Gibbs free energy
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
The thermodynamic potential, G = H - TS, whose change predicts reaction spontaneity.
Gibbs free energy in plain English
Gibbs free energy, G, is a single number that answers one very practical question: will this reaction actually happen by itself, or do I have to force it? What matters isn't G alone but how it CHANGES during a reaction (Δ G): if it goes down (negative), the reaction runs on its own and can release usable energy; if it goes up (positive), you have to push it. The recipe G = H − TS blends two instincts of nature — reactions like to release heat (that's the H, enthalpy) and they like to spread out into disorder (that's the S, entropy, scaled by temperature T). Think of it like a ball on a hill: it rolls only in the direction that lowers its overall 'free energy,' and Δ G tells you which way is downhill.
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