1. 1 For an ionic compound the lattice enthalpy is the heat energy released when one mole of solid in its standard state is formed from its ions in the gaseous state. This value cannot be determined directly and so we make use of changes for which data are available and link them together with an enthalpy cycle. This enthalpy cycle is based on the formation of the compound from its elements in their standard states. Lattice enthalpy

2. 2 Born-Haber Cycles enthalpy H magnesium chloride MgCl 2 (s) Mg 2+ (g) + 2Cl - (g) H lattice association Mg (s) + Cl 2 (g) H formation H atomisation Mg (g) + Cl 2 (g) H first ionisation energy Mg + (g) + e - + Cl 2 (g) Mg 2+ (g) + 2e - + 2Cl (g) H second ionisation energy 2 x H first electron affinity Mg 2+ (g) + 2e - + Cl 2 (g) H bond energy of chlorine

3. 3 Born-Haber Cycles enthalpy H magnesium chloride MgCl 2 (s) Mg 2+ (g) + 2Cl - (g) H lattice association Mg (s) + Cl 2 (g) H formation H atomisation Mg (g) + Cl 2 (g) H first ionisation energy Mg + (g) + e - + Cl 2 (g) Mg 2+ (g) + 2e - + 2Cl (g) H second ionisation energy 2 x H first electron affinity Mg 2+ (g) + 2e - + Cl 2 (g) H bond energy of chlorine

4. 4 Apply Hess’s Law to calculate ∆H Lattice Energy for MgCl 2 setting out the values in a methodical manner: 1 = 2 + 3 + 4 + 5 + 6 + 7 -641 = +148 + 738 + 1451 + 242 + (2x-364) + ∆H LE -641 = + 1851 + ∆H LE ∆H LE = - 1851 - 641 = - 2492 kJ mol -1