Cambridge A Level Chemistry 9701 — 2025 Oct/Nov Paper 2 · Variant 3

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] (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 Cr2O72– + 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] (d) In acidic conditions, a dynamic equilibrium is established between CrO42–(aq) and Cr2O72–(aq). 2CrO42–(aq) + 2H+(aq) Cr2O72–(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 Cr2O72–(aq) ions are orange. State what is observed when the following changes are made to an equilibrium mixture of acidified CrO42–(aq) and Cr2O72–(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]

Mark scheme: 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 2 AND methanal has permanent dipole–permanent dipole (and id-id) forces M2 IMF’s in methanal are stronger than IMF’s in CO2 1(d)(i) rate of forward and backward (reaction) are the same / equal 1 OR concentrations of reactants and products are constant 1(d)(ii) closed system 1 1(d)(iii) M1 (mixture becomes more) orange 4 M2 forward reaction is endothermic M3 (mixture becomes more) orange M4 equilibrium moves (forwards/right) to compensate for extra acid / H+ 1(e) M1 hydrolysis 2 M2 CrF4 + 2H2O → CrO2 + 4HF

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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. 250 200 150 Key radius / pm atomic radius ionic radius 100 50 0 Na Mg Al Si P S Cl 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] (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 oxidation number of A +3 Period 3 element 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]

Mark scheme: 2(a)(i) M1 nuclear charge increases (across period) AND similar shielding 2 M2 greater / stronger attraction to nucleus 2(a)(ii) M1 Al loses (3) electrons / Al is 3+, and P gains (3) electrons / P is 3– 2 M2 Al (3+) has (one) fewer shell (than P(3–)) 2(b)(i) 2 M1 (+)1 (+)2 (+)3 (+)4 (+)5 (+)6 M2 12-14 8-10 — — 0-4 0-4 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

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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 in the presence of ultraviolet light to form 2 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] (d) Fig. 3.2 shows a reaction cycle involving cyclopentane, cyclopentene and C5H9Cl . ΔH2 reaction 1 reaction 2 H2 Cl2 ΔH3 = –53 kJ mol–1 Cl reaction 3 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 enthalpy change of compound combustion, ΔHc / kJ mol–1 –3292 –3115 H2 –286 ΔH2 = ........................................... kJ mol–1 [2] (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. 100 transmittance / % 50 0 4000 3000 2000 1500 1000 500 wavenumber / cm–1 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] Table 3.3 characteristic infrared absorption range bond functional groups containing the bond (in wavenumbers) / cm–1 C–O hydroxy, ester 1040–1300 C=C aromatic compound, alkene 1500–1680 amide 1640–1690 C=O carbonyl, carboxyl 1670–1740 ester 1710–1750 C N nitrile 2200–2250 C–H alkane 2850–2950 N–H amine, amide 3300–3500 carboxyl 2500–3000 O–H hydroxy 3200–3650 [Total: 13]

Mark scheme: 3(a) simplest /lowest (whole number / integer) ratio of atoms (of each element) in a compound / molecule 1 3(b) two of: 2 OR OR 3(c)(i) homolytic 1 3(c)(ii) M1 C5H10 + Cl● → C5H9● + HCl 2 M2 C5H9● + Cl2 → C5H9Cl + Cl● 3(c)(iii) termination 1 3(d)(i) HCl (g) 1 3(d)(ii) M1 (reaction 1): –3115 –286 + 3292 = –109 (kJ mol–1) 2 M2 ∆H2 = – 53 – (–109) = (+)56 (kJ mol–1) 3(e)(i) 1 3(e)(ii) M1 absorption labelled at approx. 3000 cm–1 identified as O—H 2 M2 absorption labelled at approx. 1720 cm–1 identified as C=O

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4 Fig. 4.1 shows a possible synthesis of propene, C3H6. reaction 1 reaction 2 reaction 3 C2H5COOH C2H5CH2OH C2H5CH2Cl C3H6 NaOH 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] (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] (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. 100 80 60 relative abundance 40 20 0 10 20 30 40 50 60 70 80 m / e 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]

Mark scheme: 4(a)(i) reduction 1 4(a)(ii) PCl5 / PCl3 / SOCl2 1 OR KCl / NaCl AND conc. H2SO4 / conc.H3PO4 4(a)(ii) M1 C2H5CH2Cl + NaOH → C3H6 + H2O + NaCl 2 M2 ethanol (solvent) AND heat / reflux 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) 2 M2 but different 3D / spatial arrangement of atoms / groups 4(b)(iii) 3 M1 M2 optical isomerism M3 the methylated carbon has four different groups attached 4(b)(iv) M1 non-biodegradability 2 M2 harmful combustion products 4(c)(i) 2,3-dimethylbut-1-ene 1 4(c)(ii) (The spectrum is of) X 1 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+ 4(c)(iii) ([M+1]+ / 34.7  100 / 1.1 = 6) 1 [M+1]+ = 2.29

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