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+1/2 D+I-E-U `
Substituting the respective values, we get
`-410 = 108.8 + 1/2 xx 242.7 + 493.7 - 368.2 + U`
  Lattice energy of sodium chloride, `U=-765.65 kJ mol^(-1)`.