The relationship of free energy (G), enthalpy (H) and entropy (S) when a system changes from one equilibrium state to another: ΔG=ΔH-TΔS, where T is the absolute temperature. The law provides the conceptual basis for redox and group transfer potentials; for example, the Nernst equation or the equilibrium constant, which is ΔG=-RTln(K_eq), where R is the gas constant. The law allows prediction of the direction in which reversible reactions may proceed, dependent upon reactant and product concentrations, by quantifying and defining free energy for a spontaneous chemical change as a negative value which is independent of the path between the initial and final state. Using transition state theory, the law may be applied to rate constants and their temperature dependencies.