Question

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`.

Answer 1

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