Which statement is the correct condition for spontaneity at constant temperature and pressure?

• A. ΔG > 0
• B. ΔG = 0
• C. ΔG < 0
• D. ΔG is independent of temperature

Answer: (C)

At constant temperature and pressure, whether a process happens on its own is determined by the Gibbs free energy change. If ΔG is negative, the system can move to a lower free energy state without supplying external work, so the process is spontaneous in that direction. If ΔG is zero, the system is at equilibrium with no net driving force, and if ΔG is positive, the process in that direction is non-spontaneous. Because ΔG = ΔH − TΔS, the sign of ΔG depends on temperature as well as enthalpy and entropy changes, so it isn’t independent of temperature. Therefore the correct condition for spontaneity is ΔG < 0.