Exams › JEE Main › Chemistry
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
- -17 kcal
- +11 kcal
- -17 kcal
- +85 kcal
Correct answer: -17 kcal
Solution
To find the heat of combustion for C(s) + 2H₂(g) → CH₄(g), we need to add the heats of combustion of C(s) and 2H₂(g). Since the heat of combustion of H₂(g) is -68 kcal/mol, the heat of combustion of 2H₂(g) is 2 * -68 = -136 kcal/mol. Adding this to the heat of combustion of C(s), we get -94 + (-136) = -230 kcal/mol. However, the heat of combustion of CH₄(g) is -213 kcal/mol. Therefore, the heat of combustion of C(s) + 2H₂(g) → CH₄(g) is -213 - (-230) = 17 kcal/mol. However, the question asks for the heat of combustion, which is the negative of the heat of formation. Therefore, the heat of combustion is -17 kcal/mol.
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