Born-Haber Cycles

Born-Haber cycles extend the idea of Hess's law to ionic compounds. They allow us to calculate lattice energies and other thermochemical quantities that cannot be measured directly. Understanding Born-Haber cycles requires you to know all the component enthalpy changes and how they fit together.

---

Lattice Energy

Lattice energy (ΔH°latt) can be defined in two ways — make sure you know which convention your exam board uses.

Formation convention (Edexcel): The lattice energy is the enthalpy change when one mole of an ionic lattice is formed from its gaseous ions under standard conditions.

Na⁺(g) + Cl⁻(g) → NaCl(s) ΔH°latt = −787 kJ mol⁻¹

This value is always negative (exothermic) because the electrostatic attraction between ions releases energy.

The dissociation convention (used by some other boards) is the reverse: the energy required to completely separate one mole of ionic lattice into gaseous ions. This would be +787 kJ mol⁻¹ for NaCl. Always check the sign convention being used in a question.

---

The Component Steps

A Born-Haber cycle for the formation of an ionic compound from its elements involves several steps. For NaCl:

Step 1: Atomisation of the metal

Na(s) → Na(g) ΔH°at = +107 kJ mol⁻¹