Chemistry 9701/42 May/June 2025: question paper and mark scheme

Question

(a) (i) Calcium nitrate, Ca(NO3)2, decomposes on heating. Write an equation for the decomposition of calcium nitrate. …… [1] (ii) Describe the trend in the decomposition temperature of the Group 2 nitrates. Explain your answer. …… [3] (b) A sample of 0.333 g of strontium oxide, SrO, is completely dissolved in distilled water to form a solution of strontium hydroxide, Sr(OH)2. The resulting solution is added to a volumetric flask and made up to 250.0 cm3 with distilled water. Calculate the pH of this solution at 298 K. Give your answer to two decimal places. pH = …… [4] [Total: 8]

Question

(a) When cobalt(II) sulfate, CoSO4, is dissolved in distilled water, solution A is formed. The reaction scheme in Fig. 2.1 shows some reactions of solution A. excess NaOH(aq) NH3(aq) solution A solid B solution C [CoCl4]2– Fig. 2.1 (i) Complete Table 2.1 to show the formula and colour of each of the cobalt-containing species present in A, B and C. Identify the type of reaction forming each of B and C. Table 2.1 formula of colour of type of reaction cobalt-containing species cobalt-containing species A B C [4] (ii) Suggest a suitable reagent for the formation of [CoCl4]2– from solution A. …… [1] (b) The complex ion [CoCl4]2– has tetrahedral geometry. The 3d orbitals in an isolated Co2+ ion are degenerate. (i) Complete Fig. 2.2 to show the relative energies of the 3d orbitals in an isolated Co2+ ion and in Co2+ in a tetrahedral complex. energy isolated Co2+ ion Co2+ in a tetrahedral complex Fig. 2.2 [2] (ii) Draw a three-dimensional diagram to show the structure of the complex ion [CoCl4]2–. [1] (c) H2O2 can act as an oxidising agent or a reducing agent when reacting with species that contain manganese. Table 2.2 shows electrode potentials, E o , for some electrode reactions. Table 2.2 o electrode reaction E /V MnO2 + 2H2O + 2e– Mn(OH)2 + 2OH– –0.04 MnO2 + 4H+ + 2e– Mn2+ + 2H2O +1.22 H2O2 + 2H+ + 2e– 2H2O +1.78 H2O2 + OH– + 2e– 3OH– +0.88 O2 + 2H+ + 2e– H2O2 +0.68 Use only the species listed in Table 2.2 to suggest: • one reaction in which H2O2 acts as an oxidising agent and • one reaction in which H2O2 acts as a reducing agent. o Include the value of the standard cell potential, E cell , and an overall equation for each reaction. H2O2 acting as an oxidising agent …… H2O2 acting as a reducing agent …… [4] (d) Acidified manganate(VII) ions, MnO4–, can be used to analyse the content of iron tablets by titration. Two identical iron tablets are crushed and dissolved in distilled water. The resulting solution is made up to 150.0 cm3 with distilled water. 25.0 cm3 of this solution requires 18.60 cm3 of 0.0500 mol dm–3 acidified MnO4– to reach the end-point. All the Fe2+ ions are oxidised. The relevant half-equations are shown. MnO4– + 8H+ + 5e– Mn2+ + 4H2O Fe2+ Fe3+ + e– (i) Describe the colour change observed at the end-point of this titration. from …… to …… [1] (ii) Calculate the mass, in mg, of iron in one tablet. Assume that all the iron in the tablets is Fe2+. Show your working. mass of iron in one tablet = …… mg [4] [Total: 17]

Question

(a) Define entropy. …… [1] (b) Fig. 3.1 shows how the entropy, S, of a pure substance changes with temperature, T. entropy, S T1 T2 temperature, T Fig. 3.1 (i) Identify the process occurring at each of the temperatures T1 and T2. T1 …… T2 …… [1] (ii) Explain why the entropy change, ∆S, at T2 is bigger than the entropy change at T1. …… [1] (c) The equation for the reduction of iron(III) oxide by carbon monoxide at 450 °C is shown. Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) ∆G = –36.2 kJ mol–1 Table 3.1 shows the enthalpy of formation, ∆H f , and the entropy, S , for some substances. Table 3.1 Fe2O3(s) CO(g) Fe(s) CO2(g) ∆H f / kJ mol–1 –824.2 –110.5 0.0 –393.5 S / J K–1 mol–1 87.4 to be calculated 27.3 213.8 Use the data in Table 3.1 to calculate the entropy, S , of carbon monoxide at 450 °C. Show your working. S of CO(g) = …… J K–1 mol–1 [3] (d) Iron(II) oxide can also be reduced to iron by carbon monoxide, as shown. FeO(s) + CO(g) Fe(s) + CO2(g) ∆H = –11.1 kJ mol–1 ∆S = –15.2 J K–1 mol–1 State the effect of increasing temperature on the feasibility of this reaction. Explain your answer. …… [2] [Total: 8]

Question

(a) In aqueous solution, iodide ions react with acidified hydrogen peroxide, as shown in reaction 1. reaction 1 2I– + H2O2 + 2H+ I2 + 2H2O The rate equation for reaction 1 is shown. rate = k [I–][H2O2] (i) Explain what is meant by order of reaction. …… [1] (ii) Complete Table 4.1. Table 4.1 the order of reaction with respect to [H+] the order of reaction with respect to [I–] the order of reaction with respect to [H2O2] overall order of the reaction [2] (iii) Sketch a line on Fig. 4.1 to show the relationship between [I–] and time. [I–] 0 time Fig. 4.1 [1] (b) Nitrogen dioxide, NO2, reacts with ozone, O3, as shown in reaction 2. reaction 2 2NO2 + O3 O2 + N2O5 The rate equation for reaction 2 is shown. rate = k [NO2][O3] (i) Two experiments are carried out to measure the rate of reaction 2. In the first experiment, the initial rate is measured starting with known concentrations of NO2 and O3. In the second experiment, the concentrations of NO2 and O3 are both increased by a factor of four. Predict how the initial rate for reaction 2 would change. …… [1] (ii) The reaction mechanism for reaction 2 has two steps. Define rate-determining step. …… [1] (iii) Suggest equations for the two steps of the reaction mechanism for reaction 2. step 1 …… step 2 …… [2] (c) Dinitrogen pentoxide, N2O5, can decompose to NO2 and O2. The rate equation for this decomposition is shown. rate = k [N2O5] Fig. 4.2 shows the graph of rate against [N2O5] for this decomposition. 16.0 14.0 12.0 10.0 rate 8.0 / 10–4 mol dm–3 s–1 6.0 4.0 2.0 0.0 0.0000 0.0100 0.0200 0.0300 0.0400 0.0500 [N2O5] / mol dm–3 Fig. 4.2 (i) Use Fig. 4.2 to calculate a value for the rate constant, k, for this decomposition. k = …… [1] (ii) Use your answer to (c)(i) to calculate the half-life, t 1, in seconds, for this decomposition. 2 t 1 = …… s [1] 2 [Total: 10]

Question

(a) Chloric(I) acid, HCl O, is a weak Brønsted–Lowry acid. When chloric(I) acid is added to aqueous sodium hydroxide, an acid–base reaction takes place, as shown. HCl O + NaOH NaCl O + H2O (i) Identify the two conjugate acid–base pairs in this reaction. acid I HCl O conjugate base of acid I …… acid II …… conjugate base of acid II …… [1] (ii) The value for the acid dissociation constant, Ka, of HCl O(aq) is 3.70 × 10–8. Calculate the concentration, in mol dm–3, of HCl O(aq) at a pH of 4.51. concentration of HCl O = …… mol dm–3 [2] (iii) When a solution of HCl O(aq) is heated, chloric(V) acid and a strong acid not containing oxygen are formed as the only products. Write an equation for this reaction. …… [1] (b) (i) Define a buffer solution. …… [1] (ii) Suggest a substance that could be added to aqueous ethanoic acid to form a buffer solution. Explain your answer. …… [1] (c) Some fertilisers contain calcium dihydrogenphosphate, Ca(H2PO4)2. An aqueous solution containing dihydrogenphosphate ions, H2PO4–, can act as a buffer solution. Write two equations to show how H2PO4– ions can act as a buffer. equation 1 …… equation 2 …… [2] (d) The solubility of calcium phosphate, Ca3(PO4)2, is 1.14 × 10–7 mol dm–3 at 25 °C. (i) The expression for the solubility product, Ksp, of Ca3(PO4)2 is shown. Ksp = [Ca2+]3[PO43–]2 Calculate the value of Ksp for Ca3(PO4)2. Include units. Ksp = …… units …… [3] (ii) Some solid sodium phosphate is added to a saturated solution of Ca3(PO4)2. Predict the effect, if any, on the solubility of Ca3(PO4)2. Explain your answer. …… [1] [Total: 12]

Question

(a) Methylbenzene reacts readily with nitronium ions, NO2+. NO2+ ions are generated by the reaction between concentrated nitric acid and concentrated sulfuric acid. (i) Write an equation for the formation of the NO2+ ion. …… [1] (ii) Complete the mechanism in Fig. 6.1 for the nitration of methylbenzene to form 1-methyl-2-nitrobenzene. Include all relevant curly arrows and charges. organic intermediate 1-methyl-2-nitrobenzene CH3 CH3 NO2 + NO2 + …… Fig. 6.1 [3] (b) Phenol can be nitrated with dilute nitric acid. Explain why the nitration of phenol occurs under milder conditions than the nitration of benzene. …… [2] (c) A sample of 2-nitrophenol is reacted with sodium. Complete the equation in Fig. 6.2 for the reaction of 2-nitrophenol with sodium. OH NO2 + Na Fig. 6.2 [1] (d) 2-nitrophenol can undergo different reactions as shown in Fig. 6.3. 2-nitrophenol OH OH NH2 NO2 CH3COCl reaction 1 reaction 2 Fig. 6.3 (i) Suggest reagents and conditions for reaction 1. …… [1] (ii) Name the type of reaction for reaction 1. …… [1] (iii) Reaction 2 is carried out at room temperature. Draw the structure of the organic product from reaction 2 in Fig. 6.3. [1] (iv) Name the mechanism for reaction 2. …… [1] (e) The NO2 group in 2-nitrophenol is electron withdrawing. Suggest the relative acidities of ethanol, 2-nitrophenol, phenol and water. Explain your answer. most acidic least acidic [4] (f) Salbutamol is a pharmaceutical drug that contains a phenol functional group. salbutamol OH H N HO HO Fig. 6.4 (i) Name and classify the three other functional groups in salbutamol in Table 6.1. Table 6.1 name of functional group classification of functional group [2] (ii) Salbutamol reacts with Br2(aq) to form organic product X. Draw the structure of X. [1] (iii) Salbutamol reacts with an excess of SOCl 2 to form organic product Y. The molecular formula of Y is C13H19Cl 2NO. Draw the structure of Y. [1] [Total: 19]

Question

(a) 2-aminobutane, CH3CH(NH2)CH2CH3, exists as a mixture of two enantiomers. Define enantiomers. …… [1] (b) (i) Explain why an aqueous solution of 2-aminobutane has a pH greater than 7. Include an equation in your answer. …… [2] (ii) A 0.10 mol dm–3 solution of diethylamine, (CH3CH2)2NH, has a higher pH than a 0.10 mol dm–3 solution of 2-aminobutane, CH3CH(NH2)CH2CH3. Suggest why. …… [2] (iii) (CH3CH2)2NH reacts with ethanoic acid, CH3COOH. Complete the equation for this reaction. (CH3CH2)2NH + CH3COOH …… [1] (iv) (CH3CH2)2NH reacts with ethanoyl chloride, CH3COCl. Complete the equation for this reaction. (CH3CH2)2NH + CH3COCl …… [1] (c) Table 7.1 shows monomers that can undergo polymerisation. (i) Complete Table 7.1. Table 7.1 monomer type of polymerisation CH2C(CH3)COOH O Cl OH CH3CH(NH2)COOH [1] (ii) Ethanedioic acid, HOOCCOOH, can react with propane-1,3-diamine, H2NCH2CH2CH2NH2, to form polymer W. Draw a section of polymer W showing only one repeat unit. The new functional group formed should be displayed. [2] (iii) Poly(alkenes) biodegrade very slowly. Explain why. …… [1] [Total: 11]

Question

(a) Compound A is analysed by carbon-13 NMR and proton (1H) NMR spectroscopy. A O O O Fig. 8.1 State the reference substance and a solvent that can be used in NMR spectroscopy. reference …… solvent …… [1] (b) Predict the number of peaks in the carbon-13 NMR spectrum of A. …… [1] (c) The proton (1H) NMR spectrum of A shows peaks in four different chemical environments. Complete Table 8.1 for the proton (1H) NMR spectrum of A. Table 8.1 chemical shift number of protons on number of 1H atoms splitting pattern δ / ppm adjacent carbon atoms responsible for the peak 1.10 1.50 6 2.85 3.75 [4] [Total: 6] Table 8.2 chemical shift range environment of proton example δ / ppm alkane –CH3, –CH2–, >CH– 0.9–1.7 alkyl next to C=O CH3–C=O, –CH2–C=O, >CH–C=O 2.2–3.0 alkyl next to aromatic ring CH3–Ar, –CH2–Ar, >CH–Ar 2.3–3.0 alkyl next to electronegative atom CH3–O, –CH2–O, –CH2–Cl 3.2–4.0 attached to alkene =CHR 4.5–6.0 attached to aromatic ring H–Ar 6.0–9.0 aldehyde HCOR 9.3–10.5 alcohol ROH 0.5–6.0 phenol Ar–OH 4.5–7.0 carboxylic acid RCOOH 9.0–13.0 alkyl amine R–NH– 1.0–5.0 aryl amine Ar–NH2 3.0–6.0 amide RCONHR 5.0–12.0

Question

(a) The structures of the amino acids serine and lysine are shown in Fig. 9.1. serine lysine CH2OH CH2CH2CH2CH2NH2 H 2N C COOH H 2N C COOH H H Fig. 9.1 Draw the structure for the dipeptide, ser–lys, with molecular formula C9H19N3O4. The peptide functional group formed should be displayed. [2] (b) The isoelectric point of serine is 5.7 and of lysine is 9.7. (i) State what is meant by isoelectric point. …… [1] (ii) A mixture of serine, lysine and ser–lys is analysed by electrophoresis using a buffer at pH 5.7. – + mixture applied here Fig. 9.2 Draw and label three spots on Fig. 9.2 to indicate the predicted position of each of these three species, serine, lysine and ser–lys, after electrophoresis. Explain your answer. …… [3] (c) Fig. 9.3 shows the synthesis of compound R from compound P. Q, C6H13NO3 P O H step 1 N OH step 2 R O O NH2 Fig. 9.3 (i) Draw the structure of Q in Fig. 9.3. [1] (ii) State the reagents and conditions for steps 1 and 2 in Fig. 9.3. step 1 …… step 2 …… [2] [Total: 9] 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) The Periodic Table of Elements Group 1 2 13 14 15 16 17 18 1 2 H He hydrogen helium Key 1.0 4.0 3 4 atomic number 5 6 7 8 9 10 Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon 6.9 9.0 relative atomic mass 10.8 12.0 14.0 16.0 19.0 20.2 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon 23.0 24.3 3 4 5 6 7 8 9 10 11 12 27.0 28.1 31.0 32.1 35.5 39.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 at www.cambridgeinternational.org after the live examination series. K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.8 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 24 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 85.5 87.6 88.9 91.2 92.9 95.9 – 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 132.9 137.3 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 – – – 87 88 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson – – – – – – – – – – – – – – – – – 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 138.9 140.1 140.9 144.2 – 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.1 175.0 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium – 232.0 231.0 238.0 – – – – – – – – – – – Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge