Question

The dissociation energy of ${CH}_4$ is $400kcal$ ${mol}^{-1}$ and that of ethane is $670kcal$ ${mol}^{-1}$. The C-C bond energy is:

• A. $270kcal$
• B. $70kcal$
• C. $200kcal$
• D. $240kcal$

Answer 1

The correct option is B $70kcal$

Dissociation reaction of $C{H}_4$ is,
$C{H}_4\left(g\right)\to C\left(g\right)+4H\left(g\right)$ => $\Delta H=400{\text{Kcal mol}}^{-1}=4\times \text{(bond energy of C-H bond)}$
=> $\text{bond energy of C-H bond = 100Kcal}$
Similarly, dissociation reaction of $C_2{H}_6$ is,
$C_2{H}_6\left(g\right)\to 2C\left(g\right)+6H\left(g\right)$ => $\Delta H=670{\text{Kcal mol}}^{-1}=\text{(C-C bond energy)}+6\times \text{(C-H bond energy)}$
=> $\text{(C-C bond energy)}=670-6\times 100=70Kcal$
Hence, answer is option B.