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Heat of combustion ΔH⁰ for C (s), H₂ (g), and CH₄ (g) are -94, -68 and -213 kcal/mol, then ΔH⁰ for C(s) + 2H₂(g) → CH₄(g) is

1. -17 kcal
2. +11 kcal
3. -17 kcal
4. +85 kcal

Correct answer: -17 kcal

Solution

The heat of combustion for the reactants and products can be used to calculate the enthalpy change of the reaction. Using Hess's Law, ΔH⁰ = ΔH⁰(products) - ΔH⁰(reactants). Substituting the given values: ΔH⁰ = (-213) - [(-94) + 2(-68)] = -17 kcal.

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