OCR A Jun 2015 Paper 5 Q3

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This question looks at two reactions involving sulfur compounds. (a) Hydrogen reacts with carbon disulfide as shown below. 4H2(g) + CS2(g) CH4(g) + 2H2S(g) For this reaction, H = 234 kJ mol1 and S = 164 J K1 mol1. (i) Why does the reaction have a negative entropy change?[1] (ii) Standard entropies are shown in the table below. substance S o / J K1 mol1 CS2(g) 238 CH4(g) 186 H2S(g) 206 Calculate the standard entropy for H2. S o =J K1 mol1 [2] (iii) Explain, with a calculation, whether this reaction is feasible at 25 C. Show your working.[3] (iv) Explain, with a calculation, the significance of temperatures above 1154 C for this reaction.[2]OCR 2015 Turn over<br />
 (b) A chemist investigated methods to improve the synthesis of sulfur trioxide from sulfur dioxide and oxygen. 2SO2(g) + O2(g) 2SO3(g) The chemist:mixed together 1.00 mol SO2 and 0.500 mol O2 with a catalyst at room temperaturecompressed the gas mixture to a volume of 250 cm3 allowed the mixture to reach equilibrium at constant temperature and without changing the total gas volume. At equilibrium, 82.0% of the SO2 had been converted into SO3. (i) Determine the concentrations of SO2, O2 and SO3 present at equilibrium and calculate Kc for this reaction. Kc =units[6]OCR 2015<br />
 (ii) Explain what would happen to the pressure as the system was allowed to reach equilibrium.[1] (iii) The value of Kc for this equilibrium decreases with increasing temperature. Predict the sign of the enthalpy change for the forward reaction. State the effect on the equilibrium yield of SO3 of increasing the temperature at constant pressure. H:Effect on SO3 yield:[1] (iv) The chemist repeated the experiment at the same temperature with 1.00 mol SO2 and an excess of O2. The gas mixture was still compressed to a volume of 250 cm3. State and explain, in terms of Kc, how the equilibrium yield of SO3 would be different from the yield in the first experiment.[3] [Total: 19]OCR 2015 Turn over<br />

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