Question

The standard enthalpy of formation $\left({\Delta }_f{H}^0\right)$ at 298K for methane,$C{H}_4\left(G\right)$ is 74.8kJmol${}^{-1}$. The additional information required to determine the average energy for C-H bond formation would be

• A. the first four ionization energies of carbon and electron gain enthalpy of hydrogen
• B. the dissociation energy of hydrogen molecule,$H_2$
• C. the dissociation energy of $H_2$ and enthalpy of sublimation of carbon.
• D. latent heat of vapourization of methane.

Answer 1

The correct option is C the dissociation energy of $H_2$ and enthalpy of sublimation of carbon.

$C\left(s\right)+2H_2\left(g\right)\to C{H}_4\left(g\right)$
$\Delta H=74.8$ KJ mol${}^{-1}$
$\Delta H=$ B.E of reactants - B.E of products
$=$ 2 B.E. (H-H) + $\Delta H_{sub}\left(C\right(s\left)\right)$
- 4 x B.E (C-H)
$\therefore$ The dissociation energy of $H_2$ and enthalpy of sublimation of carbon will be required