Illustrated Glossary of Organic Chemistry

ΔG: The difference in Gibbs free energy (G) between the items on the right side of an equilibrium (such as reaction products) and the items on the left side of the same equilibrium (such as the reactants). ΔG = Gproducts - Greactants. A reaction with ΔG < 0 (an equilibrium favoring products over reactants) is said to be thermodynamically spontaneous.

This energy profile illustrates the difference between ΔG (which controls the reaction's equilibrium constant) and ΔG++ (which influences the reaction rate).