The standard enthalpy of formation (Δf H°298) for methane, CH4 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, H2.
(b) The first four ionisation energies of carbon.
(c) The dissociation energy of H2 and enthalpy of sublimation of carbon (graphite).
(d) The first four ionisation energies of carbon and electron affinity of hydrogen.
Correct answer: (c) The dissociation energy of H2 and enthalpy of sublimation of carbon (graphite).
Explanation:
To calculate average enthalpy of C – H bond in methane following information are needed
(i) dissociation energy of H2 i.e.
\(\frac{1}{2}\)H2(g) → H(g); ΔH = x(suppose)
(ii) Sublimation energy of C(graphite) to C(g)
C(graphite) → C(g); ΔH = y(Suppose)
Given
C(graphite) + 2H2(g) → CH4(g);
ΔH = 75 kJ mol-1