Question

The dissociation energy of methane and ethane are $360$ and $620kcalmo{l}^{-1}$. The value of $e_{C--C}$ in $kcalmo{l}^{-1}$ is _____.

Answer 1

The bond dissociation enthalpy is the energy needed to break one mole of the bond to give separated atoms.

In the methane case, 360 kcal/mol energy is needed to break a mole of methane gas into gaseous carbon and hydrogen atoms and it involves breaking 4 moles of C-H bonds. The average bond energy is, therefore, $\frac{360}{4}$ kcal/mol, which is $90$ kcal per mole of bonds.

In case of ethane,

$6(C-H)+C-C=620$

$6\times 90+C-C=620$

$e_{C-C}=620-540=80kcal/mol$