Given $Δ_{f} H^{\circ} (CH X_{4} (g)) = - 74.8$ , enthalpy of sublimation of graphite $= 717$ , and $H - H$ bond enthalpy $= 436$ (all in $kJ mo l^{- 1}$ ), estimate the average $C - H$ bond enthalpy in methane in $kJ mo l^{- 1}$ (nearest integer).

Numerical answer type, answer in kJ/mol, tolerance 0.5.

Answer: 415.95 kJ/mol

Atomisation:

CH X_{4} (g) C (g) + 4 H (g)

is the reverse of formation followed by sublimation of C and dissociation of

2 H X_{2}

Δ H_{a t o m} = 74.8 + 717 + 2 (436) = 1663.8 kJ

. This breaks four

C - H

bonds, so the average value is

1663.8/4 = 415.95 kJ mo l^{- 1}