Heat of combustion of methane, carbon and hydrogen are $- 212, - 94, - 68 k c a l$ ${m o l}^{- 1}$ . The heat of formation of ${C H}_{4}$ is ______ .

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Solution

$Δ H_{r e a c t i o n} = \sum Δ H_{p r o d u c t s}^{F} - \sum Δ H_{r e a c t a n t s}^{F}$
$Δ H_{r e a c t i o n} = \sum Δ H_{p r o d u c t s}^{C o m b .} - \sum Δ H_{r e a c t a n t s}^{C o m b .}$
$C + 2 H_{2} ⟶ C H_{4}$
$Δ H_{r e a c t i o n} = - 94 - 136 + 212 = - 18$ $k c a l$ $m o l^{- 1}$
$⟹ - 18 = Δ H_{C H_{4}}^{F} - 2 \times 0 - 0$ $(Δ H_{C}^{F} = 0, Δ H_{H_{2} O}^{F} = 0)$
$Δ H_{C H_{4}}^{F} = - 18$ $k c a l$ $m o l^{- 1}$

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