The Born-Haber cycle summarizes the standard enthalpy changes that occur during the formation of an ionic crystal from its gaseous constituent elements. It relates the standard enthalpy of sublimation, bond dissociation, electron affinity, and lattice energy to the standard enthalpy of atomization through Hess's law. For example, in forming NaCl, the standard enthalpy of atomization for sodium and chlorine contributes -411 kJ, which is balanced by the energy absorbed in ion formation, bond breaking, and crystal lattice formation.

1. The Born-Haber Cycle

2. The Born-Haber cycle is a special case of Hess’s law applied to ionic crystals only. Definitions Standard enthalpy of sublimation is the heat absorbed when one mole of atoms are vaporised Standard enthalpy of bond dissociation is the energy absorbed when one mole of bonds is broken The electron affinity is the energy absorbed when one mole of ions are formed from one mole of atoms The standard lattice energy is the energy absorbed when one mole of ionic crystals is formed. Standard enthalpy of atomisation is the energy absorbed when one mole of atoms are formed

3. -411 = 109 + 121 + 449 – 364 +  H l Ө Where  H l Ө = standard lattice energy of NaCl = -726 KJ/mol

4. For MgCl 2 we will need 2 x EA for 2 atoms of Cl Be careful to note if bond dissociation energy (BDE) is given or atomisation energy for chlorine or other diatomic gas e.g. BDE for chlorine is 242 KJ/mol  H a Ө for chlorine is half this value 121 KJ/mol