociates more/quicker

Mark scheme

Examiner's comment

Most candidates identified at least one feature of a dynamic equilibrium. ‘In a closed system’ was given marks most often, and other mark-worthy features were ‘the same rate for forward and reverse reactions’ and ‘concentrations do not change’. Less successful responses that were not given marks included ‘reversible reaction’ and ‘concentrations are the same’.

Notes from the mark scheme and report

• rate of forward = rate of reverse
• concentrations constant
• closed system
• DO NOT ALLOW concentration of reactants = concentration of products

Mark scheme

Examiner's comment

This long-response question was approached very well and there were some excellent and concise responses. Only the less successful responses did not identify the main trends. Candidates are well-versed with tackling this type of question and most were able to write sensible explanations. However, marks could not be given for insufficiently specific responses. For example, some candidates stated that increased pressure would shift the equilibrium in the direction with fewer moles, without stating what that direction was for this equilibrium. It was also common to see explanations that contradicted earlier statements. Responses could be improved if candidates read through what they write to see if it makes cohesive sense. Nearly half of the scripts were given the full 3 marks, the question proving to be a good discriminator.

Notes from the mark scheme and report

• low temperature because forward reaction exothermic
• high pressure because fewer gaseous moles on right
• equilibrium shift stated correctly at least once
• DO NOT ALLOW gaseous atoms

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

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

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

Some appropriate answers were seen, mostly discussing the asymmetrical shape of water. More than half didn’t score the mark. However, many were able to describe O-H being a polar bond due to the difference in electronegativity but didn’t realise that this doesn’t always mean that the molecule itself is polar. The relationship between the words electronegative, dipole and polar need more attention and how this all relates to the shapes of molecules. Some candidates said symmetrical when they meant asymmetrical.

Notes from the mark scheme and report

• dipoles do not cancel/overall dipole/unsymmetrical
• IGNORE 'polar bonds do not cancel'
• IGNORE charges uneven

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

Higher-attaining candidates often gave a very detailed responses with all stages, including warming the NaOH, use of damp litmus paper and some included an ionic equation. Quite a few lost a mark as they missed the addition of hydroxide, just warmed, but they still gained mark for testing with indicator paper. Some thought that the indicator paper would turn red or be bleached and a few gave incorrect ion test e.g. add silver nitrate, add acid. Over a third of candidates did not score on this question, with a significant proportion not even attempting it.

Notes from the mark scheme and report

• (warm with) NaOH/hydroxide
• damp litmus/indicator paper turns blue
• DO NOT ALLOW ammonium hydroxide or ammonia as reagent

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Generally, this question was well answered with a clear understanding of how and why ions are formed. However, approximately a quarter of students only gained 1 mark as they either didn’t explain electrons being lost by Mg and gained by Cl or gave no justification. A common slip was stating Cl has one electron in its outer shell. Some described bonding between Mg and Cl, which wasn’t what the question asked, but this didn’t prevent them from scoring both marks.

Notes from the mark scheme and report

• Mg loses 2 electrons AND Cl gains 1
• to obtain a full outer shell/noble gas configuration

Mark scheme

Examiner's comment

Explaining redox reactions is a common question in exam papers, however here candidates needed to do it ‘in terms of electron transfer’. Subsequently, many lost a mark as they identified oxidation and reduction in terms of oxidation numbers only. However, many gave responses both in terms of oxidation numbers and electrons. It was necessary to be specific here and say Ca had lost 2 electrons, so a few lost the mark by only referring to ‘Ca losing electrons’. Some lost marks for only describing oxidation of Ca and not reduction of H. There was some evidence that candidates were not sure of Cl’s role in the reaction (i.e. as a spectator ion) with some stating it was reduced and/or accepted electrons from Ca but then gave them to H.

Notes from the mark scheme and report

• Ca loses 2 electrons AND is oxidised
• H(+) gains electron AND is reduced
• IGNORE oxidation numbers (question asks for electron transfer)

Mark scheme

Examiner's comment

Most candidates correctly calculated the amount of HCl as 0.012 mol. However, many struggled with demonstrating that Ca is in excess. Responses often highlighted misconceptions here in terms of candidates’ understanding about excess and limiting reagents. For example, ‘Ca has a lower concentration than HCl so becomes the limiting reagent’ and ‘Not all the HCl had reacted’ Many compared moles of HCl calculated (i.e. 0.012) directly to moles of Ca (i.e. 0.01) saying that HCl was in excess, despite being told otherwise in the question. Some had the 2:1 ratio of HCl to Ca the wrong way around. Some attempted to calculate mass of Ca and HCl to use for comparison. Only a small proportion of candidates were able to access the third mark and correctly suggest that Ca was also reacting with water. Some other suggestions that were seen included: • ‘Ca reacted with oxygen or was impure’. In both cases this would mean that we would expect solid to remain. • ‘Higher concentration of HCl added’, or ‘HCl is a strong acid’, or ‘acid acts as a catalyst’. • ‘H2 evolved’ or ‘Ca reacts with hydrogen formed’. • ‘Human error’, ‘didn’t weigh Ca correctly’, ‘measured volume of HCl incorrectly’.

Notes from the mark scheme and report

• n(HCl) = 0.012 mol
• must show 2:1 ratio (0.006 mol Ca needed or 0.02 mol HCl needed)
• excess Ca reacts with the water
• Candidates often struggle to understand the concepts around limiting reagents and those in excess. Using a simple baking analogy can help to relate this to everyday life.
• For example: To make 10 pancakes you need 100 g flour, 2 eggs and 300 ml milk How many pancakes can I make if I have only 50 g flour, 2 eggs and 300 ml milk? Which is the limiting ingredient and which are in excess? The number of pancakes we can make is the theoretical yield.

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages

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

Examiner's comment

There was a lot of misunderstanding associated with this question, with many candidates failing to score any marks. Many said that nothing would change as the concentration was still the same or because the same bonds were being broken and formed. Under a quarter of students scored 1 mark, usually for making the link between the drop in volume to a change in the q and n values. A few did state that the temperature didn't change. Only a small proportion scored both marks, usually by showing by calculation that the temperature change was the same, moles was half and energy was half. Some did believe that the temperature changed, either that it decreased as less reacted or increased as there was less volume to heat. A wide variety of alternative responses were given including: ‘Enthalpy change the same regardless of mass used’ ‘Number of moles doesn’t impact energy required as it is the same bonds breaking’ ‘Energy to break and form bonds will still be the same with any volume’ ‘Amount of energy required to make the new bond would be the same’ ‘Only concentration has an effect on bond enthalpy values not volume’ ‘Decrease in volume increases concentration’ ‘Cu(NO3)2 would still run out first so enthalpy change is the same’ ‘Zn is in excess so it doesn’t matter how much volume we use because Zn and Cu(NO3)2 still 1:1 ratio.’ ‘Mole ratio is still the same’ or ‘same molar ratio’ wasn’t enough.

Notes from the mark scheme and report

• half the energy/volume AND half the moles
• temperature change the same
• ALLOW shown by calculation

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

More than half of candidates achieved both marks and most scored at least 1. The most common reason for losing marks was for not being specific, by just saying ‘concentration’ or ‘volume’, or suggesting volume and concentration for metal oxide. Some suggested use of the same gas syringe without realising that the apparatus used should not affect volume of gas produced, just how easy and accurate it is to measure.

Notes from the mark scheme and report

• any two: amount/mass/surface area of catalyst, temperature, volume or concentration of H2O2
• DO NOT ALLOW concentration/volume of catalyst

Skills: PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Over half of candidates didn’t gain any credit for their response here. Answers often focused on Markownikoff’s rule, i.e. adding H to C with most H atoms already attached, or described the stability of the product. Students need more support in understanding that the more stable carbocation will be formed in preference and result in forming more product. Many referred to the ‘secondary haloalkane’ being more stable, rather than the ‘secondary carbocation’. Some stated that 2-bromobutane is a ‘secondary carbocation’, showing some misunderstanding about the terminology used here.

Notes from the mark scheme and report

• forms via the more stable (secondary) carbocation/intermediate
• IGNORE statements of Markownikoff's rule about H joining C with most H
• IGNORE stability of the product
• We have a useful PowerPoint Presentation for teaching about Markownikoff’s rule and carbocation stability: https://ocr.org.uk/Images/250388-markownikoff-s-rule-presentation.ppt

Mark scheme

Examiner's comment

Lots of good answers from candidates were seen for this question. A few candidates attempted the explanation via a ΔG / ΔS argument and misinterpreted the comment within the question.

Notes from the mark scheme and report

• DO NOT ALLOW 'not enough activation energy'
• IGNORE 'not enough energy to react'

Mark scheme

Examiner's comment

This was a well answered question. A few candidates, incorrectly, suggested that it was heterogeneous due to the reactants and products being in different states, and did not mention the catalyst.

Notes from the mark scheme and report

• state/phase of catalyst compared with reactant
• IGNORE references to products

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 required a standard definition to be recalled. The two components are (a) one mole of an ionic lattice and (b) formed from its gaseous ion. Some candidates produced answers combining all the various definitions, most often 1 mol of gaseous ions. In general, when enthalpy definitions are required, candidates should be mindful of whether energy is given out or taken in. The phrase ‘enthalpy that accompanies’ may be most appropriate when candidates are unsure of the energy change.

Notes from the mark scheme and report

• 1 mol of ionic lattice formed from gaseous ions
• DO NOT ALLOW 'one mole of gaseous ions'

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 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

Most candidates scored the marking point. They realised that the full formulae were needed although some candidates left off the square brackets or used HA or [H]2. Care should be taken in checking the correct amount and location of H in the formula. As an acid dissociates to form a H+, it is important to acknowledge only one H+ is dissociated from the correct part of the molecule.

Notes from the mark scheme and report

• square brackets required
• DO NOT ALLOW [H+]2 in Ka expression here

Mark scheme

Examiner's comment

This question required the candidate to understand that the dissociation of water to produce H+ ions had no effect on the overall [H+] of the solution, leading to [H+] = [A-]. This was mostly answered well but some candidates used the idea of [H+] = [OH-].

Notes from the mark scheme and report

• must be in terms of concentration

Mark scheme

Examiner's comment

Nearly all candidates scored this marking point. Phenol red and Phenolphthalein were good choices of indicator as their colour changed on the sharp vertical section of the graph, depending on how the top end of the line of best fit was drawn. Occasionally malachite green and bromophenol blue were seen.

Notes from the mark scheme and report

• indicator must change on the vertical section of the candidate's own curve

Mark scheme

Examiner's comment

Candidates performed well on this question with most knowing that Cl2 kills the bacteria. Some stated that chlorine removed bacteria or purified the water.

Notes from the mark scheme and report

• kills bacteria/microorganisms
• IGNORE 'removing' bacteria

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

Examiner's comment

Most candidates were able to successfully show the overall equations were the same. Most combined the two half equations and then cancelled down to give the overall equations. A few candidates chose to describe the redox nature or use the equilibrium shifts scoring the first 2 marks. Some candidates, incorrectly, deduced the equation as H+ + OH- → H2O. Some candidates gave the 1.23V value without showing any calculation or did not refer to this part of the question at all. This was one of the most common questions where candidates omitted to provide a response on the paper.

Notes from the mark scheme and report

• E cell = 1.23 V in both
• combine half-equations to give 2H2 + O2 → 2H2O in both media

Skills: M0.0 Recognise and make use of appropriate units in calculations

Mark scheme

Examiner's comment

Most candidates scored three marks. Some used other transition metal ions such as Cu2+ or Mn2+ and candidates should be mindful of the information given in the question. Charges were sometimes incorrect, and some responses lacked the square brackets to show the complex.

Notes from the mark scheme and report

• square brackets essential for complex ions
• Co(OH)2 precipitate

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

Most candidates were able to correctly identify the alicyclic compounds. Some candidates also included Structure C showing some confusion between terms alicyclic and aromatic. This was also noted in last year’s paper on Question 13.

Notes from the mark scheme and report

• A and B only
• A molecule is either aromatic, if it contains a benzene ring, or aliphatic. Aliphatic molecules which contain a ring can also be described as alicyclic. OCR have previously offered clarification for classification of organic compounds.

Mark scheme

Examiner's comment

This question was also challenging and just under half of candidates gave the correct response. Many wrote molecular formulae underneath each structure, but less successful responses often struggled to convert from the skeletal formulae correctly. Common errors included: • just giving Structure D, and omitting Structure A • including Structure E as they miscounted the number of hydrogens in the presence of a triple C-C bond • including Structure B, on the basis that it had a double C-C bond • some less successful responses just wrote “alkene” and did not apply their responses to the hydrocarbon structures given.

Notes from the mark scheme and report

• A and D only (cycloalkane and alkene)

Mark scheme

Examiner's comment

This question appeared to be equally challenging for candidates. Common errors included names without the lowest possible numbering. The methyl group was given on the top which could have influenced candidates to start counting from there – more practice is needed to help with naming simple aromatics. Other common errors included:1-ethyl-2,4-methylbenzene (missing di-) and 1-ethyl-2,3- dimethylbenzene (miss counting groups).

Notes from the mark scheme and report

• 1-ethyl-2,4-dimethylbenzene
• smallest numbering needed – DO NOT ALLOW 1,5-dimethyl-2-ethylbenzene
• DO NOT ALLOW misspellings such as dimethy, ethly
• IGNORE alphabetical order

Mark scheme

Examiner's comment

Many candidates misunderstood the question instead explaining how a molecule shows stereoisomerism i.e. same structural arrangement but a different arrangement of atoms in space. Some gave details specific to alkenes i.e. restricted rotation around C=C and each carbon of C=C has 2 different groups attached. Just over a quarter of candidates did not score any marks here. Many that did recognise that the ‘priority groups’ are on the same side’ but did not then go on to score the second mark by identifying which groups they were referring. A significant number of candidates (including those gaining marks) described priority in terms of highest Mr or mass, suggesting that the Cahn-Ingold-Prelog priority rules of using atomic number are not well understood. Many framed their answer in terms of the methyl groups, possibly showing confusion with cis-trans isomerism. Note that same ‘plane’ was not accepted as all the molecule is planar, so all groups are in the same plane. Teachers are recommended to encourage students to stick with conventional terminology i.e. Z: same side E: opposite sides.

Notes from the mark scheme and report

• priority groups on the same side
• highest priority groups are CH3 and C2H5 (or lowest are CH3 and H)
• IGNORE references to mass/Mr of groups
• 1 mark if 'priority' not mentioned
• A useful PowerPoint presentation has been produced by OCR to help with teaching about CIP rules.

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

More than half scored both marks here. Common reasons for losing marks included adding water or steam as a reagent, using an incorrect chain length (5 carbons not 4 carbons), missing or adding hydrogens on the structure, e.g. CH3CH2CH2=CH2 or giving butane as the product.

Notes from the mark scheme and report

• acid catalyst H2SO4/H3PO4
• DO NOT ALLOW HCl (substitution) or additional reagents such as steam
• but-1-ene
• marks independent
• Useful synthetic route maps for the whole specification, both with and without reagents, can be found on Teach Cambridge.

Mark scheme

Examiner's comment

This was the most well-known out of the four reactions with more than three quarters of candidates getting both marks. The most common errors included omission of acid catalyst or adding an extra hydrogen on the carbonyl carbon of C=O. Candidates should be reminded to count the number of bonds around each carbon before moving on. A few attempted to oxidise this secondary alcohol to form a carboxylic acid.

Notes from the mark scheme and report

• K2Cr2O7 AND H2SO4 (Cr2O7 2–/H+)
• butanone
• IGNORE [O]
• DO NOT ALLOW HCl as the acid

Mark scheme

Examiner's comment

Generally well-answered with very few gaining no marks. Some lost marks for correctly giving sodium halide but forgetting the acid. Some gave the halogen rather than the halide. A few added an additional CH2 to structure. Some attempted to substitute directly with cyanide to give a nitrile product, e.g. (CH3)2CHCH2CN.

Notes from the mark scheme and report

• mark product first: (CH3)2CHCH2X
• reagent NaX AND H2SO4 consistent with product
• 1 mark for correct reagents with wrong/no product
• DO NOT ALLOW additional reagents e.g. AlCl3

Mark scheme

Examiner's comment

Once again, more than half scored both marks here but candidates found it more challenging than Question 17 (c). Many stated ‘carboxylic acid’ but did not identify the specific example to use to give their product. A common error was to omit the acid catalyst. Some had the ester the wrong way round: i.e. As seen with all parts of this question, some candidates struggled with the structural formula, resulting in some giving an additional carbon in their structure i.e. A few candidates used an acyl chloride but it was rare to see an acid anhydride suggested.

Notes from the mark scheme and report

• mark product first: a correct ester of (CH3)3COH
• reagent (carboxylic acid + acid catalyst, acyl chloride or acid anhydride) must match the ester
• DO NOT ALLOW additional reagents e.g. Cr2O7 2–

Mark scheme

Examiner's comment

Most candidates scored one mark here, with approximately a third of candidates scoring both marks. Most recognised that further or multiple substitution would occur or that there would be a variety of termination products. The second mark was harder to achieve as although many responses suggested a different position, it was not always clearly conveyed that a hydrogen on a different carbon along the chain was being substituted, e.g. ‘substitution can occur anywhere in the molecule’. The best responses gave examples to clarify their point, e.g. ‘the bromine radical can be substituted anywhere along the hydrocarbon chain making other products such as 1-bromo-2-methylpropane’. However, many gave vague answers such as poor yield or mixture of products, without explanation of how or what they might be. Some focused on HBr being formed as a product, suggesting it is toxic, or lowers the atom economy. Others highlighted ultraviolet radiation as being a limitation due to it being ‘expensive’, ‘hard to achieve’, ‘lack of sources’ or ‘hazardous’. Others suggested that the reaction needed ‘high temperatures’.

Notes from the mark scheme and report

• further substitution (or different termination products)
• substitution at different positions along the chain
• IGNORE separation of products, atom economy and yield

Mark scheme

Examiner's comment

This question was well-answered with around a half of candidates scoring 8 or 9 marks. The full range of marks was seen across the whole cohort. The most common reasons for losing marks were for the addition of extra, contradictory reagents or for missing hydrogens from structures. Bromoethane to amine: The amine structure was mostly correct with a few adding an extra hydrogen to the amine i.e. CH3CH2NH3. Most gave the correct reagent as NH3 but some omitted either ethanol or excess, so did not score here. Bromoethane to alcohol: The alcohol structure was usually correct. However, many lost the reagent mark here for the addition of acid/ H+/H2SO4 alongside the hydroxide. Bromoethane to nitrile: A suitable cyanide was often used, but again many included an acid catalyst or aqueous conditions (aq) so lost the mark here. Ideally candidates would react in ethanol to prevent hydrolysis of the haloalkane. Nitrile to carboxylic acid: Most gave the correct carboxylic acid structure, while occasionally ethanoic acid was given instead of propanoic acid. Most identified the need for acid as a reagent but some omitted water or (aq). Nitrile to amine: H2 was often seen as the reagent but sometimes without a catalyst. Many gave an incorrect structure here either with a missing C (i.e. ethylamine) or missing hydrogens on the first carbon i.e.

Notes from the mark scheme and report

• NH3 in ethanol/excess NH3 gives CH3CH2NH2
• OH–(aq) gives ethanol
• KCN in ethanol (DO NOT ALLOW HCN, CN–/H+ or aqueous)
• H+(aq) gives propanoic acid
• H2 AND Ni gives CH3CH2CH2NH2 (3 C not 2 C)
• DO NOT ALLOW OH– or NH2– as structures
• structure with H missing penalised once
• A useful resource for teaching about organic synthetic routes including functional groups, reagents and two-step processes can be found in the Topic Exploration pack on Teach Cambridge.

Mark scheme

Examiner's comment

Candidates found this difficult, with less than half gaining the mark. The most common error was incorrectly numbering the methyl group due to counting from the wrong end, giving ethyl-2- methylpentanoate.

Notes from the mark scheme and report

• ethyl 4-methylpentanoate
• DO NOT ALLOW 1-ethyl-4-methylpentanoate
• DO NOT ALLOW misspellings (methy, ethly)

Mark scheme

Examiner's comment

Most candidates were able to give the correct definition here. A common error was omission of ‘pair’ of electrons. Many also described that electrophiles are ‘species attracted to areas of high electron density’ or words to that effect, either alongside the accepted definition gaining credit or as the sole definition not gaining a mark.

Notes from the mark scheme and report

• electron pair acceptor

Mark scheme

Examiner's comment

Similar questions have been seen previously and many candidates were able to give clear and concise responses. The first marking point was the most frequently lost as although many described -NH2 as electron donating, they were not able to fully explain its role. Some understood that a lone pair was donated into the -ring but did not specify that the lone pair was on the nitrogen. Other marks were lost by not making comparison to benzene, e.g. high electron density, polarises Cl2. Some repeated the information from the question regarding phenylamine being more reactive with electrophiles but not explaining why. Lower attaining candidates often described the structure of the benzene ring or referred to phenylamine being more electronegative.

Notes from the mark scheme and report

• lone pair on N (not just NH2) partially delocalised into the π-system
• electron density of ring higher than benzene
• more susceptible to electrophilic attack/polarises Cl2 more – comparisons required
• DO NOT ALLOW two lone pairs
• IGNORE activating, charge density, electronegativity

Mark scheme

Examiner's comment

An unfamiliar question that proved challenging with only around a fifth of candidates obtaining the mark for correctly suggesting that reflux would prevent loss of volatile compounds. Many candidates suggested that reflux ensures the reaction goes to completion but here this was insufficient as esterification is an equilibrium reaction and additional information in (b)(i) indicates that there is unreacted compound G present. It was necessary to focus on the purpose for reflux rather than other ways of heating a reaction, such as the energy needed to break bonds or speed up the rate of reaction. Some less successful responses linked to oxidation reactions, presumably as they understand the importance of either reflux or distillation in this context. For example, ‘reflux is required for complete oxidation’ or ‘if distillation had been used an aldehyde would have been formed’.

Notes from the mark scheme and report

• prevents loss of volatile reactants/products by evaporation (mixture boiling dry)
• IGNORE rate or completeness of reaction

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

The best responses correctly identified that using sodium hydroxide was not an improvement and explained this either by stating that it would react with the phenol group or hydrolyse the ester group in compound H. However, most candidates appeared not to consider a reaction with H in their answer. Many focused on the neutralisation of sulfuric acid in a similar way to sodium carbonate and gave responses such as: • stronger base • no effervescence so harder to see when completely reacted • no CO2 produced so easier/safer/higher atom economy/less waste • requires double the moles compared to Na2SO4 to react

Notes from the mark scheme and report

• NaOH would react with the phenol –OH OR hydrolyse the ester (compound H)
• DO NOT ALLOW 'reacts with alcohol'

Mark scheme

Examiner's comment

Many candidates identified the need to describe the isolation and purification of a liquid in response to this question. Some struggled to remember the name of the apparatus required i.e. ‘separating funnel’ but could describe the separation of layers. The best responses used the density data provided to explain that the lower layer would contain compound H. A few gained a mark for a description of the addition of water to identify the correct layer to discard. In generally, candidates only have experience of the organic layer being less dense in their practical work which was reflected in their responses. The use of an anhydrous salt to dry the organic layer was well-known, but many candidates gave vague answers such as ‘add anhydrous salt’ failing to justify why the salt was needed. Many recognised the need to purify the resultant liquid by distillation, but not all linked to the boiling point of compound H to secure this final mark. Some who didn’t dry with an anhydrous salt said to heat to 100oC to remove water. Approximately a third of candidates gained no marks here. Candidates thought that compound H would form crystals so described recrystallisation rather than a method to purify an organic liquid. Other responses attempted to merge both methods together. This candidate scored 1 mark for this response for the use of a ‘separating funnel’. They have attempted to use the density information but have incorrectly identified that the density of H as being low which would make it the top layer. Candidates needed to know that the density of water is 1 g cm-3 to be able to make a correct comparison. There is no addition of an anhydrous salt to dry the organic layer. They have recognised the need to distil but have not given the correct temperature at which to collect the pure compound. However, if they had given the boiling point for distillation no mark would have been given as they would lose this final mark for describing the purification of a solid. It was very common to see responses which described the purification of a solid, often in addition to that of a liquid.

Notes from the mark scheme and report

• steps in order: separating funnel
• keep the bottom (organic, denser) layer
• dry with anhydrous MgSO4/CaCl2
• redistil at 222 °C
• no further credit if the method treats H as a solid (recrystallisation)
• The method to purify an organic liquid was poorly understood by many candidates. Many of the candidates answered this question using techniques to purify an organic solid which is covered in the second year of A Level, rather than an organic liquid which is covered in the first year. It is important to spend time comparing both methods and helping candidates identify when each method is required.
• OCR have produced a range of practice exam questions linked to the purification of an organic liquid (PAG5). These can be found on Teach Cambridge.

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Most candidates were aware that a bond enthalpy is a measure of the energy to break bonds. Most went on to link the positive sign to an endothermic reaction which requires energy. Some candidates linked bond enthalpies to bond formation instead. A significant number of responses described an endothermic reaction, instead of bond enthalpy, in terms of the energy required to break bonds being greater than the energy to make bonds. This was the answer to a different question and could not be given marks.

Notes from the mark scheme and report

• bond breaking is endothermic/requires energy
• IGNORE 'more energy needed to break bonds than released making bonds'

Mark scheme

Examiner's comment

Most candidates made a good attempt at working out the molecular formula of the structure as being C13H19N3O7. N and O were usually correct with mistakes most common with carbon (especially 12) and hydrogen (especially 17–20).

Notes from the mark scheme and report

• C13H19N3O7
• elements in any order

Mark scheme

Examiner's comment

This question was answered well with the correct answer of 4 being seen on most scripts, reflecting good understanding of chiral carbon centres. The commonest incorrect response was 5, presumably by including the C atom on the bottom right of the structure within the –CH(CH3)2 group.

Notes from the mark scheme and report

• 4 chiral carbons

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

The term ‘bidentate ligand’ was well-known with most candidates gaining some credit. A mark was sometimes not given if ‘lone pairs’ had been omitted. A significant number of candidates just wrote ‘a pair of electrons’, which could have been a bonded pair.

Notes from the mark scheme and report

• two lone pairs AND forms dative/coordinate bonds to a metal ion
• IGNORE 'donates two pairs of electrons' alone

Mark scheme

Examiner's comment

Candidates were expected to link proton-transfer behaviour in acids and bases to the provided equilibria. The question differentiated between candidates well. Some candidates just stated that an acid is a proton donor and a base a proton acceptor without referring to the provided equilibria. This was the answer to a much simpler question and could not be given marks. The best responses demonstrated excellent understanding within the context of the equilibria. Such candidates clearly explained how H2PO4– behaves as an acid in the forward reaction of Equilibrium 2 and as a base in the reverse reaction of Equilibrium 1.

Notes from the mark scheme and report

• equilibrium 1 (reverse): H2PO4– accepts a proton so acts as a base
• equilibrium 2: donates a proton so acts as an acid
• IGNORE references to HPO4 2– and equilibrium 3

Mark scheme

Examiner's comment

Most candidates wrote the correct systematic name of phosphorus(III) acid and the clue given in the question for the name of H3PO4 should have helped. Common errors included phosphorus(IV) acid, the same as for H3PO4, and the (III) oxidation number being placed after ‘acid’ in the name. The commonest error though, was hydrogen phosphate. Candidates are advised to use any information provided in the question, which often contains clues. This certainly would have prevented hydrogen phosphate as a response.

Notes from the mark scheme and report

• phosphoric(III) acid
• DO NOT ALLOW phosphoric acid (III) or phosphorous acid

Mark scheme

Examiner's comment

About half the candidates suggested a range of creditworthy responses with ‘different bond lengths’ and ‘decolorises bromine’ being the most common.

Notes from the mark scheme and report

• (P–N) bond lengths different, or more exothermic hydrogenation, or reacts with bromine by addition
• IGNORE C–C bond lengths
• IGNORE 'alternating single and double bonds'
• IGNORE 'more reactive' without example

Mark scheme

Examiner's comment

This was another question where valuable information: ’all N and Cl atoms are bonded to P atoms’ had been provided. Many of the structures seen ignored this information with chlorine often been shown bonded to a nitrogen atom. Nitrogen atoms were often shown with 1 bond only and chlorine atoms in the ring structure with 2 or more bonds. Most structures contained 6 or 3-membered rings. This was a difficult question, requiring candidates to use the supplied information to come up with realistic structures that met chemical bonding rules. Only about a quarter of candidates could be given any mark. The Kekulé theme in Questions 4 (c) (i) - (iv) should have prompted candidates that a Kekulé structure was likely here. Several other structures were allowed providing that they met normal chemistry bonding rules.

Notes from the mark scheme and report

• six-membered alternating P–N ring
• each P bonded to 2 Cl
• each N with 3 bonds and each P with 5 (or 3) bonds for second mark
• zero marks if N bonded to Cl or N with only 1 bond

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 novel question assessed whether candidates realised why chemists used TLC when carrying out organic reactions. A good response would identify the following key features after 20 minutes: • The C–Cl bond energy is greater than C–Br and so the reaction would be slower. • The haloalkane spot would be larger and the propan-1-ol spot smaller. • The propan-1-ol is spotted on the chromatogram to monitor the progress of the reaction. The question differentiated very well between candidates, but many did not seem to know where to start with many candidates not scoring any marks. This suggested that candidates recognised chromatography as a technique but did not appreciate its relevance in organic chemistry. Some candidates referred to pigments, recalling their early chromatography experiments in finding the colours in ink.

Notes from the mark scheme and report

• propan-1-ol spot smaller/1-chloropropane spot bigger
• C–Cl bond stronger than C–Br so reacts slower (C–Cl bond required
• DO NOT ALLOW chloride/bromide
• IGNORE halogen reactivity)
• propan-1-ol run to show product formation/monitor progress
• IGNORE 'as a control/comparison' alone

Skills: PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

Most candidates correctly described a covalent bond as a shared pair of electrons. Where this mark was dropped, it was usually because of the omission of ‘pair’ or ‘shared’, e.g. ‘shared electrons’.

Notes from the mark scheme and report

• shared pair of electrons
• IGNORE comments about attraction between nuclei and the shared pair

Mark scheme

Examiner's comment

This bond angles and shapes rewarded the well-prepared candidates, with many being given both available marks. The commonest error was NH3 shown with a tetrahedral shape and a bond angle of 109.5º. This part discriminated very well.

Notes from the mark scheme and report

• NF3 107° (ALLOW 106–108) pyramidal
• BF3 120° trigonal planar ('planar' alone insufficient)
• all 4 points for 2 marks

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

Mark scheme

Examiner's comment

Most students had the right idea here, but some lacked the detail for the marks. Some discussed repulsion in general terms, without referring the number of electron pairs. This question discriminated very well.

Notes from the mark scheme and report

• NF3 has 3 bonded pairs AND 1 lone pair AND lone pairs repel more
• BF3 has 3 bonded pairs
• IGNORE 'electrons repel'
• DO NOT ALLOW 'atoms repel'
• Many students can describe electron pair repulsion theory but many think that molecular shapes are determined by lone pairs, with bonded pairs often being ignored. Others seem to believe that lone pairs repel atoms. In this question, explanations sometimes claimed that NF3 and BF3 molecules have different shapes because a NF3 molecule has a lone pair, resulting in no credit.

Mark scheme

Examiner's comment

Most candidates knew that a strong acid completely dissociates. Only the lower-attaining candidates responded in terms of a low pH.

Notes from the mark scheme and report

• completely dissociates/ionises
• DO NOT ALLOW easily dissociates

Mark scheme

Examiner's comment

This item was much more demanding than the equation in 22(b)(i) and was often answered incorrectly. Most were unable to work out the formula of the two ammonium compounds, with NH3 often shown instead of NH4. A mark was available for 4 of the 5 formulae being correct but comparatively few were able to construct the correct balanced equation. Candidates are expected to know the formula and charge of ammonium and carbonate ions and the common acids (sulfuric, hydrochloric and nitric) and these are clearly listed in the specification.

Notes from the mark scheme and report

• any 4 formulae correct for 1 mark
• all 5 correct and balanced for 2
• ALLOW H2CO3 for CO2 + H2O

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates recognised that a volumetric flask is used to accurately prepare volumes of solutions. A common error was a conical flask, perhaps by not reading the information clearly and giving the name of the flask used in the titration itself.

Notes from the mark scheme and report

• ALLOW graduated flask

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Only just over a half of candidates obtained this mark with one of the conditions being incorrect, usually as 273 K or 100 Pa. It is important that candidates learn these standard conditions.

Notes from the mark scheme and report

• 298 K/25 °C AND 100 kPa (ALLOW 101 kPa, 1 atm)

Mark scheme

Examiner's comment

Only about half the candidates obtained this mark suggesting that this term had not been recalled by many. A common error stated that the catalyst and reactants were in the same state. Responses from lower-attaining candidates sometimes had the appearance of guesses.

Notes from the mark scheme and report

• all species (reactants AND products) in the same state/phase
• IGNORE reactants and catalyst have same state

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

Mark scheme

Examiner's comment

Again, most scored this mark. Common errors included using the mass number for number of electrons, no 4s but 3d4 instead, 4d rather than 3d, 4p2 instead of 3d2 or filling up d orbital 3d10.

Notes from the mark scheme and report

• 3d2 4s2 (ALLOW 4s before 3d)
• DO NOT ALLOW [Ar] shorthand

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

Most candidates did not gain any credit here. However, the range of responses seen highlighted some misconceptions in their understanding of how different mixtures can be separated. Many assumed that sodium chloride was in solution/aqueous, not recognising that water was not present in this reaction. Responses such as “sodium chloride will evaporate” or “remove the water” were seen. Some gave a description of the purification method for an organic liquid - the use of a separating funnel and/or distillation were common. Some suggested the use of a magnet to remove Ti despite it being a non- magnetic metal.

Notes from the mark scheme and report

• add water AND filter
• NaCl dissolves / Ti insoluble (Ti is the residue)
• Understanding how to separate mixtures is covered in both KS3 and KS4 but it is important that these concepts can be applied during further study. Asking this type of problem solving question would make a good starter activity.
• Some useful activities for separating mixtures can be found in the GCSE Chemistry B (Twenty First Century Science) Chemical analysis transition guide

Mark scheme

Examiner's comment

Responses not about observations were very common, e.g. more vigorous reaction, Ba is more reactive. Some described Ba as being less reactive. Many responses did not include the idea of rate (for example, ‘more bubbles’) or were not comparative (for example, ‘vigorous bubbling’).

Notes from the mark scheme and report

• must be an observation with idea of rate (more vigorous fizzing, dissolves faster)
• IGNORE 'more vigorous reaction', 'more bubbles', exothermic

Mark scheme

Examiner's comment

Another tricky question with less than half gaining credit. Many were able to recognise the addition of silver nitrate as the test for halide ions but did not realise that it could be used quantitatively. Many didn’t read the question carefully and assumed Na2SO4 was still present, giving a mixture of two precipitates. Some, despite recognising the formation of AgCl, could not then see how to calculate the mass of NaCl i.e. “you won’t have formation of BaSO4”. Some suggested that barium nitrate would also form a precipitate, perhaps confused by the (s) state symbol in the question.

Notes from the mark scheme and report

• AgCl precipitate would form
• mass of NaCl calculated from mass/moles of AgCl
• IGNORE incorrect formula of silver chloride

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Candidate explanations often lacked clarity even if the correct structure had been identified in (i). Most gained at least 1 mark, usually for recognising that for MgS to have a higher melting point that it must contain stronger bonds than in PCl3. Responses highlighted a range of misconceptions including the presence of intermolecular forces in ionic/metallic substances, oppositely charged atoms in ionic compounds, and thinking London forces are between atoms. Most were able to gain the conductivity mark, but some compared to PCl3 rather than Si as asked in the question. Some described ‘mobile ions’ in Al or that Si has ‘no electrons’. The use of ‘free electrons’ was seen in many responses, and we would encourage the use of ‘delocalised electrons’ for a more accurate description of metallic bonding.

Notes from the mark scheme and report

• answer must be comparative
• MgS ionic bonds vs PCl3 intermolecular forces
• Al delocalised/mobile electrons, Si none
• IGNORE 'free electrons'
• OCR have produced a KS4-KS5 transition guide for bonding and structure to support teaching of these tricky concepts.
• A bonding delivery guide is also available.

Mark scheme

Examiner's comment

Most responses did not gain credit here. Some recognised that AgI would form first but then gave an incorrect reason (such as ‘held by weaker London forces’) or their answers lacked detail (e.g., ‘weaker bonds’ without specifying which bonds). Many said AgBr because Br is more reactive or more electronegative.

Notes from the mark scheme and report

• AgI/yellow first AND C–I bond weaker / lower bond enthalpy
• IGNORE polarity and electronegativity
• ALLOW ECF from (d)(i)

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Approximately half of candidates scored both marks. Many lost marks for saying ‘energy required’ or ‘energy needed’ which demonstrated a lack of understanding that for an exothermic reaction this would be the activation energy not the enthalpy change. Some also had 1 mole of substance but this was reacted with 1 mole of oxygen or formed 1 mole of products.

Notes from the mark scheme and report

• complete combustion of 1 mol of substance
• DO NOT ALLOW 'energy required' or 1 mol of atoms
• IGNORE standard conditions

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

A significant number of candidates lost the mark for missing -an- in their answer i.e. 3-methylpent-2-ol. Others lost the mark for incorrect spelling of methyl.

Notes from the mark scheme and report

• 3-methylpentan-2-ol
• DO NOT ALLOW 2-methylpentan-3-ol or misspellings such as 3-methylpent-2-ol

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

Most candidates were able to explain why the first ionisation energy of Mg is greater than that of Sr due to the Mg’s smaller atomic radius/less shielding and therefore increased nuclear attraction. Candidates should be reminded that there is no requirement to restate the question in their answers. Terminology is important and some candidates lost marks as they referred to nuclear radius instead of atomic radius. However, most candidates did not recognise that the second ionisation energy of Sr involves removing an electron from a +1 ion and instead discussed the repulsion between electrons in the s orbital. Atomic radius instead of ionic radius was often seen when discussing the Sr+ ion. Some candidates were still referring to Mg in this part of their answer and they should be advised to reread the question between each part to remain focused on the requirement.

Notes from the mark scheme and report

• Mg outer electron closer to nucleus/less shielding so greater nuclear attraction
• second electron of Sr removed from a positive ion so greater attraction
• IGNORE repulsion between electrons in the s orbital

Mark scheme

Examiner's comment

Candidates needed to explain the correct sign and understand that entropy is a measure of the dispersal of energy in a system. Most candidates linked the increase in disorder with the number of moles increasing and therefore the + sign for ∆S. Some candidates were not given the mark as they stated that entropy increases without giving the sign. Some candidates tried to answer using ∆G statements by suggesting ∆H and T values.

Notes from the mark scheme and report

• positive AND more (gaseous) moles of products
• IGNORE explanations based on ∆G

Mark scheme

Examiner's comment

This question was answered for the most part correctly with many candidates scoring 2 marks for the explanations of the effect on the equilibrium position by the changing of the temperature and pressure. Most candidates were able to recognise the changes had opposite effects but could not score the final mark, as their response needed the concept of opposing factors, or ‘we don’t know which factor is dominant’. Some did not write anything about equilibrium and attempted answers based on rate, or loss of energy/chemicals to the surroundings.

Notes from the mark scheme and report

• higher T shifts left (forward reaction exothermic)
• higher P shifts right (fewer gaseous moles)
• cannot tell which opposing effect is dominant
• DO NOT ALLOW both shifts in the same direction

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

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

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

Examiner's comment

Most candidates scored 1 mark. Most used Cu2+ as their example and recognised Cu(OH)2 forms a blue precipitate. Some candidates either did not give the colour of the precipitate or were unable to write a balanced equation. Some candidates defined rather than described precipitation.

Notes from the mark scheme and report

• colour of precipitate AND formula plus equation, e.g. pale blue Cu(OH)2 or grey-green Cr(OH)3
• DO NOT ALLOW dark/royal blue for the precipitate
• IGNORE state symbols

Mark scheme

Examiner's comment

Only the highest-attaining candidates on the paper were able to give an example of a ligand substitution reaction with the colour of the new complex ion. Some candidates confused a precipitation reaction for a ligand substitution and some candidates were perhaps a little ambitious in giving ‘unusual’ complex ions for which they did not know the colour. Cu was often chosen over Cr, usually the formation of [CuCl4]2-, but [Cu(NH3)4(H2O)2]2+ was also chosen. With both, there were issues with charges of the complex, and with the latter some were confused with how many NH3 were coordinated. Some candidates defined rather than described ligand substitution.

Notes from the mark scheme and report

• colour AND complex formula plus equation, e.g. deep blue [Cu(NH3)4(H2O)2]2+, yellow [CuCl4]2–, purple [Cr(NH3)6]3+
• ECF on incorrect charges when linked to colour via an equation

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

Most candidates attempted this question, gaining at least 1 mark, with over half scoring 3 or more marks. Responses often lacked clarity as many candidates struggled to articulate their ideas. It was common to see lengthy responses often with unnecessary repetition and sometimes even contradictions. A good strategy adopted by some was to draw skeletal formulae for the compounds next to the data provided. This enabled them to focus their response more easily on the extent of branching. Many candidates were unable to give a clear trend for the first marking point, as asked for in the question, but were able gain credit by a lengthier comparison of all three as indicated in the extra guidance. However, this mark was often lost through incomplete explanation, not referring to boiling point at all or an attempt to compare just chain length. The most common error for the second mark was omission of ‘contact’ or ‘interaction’ with reference only to surface area or ‘packing’ of molecules. Some lost this mark for a change in number of electrons. The third marking point was the most frequently awarded. Some candidates lost the mark for not explicitly naming the intermolecular forces as London forces/induced dipole-dipole interactions or for incorrectly using van der Waals. Some lost the mark for not explicitly indicating how increased or decreased contact would affect the strength or magnitude of London forces, e.g. ‘less contact to form London forces’. The final mark was harder to obtain as it needed to be clear that energy was required to break intermolecular forces. For example, ‘less energy to break bonds’ or ‘easier to separate the molecules’ or ‘more energy to boil’ were not sufficient. This exemplar shows a clear, concise response. The candidate has drawn skeletal structures next to the table. The trend is stated first followed by a detailed explanation, presented as a bullet point list, with all 4 marking points awarded.

Notes from the mark scheme and report

• comparisons needed throughout
• must rank all three by branching
• surface contact not just surface area
• DO NOT ALLOW different numbers of electrons (all the same)
• more energy to break London forces
• IGNORE 'bonds' alone and chain length
• Responses often highlighted that candidates lacked understanding about what London forces are, e.g. indicating that they form ‘between atoms’ or referring to induced dipole-dipole forces as something else. Intermolecular forces are difficult to fully comprehend as they can’t be visualised making this a challenging topic to teach.
• OCR have produced a ‘Bonding’ teaching guide with lots of useful suggestions and resources. This includes a link to this Salters A Level chemistry revision activity on intermolecular bonding

Mark scheme

Examiner's comment

Many candidates were unable to provide reagents and conditions for this reaction. Iodo- seemed unfamiliar to some with responses including NaBr or HBr suggesting substitutions with Br are more familiar. The most common errors were to use iodide with no acid or to use iodine, sometimes in conjunction with other reagents such as AlI3 or FeI3.

Notes from the mark scheme and report

• NaI/KI AND H2SO4 (ALLOW HI or H3PO4)
• IGNORE conc/dilute

Mark scheme

Examiner's comment

About a quarter of candidates gained all 3 marks. They were able to give clear, well-structured answers with all the steps as carried out in PAG 6 practical activities. The best answers were often in the form of bullet pointed steps and often included extra details such as a hot filtration step or promoting crystallisation by scratching the surface. Many candidates lost the first mark as their responses didn’t give sufficient detail, e.g. hot solvent, not just warm and minimum volume required. Some missed the cooling step essential for formation of crystals before filtration. Some missed that the crystals need to be dried following filtration but often if only given 1 mark this was the mark given. Lots of confusion with purification with organic liquid was seen, such as references to use of a separating funnel, drying agents and distillation. Some also went on to explained how to check purity using melting point determination.

Notes from the mark scheme and report

• dissolve in minimum quantity of hot solvent
• cool AND then filter
• dry
• DO NOT ALLOW a drying agent e.g. MgSO4
• IGNORE washing and hot filtration
• A useful resource for revising the steps for purification of an organic solid is the Topic exploration pack: Experiments on organic synthesis in Learner Activity 2.

Skills: PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

Overall, this question discriminated well with most candidates gaining some credit. Most candidates were able to suggest a correct product for the reaction of salicylic acid with bromine, with just a few candidates replacing the OH group with a Br atom. Most candidates recognised that the carboxylate salt would be produced from the reaction with Na2CO3 but not all were able to recall that the phenolic OH group is too weak acid to react. Some missed an O on carboxylate i.e. -CO-Na+. Some added Na or CO3 directly to the ring. The reaction with the anhydride proved much more challenging. All sorts of weird and wonderful structures of the main ‘ester’ product were given including joining the CH3 group of propanoic anhydride to the O of the phenol group. Some used the bottom two boxes to form esters with the COOH and OH groups in turn. Many did not identify propanoic acid as a by-product even if they had correctly identified the ester product. Water was a common incorrect product. Marks were also occasionally lost for missing the benzene ring on structures or missing the -OH or - COOH groups that were left unreacted.

Notes from the mark scheme and report

• Br2 substitutes the ring (ALLOW any position, up to 4 Br)
• Na2CO3 reacts with COOH only (COO– or COONa), phenol OH unchanged
• anhydride esterifies the phenol OH giving ester + CH3CH2COOH
• IGNORE COOH forming an anhydride

Mark scheme

Examiner's comment

The majority of candidates were able to explain the term optical isomerism. However, it was still common to see incomplete responses such as ‘mirror images’ without reference to ‘non-superimposable’ or ‘non- superimposable images’ with no reference to ‘mirror’. Lots gave the definitions for stereoisomerism or an explanation for how a chiral centre arises.

Notes from the mark scheme and report

• non-superimposable mirror images
• IGNORE definition of stereoisomers

Mark scheme

Examiner's comment

Over half of candidates were unable to give the systematic name of valine, despite many being able to draw out a structure in the following question. A minority of candidates did not attempt the question. The best strategy was to use displayed formula, find the longest chain which included the COOH and label this as C number 1 to make sure of correct numbering. Common errors included 2-amino-3,3- dimethylpropanoic acid or 3-amino-2-methylbutanoic acid. Many candidates did not know how to name the amine functional group with errors including, amine, N-, nitro-, nitrile, etc. Some simply attempted to name the R group alone, e.g. ‘2-methylethyl- or ‘dimethyl’.

Notes from the mark scheme and report

• 2-amino-3-methylbutanoic acid
• DO NOT ALLOW amine/amin for amino or misspelt methyl

Mark scheme

Examiner's comment

This question was challenging for even the most able candidates with very few obtaining the correct answer of 16. Many identified the four chiral centres in compound E, often labelling these with an asterisk. However, only a small proportion were able to predict that there would be 16 possible optical isomers. Most provided an answer of four corresponding to the number of chiral centres or eight considering that each chiral centre would result in two optical isomers. They struggled to see that they needed 2n in this case where n represents the number of chiral centres. Candidates have probably seen very few, if any, examples of chiral compounds with more than two chiral centres.

Notes from the mark scheme and report

• 16 (four chiral centres)

Skills: M4.3 Understand the symmetry of 2-D and 3-D shapes

Mark scheme

Examiner's comment

Most candidates were able to recall the correct response here, especially for those more confident with drawing out the mechanism. The most common incorrect response was nucleophilic substitution. Some suggested reduction or nucleophilic addition elimination. Misspellings of ‘nucleophilic’ were often seen.

Notes from the mark scheme and report

• nucleophilic addition
• IGNORE just 'addition'

Mark scheme

Examiner's comment

This question discriminated well. Many candidates were able to demonstrate an excellent knowledge of organic reactions and it was not uncommon to see scores of at least 7 marks. This question identified which candidates had learned their synthetic routes including necessary reagents and conditions. Marks were often lost for small details such as missing Hs (check all Cs have four bonds) or not specifying that steam is required for hydration of alkenes or missing the acid needed for oxidation. Many suggested the use of NaOH or just a mixture of acids to product the diol. The minor 1,3-diol or 1,1-diol product was often seen. The sequence leading to an acyl chloride from acrolein was usually the most well answered. However, quite a few tried to use HCl to make the acyl chloride. Many lost marks for the polymer for incorrect connectivity on the aldehyde, e.g. -COH or attempting to make a polymer via connection of the aldehyde group.

Notes from the mark scheme and report

• NaBH4 gives H2C=CHCH2OH (C=C not reduced)
• steam AND acid catalyst (both needed) gives propane-1,2-diol as major product
• H+/Cr2O72– to H2C=CHCOOH
• SOCl2 to acyl chloride
• polymer with CHO side chains and end bonds
• DO NOT ALLOW –COH for aldehyde
• This topic guide provides a summary of synthetic routes. Copies of the summary posters without the conditions can be found on Teach Cambridge. This should be used in conjunction with the organic synthesis topic exploration pack.

Mark scheme

Examiner's comment

Most candidates selected N, O and F, with other responses choosing other elements seemingly at random.

Notes from the mark scheme and report

• three from N, O, F, H
• DO NOT ALLOW any other elements (CON)

Mark scheme

Examiner's comment

Most successful candidates looked for the large difference between successive ionisation, usually shown by annotations below the table. The commonest incorrect response was F, the element after the large difference. Other incorrect responses were random.

Notes from the mark scheme and report

• O (large jump after 6th ionisation energy)
• ALLOW S BOD

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

Mark scheme

Examiner's comment

Candidates found this question harder than Questions 1 (a) and (b) with S and P being the most common correct elements seen. As and Se were also allowed. Si was a common incorrect response.

Notes from the mark scheme and report

• P or S
• ALLOW S8, P4, As, Se

Mark scheme

Examiner's comment

Most candidates chose at least one of the two elements Cr and Mn, with Mn being the most common. Incorrect elements were usually other d block elements. Erratum

Notes from the mark scheme and report

• Cr (3d5 4s1) and Mn (3d5 4s2)
• IGNORE ions

Mark scheme

Examiner's comment

Most candidates correctly chose Si. As with earlier questions, there seemed to be little pattern with incorrect elements.

Notes from the mark scheme and report

• Si

Mark scheme

Examiner's comment

The answer S was well known, with SF6 being the specification example of a compound with octahedral molecules.

Notes from the mark scheme and report

• S
• ALLOW SF6

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

Mark scheme

Examiner's comment

Although most candidates did choose F, a substantial number chose Br (most common), K and H, seemingly confused between gaining and losing electrons in forming ions.

Notes from the mark scheme and report

• F

Mark scheme

Examiner's comment

This question was a good discriminator. Candidates who scored the mark usually showed their mathematical working which led to As being chosen as the correct element. Other elements were seen randomly. More candidates omitted this part of Question 1 than other parts. With questions such as this, it is always better to guess as there is no penalty for an incorrect response.

Notes from the mark scheme and report

• 1.244 × 10–22 × 6.02 × 10^23 = 74.9 so As

Skills: M0.1 Recognise and use expressions in decimal and ordinary form; M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates had difficulty in naming a compound using Roman numerals for an element which can have different oxidation numbers. For the name of RbClO4, many omitted the number entirely, showing just rubidium chlorate. Many inventive names such as rubidium chlorotetraoxide were seen. Some candidates wrote the correct VII before chlorate and many different Roman oxidation numbers were seen. Roman numerals’ use in naming compounds is part of chemical nomenclature, included in the specification.

Notes from the mark scheme and report

• rubidium chlorate(VII)
• IGNORE rubidium(VII) chlorate, chlorate(7), perchlorate

Mark scheme

Examiner's comment

Question 4 assesses candidates’ ability to apply their chemical knowledge and understanding from different parts of the specification in a novel context. Information is supplied throughout the question, and clues are sometimes presented to candidates. In part (a), candidates are introduced to Tutton’s salts and are given an example of a Tutton’s salt that forms the context of the whole question. A candidate needs to apply the information in the bullet points to predict the formula of a different Tutton’s salt. This question discriminated extremely well across different abilities and highlighted that some candidates struggled to use supplied information. This was repeated in other parts of Question 4. Just over half the candidates gave a correct formula from the information. Some candidates did not choose one of the acceptable ions shown in the first and second bullet points, and many chose S rather than Cr, despite S being in the supplied Tutton’s salt; a significant number omitted the •6H2O.

Notes from the mark scheme and report

• X from K, Rb, Cs, Fr only
• Y = Mg or Ti to Ni (not Cu, Sc or Zn)
• Z must be Cr
• e.g. K2Mg(CrO4)2•6H2O

Mark scheme

Examiner's comment

The majority of candidates did not answer this question correctly. Candidates were expected to refer back to the formula of the Tutton’s salt, spot that there was water of crystallisation present, and that this would be lost if all the solvent was boiled off. Many responded vaguely in terms of decomposition or formation of larger crystals but a mark was only awarded if there was a definite link to the water contained within the crystals.

Notes from the mark scheme and report

• prevents water of crystallisation being removed / anhydrous salt forming
• IGNORE 'all the water would be removed', prevents decomposition, larger crystals
• In the specification, Section 2.1.2a states the following: (a) the writing of formulae of ionic compounds from ionic charges, including: (i) prediction of ionic charge from the position of an element in the periodic table (ii) recall of the names and formulae for the following ions: NO3 –, CO3 2–, SO4 2–, OH–, NH4 +, Zn2+ and Ag+u.
• This section will be studied at the start of the 2 year course. Candidates need to be confident with using these common formulae. For success in chemistry, these ions must be learnt.

Mark scheme

Examiner's comment

This reaction of copper(II) ions with aqueous ammonia and the formula of the complex ion formed are part of the specification. Within this novel context, the molar mass had been provided as a clue. Less than half the candidates correctly gave the correct formula and it was noticeable how well this part discriminated across abilities. This was another example of many candidates being unable to apply their knowledge and understanding to a novel context.

Notes from the mark scheme and report

• IGNORE [Cu(NH3)4]2+

Mark scheme

Examiner's comment

Addition of NaOH(aq) to the Tutton’s salt results in two reactions: precipitation of copper(II) hydroxide and a reaction of an ammonium ion, used to show its presence as a qualitative test. As with Question 4 (c) (i), this part discriminated very well with many candidates able to be rewarded with some of the marks. The formula of copper(II) hydroxide, as Cu(OH)2 or Cu(OH)2(H2O)2 were both acceptable. This was correct more often than the responses related to the ammonium ion. The formula of the gas formed in the reaction of NaOH(aq) with the ammonium ion caused problems, with NH3 and its subsequent test with moist indicator turning blue seen much less than the reaction of Cu2+(aq) ions. Hydrogen (the ‘squeaky pop test) and oxygen (relighting a glowing split) were common incorrect responses. This was another question in which referring back to the formula of the Tutton’s salt would have revealed important clues.

Notes from the mark scheme and report

• Cu(OH)2 and NH3 (formulae required
• IGNORE names)
• moist indicator/litmus turns blue
• no ECF for a test on the wrong gas
• DO NOT ALLOW unbalanced charges or 'bleaches indicator'

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Th final part of Question 4 required candidates to identify the anion in the Tutton’s salt as sulfate, and to recall that Ba2+ ions is used for the sulfate test to form a white precipitate. Any soluble barium compound was credited with barium chloride and nitrate being the commonest seen. As with earlier parts, this part discriminated very well. Most candidates who knew that barium ions were needed also collected the mark for the white precipitate observation. Over half the candidates did not score here, the most common errors being to repeat the test for the ammonium ion, or to use silver nitrate, clear confusion with the halide test.

Notes from the mark scheme and report

• BaCl2 or Ba(NO3)2 solution gives white precipitate
• observation only from a soluble Ba2+ reagent
• DO NOT ALLOW H2SO4 added
• DO NOT ALLOW AgNO3 before the sulfate test

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

This question required candidates to apply their knowledge and understanding of bond angles and electron pair repulsion in a novel context. The best candidates rose to this challenge, securing all four marks for correct bond angles and explanations in terms of the numbers of bonded and lone pairs. The 104.5º and 120º were commonly seen and high scoring candidates provided excellent reasoning. The best explanation for 120º was in terms of three bonding regions and no lone pairs. Lower scoring responses often reasoned that bond angles are determined by lone pairs repelling the atoms, with the role of bonding pairs often being ignored.

Notes from the mark scheme and report

• H–O–N 104.5° (2 bonded pairs and 2 lone pairs, lone pairs repel more)
• O–N–O 120° (3 bonded regions)
• IGNORE names of shapes and 'electrons repel'
• 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 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

The responses to this question were highly variable. Many candidates correctly identified the disadvantage, usually in terms of CO2 release and global warming, but an advantage was less frequently seen. When it was seen it was almost universally about releasing energy. General hazard comments such as ‘harmless’; and ‘dangerous’ as too general and are never given marks at AS or A Level.

Notes from the mark scheme and report

• BOTH advantage and disadvantage needed
• IGNORE 'harmful/dangerous'
• IGNORE 'produces CO2 and H2O'

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

Candidates answered this question well. Where candidates had not been given marks, it was usually because they omitted the term ‘ions’ or did not state that an ionic bond is between metals and non- metals.

Notes from the mark scheme and report

• 'attraction' essential
• IGNORE references to metal and non-metal

Mark scheme

Examiner's comment

Many candidates answered this question well, with most identifying the model of metallic bonding as fixed positive ions and mobile delocalised electrons. The question did ask for bonding and structure and the giant feature of the structure was often omitted. Unfortunately, some candidates contradicted a correct metallic bonding statement by including descriptors of intermolecular forces, covalent bonding or attraction between electrons and the nucleus rather than with positive ions. Less successful responses demonstrated less understanding: some didn’t realise that Ca is a metal and conductivity explanations were often given as ions moving in the molten and not in the solid state, clear confusion with ionic bonding. Full marks were only given for showing the correct charges on the Ca2+ cations (+ was insufficient) and for explaining conductivity in terms of electron movement, rather than the common ‘the electrons carry charge’ and ‘the electrons are free’. Overall, this relatively simple question discriminated very well and demonstrated how well the candidates understood metal bonding and structure.

Notes from the mark scheme and report

• 'giant metallic' gains both structure and bonding marks
• attraction between nucleus and electrons is a CON
• idea of electron movement required
• IGNORE 'electrons are free' and 'electrons carry charge'

Mark scheme

Examiner's comment

Candidates answered this question well and most were given the mark. Where candidates didn’t receive credit, it was mainly because they used the term ‘same atom’ instead of ‘same element’. Some less successful responses responded with completely incorrect chemistry and had clearly not learnt this specification content.

Notes from the mark scheme and report

• 'atom' is insufficient for element
• IGNORE 'species'

Mark scheme

Examiner's comment

Most candidates scored this mark. Some candidates referred to acids having or containing H+ ions rather than indicating that H+ ions are donated or released. A small number of candidates gave simplistic responses in terms of pH only.

Notes from the mark scheme and report

• releases H+ ions/proton donor

Mark scheme

Examiner's comment

Again, this was well answered by most candidates. A few candidates gave responses in terms of pH or indicated that there were more OH- ions present.

Notes from the mark scheme and report

• partially dissociates/ionises
• IGNORE 'poor proton donor' and 'doesn't easily dissociate'
• Some candidates described a solution with a low concentration of H+ ions, demonstrating a confusion between concepts of weak and dilute in reference to acids. Visual picture cards can be very helpful in assessing understanding of these concepts. This topic is covered at GCSE and resources can be found here: http://www.ocr.org.uk/Images/363946-ph-scale-lesson- element.doc

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

Most candidates managed to score at least 1 mark for this question. The most common reason for losing a mark, despite demonstrating a good understanding of redox, was stating that H changed from +2 to 0 (need to give oxidation number per atom). Other errors seen included only giving change for Sr, descriptions in terms of electrons rather than oxidation numbers, Sr change from 0 to +1 (linked to SrOH), oxygen being reduced rather than H and mixing up oxidation/reduction or not specifying.

Notes from the mark scheme and report

• Sr 0 to +2 oxidation, H +1 to 0 reduction
• '+' sign required
• 1 mark if all numbers correct but oxidation/reduction reversed or not given

Mark scheme

Examiner's comment

Most candidates gained some marks here although a significant proportion were unable to score all 3 marks covering atomic radius, shielding, nuclear attraction/IE. The mark most often missed was for shielding. Some candidates did not answer the question asked and gave the trend down the group so could not be given marks unless they made it clear Sr is below Ca in the group. Care must be taken to answer question asked not similar questions they have seen before. The best responses were those with direct comparative statements, e.g. “Ca has a smaller atomic radius than Sr”. It is worth noting that harder/easier to lose electrons didn’t gain marks, but was seen fairly frequently, as response needs to be in terms of energy required or linked to nuclear attraction.

Notes from the mark scheme and report

• comparison needed for each mark
• Ca smaller radius/fewer shells, less shielding, greater nuclear attraction/higher ionisation energy
• IGNORE nuclear charge
• IGNORE fewer orbitals/sub-shells

Mark scheme

Examiner's comment

Most candidates were able to give the formula of the barium compound as BaSO4. However, they did not recognise that this would cause a white ppt to be seen, presumably as not in the context of qualitative ions testing. Many candidates said they would see bubbling/fizzing. Some gave a colour change as they were possibly considering what would be seen if an indicator is present. Others mentioned a precipitate but with no colour given. Some candidates gave the incorrect formula, such as Ba2SO4 or Ba(SO4)2, again showing the importance of practising writing formulae. In addition, some candidates wrote out the whole equation for the reaction.

Notes from the mark scheme and report

• white precipitate
• BaSO4
• We have produced a topic support pack to assist with learning about the reaction of group 2 elements and their compounds: http://www.ocr.org.uk/Images/364103-chemistry-of-group- 2.docx

Skills: PAG4 Qualitative analysis of ions

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

Examiner's comment

Less than half of all candidates managed to gain all 3 marks giving the correct enthalpy profile diagram. Many demonstrated a lack of understanding about what an enthalpy profile diagram shows as seen by labelling of curves for Ea/Ec rather than the enthalpy change or arrows for ∆H extending to the x-axis. Other errors also seen included, -ΔH, double headed ΔH arrow, activation energy lines starting at products, endothermic profiles, Ea and Ec the wrong way round and missing or incorrect labels. Some candidates found it tricky to give activation lines for both catalysed and uncatalysed reactions on the same diagram, these often overlapped or did not extend to the top of the energy barrier.

Notes from the mark scheme and report

• products below reactants with ∆H arrow downwards (DO NOT ALLOW –∆H or double-headed arrow)
• Ea and Ec arrows from reactant level to top of curve, Ec < Ea
• ECF for Ea/Ec on endothermic diagram

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

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

Most candidates gave the correct response to this question. Incorrect responses included use of high temperatures and/or catalyst.

Notes from the mark scheme and report

• UV
• ALLOW sunlight
• IGNORE temperature

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

This question was not answered well with over half the candidates failing to score any marks. While some candidates seemed familiar with the techniques required, describing the process to separate the layers, they often struggled to name the separating funnel. Common approaches were to attempt to ‘filter’ the layers or to use heat (via evaporation or distillation) to drive off the water. Some attempted to use Na2CO3 or NaOH to dry the organic layer – perhaps confusing neutralisation of any remaining acid. Although distillation appeared frequently many did not give the temperature so did not gain marks. The order of the procedure was also not always clear with distillation before using a drying agent. Some described attempts to crystallise the organic layer. The range of answers suggests students may need more practical experience with separating organic liquids.

Notes from the mark scheme and report

• separating funnel to separate layers
• dry organic layer with anhydrous salt (MgSO4, CaCl2)
• distil and collect fraction at 91 °C

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Many candidates recognised the need for 2NO2 in the first step, with slightly less then adding an appropriate second step. Errors included unbalanced species and unbalanced charges. A wide range of suggested possible mechanisms were seen, including several which led to multiple correct versions of the overall equation. Both balanced equations needed to combine to form the overall equation. However, a few inventive candidates gave a first step of NO2 + NO2 → 2NO2, followed by the full balanced equation for the second step – this was not given marks.

Notes from the mark scheme and report

• step 1 must have 2NO2 only on the left
• second mark dependent on the first
• steps must add up to the overall equation

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

The majority of candidates gave the required definition correctly, frequently going beyond to include ideas about standard states and conditions. The smaller number of incorrect responses covered a range of errors, including assuming that 1 mole of elements were reacting and (perhaps as a result of later work on lattice enthalpies) bringing in a requirement for the compound or the elements to be gaseous.

Notes from the mark scheme and report

• 1 mole of compound formed from its elements
• DO NOT ALLOW 1 mole of element
• DO NOT ALLOW 'gaseous elements'

Mark scheme

Examiner's comment

Candidates needed to explain that entropy is a measure of the dispersal of energy in a system which is greater, the more disordered a system. A good number of correct answers but quite a few did not get across the required idea of ‘a measure of’.

Notes from the mark scheme and report

• measure of dispersal of energy/disorder

Mark scheme

Examiner's comment

Many candidates realised the significance of a negative free energy change, but they seemed to struggle to link this to the previous question, giving a more general answer about feasibility, rather than using their values of entropy change and enthalpy change to determine feasibility. If temperature of feasibility was calculated as -9110K many still thought that meant a temperature existed below this which was feasible.

Notes from the mark scheme and report

• ∆S positive AND ∆H negative
• so ∆G negative at all temperatures
• ALLOW ECF from signs in (c)(ii)

Mark scheme

Examiner's comment

Some very wordy responses to this straightforward question were seen, with many candidates going onto the extra pages. Candidates are reminded that keeping responses concise and to the point can make their answer clearer and potentially avoid contradicting themselves in the process. There were a good number of excellent answers seen, although many candidates were distracted by arguments about the stability of full shells or subshells rather than explaining why nuclear attraction would be greater. Some students mixed up the explanation for Be/Li with the difference in ionisation energies between groups 5 and 6 (electron pair repulsion).

Notes from the mark scheme and report

• comparison needed for each mark
• He: outer electron in closer/lower shell
• Be: more protons with same shell/similar shielding
• greater nuclear attraction
• IGNORE 'effective nuclear charge', atomic number increases, just 'greater attraction'
• IGNORE 'different sub-shell'

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

Most candidates correctly described the ammonia reacting with the glycolic acid or with hydrogen ions, although some thought that ammonia was acidic. Many of them then went on to say that “the equilibrium will move to the right” without realising that the equilibrium had not itself appeared within the question, and so they needed to write it out to gain marks. A few candidates thought that ammonia was an acid, due to the 3 x Hs in the molecule.

Notes from the mark scheme and report

• NH3/OH– reacts with H+ (or the acid HA)
• equilibrium equation HA ⇌ H+ + A– must be shown AND shifts right

Mark scheme

Examiner's comment

Candidates coped well with this question, but many candidates did not gain the mark due to ambiguous statements. Some identified the forward reaction as endothermic, but stated that Kp decreased which was given in the question. Others simply stated that the forward reaction was endothermic as the reverse reaction was exothermic.

Notes from the mark scheme and report

• equilibrium shifts left as T decreases AND forward reaction endothermic

Mark scheme

Examiner's comment

Many candidates gave the correct reason to agree with student 2. Those who agreed with student 1 did not see the equation as a heterogeneous equilibrium system. There were a small number of responses agreeing with student 2 but for the wrong reason – such as a confusion about how the position of equilibrium can change when the value of Kp stays constant. Candidates are advised to read through and address all parts of the question as a minority of students didn’t identify which student was correct but gave a correct explanation.

Notes from the mark scheme and report

• Student 2 correct AND same number of moles of GAS on each side (solids ignored)

Mark scheme

Examiner's comment

Very few candidates made two valid statements where both clearly indicated an idea of relative rate – in almost all cases one of the descriptions would be about quantity of gas rather than rate of gas production. Some candidates identified a precipitate being formed, colour change, or gave a general answer of the reaction happening quicker.

Notes from the mark scheme and report

• both needed for 1 mark: Ca fizzes faster AND Ca dissolves more quickly
• IGNORE 'more bubbles' (rate needed)
• IGNORE exothermic

Mark scheme

Examiner's comment

There were some good answers to this standard question, but candidates did not gain the mark due to missing one of the three key ideas of “donation, electron pairs and forming two coordinate bonds”.

Notes from the mark scheme and report

• donates two electron/lone pairs AND forms two coordinate bonds (to the metal ion)

Mark scheme

Examiner's comment

The majority of candidates were able to correct name hydrocarbon A as 3-methylhex-2-ene. A number of responses used incorrect numbering or suggested 3-methylhexan-2-ene as the name.

Notes from the mark scheme and report

• DO NOT ALLOW 3-methyhex-2-ene, 3-methhex-2-ene or 3-methylhexan-2-ene
• IGNORE E/Z or cis/trans

Mark scheme

Examiner's comment

The majority of candidates were able to correctly define a structural isomer.

Notes from the mark scheme and report

• 'same formula' insufficient (must be molecular)
• 'different arrangement of atoms' insufficient (must be structural)
• DO NOT ALLOW any reference to spatial/space

Mark scheme

Examiner's comment

This definition was well known by candidates with the majority of responses given the mark. Some candidates omitted the reference to structural formula.

Notes from the mark scheme and report

• same structural formula AND different arrangement in space
• DO NOT ALLOW same empirical/general formula
• IGNORE same molecular formula
• naming E/Z or optical isomerism is not sufficient

Mark scheme

Examiner's comment

The use of 2,4-dinitrophenylhydrazine as a test for the carbonyl group is well known by candidates at this level. The majority of the cohort correctly identified this test and the subsequent analysis of the melting point of the products as a method of identifying each compound. Lower ability candidate responses made reference to analysis of the boiling points or omitted the reference to analysis of the melting points.

Notes from the mark scheme and report

• 2,4-DNP(H) AND orange/yellow/red precipitate
• take melting point of the derivative
• compare with known values
• DO NOT ALLOW boiling points or chromatograms

Skills: PAG6 Synthesis of an organic solid; PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Almost all candidates were able to correctly describe the use of Tollens’ reagent as a test for an aldehyde fictional group

Notes from the mark scheme and report

• Tollens' reagent AND silver mirror
• ALLOW acidified dichromate turning green

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Few candidates knew the test for a phenol group. Frequent incorrect responses involved the production of a gas with a carbonate or stating that bromine water is decolourised but failing to state that a white precipitate is also formed. A significant number of candidates also stated that the reaction with sodium hydroxide confirms the presence of the phenol group.

Notes from the mark scheme and report

• indicator shows acidity AND no reaction with carbonate
• ALLOW bromine giving white precipitate
• ALLOW FeCl3 giving violet/purple colour

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

This question proved challenging to candidates with few scoring all 3 marks. Where no marks were given, this was frequently because candidates did not state the number of carbon environments in compounds J, K and L. Candidates who were given 1 or two marks frequently stated the incorrect number of peaks that would be observed.

Notes from the mark scheme and report

• J has 6 peaks, K has 5 peaks, L has 8 peaks
• IGNORE chemical shifts

Skills: M4.3 Understand the symmetry of 2-D and 3-D shapes

Mark scheme

Examiner's comment

The most able candidates scored well on what proved a difficult question for many. Although the vast majority of candidates knew about the reasons behind compounds K and L’s increased reactivity many were unable to express themselves clearly to gain credit. Often answers lacked the specific detail about the lone pair on the oxygen atom in the –OH group being delocalised into the ring. Less successful answers discussed electrons being supplied to the ring, the -OH group providing the electrons to the ring or just that –OH is an activator. More successful answers expressed the increase in electron density and the subsequent increase in compound K and L’s susceptibility to electrophilic attack. This response correctly states that the electron density is higher which result in K and L being more susceptible to electrophilic attack and receives 2 marks. To score the third mark, the candidate needed to identify that that the lone pair is on the oxygen of the OH group.

Notes from the mark scheme and report

• a lone pair on O delocalised into the ring (DO NOT ALLOW two lone pairs)
• electron density higher
• ring more susceptible to electrophilic attack/polarises Cl2 more
• IGNORE activating, charge density
• IGNORE 'react more readily' (given in question)

Mark scheme

Examiner's comment

Candidates were familiar with the reagents required in these two reactions.

Notes from the mark scheme and report

• HNO3 with H2SO4 for nitration
• Sn and HCl for reduction
• IGNORE concentrations
• IGNORE NaOH after Sn/HCl
• IGNORE H2

Mark scheme

Examiner's comment

This question proved challenging with only the most able being given full marks. The reagents and conditions were not well known and candidates did not include water in their equations to make sure they were balanced.

Notes from the mark scheme and report

• stage 1: acid catalyst H2SO4/H3PO4 (heat) eliminates H2O to give phenylethene (DO NOT ALLOW HCl)
• stage 2: steam with acid catalyst gives 1-phenylethanol (Markownikoff)
• equations with structures needed

Mark scheme

Examiner's comment

Most candidates did not secure a mark in this question. Many candidates used butan-1-ol in their equations or used benzoic acid rather than benzoic anhydride as the reactant. The most able candidates suggested that the benzoic acid product would then further react with butan-2-ol to produce a second ester molecule and water. This was an acceptable alternative response.

Notes from the mark scheme and report

• (C6H5CO)2O + butan-2-ol → C6H5COOCH(CH3)CH2CH3 + C6H5COOH
• ester structure 1 mark, balanced equation 1 mark

Mark scheme

Examiner's comment

Candidates found the explanation difficult, and the responses showed some misconceptions. For example, many suggested that NH3 and NH4NO3 both have hydrogen bonds. Those identifying that NH4NO3 has ionic bonding usually compared the greater strength of ionic bonding over intermolecular forces in NH3. Unfortunately, many candidates described the ionic bonds as acting between molecules. This question proved to be one of the most difficult on the paper. Understanding could be improved by first considering the following. • What is the type of bonding? • What are particles in the structure? Candidates need to be very careful when describing the two types of structure containing covalent bonds: • Simple molecular with strong covalent bonds within the molecules and weaker intermolecular bonds between the molecules in the structure Giant covalent with strong covalent bonds between the atoms in the structure. than in NH3, or that NH4NO3 has more hydrogen bonds. This response was given 0 marks.

Notes from the mark scheme and report

• NH3 simple molecular with intermolecular forces AND NH4NO3 ionic
• ionic bonds stronger/need more energy to break than intermolecular forces
• A good understanding of structure and bonding continues to be difficult for candidates, demonstrated by many incorrect explanations for the different boiling points. This is a key misconception.

Mark scheme

Examiner's comment

Considering the large number of candidates describing NH4NO3 as molecular in Question 1 (a) (i), most candidates identified here that NH4NO3 contains NH4+ and NO3– ions. Most then went on to show that the nitrogen atoms in the ions have different oxidation numbers: –3 and +5 respectively. Candidates were only given marks if both signs had been included and this was usually the case. –4 and +6 were common incorrect responses, presumably by ignoring the charges on the ions. Comparatively few candidates grouped the nitrogen atoms together and suggested that they had the same oxidation number.

Notes from the mark scheme and report

• N in NH4+ is –3 AND N in NO3– is +5
• both signs essential

Mark scheme

Examiner's comment

This novel question required candidates to apply their knowledge and understanding of proton NMR spectroscopy to explain different chemical shifts. Most candidates used the Data Sheet in their answers, and most were then able to relate the chemical shift at 𝛿 = 2.5 ppm to an adjacent C=O group. Fewer candidates were then able to interpret the unexpected chemical shift at 𝛿 = 3.6 ppm to the presence of two adjacent C=O groups. Less successful responses often resorted to the Data Sheet a second time, suggesting the presence of an adjacent HC–O group, despite none being present in the structure. It was encouraging to see how many candidates were able to explain the two chemical shifts correctly, suggesting that most candidates are comfortable with this concept.

Notes from the mark scheme and report

• environment 2 adjacent to one C=O (HC–C=O)
• environment 3 between two C=O
• DO NOT ALLOW H–C=O or HC–O (simply read from data sheet)
• IGNORE 'next to 2 Os'

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

Mark scheme

Examiner's comment

This question asked candidates to explain the different rates from a novel experiment carried out at 50ºC and 90ºC, and to predict the ∆H sign for the forward reaction. Candidate explanations for the rates were often superficial, solely in terms of greater energy at 90ºC. Many responses referred neither to the different frequency of collisions nor the greater number of particles exceeding the activation energy at 90ºC. Most candidates predicted that ∆H would have a negative sign. Candidates were expected to link the evidence from the absorbance data in the graph to less [Co(H2O)6]2+ being present at 90ºC. When experimental information has been presented, candidates are advised to look for the evidence responsible in their explanations.

Notes from the mark scheme and report

• at 90 °C faster rate AND more frequent collisions
• more particles have energy ≥ Ea
• equilibrium [Co(H2O)6]2+ is lower at 90 °C so forward reaction exothermic/∆H negative
• IGNORE more successful collisions

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

Mark scheme

Examiner's comment

Almost all candidates realised that Cl– ions would react with the added AgNO3 at time = t1.

Notes from the mark scheme and report

• Cl– reacts with Ag+ to form AgCl (precipitate)
• IGNORE chlorine for chloride

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