G = H - TS. delta_G = delta_H - T*delta_S. Spontaneity criterion: delta_G < 0. The sign of delta_G depends on the balance between enthalpy and entropy terms. Four scenarios: (1) delta_H < 0, delta_S > 0: always spontaneous (both terms favour it). (2) delta_H > 0, delta_S < 0: never spontaneous. (3) delta_H < 0, delta_S < 0: spontaneous at low T (enthalpy-driven, e.g., freezing). (4) delta_H > 0, delta_S > 0: spontaneous at high T (entropy-driven, e.g., melting, decomposition). Crossover temperature: T = delta_H/delta_S. At this temperature, delta_G = 0 (equilibrium).