$word_{count}$: 200

The Born-Haber cycle calculates lattice energy indirectly using Hess's law. For MX (e.g., NaCl): start with elements in standard states, apply steps to reach the ionic solid. Steps: (1) Sublimation of metal: M(s) -> M(g), +delta_H_{sub}. (2) Ionisation: M(g) -> M+(g) + e-, +IE. (3) Bond dissociation: 1/2 X2(g) -> X(g), +1/2 D. (4) Electron affinity: X(g) + e- -> X-(g), EA (usually negative). (5) Lattice formation: M+(g) + X-(g) -> MX(s), -U. By Hess's law: $delta_{Hf}$ = delta_H_{sub} + IE + 1/2 D + EA - U. Solving for U: U = delta_H_{sub} + IE + 1/2 D + EA - $delta_{Hf}$. For MX2 (e.g., MgCl2): include IE1 + IE2 for the metal, D (full dissociation for 2 atoms of X), and 2 x EA. Lattice energy is always positive (endothermic to separate ions). Higher lattice energy = more stable compound. Lattice energy increases with higher ion charges and smaller ionic radii (Coulomb's law: U proportional to q+q-/r).