June 2025: AS Paper 1, Breadth in chemistry

Answer

D

Mark scheme

Examiner's comment

Most candidates gave the correct answer. B was the most common incorrect response, suggesting they had not multiplied by 2 to consider that the titre was calculated from two separate readings.

Notes from the mark scheme and report

• Section M1.3 of the Mathematical Skills Handbook gives details of calculating relative uncertainties, including values calculated by difference.

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

Answer

C

Mark scheme

Examiner's comment

Most candidates answered this correctly, showing good knowledge of electron configuration.

Answer

C

Mark scheme

Examiner's comment

Many candidates gave C as the correct response suggesting they were well-prepared for a calculation involving Avogadro’s constant. A was a very common incorrect response which gives the number of molecules of ammonia instead of hydrogen atoms.

Answer

D

Mark scheme

Examiner's comment

Most candidates correctly calculated atom economy. They usually showed their working alongside the options which is good practice.

Answer

A

Mark scheme

Examiner's comment

Many candidates gave the correct response of A. B was the most common incorrect response, presumably assuming the 2:3 ratio of atoms is closest to 50:50 (and not taking into account relative atom masses).

Answer

C

Mark scheme

Examiner's comment

A significant number of candidates were unable to apply the oxidation number rules correctly to arrive at C as the correct response. Many candidates gave D as the response by forgetting to calculate per S atom. Some chose B, perhaps confusing this with the charge on a sulfide ion.

Answer

B

Mark scheme

Examiner's comment

Many candidates wrote oxidation numbers below the equations, which is generally good practice. However, in this case, the incorrect options can be quickly excluded as they include an element with an oxidation state of zero, which clearly changes in the reaction. Over half obtained the correct response of B (a neutralisation reaction).

Answer

B

Mark scheme

Examiner's comment

Most candidates identified BCl3 as the trigonal planar molecule, suggesting that knowledge of bond shape examples is good.

Answer

C

Mark scheme

Examiner's comment

This question was answered well with most candidates correctly selecting C.

Answer

A

Mark scheme

Examiner's comment

Just under half of candidates did not spot that H2S would be polar. There was no clear distractor, suggesting that many responses may have been guesses.

Answer

D

Mark scheme

Examiner's comment

Over half of candidates correctly calculated the enthalpy change of combustion. A was the most common incorrect response, due to the Hess cycle being the wrong way round. In some instances, candidates corrected this error by appreciating the reaction would be exothermic and therefore picked the negative value instead.

Answer

B

Mark scheme

Examiner's comment

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

Answer

B

Mark scheme

Examiner's comment

The correct option of B was missed by a quarter of candidates. C was the main distractor, ignoring that a catalyst has no effect on the position of equilibrium.

Answer

C

Mark scheme

Examiner's comment

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

Answer

C

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

Answer

D

Mark scheme

Examiner's comment

This was well-answered with most candidates identifying D as a propagation step.

Answer

C

Mark scheme

Examiner's comment

More than half of candidates gained this mark. However, the question did not discriminate very well, suggesting that some candidates may have guessed. Students should be encouraged to draw structures out as working – those who did often gave the correct response.

Answer

B

Mark scheme

Examiner's comment

This question was generally well-answered.

Answer

D

Mark scheme

Examiner's comment

A significant number did not recognise the CH3CH2 group in D would give the fragment ion at m/z = 29.

Answer

B

Mark scheme

Examiner's comment

Roughly half of candidates gained this mark, with D being the most common incorrect response (mistaking the peaks at around 3000cm-1 as a broad peak caused by a O-H bond in a carboxylic acid). Section B overview Section B includes short response question styles (structured questions, problem solving, calculations, practical) and extended response questions. This section of the paper is worth 50 marks.

Mark scheme

Examiner's comment

This question proved low scoring, with many candidates not using the information in the question and giving species other than sulfur. ‘Ar’ was the most common error, which lost both marks as 18 was usually then given as the proton number (incorrectly assumed from the species having 18 electrons).

Notes from the mark scheme and report

• look for the 2– charge on 32S2–
• 4 correct for 2 marks, 3 correct for 1

Mark scheme

Examiner's comment

Candidates answered this question very well. A few mistyped the numbers into their calculator after giving the correct calculation and even fewer made rounding errors. This type of question has featured on previous examinations, and it was encouraging to see how well it was answered.

Notes from the mark scheme and report

• answer to 2 decimal places (24.34)
• 1 mark ECF if percentages used with wrong isotopes once or one percentage transposed

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

Mark scheme

Examiner's comment

The majority of candidates were able to link movement of electrons to the ability of magnesium to conduct electricity. However, sometimes ‘electrons carry charge’ was not developed to explicitly state that they are mobile. Many diagrams were unclearly drawn and unlabelled, with the charge of the Mg2+ ion incorrectly shown.

Notes from the mark scheme and report

• regular arrangement of labelled Mg2+ ions (DO NOT ALLOW + or 3+
• DO NOT ALLOW nuclei/protons/positive atoms)
• electrons labelled and described as delocalised
• idea of electron movement required
• DO NOT ALLOW ions can move
• IGNORE 'electrons are free'

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

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

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

Following browsing of candidate responses, marks were given for correctly writing the Kc expression, substituting the terms with values, and finally, giving the calculated response to three significant figures and standard form. Most candidates scored either 2 or 3 marks. However, this question highlights the need for candidates to read the question carefully, with many responses not giving the Kc expression, or not giving the answer in standard form, or both.

Notes from the mark scheme and report

• square brackets required
• 3 sig figs AND standard form (DO NOT ALLOW 0.0261 × 10^0)
• ECF from inverted Kc only (3.83 × 10^1, 2 marks)
• IGNORE units

Skills: M0.1 Recognise and use expressions in decimal and ordinary form; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Under half of candidates scored this mark, suggesting students would benefit from practising drawing repeat polymer units. A structure with six carbons in the chain was a common error.

Notes from the mark scheme and report

• CH3 and Cl must be on the same C atom
• side bonds required
• IGNORE brackets and n

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 was well answered by the majority of candidates. Where the mark was missed, it was either through incorrect products or incorrect balancing of oxygen.

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

The majority of candidates scored 2 or 3 marks on this question. Occasionally some gave carbonyl structures for E and G failing to recognise that full oxidation would take place. Other errors were additional H atoms (especially on the carbonyl carbon for D) or too many carbon atoms for E. A more common error was incorrect connectivity for the OH group especially in the carboxylic acid from alcohol G. This question was a good discriminator. Question 25 (b Just under half of candidates scored this with a wide range of incorrect responses. Most candidates did not appreciate that in order to form two isomeric alkenes a secondary alcohol was necessary, so that water could be eliminated from either side of the OH group.

Notes from the mark scheme and report

• reflux so primary alcohols E and G give carboxylic acids
• D gives the ketone
• F (tertiary) no reaction
• take care with branching for E
• penalise –HO connectivity once

Mark scheme

Examiner's comment

ocr.org.uk/science Oxford Cambridge and RSA CHEMISTRY A OCR Level 3 Advanced Subsidiary GCE in Chemistry A H032 For first teaching in 2015 Examiners’ report Version 1 H032/01 Summer 2025 series AS LEVEL Contents Contents ................................................................................................................................................... 2 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 ........................................................................................................................................ 11 Question 12 ........................................................................................................................................ 12 Question 13 ........................................................................................................................................ 12 Question 14 ........................................................................................................................................ 13 Question 15 ........................................................................................................................................ 14 Question 16 ........................................................................................................................................ 15 Question 17 ........................................................................................................................................ 15 Question 18 ........................................................................................................................................ 16 Question 19 ........................................................................................................................................ 16 Question 20 ........................................................................................................................................ 17 Section B overview ................................................................................................................................. 18 Question 21 (a) ................................................................................................................................... 18 Question 21 (b) ................................................................................................................................... 19 Question 21 (c) ................................................................................................................................... 20 Question 21 (d) (i) ............................................................................................................................... 21 Question 21 (d) (ii) .............................................................................................................................. 23 Question 22 (a) .................................................................................................................................. 24 Question 22 (b) (i) .............................................................................................................................. 25 Question 22 (b) (ii) ............................................................................................................................. 25 Question 22 (c) ................................................................................................................................... 26 Question 23 (a) .................................................................................................................................. 27 Question 23 (b) ................................................................................................................................... 28 Question 23 (c) .................................................................................................................................. 29 Question 24 (a) (i) ............................................................................................................................... 30 Question 24 (a) (ii) .............................................................................................................................. 30 Question 24 (b) .................................................................................................................................. 31 Question 24 (c) (i) ............................................................................................................................... 32 Question 24 (c) (ii) .............................................................................................................................. 33 Question 25 (a) .................................................................................................................................. 34 Question 25 (b .................................................................................................................................... 35 Question 25 (c) ................................................................................................................................... 36 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 25 (b Just under half of candidates scored this with a wide range of incorrect responses.

Mark scheme

Examiner's comment

Many candidates followed a well drilled method to apply the ideal gas equation to this calculation (including writing each term down and converting where necessary): • Moles of alcohol F used • Multiplication by 4 to give moles of CO2 • Rearranging the ideal gas equation • Substituting the correct values using the correct conversion to Kelvin for temperature • Converting from m3 to cm3 and giving the response to an appropriate number of significant figures (3 in this case) Most candidates scored at least 3 marks on this question. Common errors were using the number of moles of alcohol instead of the number of moles of carbon dioxide in the rearranged ideal gas equation, converting from m3 to cm3 incorrectly in the final step (usually only multiplying by 1,000 rather than 1,000,000) and not giving the response to an appropriate number of significant figures. Occasionally, candidates did not convert to Kelvin or used the pressure in kPa instead of Pa. This response shows an example of clear working. Values are assigned to each term and moles calculated correctly. Rearrangement and substitution of values is shown and correct. The appropriate number of significant figures is given, but the response only converted to dm3 not cm3. This response was given 4 marks.

Notes from the mark scheme and report

• must use pV = nRT
• common errors: no ×4 ratio (7240), no conversion to K (7760), not 3 sig figs (28943.8), using 25.9 as n
• DO NOT ALLOW 150 kPa unconverted in the final calculation
• Candidates should understand that the lowest level of significant figures in the raw data provided for a calculation will determine the number of significant figures that should be given in the final answer. More details can be found in Section M1.1 of the Mathematical Skills Handbook.

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