| Step | ΔH (kJ/mol) |
|---|
A Born-Haber cycle is an application of Hess's law to the formation of an ionic compound from its elements. It breaks the overall formation into a sequence of hypothetical steps — each with a known enthalpy change.
The key idea: lattice enthalpy cannot be measured directly, so we calculate it from the other steps using Hess's law.
Common mistake: Forgetting to double the atomization or electron affinity for compounds like MgO (Mg²⁺ needs 2 IEs) or CaCl₂ (2 Cl atoms need 2 EAs).
Sign convention: Lattice enthalpy of formation is always exothermic (negative). IB may ask for lattice dissociation enthalpy — same magnitude, opposite sign.
Data Booklet: All values except lattice enthalpy are given. You calculate lattice enthalpy by closing the cycle.
Select a compound and build the cycle to see comparative data.