Question

The standard enthalpy of formation $\left(\Delta_{f}H^{o}_{298}\right)$ for methane, $CH_{4}$ is $-74.9 \,kJ\,mol^{-1}$. In order to calculate the average energy given out in the formation of a C — H bond from this it is necessary to know which one of the following ?

• A. the dissociation energy of the hydrogen molecule, $H_2$
• B. the first four ionisation energies of carbon
• C. the dissociation energy of $H_2$ and enthalpy of sublimation of carbon (graphite)
• D. the first four ionisation energies of carbon and electron affinity of hydrogen

Answer 1

Answer: (C)

To calculate average bond energy of $( C - H )$ bond, dissociation energy of $H _{2}$ and enthalpy of sublimation of carbon (graphite) is needed.