The standard enthalpy of formation steadily changes from −18.0 kcal/mol to −40.0 kcal/mol as we go from CH₄ through C₂H₆ to C₅H₁₂. The standard free energy of formation, however, shows the opposite tr

Question

The standard enthalpy of formation steadily changes from −18.0 kcal/mol to −40.0 kcal/mol as we go from CH₄ through C₂H₆ to C₅H₁₂. The standard free energy of formation, however, shows the opposite trend, rising from −12.0 kcal/mol for CH₄ toward positive values for C₅H₁₂. Why?

TestHub · Accepted Answer

nC(s) + (n+1)H₂(g) → CₙH₂ₙ₊₂ consumes n+1 moles of gas, a number that grows with n, so ΔS of formation becomes more and more negative. Since ΔG = ΔH − TΔS, the −TΔS term becomes steadily more positive and overrides the increasingly negative ΔH, making ΔG rise along the series. Matches (b).

Chemistry - Thermodynamics - II Question with Solution | TestHub

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