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G = H - TS. delta_G = delta_H - T*delta_S (at constant T). Spontaneity criterion at constant T and P: delta_G < 0 (spontaneous), delta_G = 0 (equilibrium), delta_G > 0 (non-spontaneous). Four scenarios: (1) delta_H < 0, delta_S > 0: always spontaneous — both terms favour negative delta_G. (2) delta_H > 0, delta_S < 0: never spontaneous — both terms oppose. (3) delta_H < 0, delta_S < 0: spontaneous at low T (enthalpy-driven), non-spontaneous at high T. (4) delta_H > 0, delta_S > 0: spontaneous at high T (entropy-driven), non-spontaneous at low T. Crossover temperature: T = delta_H/delta_S (where delta_G = 0). Common trap: forgetting to convert kJ to J when delta_H is in kJ and delta_S is in J/K. delta_G_standard vs delta_G: delta_G_standard is for standard conditions (constant for a reaction at given T). delta_G = delta_G_standard + RT ln Q (actual conditions). At equilibrium: Q = K, delta_G = 0. Therefore delta_G_standard = -RT ln K. If K > 1: delta_G_standard < 0. If K < 1: delta_G_standard > 0. Gibbs energy is the maximum non-PV work obtainable from a process.