OCR A Jun 2010 Paper 5 Q5

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5 Ammonia is one of our most important chemicals, produced in enormous quantities because of its role in the production of fertilisers. 12 Much of this ammonia is manufactured from nitrogen and hydrogen gases using the Haber process. The equilibrium is shown below. N2(g) + 3H2(g) 2NH3(g) H = 92 kJ mol1 (a) (i) Write an expression for Kc for this equilibrium. [1] (ii) Deduce the units of Kc for this equilibrium.[1] (b) A research chemist was investigating methods to improve the synthesis of ammonia from nitrogen and hydrogen at 500 C.The chemist mixed together nitrogen and hydrogen and pressurised the gases so that their total gas volume was 6.0 dm3. The mixture was allowed to reach equilibrium at constant temperature and without changing the total gas volume. The equilibrium mixture contained 7.2 mol N2 and 12.0 mol H2. At 500 C, the numerical value of Kc for this equilibrium is 8.00102. Calculate the amount, in mol, of ammonia present in the equilibrium mixture at 500 C.OCR 2010 equilibrium amount of NH3 =mol [4]<br />
 13 (c) The research chemist doubled the pressure of the equilibrium mixture whilst keeping all other conditions the same. As expected the equilibrium yield of ammonia increased. (i) Explain in terms of le Chateliers principle why the equilibrium yield of ammonia increased.[2] (ii) Explain in terms of Kc why the equilibrium yield of ammonia increased.[3] (d) For the industrial manufacture of ammonia, nitrogen and hydrogen gases are required in large quantities from readily available resources. Various methods have been developed to obtain hydrogen gas for this process. (i) Much of the hydrogen is obtained by reacting together natural gas (methane) and steam. Construct an equation for this reaction.[1] (ii) Natural gas is a fossil fuel and the annual production of ammonia accounts for about 2% of all methane consumption. In the future, as fossil fuels become more depleted, the use of methane for ammonia production may become too expensive. Suggest another process that might be used in the future to obtain hydrogen gas for the Haber process.[1]OCR 2010 Turn over<br />
 (e) In the industrial production of ammonia, a temperature in the range 400500 C is used. 14 N2(g) + 3H2(g) 2NH3(g) H = 92 kJ mol1 Standard entropies of N2(g), H2(g) and NH3(g) are given in the table below. substance S / J K1 mol1 N2(g) 191 H2(g) 131 NH3(g) 192 (i) Show that the formation of ammonia from nitrogen and hydrogen gases should be feasible at room temperature (25 C). [6] (ii) Explain, in terms of entropy, why this reaction is not feasible at very high temperatures.[2] (iii) Suggest why a temperature of 400500 C is used for ammonia production, despite the reaction being feasible at room temperature.[1] [Total: 22]OCR 2010<br />

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