Cambridge A Level Chemistry 9701 — 2025 Oct/Nov Paper 2 · Variant 1

This document has 16 pages. Any blank pages are indicated. [Turn over Cambridge International AS & A Level * 9 1 8 7 0 3 2 9 6 7 * CHEMISTRY 9701/21 Paper 2 AS Level Structured Questions October/November 2025 1 hour 15 minutes You must answer on the question paper. No additional materials are needed. INSTRUCTIONS ● Answer all questions. ● Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs. ● Write your name, centre number and candidate number in the boxes at the top of the page. ● Write your answer to each question in the space provided. ● Do not use an erasable pen or correction fluid. ● Do not write on any bar codes. ● You may use a calculator. ● You should show all your working and use appropriate units. INFORMATION ● The total mark for this paper is 60. ● The number of marks for each question or part question is shown in brackets [ ]. ● The Periodic Table is printed in the question paper. ● Important values, constants and standards are printed in the question paper. DC (KS) 357208 © UCLES 2025 ,










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2 9701/21/O/N/25 © UCLES 2025 1 Chromium, Cr, and its compounds are widely used in many chemical reactions. (a) Cr exists as four stable isotopes. (i) The most common isotope of Cr is chromium-52. Determine the number of protons, neutrons and electrons in an atom of chromium-52. number of protons … neutrons … electrons … [1] (ii) Describe how an atom of chromium-54 differs from an atom of chromium-52. Refer to numbers of particles in your answer. … … [1] (iii) The relative isotopic masses of the isotopes of Cr can be determined using mass spectrometry. State what other information is needed to calculate the relative atomic mass, Ar , of Cr. … [1] (iv) Atoms of 52Cr, 53Cr and 54Cr make up more than 95% of naturally occurring chromium atoms. The Ar of naturally occurring Cr is 51.996. Suggest what these statements imply about the relative isotopic mass of the fourth stable isotope of chromium. … [1] (b) The shorthand electronic configuration of chromium is [Ar] 3d54s1. (i) Complete the full electronic configuration of chromium. … 3d54s1 [1] (ii) Deduce the total number of unpaired electrons in an atom of chromium. … [1] * 0000800000002 * ,










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3 9701/21/O/N/25 © UCLES 2025 [Turn over (c) Acidified dichromate(VI) ions will convert methanal, CH2O, to carbon dioxide. The movement of electrons to or from relevant species is shown in the following half-equations. half-equation 1 CH2O + H2O CO2 + 4H+ + 4e– half-equation 2 Cr2O7 2– + 14H+ + 6e– 2Cr3+ + 7H2O (i) Identify the species that is reduced in half-equation 2. Explain your answer. … [1] (ii) The oxidation state of the carbon atom in methanal is 0. Calculate the oxidation state of carbon in carbon dioxide. … [1] (iii) Construct the ionic equation for the reaction of dichromate(VI) ions with methanal in acidic conditions. … [2] (iv) Methanal is a liquid at –30 °C but carbon dioxide is a gas at this temperature. Explain why. … … … … [2] * 0000800000003 * ,










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4 9701/21/O/N/25 © UCLES 2025 (d) In acidic conditions, a dynamic equilibrium is established between CrO4 2–(aq) and Cr2O7 2–(aq). 2CrO4 2–(aq) + 2H+(aq) Cr2O7 2–(aq) + H2O(l) ΔH > 0 (i) State what is meant by dynamic equilibrium. … … [1] (ii) Identify the condition necessary to establish dynamic equilibrium. … [1] (iii) CrO4 2–(aq) ions are yellow and Cr2O7 2–(aq) ions are orange. State what is observed when the following changes are made to an equilibrium mixture of acidified CrO4 2–(aq) and Cr2O7 2–(aq) ions. Explain your answers. • The equilibrium mixture is warmed gently. observation … explanation … … … • Dilute HCl (aq) is added to the equilibrium mixture. observation … explanation … … … [4] (e) Chromium(IV) fluoride, CrF4, is a covalent molecule that shows similar chemical properties to SiCl 4. Suggest the type of reaction that occurs when CrF4 is placed in water. Construct a relevant equation for this reaction. type of reaction … equation … [2] [Total: 20] * 0000800000004 * ,










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5 9701/21/O/N/25 © UCLES 2025 [Turn over BLANK PAGE * 0000800000005 * ,










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6 9701/21/O/N/25 © UCLES 2025 2 The Period 3 elements show trends in physical and chemical properties across the period. (a) Fig. 2.1 shows the variation in atomic and ionic radii of the Period 3 elements Na to Cl . The ionic radius of Si is not shown. 0 Na Mg Al Si P S Cl 50 100 150 250 200 radius / pm atomic radius ionic radius Key Fig. 2.1 (i) Explain the trend shown in the atomic radii of the Period 3 elements Na to Cl . … … … … [2] (ii) Explain why there is a large difference in the ionic radii of Al and P. … … … … [2] * 0000800000006 * ,










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7 9701/21/O/N/25 © UCLES 2025 [Turn over (b) Table 2.1 gives some information about some of the Period 3 oxides. Row B gives the pH of the solution that forms when the Period 3 oxide is added to water. Table 2.1 formula of Period 3 oxide Na2O MgO Al 2O3 SiO2 P4O10 SO3 A oxidation number of Period 3 element +3 B pH of solution — — (i) Complete Table 2.1. [2] (ii) State why there is no data given in row B for Al 2O3 and SiO2. … [1] (c) (i) Write an equation for the reaction of Na2O with dilute hydrochloric acid. … [1] (ii) Construct an equation for the reaction of Al 2O3 with a base to form NaAl O2. … [1] (d) Group 2 nitrates decompose on heating to form oxides. (i) State the trend in thermal stability of the Group 2 nitrates down the group. … [1] (ii) Identify the other products of the thermal decomposition of Group 2 nitrates. … [1] [Total: 11] * 0000800000007 * ,










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8 9701/21/O/N/25 © UCLES 2025 3 Cycloalkanes show similar chemical properties to alkanes but have the same empirical formula as alkenes. (a) Define empirical formula. … … [1] (b) Cyclopentane, C5H10, has four cyclic structural isomers. One of these isomers is C, shown in Fig. 3.1. Complete Fig. 3.1 to show two other cyclic structural isomers of C5H10. cyclopentane C Fig. 3.1 [2] (c) Cyclopentane reacts with Cl 2 in the presence of ultraviolet light to form C5H9Cl. (i) The reaction is initiated by the bond fission of Cl 2. State the type of bond fission shown in the initiation step. … [1] (ii) Complete the equations to show the two propagation steps that follow the initiation step. propagation 1 C5H10 + … C5H9• + … propagation 2 C5H9• + … … [2] (iii) The final step is shown. C5H9• + Cl • C5H9Cl Give the name for this step in the reaction. … [1] * 0000800000008 * ,










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9 9701/21/O/N/25 © UCLES 2025 [Turn over (d) Fig. 3.2 shows a reaction cycle involving cyclopentane, cyclopentene and C5H9Cl . ΔH2 reaction 2 Cl2 reaction 1 H2 reaction 3 ΔH3 = –53 kJ mol–1 Cl Fig. 3.2 (i) Identify a suitable reagent for reaction 3. … [1] (ii) Use the data in Fig. 3.2 and in Table 3.1 to calculate the enthalpy change of reaction 2, ΔH2. Table 3.1 compound enthalpy change of combustion, ΔHc / kJ mol–1 –3292 –3115 H2 –286 ΔH2 = … kJ mol–1 [2] * 0000800000009 * ,










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10 9701/21/O/N/25 © UCLES 2025 (e) Cyclopentene, C5H8, reacts with hot concentrated acidified KMnO4 to form compound W, C5H8O4. (i) Draw the structure of W. [1] (ii) The infrared spectrum of W is shown in Fig. 3.3. 4000 0 50 transmittance / % 100 3000 2000 1500 wavenumber / cm–1 1000 500 Fig. 3.3 Identify two absorptions in the infrared spectrum of W that would not be present in the infrared spectrum of cyclopentene. • Write 1 or 2 on Fig. 3.3 against each of these two absorptions. • Complete Table 3.2 to show which bond is responsible for each absorption that you have identified in Fig. 3.3. Table 3.2 absorption 1 2 bond responsible [2] * 0000800000010 * ,










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11 9701/21/O/N/25 © UCLES 2025 [Turn over Table 3.3 bond functional groups containing the bond characteristic infrared absorption range (in wavenumbers) / cm–1 C–O hydroxy, ester 1040–1300 C=C aromatic compound, alkene 1500–1680 C=O amide carbonyl, carboxyl ester 1640–1690 1670–1740 1710–1750 C N nitrile 2200–2250 C–H alkane 2850–2950 N–H amine, amide 3300–3500 O–H carboxyl hydroxy 2500–3000 3200–3650 [Total: 13] * 0000800000011 * ,









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12 9701/21/O/N/25 © UCLES 2025 4 Fig. 4.1 shows a possible synthesis of propene, C3H6. C2H5COOH C2H5CH2OH reaction 1 reaction 2 reaction 3 NaOH C2H5CH2Cl C3H6 Fig. 4.1 (a) (i) Identify the type of reaction that occurs in reaction 1. … [1] (ii) Suggest a suitable reagent for reaction 2. … [1] (iii) Reaction 3 is an elimination reaction. Write an equation for this reaction and identify the solvent and conditions used. equation … solvent and conditions … [2] (iv) C2H5CH2OH can be directly converted to C3H6. Suggest the reagent and conditions for this conversion. … [1] (b) Under suitable conditions, propene polymerises to form poly(propene). Poly(propene) exhibits stereoisomerism. (i) Identify the type of polymerisation that forms poly(propene) from propene. … [1] (ii) Define stereoisomerism. … … [2] * 0000800000012 * ,









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13 9701/21/O/N/25 © UCLES 2025 [Turn over (iii) Draw a section of poly(propene), showing two repeat units. Use your diagram to identify the type of stereoisomerism shown by poly(propene). Explain your answer. section of poly(propene) … … … … [3] (iv) State two difficulties associated with the disposal of poly(propene). 1 … 2 … [2] * 0000800000013 * ,









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14 9701/21/O/N/25 © UCLES 2025 (c) Under different conditions, two molecules of propene can combine to form compounds X and Y. X Y (i) Name X. … [1] (ii) Fig. 4.2 shows the mass spectrum of either compound X or compound Y. 10 20 30 40 50 m / e 60 70 80 0 20 40 60 relative abundance 80 100 Fig. 4.2 Identify which of X and Y gives this mass spectrum. Give one reason for your answer, referring to the fragmentation pattern. … … … [1] (iii) The molecular ion peak in the spectrum in Fig. 4.2 has relative abundance 34.7. Calculate the relative abundance of the [M+1]+ peak in this spectrum. relative abundance of [M+1]+ peak = … [1] [Total: 16] * 0000800000014 * ,









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15 9701/21/O/N/25 © UCLES 2025 Important values, constants and standards molar gas constant R = 8.31 J K–1 mol–1 Faraday constant F = 9.65 × 104 C mol–1 Avogadro constant L = 6.02 × 1023 mol–1 electronic charge e = –1.60 × 10–19 C molar volume of gas Vm = 22.4 dm3 mol–1 at s.t.p. (101 kPa and 273 K) Vm = 24.0 dm3 mol–1 at room conditions ionic product of water Kw = 1.00 × 10–14 mol2 dm–6 (at 298 K (25 °C)) specific heat capacity of water c = 4.18 kJ kg–1 K–1 (4.18 J g–1 K–1) * 0000800000015 * ,









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16 9701/21/O/N/25 © UCLES 2025 To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series. Group The Periodic Table of Elements 1 H hydrogen 1.0 2 He helium 4.0 1 2 13 14 15 16 17 18 3 4 5 6 7 8 9 10 11 12 3 Li lithium 6.9 4 Be beryllium 9.0 atomic number atomic symbol Key name relative atomic mass 11 Na sodium 23.0 12 Mg magnesium 24.3 19 K potassium 39.1 20 Ca calcium 40.1 37 Rb rubidium 85.5 38 Sr strontium 87.6 55 Cs caesium 132.9 56 Ba barium 137.3 87 Fr francium – 88 Ra radium – 5 B boron 10.8 13 Al aluminium 27.0 31 Ga gallium 69.7 49 In indium 114.8 81 Tl thallium 204.4 6 C carbon 12.0 14 Si silicon 28.1 32 Ge germanium 72.6 50 Sn tin 118.7 82 Pb lead 207.2 22 Ti titanium 47.9 40 Zr zirconium 91.2 72 Hf hafnium 178.5 104 Rf rutherfordium – 23 V vanadium 50.9 41 Nb niobium 92.9 73 Ta tantalum 180.9 105 Db dubnium – 24 Cr chromium 52.0 42 Mo molybdenum 95.9 74 W tungsten 183.8 106 Sg seaborgium – 25 Mn manganese 54.9 43 Tc technetium – 75 Re rhenium 186.2 107 Bh bohrium – 26 Fe iron 55.8 44 Ru ruthenium 101.1 76 Os osmium 190.2 108 Hs hassium – 27 Co cobalt 58.9 45 Rh rhodium 102.9 77 Ir iridium 192.2 109 Mt meitnerium – 28 Ni nickel 58.7 46 Pd palladium 106.4 78 Pt platinum 195.1 110 Ds darmstadtium – 29 Cu copper 63.5 47 Ag silver 107.9 79 Au gold 197.0 111 Rg roentgenium – 30 Zn zinc 65.4 48 Cd cadmium 112.4 80 Hg mercury 200.6 112 Cn copernicium – 114 Fl flerovium – 116 Lv livermorium – 7 N nitrogen 14.0 15 P phosphorus 31.0 33 As arsenic 74.9 51 Sb antimony 121.8 83 Bi bismuth 209.0 8 O oxygen 16.0 16 S sulfur 32.1 34 Se selenium 79.0 52 Te tellurium 127.6 84 Po polonium – 9 F fluorine 19.0 17 Cl chlorine 35.5 35 Br bromine 79.9 53 I iodine 126.9 85 At astatine – 10 Ne neon 20.2 18 Ar argon 39.9 36 Kr krypton 83.8 54 Xe xenon 131.3 86 Rn radon – 113 Nh nihonium – 115 Mc moscovium – 117 Ts tennessine – 118 Og oganesson – 21 Sc scandium 45.0 39 Y yttrium 88.9 57–71 lanthanoids 89–103 actinoids 57 La lanthanum 138.9 89 Ac lanthanoids actinoids actinium – 58 Ce cerium 140.1 90 Th thorium 232.0 59 Pr praseodymium 140.9 91 Pa protactinium 231.0 60 Nd neodymium 144.2 92 U uranium 238.0 61 Pm promethium – 93 Np neptunium – 62 Sm samarium 150.4 94 Pu plutonium – 63 Eu europium 152.0 95 Am americium – 64 Gd gadolinium 157.3 96 Cm curium – 65 Tb terbium 158.9 97 Bk berkelium – 66 Dy dysprosium 162.5 98 Cf californium – 67 Ho holmium 164.9 99 Es einsteinium – 68 Er erbium 167.3 100 Fm fermium – 69 Tm thulium 168.9 101 Md mendelevium – 70 Yb ytterbium 173.1 102 No nobelium – 71 Lu lutetium 175.0 103 Lr lawrencium – * 0000800000016 * ,









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This document consists of 12 printed pages. © Cambridge University Press & Assessment 2025 [Turn over Cambridge International AS & A Level CHEMISTRY 9701/21 Paper 2 AS Structured Questions October/November 2025 MARK SCHEME Maximum Mark: 60 Published This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge International will not enter into discussions about these mark schemes. Cambridge International is publishing the mark schemes for the October/November 2025 series for most Cambridge IGCSE, Cambridge International A and AS Level components, and some Cambridge O Level components.

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 2 of 12 Generic Marking Principles These general marking principles must be applied by all examiners when marking candidate answers. They should be applied alon gside the specific content of the mark scheme or generic level descriptions for a question. Each question paper and mark scheme will also comply with these marking principles. GENERIC MARKING PRINCIPLE 1: Marks must be awarded in line with: • the specific content of the mark scheme or the generic level descriptors for the question • the specific skills defined in the mark scheme or in the generic level descriptors for the question • the standard of response required by a candidate as exemplified by the standardisation scripts. GENERIC MARKING PRINCIPLE 2: Marks awarded are always whole marks (not half marks, or other fractions). GENERIC MARKING PRINCIPLE 3: Marks must be awarded positively: • marks are awarded for correct/valid answers, as defined in the mark scheme. However, credit is given for valid answers which go beyond the scope of the syllabus and mark scheme, referring to your Team Leader as appropriate • marks are awarded when candidates clearly demonstrate what they know and can do • marks are not deducted for errors • marks are not deducted for omissions • answers should only be judged on the quality of spelling, punctuation and grammar when these features are specifically assessed by the question as indicated by the mark scheme. The meaning, however, should be unambiguous. GENERIC MARKING PRINCIPLE 4: Rules must be applied consistently, e.g. in situations where candidates have not followed instructions or in the application of generic level descriptors.

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 3 of 12 GENERIC MARKING PRINCIPLE 5: Marks should be awarded using the full range of marks defined in the mark scheme for the question (however; the use of the full mark range may be limited according to the quality of the candidate responses seen). GENERIC MARKING PRINCIPLE 6: Marks awarded are based solely on the requirements as defined in the mark scheme. Marks should not be awarded with grade thre sholds or grade descriptors in mind. Science-Specific Marking Principles 1 Examiners should consider the context and scientific use of any keywords when awarding marks. Although keywords may be present, marks should not be awarded if the keywords are used incorrectly. 2 The examiner should not choose between contradictory statements given in the same question part, and credit should not be awarded for any correct statement that is contradicted within the same question part. Wrong science that is irrelevant to the question should be ignored. 3 Although spellings do not have to be correct, spellings of syllabus terms must allow for clear and unambiguous separation fro m other syllabus terms with which they may be confused (e.g. ethane / ethene, glucagon / glycogen, refraction / reflection). 4 The error carried forward (ecf) principle should be applied, where appropriate. If an incorrect answer is subsequently used in a scientifically correct way, the candidate should be awarded these subsequent marking points. Further guidance will be included in the mark scheme where necessary and any exceptions to this general principle will be noted. 5 ‘List rule’ guidance For questions that require n responses (e.g. State two reasons …): • The response should be read as continuous prose, even when numbered answer spaces are provided. • Any response marked ignore in the mark scheme should not count towards n. • Incorrect responses should not be awarded credit but will still count towards n. • Read the entire response to check for any responses that contradict those that would otherwise be credited. Credit should not be awarded for any responses that are contradicted within the rest of the response. Where two responses contradict one another, this should be treated as a single incorrect response. • Non-contradictory responses after the first n responses may be ignored even if they include incorrect science.

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 4 of 12 6 Calculation specific guidance Correct answers to calculations should be given full credit even if there is no working or incorrect working, unless the question states ‘show your working’. For questions in which the number of significant figures required is not stated, credit should be awarded for correct answers when rounded by the examiner to the number of significant figures given in the mark scheme. This may not apply to measured values. For answers given in standard form (e.g. a  10n) in which the convention of restricting the value of the coefficient (a) to a value between 1 and 10 is not followed, credit may still be awarded if the answer can be converted to the answer given in the mark scheme. Unless a separate mark is given for a unit, a missing or incorrect unit will normally mean that the final calculation mark is not awarded. Exceptions to this general principle will be noted in the mark scheme. 7 Guidance for chemical equations Multiples / fractions of coefficients used in chemical equations are acceptable unless stated otherwise in the mark scheme. State symbols given in an equation should be ignored unless asked for in the question or stated otherwise in the mark scheme.

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 5 of 12 Annotations guidance for centres Examiners use a system of annotations as a shorthand for communicating their marking decisions to one another. Examiners are trained during the standardisation process on how and when to use annotations. The purpose of annotations is to inform the standard isation and monitoring processes and guide the supervising examiners when they are checking the work of examiners within their team. The meaning of annotations and how they are used is specific to each component and is understood by all examiners who mark the component. We publish annotations in our mark schemes to help centres understand the annotations they may see on copies of scripts. Note that there may not be a direct correlation between the number of annotations on a script and the mark awarded. Similarly, the use of an annotation may not be an indication of the quality of the response. The annotations listed below were available to examiners marking this component in this series. Annotations Annotation Meaning Correct point or mark awarded Incorrect point or mark not awarded Unclear Information missing or insufficient for credit Benefit of the doubt given Contradiction in response otherwise markworthy, mark not given Part of the correct answer has been seen. Full credit has not been awarded. Error carried forward applied Incorrect or insufficient point ignored while marking the rest of the response Rounding error

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 6 of 12 Annotation Meaning Repetition Blank page or part of script seen Error in number of significant figures Transcription error

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 7 of 12 Question Answer Marks 1(a)(i) 24 protons 28 neutrons 24 electrons 1 1(a)(ii) (54Cr has two) more neutrons (than 52Cr) 1 1(a)(iii) (relative) abundance (of each isotope) 1 1(a)(iv) The fourth isotope has any value of RIM less than 51.996 1 1(b)(i) 1s2 2s2 2p6 3s2 3p6 (3d5 4s1) 1 1(b)(ii) 6 1 1(c)(i) Cr / Cr2O72– AND it gains electrons 1 1(c)(ii) (+)4 1 1(c)(iii) Due to an issue with question 1(c)(iii), the question has been removed from the question paper. N/A 1(c)(iv) M1 CO2 has only instantaneous / temporary dipole–induced dipole forces AND methanal has permanent dipole–permanent dipole (and id-id) forces M2 IMF’s in methanal are stronger than IMF’s in CO2 2

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 8 of 12 Question Answer Marks 1(d)(i) rate of forward and backward (reaction) are the same / equal OR concentrations of reactants and products are constant 1 1(d)(ii) closed system 1 1(d)(iii) M1 (mixture becomes more) orange M2 forward reaction is endothermic M3 (mixture becomes more) orange M4 equilibrium moves (forwards/right) to compensate for extra acid / H+ 4 1(e) M1 hydrolysis M2 CrF4 + 2H2O → CrO2 + 4HF 2

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 9 of 12 Question Answer Marks 2(a)(i) M1 nuclear charge increases (across period) AND similar shielding M2 greater / stronger attraction to nucleus 2 2(a)(ii) M1 Al loses (3) electrons / Al is 3+, and P gains (3) electrons / P is 3– M2 Al (3+) has (one) fewer shell (than P(3–)) 2 2(b)(i) (+)1 (+)2 (+)3 (+)4 (+)5 (+)6 12-14 8-10 — — 0-4 0-4 2 2(b)(ii) they are insoluble 1 2(c)(i) Na2O + 2HCl → 2NaCl + H2O 1 2(c)(ii) Al2O3 + 2NaOH → 2NaAlO2 + H2O 1 2(d)(i) (thermal stability) increases (down the group) 1 2(d)(ii) NO2 AND O2 1 M1 M2

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 10 of 12 Question Answer Marks 3(a) simplest /lowest (whole number / integer) ratio of atoms (of each element) in a compound / molecule 1 3(b) two of: OR OR 2 3(c)(i) homolytic 1 3(c)(ii) M1 C5H10 + Cl● → C5H9● + HCl M2 C5H9● + Cl2 → C5H9Cl + Cl● 2 3(c)(iii) termination 1 3(d)(i) HCl (g) 1 3(d)(ii) M1 (reaction 1): –3115 –286 + 3292 = –109 (kJ mol–1) M2 ∆H2 = – 53 – (–109) = (+)56 (kJ mol–1) 2 3(e)(i) 1 3(e)(ii) M1 absorption labelled at approx. 3000 cm–1 identified as O—H M2 absorption labelled at approx. 1720 cm–1 identified as C=O 2

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 11 of 12 Question Answer Marks 4(a)(i) reduction 1 4(a)(ii) PCl5 / PCl3 / SOCl2 OR KCl / NaCl AND conc. H2SO4 / conc.H3PO4 1 4(a)(ii) M1 C2H5CH2Cl + NaOH → C3H6 + H2O + NaCl M2 ethanol (solvent) AND heat / reflux 2 4(a)(iv) conc. H2SO4 OR conc. H3PO4 OR Al2O3 AND heat 1 4(b)(i) addition 1 4(b)(ii) M1 (molecules that have the) same structural formula (and same molecular formula) M2 but different 3D / spatial arrangement of atoms / groups 2 4(b)(iii) M1 M2 optical isomerism M3 the methylated carbon has four different groups attached 3 4(b)(iv) M1 non-biodegradability M2 harmful combustion products 2 4(c)(i) 2,3-dimethylbut-1-ene 1

9701/21 Cambridge International AS & A Level – Mark Scheme PUBLISHED October/November 2025 © Cambridge University Press & Assessment 2025 Page 12 of 12 Question Answer Marks 4(c)(ii) (The spectrum is of) X AND 41 is [CH2=CCH3]+ / 41 is C3H5+ OR 43 is [CH3CHCH3]+ / 43 is C3H7+ OR X would NOT have significant peak at 54 / [(CH3)2C=C+] / C4H6+ OR X would NOT have significant peak at 42 due to [(CH3)2C+] / C3H6+ 1 4(c)(iii) ([M+1]+ / 34.7  100 / 1.1 = 6) [M+1]+ = 2.29 1