The Born-Haber cycle calculates lattice energy (U) indirectly using Hess's law. Steps for MX (e.g., NaCl): (1) Sublimation of metal: M(s) -> M(g), delta_H_{sub}. (2) Ionisation: M(g) -> M+(g) + e-, IE. (3) Dissociation of X2: 1/2 X2(g) -> X(g), 1/2D. (4) Electron affinity: X(g) + e- -> X-(g), EA. (5) Lattice formation: M+(g) + X-(g) -> MX(s), -U. Sum = $delta_{Hf}$(MX). So U = delta_H_{sub} + IE + 1/2D + EA - $delta_{Hf}$. Note: EA is usually negative (exothermic), $delta_{Hf}$ is usually negative. Lattice energy is always positive (energy needed to SEPARATE the lattice). Higher U means more stable ionic compound.