22 9 An equation for the formation of ammonia using the Haber process is shown. N2(g) + 3H2(g)  2NH3(g) (a) (i) Calculate the enthalpy change for the forward reaction shown in the equation, selecting from the bond enthalpies in the table. Include a sign in your answer. (3) Bond Mean bond enthalpy / kJ mol–1 N N 158 N N 410 N N 945 N H 391 H H 436 Turn over 23 (ii) A data book gives the standard enthalpy change of formation of ammonia as –46.1 kJ mol–1. Give two reasons for the difference between this value and the value that you calculated in (a)(i). (2) Reason 1 .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Reason 2 .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (iii) What is the percentage atom economy, by mass, for ammonia in the forward reaction? N2(g) + 3H2(g)  2NH3(g) (1) A 17.6 % B 50.0 % C 82.4 % D 100 % (iv) What is the equilibrium expression for Kc? (1) A Kc = [N ][3H ] [2NH ] 2 2 3 B Kc = [2NH ] [N ][3H ] 3 2 2 C Kc = [NH ] [N ][H ] 3 2 2 2 3 D Kc = [N ][H ] [NH ] 2 2 3 3 2 24 (b) In the chemical industry, many processes involve reversible reactions. The product is often removed before equilibrium is attained. Give three reasons why the product may be removed before its maximum concentration is achieved. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 25 (c) Ammonia is stable in air but can be oxidised on the surface of a copper catalyst. An equation for this reaction is 4NH3(g) + 5O2(g) → 6H2O(g) + 4NO(g) ΔrH = –905.2 kJ mol–1 The catalyst is usually warmed to approximately 300 °C to start the reaction, but after a short reaction time the copper catalyst often melts. (i) Give a reason why the catalyst is warmed and a reason why the catalyst may melt. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 26 (ii) Complete the reaction profile for this catalysed oxidation of ammonia, showing the enthalpy change, ΔrH. (2) Reactants Reaction path Enthalpy 27 (iii) Describe the processes that occur on the surface of a heterogeneous catalyst during the oxidation of ammonia in air. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 9 = 17 marks) TOTAL FOR PAPER = 80 MARKS 28