Gibbs free energy

Meaning: Gibbs free energy, G = H − TS, measures the energy available to do useful work at constant temperature and pressure. The exam uses its change, ΔG, to predict direction: ΔG < 0 is spontaneous (exergonic), ΔG > 0 is nonspontaneous (endergonic), and ΔG = 0 is equilibrium. Releasing heat (ΔH < 0) and increasing disorder (ΔS > 0) both push ΔG negative. When ΔH and ΔS have the same sign, temperature decides the outcome.

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).

On the MCAT: Given ΔH and ΔS, decide whether a reaction is spontaneous at low or high T, or find the crossover T = ΔH/ΔS where ΔG = 0. Also ΔG = ΔG° + RT ln Q, with R = 8.314 J/(mol·K): if Q < Keq, ΔG is negative and the reaction proceeds forward. Coupling a favorable reaction such as ATP hydrolysis to an unfavorable one works because ΔG values add.

Common confusion: Spontaneous does not mean fast. ΔG says nothing about rate, which depends on activation energy. Also, ΔG° (standard conditions, with ΔG° = −RT ln Keq) is not the same as ΔG, which depends on current concentrations through Q. A reaction with positive ΔG° can still run forward if Q is small enough.