Process Flow

Key Annotations

• Endothermic steps: 1 (sublimation), 2 (bond dissociation), 3 (ionisation energy) — always +ve
• Exothermic steps: 4 (electron affinity of halogens) and 5 (lattice formation) — always −ve
• Hess's Law: Sum of steps 1→5 = Direct formation ($\Delta Hf$)
• The cycle "closes" — both paths from Na(s)+½$Cl_{2}$(g) to NaCl(s) give the same $\Delta H$

Extension: MgO cycle

For MgO, step 3 has TWO ionisation energies ($IE_{1}$+$IE_{2}$) and step 4 has TWO electron affinities ($EA_{1}$+$EA_{2}$ for O). The second EA of O (+780 kJ/mol) is endothermic — but the huge lattice energy (~−3791 kJ/mol) more than compensates.