The enthalpy of formation steadily changes from −17.89 Kcal/mol to −49.82 Kcal/mol as we go from CH₄, C₂H₆ to C₈H₁₈. The value of ΔG however shows opposite trend, from −12.12 Kcal/mol for CH₄ to 4.14

Question

The enthalpy of formation steadily changes from −17.89 Kcal/mol to −49.82 Kcal/mol as we go from CH₄, C₂H₆ to C₈H₁₈. The value of ΔG however shows opposite trend, from −12.12 Kcal/mol for CH₄ to 4.14 Kcal/mol for C₈H₁₈. Why?

TestHub · Accepted Answer

For nC(s) + (n+1)H₂(g) → CₙH₂ₙ₊₂, the number of gas moles consumed (n+1) grows with n, so ΔS of formation becomes increasingly negative. With ΔG = ΔH − TΔS, the −TΔS term becomes steadily more positive and eventually outweighs the increasingly negative ΔH, making ΔG rise along the series. Matches (c).

Chemistry - Thermodynamics - II Question with Solution | TestHub

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