A thermodynamic state property useful to determine the spontaneity of a reaction within a system (without regard to the surroundings as is the case with entropy changes) and the direction of the reaction. The change in Gibbs energy, ΔG, is equal to ΔH – TΔS, where ΔH is the change in enthalpy (cal/mole), T is the absolute temperature (K), and ΔS is the change in entropy (cal deg^-1 mole^-1). A substance reacts if the change in Gibbs energy is negative going from the initial state to the final state. Gibbs energy is often referred to as Gibbs free energy.
See enthalpy, entropy, state function