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For which of the following processes does the Gibbs free energy of the system decrease (delta G < 0)?

1. Combustion of propane at 1 bar and 500 K
2. Vaporisation of any liquid at 1 atm and above its normal boiling point
3. Fusion of H2O at 1 atm and -15 deg C if its normal melting point is 0 deg C
4. Vaporisation of H2O at 100 deg C and 1 bar if its normal boiling point is 100 deg C

Correct answer: Combustion of propane at 1 bar and 500 K

Solution

Combustion of propane is highly exothermic with large positive delta S, so delta G = delta H - T*delta S is very negative at any temperature — option A is correct. Vaporisation above the boiling point (B) also gives delta G < 0. Water fusion at -15 deg C (below melting point) is non-spontaneous (C, delta G > 0). Vaporisation of water exactly at its normal boiling point and 1 bar means delta G = 0 (D).

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