Gibbs free energy

Will a reaction go forward on its own, or does it need a constant push? Gibbs free energy is the single accountant that settles the question. It bundles together the two things nature cares about — how much heat is released and how much disorder is created — into one number, the change called delta G. The rule is simple: if delta G is negative, the reaction is favorable and can proceed on its own; if positive, it is uphill and will not go spontaneously the way it is written.

The bookkeeping is captured by one short equation: delta G = delta H minus T times delta S. Here delta H is the enthalpy change (the heat given off or taken in, mostly the difference in bond strengths between products and reactants), delta S is the entropy change (how much more or less spread out and disordered things become), and T is the absolute temperature in kelvin. The minus sign and the temperature mean that at high temperature, the entropy term grows in importance — which is why some reactions that are stuck at room temperature suddenly become favorable when heated. A reaction with a negative delta G is called exergonic; a positive one, endergonic.

In organic chemistry, delta G is what ties acid-base equilibria, ring openings, and every reversible step to a position of equilibrium: a more negative delta G means the equilibrium lies further toward products. But there is a crucial honesty here — delta G tells you whether a reaction can happen and where it will settle, but says nothing about how fast. A reaction can be strongly favorable (large negative delta G) and yet sit untouched for years because the path to get there is slow. Speed is a separate question, governed by activation energy, not by delta G.