OCR A Jun 2014 Paper 5 Q2

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This question looks at different aspects of entropy. (a) Three processes are given below. For each process, state and explain whether the change would be accompanied by an increase or decrease in entropy. (i) The freezing of water. increase or decreaseexplanation[1] (ii) The reaction of calcium carbonate with hydrochloric acid. increase or decreaseexplanation[1] (iii) The formation of O3(g) from O2(g). increase or decreaseexplanation[1] (b) The enthalpy and entropy changes of a reaction both have a negative sign. Discuss how the feasibility of this reaction will change as the temperature increases.[2]OCR 2014<br />
 (c) The metal tungsten is obtained on a large scale from its main ore, wolframite. Wolframite contains tungsten(VI) oxide, WO3. Tungsten is extracted from wolframite by reduction with hydrogen: WO3(s) + 3H2(g) W(s) + 3H2O(g) H = +115 kJ mol1 Standard entropies are given in the table below. Substance S / J K1 mol1 WO3(s) 76 H2(g) 131 W(s) 33 H2O(g) 189 (i) Calculate the free energy change, G, in kJ mol1, for this reaction at 25 C. Show your working. (ii) Calculate the minimum temperature, in K, at which this reaction becomes feasible. G at 25 C =kJ mol1 [2] Show your working. minimum temperature =K [2] [Total: 9] Turn overOCR 2014<br />

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