The standard enthalpy of formation $\left({\mathrm{\Delta }}_{\mathrm{f}}{H^{\circ }}_{298\mathrm{K}}\right)$ for methane, ${\mathrm{CH}}_4$ is $-74.9\mathrm{kJ}{\mathrm{mol}}^{-1}$. In order to calculate the average energy given out in the formation of a $\mathrm{C}-\mathrm{H}$ bond from this it is necessary to know which one of the following?

We are given that

$\mathrm{C}\text{ (graphite) }+2{\mathrm{H}}_2\left(\mathrm{g}\right)⟶{\mathrm{CH}}_4\left(\mathrm{g}\right); {\mathrm{\Delta }}_{\mathrm{f}}{H^{\circ }}_{298\mathrm{K}}=-74.9\mathrm{kJ}{\mathrm{mol}}^{-1}$

To calculate the bond enthalpy of $\mathrm{C}-\mathrm{H}$ bond, we require the enthalpy change of the reaction

 ${\mathrm{CH}}_4⟶\mathrm{C}\left(\mathrm{g}\right)+4\mathrm{H}\left(\mathrm{g}\right)$

for which we must require the enthalpy change for the reactions

$\begin{array}{l}\mathrm{C}\text{ (graphite) }⟶\mathrm{C}\left(\mathrm{g}\right);\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{\mathrm{\Delta }}_{\mathrm{sub}}{H}^{\circ }\left(\mathrm{C}\right)=?\\ {\mathrm{H}}_2\left(\mathrm{g}\right)⟶2\mathrm{H}\left(\mathrm{g}\right);\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{\mathrm{\Delta }}_{\mathrm{at}}{H}^{\circ }\left({\mathrm{H}}_2\right)=?\end{array}$