change in Gibbs free energy

Meaning: ΔG is the criterion for whether a process is thermodynamically favorable at constant temperature and pressure. It balances the drive toward lower enthalpy against the drive toward higher entropy. ΔG < 0: spontaneous (exergonic). ΔG > 0: nonspontaneous (endergonic), so the reverse is spontaneous. ΔG = 0: equilibrium.

Equation: ΔG = ΔH − TΔS, where ΔG and ΔH are in J/mol, T is absolute temperature in K, and ΔS is in J/(mol·K). Related forms: ΔG = ΔG° + RT ln Q and ΔG° = −RT ln Keq, with R = 8.314 J/(mol·K).

On the MCAT: Expect sign analysis. ΔH < 0 with ΔS > 0 is spontaneous at all temperatures, ΔH > 0 with ΔS < 0 is nonspontaneous at all temperatures, and the two mixed cases change with temperature. You may also compare Q with Keq (Q < Keq gives ΔG < 0 for the forward direction), or add ΔG values for coupled reactions, as when ATP hydrolysis drives an unfavorable step.

Common confusion: Spontaneous does not mean fast. ΔG says nothing about rate, and a catalyst does not change it. Also keep ΔG° (standard conditions) separate from ΔG (actual conditions): a reaction with ΔG° > 0 can still run forward if Q is small enough to make ΔG negative.