Gibbs free energy is the energy available in a substance to do work. However, this work does not involve mechanical work, meaning the substance does not expand or contract to push on something. It refers to the 'chemical work' involved in chemical reactions. One could think of chemical work as the energy involved in transforming one chemical into another. The Gibbs free energy is a chemical potential energy in a substance. It is defined by the equation:
G = H - TS
Where G is the Gibbs free energy, H is the enthalpy, T is the temperature, and S is the entropy. Enthalpy and entropy are thermodynamic properties of a system related to the system's internal energy and its degree of disorder.