Cambridge A Level Chemistry 9701 — 2021 Oct/Nov Paper 2 · Variant 3

1 Sulfides are compounds that contain sulfur but not oxygen. (a) Carbon disulfide, CS2, is a volatile liquid at room temperature and pressure. (i) State the meaning of volatile. ....................................................................................................................................... [1] (ii) Draw a ‘dot-and-cross’ diagram of the CS2 molecule. [2] (iii) Suggest the bond angle in a molecule of CS2. ....................................................................................................................................... [1] (iv) CS2 is a liquid under room conditions, while CO2 is a gas. Explain what causes the difference in the physical properties between CS2 and CO2. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (b) The enthalpy change of combustion of CS2(l) is represented by the following equation. ∆Hc CS2(l) + 3O2(g) CO2(g) + 2SO2(g) (i) Define enthalpy change of combustion. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) The table shows the enthalpy changes of formation of CS2(l), CO2(g) and SO2(g). enthalpy change of compound formation, ∆Hf / kJ mol–1 CS2(l) +89.7 CO2(g) –394 SO2(g) –297 Use the data in the table to calculate the enthalpy change of combustion, ∆Hc, of CS2(l), in kJ mol–1. Show your working. ∆Hc of CS2(l) = .............................. kJ mol–1 [2] (c) Hydrogen sulfide gas, H2S(g), is slightly soluble in water. It acts as a weak acid in aqueous solution. (i) State the meaning of weak acid. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Give the formula of the conjugate base of H2S. ....................................................................................................................................... [1] (iii) H2S(aq) reacts slowly with oxygen dissolved in water. The reaction is represented by the following equation. H2S(aq) + 12O2(aq) → H2O(l) + S(s) Explain, with reference to oxidation numbers, why this reaction is a redox reaction. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (d) The compound As2S3 is a common mineral. When As2S3 is heated strongly in air, it forms a mixture of products, as shown. 2As2S3(s) + 9O2(g) → As4O6(s) + 6SO2(g) (i) A sample containing 0.198 g As2S3 is placed in 0.100 dm3 of pure oxygen, an excess, in a reaction chamber connected to a gas syringe at room temperature. The reactants are heated until no further change is observed. The products are then allowed to cool to room temperature. Calculate the volume, in dm3, of gas present at the end of the experiment. The molar volume of gas is 24.0 dm3 mol–1 under these conditions. Assume that the pressure is constant throughout the experiment. Show your working. volume of gas remaining = ............................. dm3 [4] (ii) State the environmental consequences of releasing SO2(g) into the atmosphere. ....................................................................................................................................... [1] (iii) SO2(g) can be removed from the air by reacting it with NaOH(aq). Construct an equation for the reaction of SO2(g) with NaOH(aq). Include state symbols. ....................................................................................................................................... [2] [Total: 21]

Mark scheme: 1(a)(i) easily vaporised / easily evaporates / turns to gas easily 1 1(a)(ii) M1 bonding pairs M2 Correct number of remaining outer electrons 2 1(a)(iii) 180° 1 1(a)(iv) M1 CS2 has more electrons M2 So stronger induced dipole (forces) (between molecules) 2 1(b)(i) M1 (enthalpy/energy change when) 1 mole of a compound M2 burns/combusts/reacts in excess oxygen/O2 OR completely burns/ completely combusts/completely reacts in oxygen/O2 2 1(b)(ii) M1 (–394 +2 (–297) – (+89.7) M2 = –1080 (kJ mol–1) 2 1(c)(i) weak [acid] partially dissociates/partially ionises (into H+ ions/protons) 1 1(c)(ii) HS– 1 Question Answer Marks 1(c)(iii) M1 S (increases) oxidation number –2 → 0 so oxidation / or is oxidised M2 O (decreases) O.N. 0 → –2 so reduction / is reduced 2 1(d)(i) M1 moles of As2S3 = 0.198 / 246.1 / 8.05 × 10–4 M2 moles SO2 (using moles of As2S3 as limiting factor) = 2.41(36) × 10–3 moles (6 / 2 × 8.05 × 10–4) Volume SO2 = 2.41(36) × 10–3 × 24 = 0.0579 dm3 M3 Moles O2 used in reaction = 8.05 × 10–4 × 9 / 2 = 3.62 × 10–3 Volume O2 used in reaction = 3.62 × 10–3 × 24 = 0.0869 dm3 M4 Final total volume gas = (0.1 – 0.0869) + 0.0579 = [0.0131 + 0.0579] = 0.071(0) dm3 M4 ONLY award 4th mark if the final answer rounds to 0.071 Answer to minimum of 2 sig figs MAX 3 for using ecf from M1 to M2 to M3 and M4 Award all 4 marks if final answer rounds to 0.071 4 1(d)(ii) acid rain 1(d)(iii) M1 SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(l) AND correct species and balancing M2 State symbols 2

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2 The reaction scheme shows some reactions of calcium. reaction 1 Ca(s) Ca(NO3)2(aq) Ca(OH)2(aq) HNO3 NaOH(aq) heat reaction 2 CO2(g) CaO(s) CaCO3(s) + H2O(l) reaction 4 C(s) reaction 3 excess CO2(g) CaC2(s) Ca(HCO3)2(aq) (a) (i) Reaction 1 produces Ca(NO3)2 and one other product. Identify the other product. ....................................................................................................................................... [1] (ii) Construct an equation for the thermal decomposition of Ca(NO3)2(s). ....................................................................................................................................... [1] (iii) State the trend in the thermal stability of the Group 2 nitrates down the group. ....................................................................................................................................... [1] (iv) In reaction 3, excess CO2 is bubbled through water containing CaCO3. A solution of Ca(HCO3)2(aq) forms. Construct an equation for reaction 3. ....................................................................................................................................... [1] (b) Describe how Ca(OH)2 is used in agriculture. .................................................................................................................................................... .............................................................................................................................................. [1] (c) In reaction 4, calcium carbide, CaC2, is formed from CaO. CaC2 contains the C22– anion. Each carbon in C22– is sp hybridised. (i) Describe how sp hybridised orbitals are formed. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Sketch a diagram to show how two sp hybrid orbitals can form a sigma (σ) bond. [2] (d) The flowchart shows some reactions of CaC2. reaction 5 CaC2 CaCN2 CaCO3 + NH3 N2 H2O NaCl and C NaCN (i) Reaction 5 can be used to prepare NH3. CaCN2 + 3H2O → CaCO3 + 2NH3 Calculate the minimum mass, in tonnes, of calcium cyanamide, CaCN2, that is required to produce 1.50 × 106 tonnes of NH3. Show your working. 1 tonne = 1.00 × 106 g minimum mass of CaCN2 = .............................. tonnes [2] (ii) Draw the structure of the organic products formed in the following reactions. NaCN H2SO4(aq) CH3CH2Br dissolved heat under in ethanol reflux O NaCN and HCN [3] [Total: 13]

Mark scheme: 2(a)(i) hydrogen / H2 1 2(a)(ii) Ca(NO3)2 → CaO + 2NO2 + ½O2 1 2(a)(iii) (thermal stability) increases 1 2(a)(iv) CaCO3 + H2O + CO2 → Ca(HCO3)2 1 2(b) reduces acidity of soil 1 2(c)(i) Mixing / overlap / combination of one / an s and one / a p orbital 1 2(c)(ii) Sketch a diagram to show HOW two sp hybrid orbitals can form a SIGMA bond M1 M2 2 2(d)(i) M1 moles of NH3 = 1.50 × 106 × 106 ÷ 17 = 8.82 × 1010 M2 mass of CaCN2 = 12 6 M1 80.1 10 × × = 3.53 × 106 2 2(d)(ii) CH3CH2CN CH3CH2CO2H CH3C(OH)(CN)CH3 3

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3 Phosphorus is a reactive Period 3 element. (a) Phosphorus has several allotropes. Details of two allotropes are given. allotrope of phosphorus formula melting point / °C white P4 44 red P 590 (i) White phosphorus and red phosphorus both have covalent bonding. Suggest the types of structure shown by white phosphorus (P4) and red phosphorus (P). Explain why red phosphorus (P) has a higher melting point than white phosphorus (P4). structure of P4 ...................................................................................................................... structure of P ....................................................................................................................... explanation .......................................................................................................................... ............................................................................................................................................. ............................................................................................................................................. [3] (ii) Red phosphorus (P) forms when white phosphorus (P4) is exposed to sunlight. 14 P4(s) → P(s) ∆H = –17.6 kJ mol–1 white red Use this information to draw a reaction pathway diagram to show the formation of red phosphorus (P) from white phosphorus (P4). enthalpy P4 progress of reaction [1] (b) Some reactions of P4(s) are shown in the reaction scheme. excess Cl 2(g) P4(s) PCl 5(s) excess O2(g) H2O(l) reaction 1 reaction 2 P4O10(s) Q H2O(l) (i) State the oxidation number of phosphorus in P4O10. ....................................................................................................................................... [1] (ii) Deduce the identity of Q and hence construct chemical equations for reactions 1 and 2. reaction 1 PCl 5 + .......H2O → ............................................................................................... reaction 2 P4O10 + .......H2O → .............................................................................................. [2] (c) Triphenylphosphine is used in a type of reaction known as a Wittig reaction. triphenylphosphine where = –C6H5 P (i) Give the empirical formula of triphenylphosphine. ....................................................................................................................................... [1] In a Wittig reaction, an aldehyde reacts with a halogenoalkane to form an alkene. The conversion is shown in the following unbalanced equation. O R2 triphenylphosphine + R2 I strong base R1 H R1 H Compound H can be made from propanal, C2H5CHO. Stage 3 in the reaction scheme is a Wittig reaction. NaBH4 stage 1 C2H5CHO G red phosphorus and I2 stage 2 G C2H5CH2I triphenylphosphine stage 3 C2H5CH2I + C2H5CHO H strong base(Wittig reaction) (ii) State the types of reaction that occur in stages 1 and 2. stage 1 ................................................................................................................................ stage 2 ................................................................................................................................ [2] (iii) Draw the structures of G and H in the boxes provided. G H [2] (d) Identify the organic products formed when compound J, shown below, is heated with hot concentrated acidified manganate(VII) ions. J hot concentrated MnO4– H2SO4 [2] [Total: 14]

Mark scheme: 3(a)(i) M2 giant molecular M3 weak IMFs (overcome) in P4 AND strong (covalent) bonds (broken) in P 3(a)(ii) 1 3(b)(i) (+)5/V 1 3(b)(ii) PCl5 + 4H2O → H3PO4 + 5HCl P4O10 + 6H2O → 4H3PO4 2 3(c)(i) C18H15P 1 3(c)(ii) stage 1 = reduction stage 2 = substitution 2 Question Answer Marks 3(c)(iii) G = C2H5CH2OH H = C2H5CH=CHC2H5 2 3(d) CH3CH2CO2H CH3COCH3 2

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4 Compound B is a liquid with a fruity smell. B O Br O The reaction scheme shows how B can be made from ethanol, C2H5OH. O O reaction 1 reaction 2 C2H5OH Br [O] HO HO C2H5OH reaction 3 and H2SO4 B O Br O (a) (i) Reaction 1 is an oxidation reaction. Give the reagent(s) and conditions required for reaction 1. reagent(s) ............................................................................................................................ conditions ............................................................................................................................ ............................................................................................................................................. [2] (ii) Construct an equation to represent reaction 1. Use [O] to represent an oxygen atom from the oxidising agent in this reaction. ....................................................................................................................................... [1] (iii) Suggest the type of reaction that occurs in reaction 2. ....................................................................................................................................... [1] (iv) H2SO4 acts as a homogeneous catalyst in reaction 3. Explain why H2SO4 is described as homogeneous. ............................................................................................................................................. ....................................................................................................................................... [1] (b) Reaction 2 needs to take place in the absence of water to prevent formation of compound C. C O OH HO If C is present in the reaction mixture of reaction 3, a different compound, compound D, will also form. Compound D has two identical functional groups. The infrared spectrum of D shows strong absorptions at 1100 cm–1 and 1720 cm–1, but no absorption due to O–H bonds. Use the Data Booklet to identify the functional group present in D. Explain your answer as fully as you can. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [3] (c) Some other reactions of C are shown. C O OH HO Na2CO3 E F SOCl 2 G (CH2OH)2 (CH2Cl )2 (i) Draw the structure of E. [1] (ii) Suggest why NaBH4 is not a suitable reagent to make F, (CH2OH)2, from C. Explain your answer. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) Construct an equation for the reaction of (CH2OH)2 with SOCl 2 to form G, (CH2Cl )2. ....................................................................................................................................... [1] (d) Explain why C is very soluble in water. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 12]

Mark scheme: 4(a)(i) potassium/sodium dichromate [(VI)] / K2Cr2O7 / Na2Cr2O7 acidified AND (heat) under reflux 2 4(a)(ii) C2H5OH + 2[O] → CH3CO2H + H2O 1 4(a)(iii) substitution 1 4(a)(iv) in the same phase / in same state 1 4(b) M1 ester M2 1100 cm–1 linked to C—O AND 1720 cm–1 linked to C=O M3 No COOH / carboxylic acid and No OH / alcohol in D (but present in C) OR COOH / carboxylic acid and OH / alcohol reacted /lost (in C to form D) 3 4(c)(i) OR Na+ 1 4(c)(ii) Not a strong (enough) reducing agent 1 Question Answer Marks 4(c)(iii) Construct an equation (CH2OH)2 + SOCl2 → (CH2Cl)2 + SO2 + H2O 1 4(d) Forms hydrogen bonds with water 1

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