exergonic

Meaning: Exergonic describes a reaction or process whose Gibbs free energy change is negative (ΔG < 0). The products have lower free energy than the reactants, so the process can proceed without a net input of free energy. The released free energy can be harnessed, for example ATP hydrolysis, which is exergonic, is used to drive endergonic reactions through coupling. Its opposite is endergonic (ΔG > 0). At equilibrium, ΔG = 0.

Equation: ΔG = ΔH − TΔS, where ΔG is the change in Gibbs free energy (J/mol), ΔH is the change in enthalpy (J/mol), T is absolute temperature (K), and ΔS is the change in entropy (J/(mol·K)). Also ΔG°′ = −RT ln Keq, where R = 8.314 J/(mol·K); a negative ΔG°′ means Keq > 1.

On the MCAT: Passages give ΔG values or energy diagrams and ask which step is spontaneous, or whether a coupled pair has a net negative ΔG. For coupled reactions, add the ΔG values; the overall process is exergonic if the sum is negative. On a reaction coordinate diagram, an exergonic reaction ends lower than it starts.

Common confusion: Students equate exergonic with fast, or with exothermic. ΔG says nothing about rate; that depends on activation energy, and enzymes lower it without changing ΔG. Exothermic refers to ΔH < 0 only. A reaction with ΔH > 0 can still be exergonic if TΔS is large enough.