ΔG = ΔH − TΔS
Gibbs free energy (G) determines whether a process occurs spontaneously at constant temperature and pressure. ΔG = ΔH − TΔS combines enthalpy (heat) and entropy (disorder). If ΔG < 0: spontaneous (exergonic). If ΔG > 0: non-spontaneous (endergonic). If ΔG = 0: equilibrium. Four thermodynamic scenarios: (1) ΔH < 0, ΔS > 0 → always spontaneous; (2) ΔH > 0, ΔS < 0 → never spontaneous; (3) ΔH < 0, ΔS < 0 → spontaneous at low T; (4) ΔH > 0, ΔS > 0 → spontaneous at high T. The crossover temperature T = ΔH/ΔS is where ΔG = 0. For non-standard conditions: ΔG = ΔG° + RT ln(Q), linking to the reaction quotient. At equilibrium, ΔG = 0 and Q = K, giving ΔG° = −RT ln(K).