Calculate the heat of formation of methane in kcal mol⁻¹ using the following thermochemical reactions:
C(s) + O₂(g) $⟶$ CO₂(g) ; $Δ H = - 94.2 k c a l m o l^{- 1}$
H₂(g) + $\frac{1}{2}$ O₂(g) $⟶$ H₂O(l) ; $Δ H = - 68.3 k c a l m o l^{- 1}$
CH₄(g) + 2O₂(g) $⟶$ CO₂(g) + 2H₂O(l) ; $Δ H = - 210.8 k c a l m o l^{- 1}$

Question

Calculate the heat of formation of methane in kcal mol⁻¹ using the following thermochemical reactions:

C(s) + O₂(g) $⟶$ CO₂(g) ; $Δ H = - 94.2 k c a l m o l^{- 1}$

H₂(g) + $\frac{1}{2}$ O₂(g) $⟶$ H₂O(l) ; $Δ H = - 68.3 k c a l m o l^{- 1}$

CH₄(g) + 2O₂(g) $⟶$ CO₂(g) + 2H₂O(l) ; $Δ H = - 210.8 k c a l m o l^{- 1}$

TestHub · Accepted Answer

C(s) + O₂(g) ⟶ CO₂(g) + 94.2 kcal …(1)

H₂(g) + ½O₂(g) ⟶ H₂O(l) + 68.3 kcal …(2)

CH₄(s) + 2O₂(g) ⟶ CO₂(g) + 2H₂O(l) + 210.8 kcal …(3)

The required equation is

C(g) + 2H₂(g) ⟶ CH₄(g)

so, $Δ H = Δ H_{1} + 2 \times Δ H_{2} - Δ H_{3} = - 20 k c a l m o l^{- 1}$ .

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