14 6 This question is about entropy. (a) Some hydrated barium hydroxide is added to ammonium thiocyanate in a flask which is placed on a few drops of water on a wooden block. After the addition, the contents are stirred and then the flask can be lifted up with the wooden block attached, as shown. hydrated barium hydroxide ammonium thiocyanate table table water wooden block wooden block flask with mixed contents The equation for the reaction is Ba(OH)2.8H2O(s) + 2NH4SCN(s) → Ba(SCN)2(aq) + 2NH3(g) + 10H2O(l) (i) Give two reasons why you would expect ΔS d system to be positive. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Explain why the wooden block is lifted up by the flask. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 15 (b) What is the standard molar entropy change, ΔS d system , in J K–1 mol–1, for the reaction shown? 2Mg(s) + O2(g) → 2MgO(s) Substance Standard molar entropy, S d / J K–1 mol–1 Mg(s) 32.7 O2(g) 205.0 MgO(s) 26.9 (1) A +210.8 B –210.8 C +216.6 D –216.6 (Total for Question 6 = 5 marks)