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 = $\frac{delta_H}{delta_S}$. At this temperature, $delta_G$ = 0 (equilibrium).