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.

The heat of formation of crystalline sodium chloride is -410 kJ mol^(-

1.	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.
Answer» Solution :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)`.