Question

The $H - H$ bond energy is $430 \,kJ\, mol ^{-1}$ and $Cl-Cl$ bond energy is $240 \,kJ \,mol ^{-1}, \Delta H$ for $HCl$ is $-90\, kJ .$ The $H - Cl$ bond energy is about :

• A. $180\, kJ\, mol ^{-1}$
• B. $360\, kJ\, mol ^{-1}$
• C. $213\, kJ\, mol ^{-1}$
• D. $425\, kJ\, mol ^{-1}$

Answer 1

Answer: (D)

The formation of one mole of $HCl$ can be represented as,
$\frac{1}{2}( H - H )+\frac{1}{2}( Cl - Cl ) \longrightarrow H - Cl ; $
$\Delta H=-90\, kJ$
Hence, for the reaction,
Heat evolved = heat evolved in bond formation
- heat required for bond breaking
$\therefore 90 \,kJ =[ H - Cl$ bond energy]
$-\left(\frac{1}{2} \times 430+\frac{1}{2} \times 24\right)$
$\therefore ( H - Cl \text { bond energy })=90+(215+120)$
$=425 \,kJ\, mol ^{-1} .$