ThermodynamicsHard

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?

Options

A.As the number of carbon atoms increases, the number of possible isomers increases. This reverses the expected trend of ΔG values.
B.The increase in the number of C–C bonds in relation to the number of C–H bonds modifies the trend of ΔG values in relation to ΔH values.
C.In the formation of CnH2n+2 from n carbon atoms and (n + 1) hydrogen molecules there is a large decrease in entropy. This is reflected in the ΔG values.
D.No simple reason is possible.

Solution

nC(s) + (n + 1) H2 (g) → CnH2n + 2 (g)

with increase in n, the decrease in entropy increases.

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