$word_{count}$: 220

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 = $\frac{delta_H}{delta_S}$ (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.