JEE MAIN - Chemistry (2006 - No. 1)
The standard enthalpy of formation $$\Delta _fH^o$$ at 298 K for methane, CH4(g), is –74.8 kJ mol–1. The additional information required to determine the average energy for C – H bond formation would be :
the dissociation energy of H2 and enthalpy of sublimation of carbon
latent heat of vapourization of methane
the first four ionization energies of carbon and electron gain enthalpy of hydrogen
the dissociation energy of hydrogen molecule, H2
Explication
The standard enthalpy of formation of $$C{H_4}$$ is given by the equation :
$$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$ $$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$ $$C\left( s \right) + 2{H_2}\left( g \right) \to C{H_4}\left( g \right)$$
In order to calculate average energy for C – H bond formation we should know the following data.
C(graphite) $$ \to $$ C(g) ; $$\Delta H_f^o$$ = enthalpy of sublimation of carbon
H2(g) $$ \to $$ 2H(g) ; $$\Delta $$H = bond dissociation energy of H2
$$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$ $$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$ $$C\left( s \right) + 2{H_2}\left( g \right) \to C{H_4}\left( g \right)$$
In order to calculate average energy for C – H bond formation we should know the following data.
C(graphite) $$ \to $$ C(g) ; $$\Delta H_f^o$$ = enthalpy of sublimation of carbon
H2(g) $$ \to $$ 2H(g) ; $$\Delta $$H = bond dissociation energy of H2