If a reaction is run at a constant temperature, T, the equation is delta G equals delta H minus T times delta S. This is the measurement of whether a chemical reaction is likely to occur with no additional energy input.
Delta G is the change in the Gibbs free energy of a system. Delta H is the change in enthalpy of a system; enthalpy is the heat transfer in a reaction. Entropy is a measurement of the energy unavailable for work, and the change in entropy is represented by delta S. If the change in Gibbs free energy is less than zero, the reaction occurs spontaneously. These reactions are called exergonic reactions.