Spontaneity is determined by Gibbs free energy ΔG. Which condition indicates a spontaneous process at constant temperature and pressure?

• A. ΔG < 0
• B. ΔG > 0
• C. ΔG = ΔH
• D. ΔG = 0 at equilibrium

Answer: (A)

Spontaneity at constant temperature and pressure is determined by the sign of Gibbs free energy change, ΔG. When ΔG is negative, the process can proceed on its own and do useful work; when ΔG is positive, the process won’t occur without external input; and when ΔG is zero, the system is at equilibrium with no net change.

In this context, a negative ΔG indicates spontaneity, so the correct condition is ΔG < 0. The relation ΔG = ΔH − TΔS shows how enthalpy and entropy changes, at a given temperature, determine spontaneity: if the temperature-entropy term dominates (and ΔG becomes negative), the process runs spontaneously. Conversely, a positive ΔG means the process is non-spontaneous under those conditions, and ΔG = 0 corresponds to equilibrium, not spontaneous progress in one direction.