Cambridge A Level Chemistry 9701 — 2022 Oct/Nov Paper 2 · Variant 2

1 Species such as NH4+, CO32– and PO43– are examples of molecular ions. (a) Ionic and covalent bonds both involve an electrostatic attraction between different species. Identify the species that are electrostatically attracted to one another in: ● an ionic bond .................................................................................................................................................... ● a covalent bond. .................................................................................................................................................... [2] (b) Complete Table 1.1 to show the total numbers of protons and electrons in the molecular ions NH4+, CO32– and PO43–. Table 1.1 molecular total number total number ion of protons of electrons NH4+ CO32– PO43– [3] (c) NH4+ is a Brønsted–Lowry acid. (i) Define Brønsted–Lowry acid. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) When NH4+(aq) is heated with NaOH(aq), a pungent gas is produced. Write an ionic equation for this reaction. ....................................................................................................................................... [1] (iii) The nitrogen atom in NH4+ is sp3 hybridised. sp3 orbitals form from the mixing of one 2s and three 2p orbitals. Sketch the shapes of a 2s and a 2px orbital on the axes in Fig. 1.1. z z y y x x 2s 2px Fig. 1.1 [2] (d) There are many naturally occurring hydrated compounds that contain the anion PO43–. (i) Name the anion PO43–. ....................................................................................................................................... [1] (ii) Struvite is a soft hydrated mineral with Mr = 245.3. The anhydrous form of the mineral has the formula NH4MgPO4. Calculate the number of molecules of water of crystallisation in struvite. Give your answer to the nearest integer. Show your working. number of molecules of water of crystallisation = .............................. [2] (e) OH–(aq) reacts with 2-bromo-2-methylpropane in an SN1 reaction. The molecular ion (CH3)3C+ forms as the intermediate in this reaction. (i) Draw the mechanism for the SN1 reaction of OH– with 2-bromo-2-methylpropane. Include charges, dipoles, lone pairs of electrons and curly arrows as appropriate. Draw the structures of the organic reactant and organic product. H3C + CH3 C CH3 [3] (ii) 2-bromo-2-methylpropane is a tertiary bromoalkane. Define tertiary bromoalkane. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Organic compound M forms when 2‑bromo‑2‑methylpropane is heated with ethanolic OH–. Draw the structure of M. [1] [Total: 17]

Mark scheme: Question Answer Marks 1(a) M1 ionic = oppositely charged ions / cations AND anions 1 M2 covalent = shared pair(s) of electrons AND two nuclei 1 1(b) NH4+ 11 p+ AND 10 e– 1 CO32– 30 p+ AND 32 e– 1 PO43– 47 p+ AND 50 e– 1 1(c)(i) proton / H+ donor 1 1(c)(ii) ionic equation NH4+ + OH– → NH3 + H2O 1 1(c)(iii) 2 2s 2px 1(d)(i) phosphate(V) 1 1(d)(ii) (molecular mass of NH4MgPO4) = 137.3 1 (245.3 – Mr(NH4MgPO4) ÷ 18.0) 1 = 6 1(e)(i) HO– 1 CH3 CH3 H3 C + CH3 d- Br Cd+ CH3 C H O C CH3 CH3 CH3 CH3 OR (CH3)3COH M1 correct dipole in on δ+C—Brδ– (in tBuBr) AND curly arrow from C—Br to Br M2 curly arrow from electron pair on O of –:OH / :OH– to C+ in carbocation 1 M3 tBuOH product 1 1(e)(ii) The C—Br carbon is bonded to three other carbon atoms 1 owtte 1(e)(iii) 1

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2 The chlorides of some of the Period 3 elements are shown in Table 2.1. Table 2.1 Period 3 chloride NaCl Al Cl 3 SiCl 4 PCl 5 PCl 3 SCl 2 bonding C C structure S S oxidation state of Period 3 element (a) Complete Table 2.1. ● Identify the bonding shown by each chloride under standard conditions. Use C = covalent, I = ionic, M = metallic. ● Identify the structure shown by each chloride under standard conditions. Use G = giant, S = simple. ● Deduce the oxidation state of the Period 3 element in each chloride. [4] (b) Write equations for the reactions of NaCl and PCl 5 with water. Include state symbols in both equations. NaCl ........................................................................................................................................... PCl 5 ............................................................................................................................................ [3] (c) In the gas phase, Al Cl 3(g) exists at equilibrium with Al 2Cl 6(g) as shown. equation 1 2Al Cl 3(g) Al 2Cl 6(g) ∆Hr = –63 kJ mol–1 (i) Complete the dot-and-cross diagram to show the bonding in Al 2Cl 6. Cl Al Cl [2] (ii) State the effect of an increase in temperature on the equilibrium mixture in equation 1. Explain your answer. ............................................................................................................................................. ....................................................................................................................................... [1] (d) A 3.30 g sample of a Period 3 chloride is heated to 500 K in a sealed flask. At this temperature, the chloride is a gas of volume 250 cm3 and the pressure in the flask is 323 kPa. Use the ideal gas equation pV = nRT to calculate the Mr of the Period 3 chloride. Deduce its formula. Mr = .............................. formula of Period 3 chloride = .............................. [3] (e) (i) An excess of Cl –(aq) is added to 1 cm3 of Br2(aq). Describe what is observed. Explain your answer. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) SCl 2 has Mr = 103.1 and is a liquid at room temperature. SBr2 has Mr = 191.9 and is a gas at room temperature. Explain the difference in the physical state of SCl 2 and SBr2. Give your answer in terms of intermolecular forces. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (f) Bismuth is a dense metal in the same group as phosphorus. (i) Draw a labelled diagram to show the bonding in bismuth metal. [2] (ii) Bismuth reacts with chlorine to form BiCl 3. BiCl 3 is a solid at room temperature. It melts when heated gently. BiCl 3 reacts vigorously with water at room temperature to form an acidic solution. Suggest the type of bonding and structure shown by BiCl 3. Explain your answer. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] [Total: 21]

Mark scheme: 2(a) 1 NaCl AlCl3 SiCl4 PCl5 I I OR C C C 1 G G S S 1 +1 +3 +4 +5 1 +3 +2 2(b) M1 NaCl(s) → Na+(aq) + Cl –(aq) 1 PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(aq) 1 M2 balanced equation M3 all state symbols for PCl5 equation 1 2(c)(i) 1 Cl Cl Cl Al Al Cl Cl Cl ‘dot-and-cross’ diagram to show 1 M1 correct number and connectivity of Cl and Al atoms M2 2 correct dative bonds (••) between Cl and Al of AlCl3 neighbour AND correct number of non-bonding electrons around each atom of Al2Cl6 2(c)(ii) reaction is exothermic so equilibrium shifts to the LHS 1 OR equilibrium shifts to LHS because forward reaction is exothermic OR equilibrium shifts to LHS because backward reaction is endothermic OR endothermic reaction is favoured so more reactants / are Al Cl3 is made 2(d) M1 value of n based on re-arranged equation for ‘n’ and substitution of correct values 1 3.23 ×105 250 ×10 –6 n = = 0.0194(34416) 8.31 500 M2 calculated value of Mr based on M1 1 Mr = 3.30 ÷ n = 170 M3 period 3 chloride which matches Mr in M2 1 (formula) SiCl4 2(e)(i) M1 orange colour (of solution) remains / no visible reaction 1 M2 1 bromine/Br2 cannot oxidise chloride/Cl– OR bromine/Br2 is not a strong enough oxidising agent (to oxidise chloride/Cl–) 2(e)(ii) M1 sum of instantaneous dipole – dipole attractions and permanent dipole attractions in SCl2 is greater than those in SBr2 1 OR permanent dipole-permanent dipole (forces) in SCl2 are stronger OR intermolecular forces are stronger in SCl2 ora M2 SCl2 (molecule / bond) is more polar than SBr2 1 OR Cl is more electronegative than Br so SCl2 has stronger (permanent) dipoles ora 2(f)(i) 1 ion delocalised electrons M1 diagram showing minimum of 4 particles (in total in 2 rows) • circles containing Bin+ do not have to be labelled • must be labelled as ‘ion’ OR empty circles / circles with Bi must be labelled + ion / positive ion / cation / Bin+ AND • with the circles surrounded by electrons shown as e– / – OR little circles labelled electrons M2 label / legend showing delocalised electrons 1 2(f)(ii) M1 1 (structure =) simple/molecular, because it has a low melting/boiling point owtte M2 1 (bonding =) covalent, because it is hydrolysed

More questions on Periodicity of physical properties of the elements in Period 3

3 Organic compounds can be distinguished using chemical tests and analytical techniques. (a) Table 3.1 shows four pairs of organic compounds. Table 3.1 positive result of organic compounds reagent chemical test on identified compound A1 A2 O O H B1 B2 O O C1 C2 O O O D1 D2 CH3 OH OH (i) Complete Table 3.1 to: ● identify a reagent which can distinguish between the compounds in each pair ● give the positive result of the chemical test and identify which compound shows this result. Use a different reagent for each test. [8] (ii) A1 and A2 are structural isomers. Define structural isomers. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Give the systematic name of B2. ....................................................................................................................................... [1] (iv) Deduce the molecular formula of D1. ....................................................................................................................................... [1] (b) D2 forms polymer Z when heated gently. (i) Identify the type of polymer that forms from D2. ....................................................................................................................................... [1] (ii) Draw one repeat unit of polymer Z. [2] (c) Organic compound E contains three carbon atoms. E reacts with cold dilute acidified KMnO4(aq) to form a single compound F with Mr = 154.9. Fig. 3.1 shows the infrared spectrum of E. Fig. 3.2 shows the infrared spectrum of F. E 100 transmittance / % 50 0 4000 3000 2000 1500 1000 500 wavenumber / cm–1 Fig. 3.1 F 100 transmittance / % 50 0 4000 3000 2000 1500 1000 500 wavenumber / cm–1 Fig. 3.2

Mark scheme: 3(a)(i) 2 M1 M2 Tollens’ OR Fehling’s OR K2Cr2O7 OR KMnO4 A1 gives silver mirror OR brick-red ppt OR (orange solution) turns green OR (purple solution) turns colourless OR alkaline I2(aq) with A2 gives yellow ppt 2 M3 alkaline I2(aq) M4 B1 gives yellow ppt 2 M5 2,4-DNPH M6 C1 gives red/orange/yellow ppt 2 M7 Br2(aq) M8 D2 turns it (orange to) colourless 3(a)(ii) molecules with the same molecular formula 1 AND different structural formulae owtte 3(a)(iii) pentan-3-one 1 3(a)(iv) molecular formula 1 C5H10O 3(b)(i) addition 1 3(b)(ii) 1 M1 correct carbon backbone of one repeat unit including ‘dangling’ bonds M2 rest of structure correct 1 3(c)(i) C=C (bond) / alkene 1 AND (absorption within) 1500–1680 cm–1 (present in E’s IR spectrum only) 3(c)(ii) hydroxyl / alcohol 1 AND (broad absorption within) 3200–3650 cm–1 (present in F’s IR spectrum only) 3(c)(iii) 120.9 / 121 1 3(c)(iv) 1 3(c)(v) (1)H2O + (1)[O] + C3H5Br → C3H7BrO2 1 3(d) M1 one portion of A1: NaBH4 AND → propan-1-ol / CH3(CH2)2OH 1 M2 second portion of A1: 1 acidified K2Cr2O7 AND → propanoic acid / C2H5CO2H M3 two products: 1 conc(entrated mineral) acid / H+ / H2SO4 (catalyst)

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