The heat of formation of crystalline sodium chloride is `-410 kJ mol^(-1)`. The heat of sublimation of sodium metal is `108.8 kJ mol^(-1)`. The heat of dissociation of chlorine gas into atoms is `242.7 kJ mol^(-1)`. The ionisation energy of `Na` and electron affinity of `Cl` are `493. 7 kJ` and `-368.2 kJ` respectively. Calculate the lattice energy of `NaCl`.

Explanation

Based on the Hess law, Born-Haber cycle is constructed. `Q=+S+(1)/(2)D+1-E-U`
Substituting the respective values, we get
`-410=108.8+(1)/(2)xx242.7+493.7-368.2+U`
Lattice energy of sodium chloride, `U=-765.65kJmol^(-1)`.