# The standard enthalpy of formation $\dpi{100} (\Delta H^{\circ}_{f})$ at 298 K for methane, $\dpi{100} CH_{4(g)}$ is  –74.8 kJ mol-1 . The additional information required to determine the average energy for C – H bond formation would be   Option 1) the dissociation energy of $H_2$ and enthalpy of sublimation of carbon Option 2) latent heat of vaporisation of methane Option 3) the first four ionisation energies of carbon and electron gain enthalpy of hydrogen Option 4) the dissociation energy of hydrogen molecule, $H_2$.

As we learnt in

Bond dissociation enthalpy -

It is the average of enthalpy required to dissociate the said bond present in different gaseous compound in to free atoms in gaseous state.

- wherein

$N_{2}+Bond\, Energy\rightarrow 2N$

Enthalpy of Sublimation -

Amount of enthalpy change to sublimise 1 mole solid into 1 mole vapour at a temperature below its melting point

- wherein

$H_{2}O_{(s)}\rightarrow H_{2}O_{(g)}$

$\Delta H_{sublimation}= 46.6\, kj/mol$

To calculate average enthappy of C - H bond in methane following information are needed.

i) dissociation energy of H2 i.e

$\frac{1}{2}H_{2}\rightarrow H(g), \Delta H = x (suppose)$

ii) Sublimation energy of C (graphite) to C (g)

C (graphite) $\rightarrow$ C (g), $\rightarrow \Delta$H = y (suppose)

Given

C (graphite) + 2H2(g)$\rightarrow$ CH4(g), $\Delta$H = 75 KJmol-1

Option 1)

the dissociation energy of $H_2$ and enthalpy of sublimation of carbon

this is correct option

Option 2)

latent heat of vaporisation of methane

this is incorrect option

Option 3)

the first four ionisation energies of carbon and electron gain enthalpy of hydrogen

this is incorrect option

Option 4)

the dissociation energy of hydrogen molecule, $H_2$.

this is incorrect option

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