13108 [Turn over 12 Magnesium oxide is formed from its elements as follows: Mg(s)  + 1 2O2(g)  →  MgO(s)         ΔfHO — = −602 kJ mol−1 (a) (i) Use the values in the table to calculate the entropy change for the reaction. Substance MgO(s) O2(g) Mg(s) Entropy /J K−1 mol−1 26.9 205 32.7 [2] (ii) Write the equation for free energy change. [1] (iii) Calculate the free energy change for the formation of magnesium oxide at 298 K. State the units. Give your answer to 3 significant figures. [3] (iv) Explain why this reaction is feasible at 298 K. [1] 13108 (b) The following enthalpy changes can be used to calculate the lattice enthalpy of magnesium oxide. Mg(s)  →  Mg(g) + 148 kJ mol−1 Mg(g)  →  Mg2+(g)  +  2e− + 2189 kJ mol−1 O2(g)  →  2O(g) + 498 kJ mol−1 O(g)  +  2e−  →  O2–(g) + 657 kJ mol−1 Mg(s)  + 1 2O2(g)  →  MgO(s) − 602 kJ mol−1 (i) Define lattice enthalpy. [2] (ii) Calculate the lattice enthalpy of magnesium oxide. [2] (iii) Write an equation for the second electron affinity of oxygen. Explain whether this process is exothermic or endothermic. [2] 13108 [Turn over (c) A theoretical value for the enthalpy of solution of magnesium oxide may be calculated using a Hess’s Law diagram. (aq) Mg2 (g) O2 (g) A B MgO(s) Mg2 (aq) O2 (g) enthalpy of solution C Mg2 (aq) O2 (i) Name the enthalpy changes labelled A and B. A B [2] (ii) Suggest why an actual value for the enthalpy of solution of magnesium oxide is not found in any data books. [1]