Question

Dissociation of water takes place in two steps :
$H_2O \to H + OH$; $\Delta H = +497.8\, kJ$
$OH \to H + O$; $\Delta H = 428.5 \,kJ$
What is the bond energy of $O — H$ bond ?

• A. $463.15\, kJ\, mol^{-1}$
• B. $428.5\, kJ\, mol^{-1}$
• C. $69.3\, kJ\, mol^{-1}$
• D. $926.3\, kJ\, mol^{-1}$

Answer 1

Answer: (A)

In given 2 reactions, $\Delta H$ is basically representing bond energies of $H - O$ bond. So, our answer should be average of these two $\Delta H$ values.
Hence, Average of two bond dissociation energies: $\frac{497.8+428.5}{2}=463.15\, kJ mol ^{-1}$