The lattice energy of solid NaCl is 180 kcal mol−1. The dissolution of the solid in water in the form of ions is endothermic to the extent of 1 kcal mol−1. If the hydration energies of Na+ and Cl− are in the ratio 6:5, then the enthalpy of hydration of Na+ ion is:
ΔHsolution=ΔHlattice+ΔHhydration
∴1 kcal=180+ΔHhydration
ΔHhydration=−179
ΔH(Na+hydration)+ΔH(Cl−hydration)=−179
ΔH(Na+)+56ΔH(Na+)=−179
ΔH(Na+hydration)≈−97.5 kcal