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:
425 KJ mol−1
H2+Cl2→2HCl
One each of H−H and Cl−Cl bonds are broken whereas 2 H−Cl bonds are formed. Bond breaking requires energy and bond formation is always exothermic.
[BEH−H+BECl−Cl]−[2BEH−Cl]=ΔHR
[430+240]−[2BEH−Cl]=180
or 670−2(BE)H−Cl=850
⟹BEH−Cl=425 kJ mol−1