The heat of formation of crystalline sodium chloride is `-410 kJ mol^(-1)`. The heat of sublimation of sodium metal is `180.8 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+1/2D+I-E+U`
Substituting the respective values, we get `410=108.8+1/2xx242.7+493.7-368.2+U`
Lattice energy of sodium chloride, `U=-765.65KJmol^(-1)`.