The bond enthalpies of H−H, Cl−Cl and H−Cl are 435,243 and 431 kJ mol−1, respectively. The enthalpy of formation of HCl(g) will be:
Given that bond energies of H–H and Cl−Cl are 430 kJ mol−1 and 240 kJ mol−1 respectively and ΔfH for HCl is -90 kJ mol−1. The bond enthalpy of HCl is: