Edexcel Jun 2013 (R) Paper 4 Q11

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 11 This question is about the equilibrium reaction below. Fe2+(aq) + Ag+(aq) U Fe3+(aq) + Ag(s) The equilibrium is reached slowly. *(a) Describe the changes you would see if aqueous solutions of iron(II) sulfate and silver nitrate were mixed and allowed to stand for a few hours. (2)(b) The concentration of silver ions in the equilibrium mixture can be found by titration with potassium thiocyanate. Silver thiocyanate precipitates. Ag+(aq) + CNS(aq) U AgCNS(s) When all the silver ions have reacted, a deep red complex ion of iron(III) thiocyanate forms. 10 In an experiment, 25.0 cm3 of 0.100 mol dm3 silver nitrate solution was added to 25.0 cm3 of 0.100 mol dm3 of iron(II) sulfate solution, mixed thoroughly, and allowed to stand overnight in an air-tight container. 10.0 cm3 samples of the reaction mixture were then titrated with 0.0200 mol dm3 potassium thiocyanate solution. The average titre was 5.60 cm3. (i) The initial concentrations of silver ions and iron(II) ions in the reaction mixture are the same. Calculate this initial concentration in mol dm3. (1) *P42984A01024*<br /> (ii) Calculate the number of moles of silver ions in the 10.0 cm3 sample at equilibrium and hence calculate the equilibrium concentration of silver ions in the mixture. (iii) Deduce the equilibrium concentration of iron(II) ions. (iv) Hence calculate the equilibrium concentration of iron(III) ions. (v) Write the expression for the equilibrium constant, Kc, for the reaction Fe2+(aq) + Ag+(aq) U Fe3+(aq) + Ag(s) Calculate its value and give your answer, with appropriate units, to three significant figures. (2) (1) (1) (4) *P42984A01124* 11 Turn over<br /> (c) (i) The relationship between the total entropy change for a reaction and the equilibrium constant is S 9 total = R ln K Calculate the total entropy change for this reaction, giving a sign and appropriate units. [R = 8.31 J mol1 K1] (ii) Fe2+(aq) + Ag+(aq) U Fe3+(aq) + Ag(s)  S 9 system for this reaction is 208.3 J mol1 K1 Use ideas about entropy to explain why this value is negative. (2) (2)(iii) Calculate the entropy change of the surroundings, S 9 surroundings. (1) 12 *P42984A01224*<br /> (iv) Use your answer to (c)(iii) to calculate the standard enthalpy change, H9, for this reaction at 298 K. Hence state and explain the effect of increasing temperature on the value of Stotal. (3)*(d) After the samples from the original mixture for the titration are taken, the remainder was filtered and then allowed to stand overnight, in an air-tight container at the same temperature. Another 10.0 cm3 sample was taken and titrated. How, if at all, would you expect the titre to change? Justify your answer. A calculation is not required. (2)(Total for Question 11 = 21 marks) *P42984A01324* 13 Turn over<br />

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