23.1.4—Calculations: Born–Haber cycles

Use Hess’s law: the algebraic sum of the enthalpy changes around the cycle equals the standard enthalpy of formation. Rearrange only after deciding whether each step forms or separates a species.

Keep a sign table and isolate the unknown lattice energy or electron affinity. Check units and compare the magnitude with the expected ionic attraction.

If all steps except lattice formation are known, ΔHlatt = ΔHf° − (atomisation + ionisation + electron-affinity terms). A negative formation lattice energy is expected in the formation direction.

Do not change signs because a value “looks too large”, and do not use lattice dissociation data as formation data without reversal.