Question

Heat of combustion of methane, carbon and hydrogen are $-212,-94,-68kcal$ ${mol}^{-1}$. The heat of formation of ${CH}_4$ is ______ .

Answer 1

$\Delta H_{reaction}=\sum \Delta H_{products}^F-\sum \Delta H_{reactants}^F$

$\Delta H_{reaction}=\sum \Delta H_{products}^{Comb.}-\sum \Delta H_{reactants}^{Comb.}$

$C+2H_2⟶C{H}_4$

$\Delta H_{reaction}=-94-136+212=-18$ $kcal$ $mo{l}^{-1}$

$⟹-18=\Delta H_{C{H}_4}^F-2\times 0-0$ $(\Delta H_C^F=0,\Delta H_{H_{2}O}^F=0)$

$\Delta H_{C{H}_4}^F=-18$ $kcal$ $mo{l}^{-1}$