Q1(b). Born–Haber cycle for NaCl at 298 K

Answer in brief

Energy demanding: sublimation, dissociation, ionisation. Energy evolving: electron gain, lattice formation; ΔfH° = −411 kJ mol⁻¹.

Key points

• Steps: sublimation of Na +108, half dissociation of Cl₂ +121, ionisation of Na +496, electron gain of Cl −349, lattice formation −787 kJ mol⁻¹.
• Enthalpy is a state function (Hess’s law): ΔfH° = 108 + 121 + 496 − 349 − 787 = −411 kJ mol⁻¹.
• Making the gaseous ions costs +376 kJ mol⁻¹ overall, because the electron gain of Cl returns less than the ionisation of Na takes.
• The lattice enthalpy (−787 kJ mol⁻¹) more than repays this cost; it is the lattice, not the electron transfer, that makes NaCl(s) stable.