8   5 This question is about the decomposition of dinitrogen tetroxide. The reaction eventually reaches equilibrium. N2O4(g)  2NO2(g) colourless brown (a) The table shows values of ΔG at different temperatures for this reaction. Temperature / K ΔG / kJ mol–1 350 –4.0 400 –13 450 –22 500 –31 550 –40 (i) Plot a graph of ΔG against temperature. (2) ΔG / kJ mol–1 Temperature / K Turn over 9   (ii) Calculate the entropy change of the system, ΔSsystem , in J K–1 mol–1, using your straight line from the graph in (a)(i) and the equation shown. ΔG = –TΔSsystem + ΔH (3) (iii) What feature of the graph in (a)(i) gives the enthalpy change of the reaction? (1) A intercept of the x‑axis B (intercept of the x‑axis) × –1 C intercept of the y‑axis D (intercept of the y‑axis) × –1 10   (b) What happens to the position of the equilibrium and the colour of the mixture when the pressure is increased? The volume of the system remains constant. N2O4(g)  2NO2(g) colourless brown (1) A B C D (Total for Question 5 = 7 marks) Position of equilibrium Change in colour moves to the right mixture gets lighter moves to the right mixture gets darker moves to the left mixture gets lighter moves to the left mixture gets darker Turn over 11   6 Benzoic acid is a weak acid found in cranberries. O C OH C6H5COOH – benzoic acid (a) Which of these answers identifies the types of species present when benzoic acid is mixed with nitric acid? [Ka of benzoic acid = 6.3 × 10–5 mol dm–3;          Ka of nitric acid = 40 mol dm–3] (1) C6H5COOH + HNO3  C6H5COOH2 + + NO3 – A B C D (b) The ionic salts sodium benzoate and potassium benzoate are both used as food preservatives. Explain why the melting temperature of sodium benzoate is higher than the melting temperature of potassium benzoate. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... acid base acid base acid base base acid base acid base acid base acid acid base 12   (c) The value of Ka for benzoic acid = 6.28 × 10–5 mol dm–3. (i) Write the expression for the acid dissociation constant, Ka , of benzoic acid. (1) (ii) Calculate the mass of benzoic acid needed to prepare 250 cm3 of a solution with a pH = 3.51 (4) Turn over 13   (d) Weak acids such as benzoic acid can be neutralised by sodium hydroxide solution. C6H5COOH(aq) + NaOH(aq) → C6H5CO2Na(aq) + H2O(l) (i) Which of these could be used to show the end‑point of a titration of benzoic acid with sodium hydroxide solution? (1) A bromothymol blue B litmus C methyl orange D phenolphthalein (ii) Another weak acid found in cranberries is quinic acid, C6H7(OH)4COOH. It is neutralised by sodium hydroxide solution in a similar way to benzoic acid. A 25.0 cm3 sample of 0.500 mol dm–3 quinic acid solution was neutralised under standard conditions in a polystyrene cup using 25.0 cm3 of 0.800 mol dm–3 of sodium hydroxide solution. This resulted in a temperature rise of 2.9 °C. Calculate the standard enthalpy change of neutralisation, ΔneutH d, of quinic acid in kJ mol–1. [Assume the density of both solutions is 1.0 g cm–3. specific heat capacity of solution formed = 4.18 J g–1 °C–1] (3) 14   (iii) The standard enthalpy change of neutralisation of the weak acid HCN by sodium hydroxide is –11.7 kJ mol–1 while that of the strong acid HCl is –57.9 kJ mol–1. Explain the difference between these values. 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(Total for Question 6 = 14 marks) Turn over 15   7 This question is about chromium and chromium compounds. (a) Naturally occurring chromium has four isotopes, 50Cr, 52Cr, 53Cr and 54Cr. State what is meant by the term isotopes. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (b) Both chromium and calcium can form ions with a +2 charge. (i) Complete the electronic configuration of a Cr2+ ion. (1) 1s2 . .............................................................................................................................................................................................................................................. (ii) Explain which of chromium or calcium most easily forms a +2 ion using all of the data in the table. Element Atomic number 1st ionisation energy / kJ mol–1 2nd ionisation energy / kJ mol–1 Metallic radius / nm Chromium 24 653 1592 0.129 Calcium 20 590 1145 0.197 (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 16   (c) Chromium(III) sulfate, Cr2(SO4)3 , dissolves in water to form the complex ion [Cr(H2O)6]3+(aq). (i) State the colour of this complex ion. (1) .................................................................................................................................................................................................................................................................................... (ii) Explain why the aqueous solution of this complex ion has an acidic pH by considering the interaction between the metal ion and the ligands. 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Turn over 17   (d) A student researching the role of dichromate(VI) ions, Cr2O7 2– , as an oxidising agent made the statement shown. ‘Standard electrode potential data shows that it is never feasible for a 1.00 mol dm–3 solution of potassium dichromate(VI) to oxidise the chloride ions in hydrochloric acid.’ Comment on this statement using the data and equilibria shown. Equilibrium 1 Cr2O7 2–(aq) + 14H+(aq) + 6e–  2Cr3+(aq) + 7H2O(l) E d = +1.33 V Equilibrium 2 Cl2(aq) + 2e–  2Cl–(aq) E d = +1.36 V (4) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 7 = 13 marks) 18   8 * Transition metals and their compounds can act as catalysts in many reactions such as the ones shown: • platinum, Pt, in the catalytic converters of vehicles • manganese(II) ions, Mn2+(aq), in the oxidation of ethanedioate ions, C2O4 2–(aq), by manganate(VII) ions, MnO4 –(aq). Compare and contrast the role of the catalysts in these reactions. 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(Total for Question 8 = 6 marks)