14   5 This question is about enthalpy changes and energy changes. (a) Magnesium carbonate reacts with dilute hydrochloric acid at room temperature. MgCO3(s) + 2HCl(aq) → MgCl2(aq) + CO2(g) + H2O(l) When the reaction is carried out in a sealed container with a constant volume, the heat energy change is not the same as the enthalpy change for this reaction. Give a reason why this is so. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (b) State what is meant by the standard enthalpy change of formation of aluminium oxide, Al2O3(s). Include standard conditions. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 15   (c) Use the data in the table to answer the questions. Enthalpy change Value / kJ mol–1 Enthalpy change of hydration of K+ –322 Enthalpy change of hydration of Ca2+ –1650 Enthalpy change of solution of KCl +17.2 Lattice energy of KCl –711 (i) Name the two properties of ions that affect the value of their enthalpy change of hydration. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Calculate the enthalpy change of hydration for chloride ions by completing the energy cycle, including labels, and using the data in the table. (3) KCl(s) + aq K+(aq) + Cl–(aq) ΔsolnH enthalpy change of hydration for Cl– ions .............................................................. kJ mol–1 (Total for Question 5 = 9 marks)