The formation of one mole of HCl can be represented as, 21(H−H)+21(Cl−Cl)⟶H−Cl; ΔH=−90kJ
Hence, for the reaction,
Heat evolved = heat evolved in bond formation
- heat required for bond breaking ∴90kJ=[H−Cl bond energy] −(21×430+21×24) ∴(H−Cl bond energy )=90+(215+120) =425kJmol−1.