Calculation of Enthalpy of Reaction

The enthalpy of reaction can be found out by finding the difference in the bond energies of reactants and products.
ΔH   =   Σ Bond Energy of Reactants – Σ Bond energy of Products.
Example:
Calculate DH for the following reaction.
H₂ (g)  +  ½ O₂ (g) --------->  H2O
Given
H – H  =  433 KJ/mol.
O – O =   499 KJ/mol.
and        O – H =   460 KJ/mol.
Solution:
ΔH  =   Σ B.E of H₂ and ½ O₂ - Σ B.E of H₂O
=  (B. E of H₂ + ½  B. E. of O₂ ) – (B. E. of H2O)
=  (B. E. of H – H + ½  B. E. of O – O) – (B. E. of 2 .O – H)
= (433 + ½ x 499) – (2  x 460)
=  (433 + 249.5) – (920)
=   682.5 – 920
ΔH  =  – 237.5 KJ/Mol.