The heats of formation of `CO_(2(g)), H_(2)O_((l)) and CH_(4(g))` are -94.0, -68.4 and -17.9 kcal `"mol"^(-1)` respectively. The heat of combustion of `CH_(4(g))` in kcal `"mol"^(-1)` is ______.

Answer

`-212.9`

Explanation

`C_((s)) + O_(2(g)) to CO_(2(g)) , DeltaH = -94.0"kcal mol"^(-1) " " ….(i)`
`H_(2(g)) +(1)/(2) O_(2(g)) to H_(2)O_((l)), DeltaH = -68.4" kcal mol"^(-1) ……(ii)`
`C_((s)) +2H_(2(g)) to CH_(4(g)) , DeltaH = -17.9" kcal mol"^(-1)….(iii)`
Required equation:
`CH_(4(g)) +2O_(2(g)) to CO_(2(g)) +2H_(2)O_((i)) `,
is obtained by reversing equation (iii),multiplying equation (ii) by 2and adding both these equations to equation (i).
`Delta_(c )H(CH_(4))= [17.9 + (2 xx -68.4) +(-94.0)]`
`= -212.9" kcal mol"^(-1)`
Therefore, heat of combustion of `CH_(4(g))` is -212.9 kcal `"mol"^(-1)`.