Question
The standard enthalpy of formation $$\left( {{\Delta _f}{H^ \circ }_{298}} \right)$$ for methane, $$C{H_4}$$ is $$ - 74.9\,kJ\,mo{l^{ - 1}}.$$ 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 :
The dissociation energy of $${H_2}$$ and enthalpy of sublimation of carbon (graphite).
Solution :
To calculate average enthalpy of $$C – H$$ bond in methane following informations are needed
(i) dissociation energy of $${H_2}$$ i.e.
$$\frac{1}{2}{H_2}\left( g \right) \to H\left( g \right);\,\Delta H = x\left( {{\text{suppose}}} \right)$$
(ii) Sublimation energy of $$C\left( {graphite} \right)$$ to $$C\left( g \right)$$
$$\eqalign{
& C\left( {graphite} \right) \to C\left( g \right);\Delta H = y\left( {{\text{suppose}}} \right) \cr
& {\text{Given}} \cr
& C\left( {graphite} \right) + 2{H_2}\left( g \right) \to C{H_4}\left( g \right); \cr
& \Delta H = 75\,kJ\,mo{l^{ - 1}} \cr} $$