Cambridge A Level Chemistry 9701 — 2020 Feb/March Paper 2 · Variant 2

1 Group 2 metals form alkaline solutions in water. (a) (i) Write the equation for the reaction of calcium oxide with water. ....................................................................................................................................... [1] (ii) Identify the ion that causes an aqueous solution to be alkaline. ....................................................................................................................................... [1] (b) The table shows the melting points of some Group 2 metal oxides. compound melting point / °C MgO 2825 CaO 2613 SrO 2531 BaO 1923 Explain the trend in the melting points of the oxides down Group 2. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [2] (c) Oxygen reacts readily with some metals, but each Group 2 metal requires strong heating to start the reaction with oxygen. Suggest why strong heating is required to start these reactions. .................................................................................................................................................... .............................................................................................................................................. [1] (d) Beryllium oxide reacts with hydrochloric acid to form molecules of BeCl 2. Deduce the bond angle in BeCl 2. .............................. [1] (e) Unlike the other oxides of Group 2 metals, beryllium oxide is amphoteric. (i) Give the meaning of the term amphoteric. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Beryllium oxide and aluminium oxide have similar chemical properties. The Be(OH)42– anion is a product of the reaction between beryllium oxide and excess concentrated OH–(aq). Construct an equation for this reaction. ....................................................................................................................................... [1] (f) Magnesium oxide reacts reversibly with chlorine according to the following equation. 2MgO(s) + 2Cl 2(g) 2MgCl 2(s) + O2(g) Under certain conditions, a dynamic equilibrium is established. (i) State two features of a reaction that is in dynamic equilibrium. 1 .......................................................................................................................................... 2 .......................................................................................................................................... [2] (ii) The equilibrium constant, Kp, is given by the following expression. pO2 Kp = 2 pCl 2 At 1.00 × 105 Pa and 500 K, 70% of the initial amount of Cl 2(g) has reacted. Calculate Kp and state its units. Kp = .............................. units = .............................. [3] (g) Magnesium peroxide, MgO2, is made in the following reaction. MgO(s) + H2O2(l) MgO2(s) + H2O(l) ∆H = –96 kJ mol–1 enthalpy change of compound formation, ∆Hf / kJ mol–1 MgO(s) –602 H2O2(l) –188 H2O(l) –286 (i) The peroxide ion is O22–. Deduce the average oxidation number of oxygen in the peroxide ion. ....................................................................................................................................... [1] (ii) Define the term enthalpy change of formation. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (iii) Use the data given to calculate the enthalpy change of formation of MgO2(s). ∆Hf MgO2(s) = .............................. kJ mol–1 [2] (iv) Magnesium peroxide decomposes slowly to form magnesium oxide and oxygen. MgO2(s) MgO(s) + 12O2(g) Use your answer to (g)(iii) and the data in the table to calculate the enthalpy change of this reaction. If you were unable to obtain an answer to (g)(iii), use the value ∆Hf = –550 kJ mol–1. This is not the correct answer. enthalpy change of reaction = .............................. kJ mol–1 [1] [Total: 19]

Mark scheme: 1(a)(i) CaO + H2O → Ca(OH)2 1 1(a)(ii) OH– / hydroxide 1 1(b) M1 (decreasing melting point down the group because) lower forces of attraction / weaker bonds (between cations and anions / oxide / O2–) M2 larger cations and constant charge OR decreasing charge density of cation (down group) 2 1(c) high(er) activation energy / heating overcomes activation energy 1 1(d) 180(°) 1 1(e)(i) reacts with / behaves as both acid and base 1 1(e)(ii) BeO + 2OH– + H2O → Be(OH)42– 1 1(f)(i) M1 equal rates of forward and backward reactions M2 closed system OR macroscopic properties unchanging 2 1(f)(ii) M1 Cl2 O2 initial x 0 mol equilibrium 0.3x 0.35x mol mol fraction 6 13 7 13 3 M2 ( ) 7 5 13 p 2 6 13 100000 2.53 10 100000 K − × = = × × M3 Pa–1 Question Answer Marks 1(g)(i) –1 1 1(g)(ii) M1 (enthalpy / energy change) when one mole of a compound / substance is formed M2 from its elements in their standard states 2 1(g)(iii) –(–602 + –188) + (ΔHf [MgO2] + –286) = –96 ΔHf[MgO2] = –600 (kJ mol–1) 2 1(g)(iv) – (–600) – (+602) = –2 (kJ mol–1) 1

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2 The Group 17 elements, chlorine, bromine and iodine, are non-metals that show trends in their physical and chemical properties. (a) Describe the trend in the colour of the Group 17 elements down the group. .................................................................................................................................................... .............................................................................................................................................. [1] (b) The Group 17 elements can oxidise many metals to form halides. (i) Describe the relative reactivity of the elements in Group 17 as oxidising agents. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Chlorine reacts with hot tin metal to form tin(IV) chloride, SnCl 4. SnCl 4 is a colourless liquid at room temperature that reacts vigorously with water to form an acidic solution. Suggest the type of structure and bonding shown by SnCl 4. Explain your answer. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (c) The Group 17 elements form soluble halides with sodium. (i) Describe what is seen when dilute AgNO3(aq) is added to NaBr(aq) followed by aqueous ammonia. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) NaCl reacts with concentrated H2SO4 to form HCl and NaHSO4. Explain the difference between the reactions of concentrated H2SO4 with NaCl and with NaI. Your answer should refer to the role of the sulfuric acid in each reaction. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [3] (d) The hydrogen halides are useful reagents in organic and inorganic reactions. (i) Describe and explain the trend in the boiling points of the hydrogen halides, HCl, HBr and HI. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Describe and explain the trend in the thermal stabilities of the hydrogen halides, HCl, HBr and HI. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (e) Lucas’s reagent is a mixture of HCl and ZnCl 2. Primary, secondary and tertiary alcohols can be distinguished by their reaction with Lucas’s reagent. Alcohols react with the HCl in Lucas’s reagent to form halogenoalkanes. ZnCl 2 acts as a homogeneous catalyst for these reactions. (i) Explain the meaning of the term homogeneous. ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Pentan-3-ol, C2H5CH(OH)C2H5, reacts slowly with HCl to form a secondary halogenoalkane. Complete the equation for this reaction using structural formulae. C2H5CH(OH)C2H5 + ���������������������������������������������������������������������������������������������������� [1] (iii) The fastest reaction shown by Lucas’s reagent is with a tertiary alcohol. Draw the structure of the tertiary alcohol that is an isomer of pentan-3-ol. [1] (iv) Tertiary alcohols tend to react with Lucas’s reagent using the same mechanism as in their reaction with HCl. Suggest the type of reaction shown by tertiary alcohols with Lucas’s reagent. ....................................................................................................................................... [1] [Total: 17]

Mark scheme: 2(a) darker / stronger / deeper down the group 1 2(b)(i) weaker oxidising agents / (relative reactivity as oxidising agents) decreases down the group 1 2(b)(ii) M1 (structure =) simple / molecular, because it has a low melting / boiling point M2 (bonding =) covalent, because it is hydrolysed 2 2(c)(i) M1 cream ppt / solid M2 (ppt / solid) partially dissolves in (aqueous) ammonia 2 Question Answer Marks 2(c)(ii) M1 Acid behaviour of H2SO4 H2SO4 acts as an acid with Cl – OR acid / base reaction with Cl – M2 Oxidising behaviour of H2SO4 H2SO4 acts as an oxidising agent with I– OR H2SO4 does not oxidise Cl – M3 Products formed (for iodide reaction) I2 / S / SO2 / H2S is formed OR (for chloride reaction) (only) HCl is formed OR Comparison of oxidising strength H2SO4 not strong enough to / cannot oxidise Cl – (to Cl2) OR I– more powerful reducing agent than Cl – 3 2(d)(i) M1 increases (down the group) because of increasing VdW M2 because of increasing number of electrons 2 2(d)(ii) M1 less stable (down the group) / decreases M2 lower H–Hal bond enthalpy / energy 2 2(e)(i) in the same phase / state 1 2(e)(ii) C2H5CH(OH)C2H5 + HCl → C2H5CH(Cl)C2H5 + H2O 1 2(e)(iii) 1 2(e)(iv) substitution 1

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3 Glycerol, CH2(OH)CH(OH)CH2OH, is widely used in the food industry and in pharmaceuticals. (a) A series of reactions starting from glycerol is shown. P Q CH2OH HOOC COOH NaCN and HCN H C OH C O NC C OH reaction 1 reaction 2 HOOC CH2OH COOH (i) Suggest the reagent(s) and conditions for reaction 1. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) Name the reaction mechanism for reaction 2. ....................................................................................................................................... [1] (iii) Give the observation you would make when 2,4-dinitrophenylhydrazine is added to P. ....................................................................................................................................... [1] (iv) Q does not show optical isomerism. Explain why. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] (v) When Q is heated with excess aqueous ethanoic acid in the presence of a catalytic amount of sulfuric acid, two reactions take place to form compound R. R COOH HOOC C OCOCH3 COOH Identify the two types of reaction that occur. 1 .......................................................................................................................................... 2 .......................................................................................................................................... [2] (b) Glycerol can be used as a starting material in the manufacture of nitroglycerine, C3H5N3O9. Nitroglycerine decomposes rapidly on heating to form a mixture of gases. 4C3H5N3O9(l) 12CO2(g) + 10H2O(g) + 6N2(g) + O2(g) A sample of nitroglycerine decomposes, releasing 1.06 dm3 of O2(g) at 850 K and 1.00 × 105 Pa. (i) Calculate the mass of nitroglycerine that decomposes. mass of nitroglycerine = .............................. g [3] (ii) Calculate the total volume of gas released by this decomposition at 850 K and 1.00 × 105 Pa. total volume of gas = .............................. dm3 [1] (c) Fats are compounds made from glycerol and unsaturated carboxylic acids. 4-pentenoic acid is an example of an unsaturated carboxylic acid. 4-pentenoic acid (CH2)2COOH H H H (i) Give the molecular formula of 4-pentenoic acid. ....................................................................................................................................... [1] (ii) Draw the repeat unit of the addition polymer that can be formed from 4-pentenoic acid. [1] (iii) Unsaturated acids are often brominated before being added to soft drinks. Complete the mechanism for the addition of Br2 to 4-pentenoic acid. ●● Include the structures of the intermediate and the product of the reaction. ●● Include all charges, partial charges, lone pairs and curly arrows. In the mechanism, R has been used to represent (CH2)2COOH. R H H H Br Br [4]

Mark scheme: 3(a)(i) M1 acidified / H+ Cr2O72– / (potassium / sodium) dichromate OR manganate(VII) / MnO4– / KMnO4 M2 (heat under) reflux 2 3(a)(ii) nucleophilic addition 1 3(a)(iii) yellow / orange / red ppt / solid 1 3(a)(iv) it does not have four different (groups of) atoms attached to (central) carbon OR it does not have a chiral carbon / centre OR it has two identical / COOH groups attached to (central) carbon OR mirror image is super(im)posable 1 3(a)(v) M1 hydrolysis M2 esterification / condensation 2 3(b)(i) M1 no. of mol O2 = ( ) 5 3 1.00 10 1.06 10 8.31 850 − × × × × M2 no. of mol of nitroglycerine = 4 × 0.0150 = 0.0600 (mol) M3 mass of nitroglycerine = 0.0600 × 227 = 13.6(2) (g) 3 3(b)(ii) 1.06 × 29 = 30.7(4) dm3 1 3(c)(i) C5H8O2 1 Question Answer Marks 3(c)(ii) 1 3(c)(iii) M1 curly arrow from C=C double bond to Br M2 correct dipole in Br2 AND curly arrow from Br—Br to Brδ– M3 correct intermediate AND curly arrow from lone pair on Br– to C+ M4 correct product 4 Question Answer Marks 3(d)(i) M1 M2 (two) different groups on each C atom in the C=C / end of the C=C double bond M3 no / restricted rotation about C=C 3 3(d)(ii) H2 / hydrogen 1 3(d)(iii) M1 / M2 absorptions seen in both spectra (any two): (same) both show an absorption at 1680–1730 (cm–1) because of C=O (same) both show an absorption at 1040–1300 (cm–1) because of C–O (same) both show an absorption at 2500–3000 (cm–1) because of RCO2–H / O–H in RCO2H / carboxyl(ic acid) M3 absorption only seen in spectrum of T: (different) T shows an absorption at 1500–1680 (cm–1) because of C=C (different) T shows an absorption at 3000–3100 (cm–1) because of (C=)C–H 3

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