Free Energy (G): The energy in a system that can be used to do work; changes in free energy (ΔG\Delta GΔG) predict the spontaneity of a process. First Law of Thermodynamics: The principle of conservat...Free Energy (G): The energy in a system that can be used to do work; changes in free energy (ΔG\Delta GΔG) predict the spontaneity of a process. First Law of Thermodynamics: The principle of conservation of energy applied to thermodynamic systems, stating that the total energy of an isolated system is constant.