ギブス自由エネルギー

The second law of thermodynamics states that for all spontaneous processes, the entropy of the universe—the sum of the entropy of the system and the entropy of the surroundings—increases. While the entropy of the system can be calculated from standard molar entropies, the entropy of the surroundings is more difficult to calculate or measure. Therefore, J. Willard Gibbs defined a new thermodynamic function, which allows spontaneity to be determined solely through the entropy and enthalpy of the system and not the surroundings. Recall that under constant pressure and temperature conditions, the ΔS of the surroundings is equal to the negative ΔH of the system divided by the temperature, T. This term can be substituted into the equation representing the second law. When both sides are multiplied by negative T, the equation now becomes: −TΔSuniv = ΔHsys − TΔSsys. The thermodynamic functions on the right side of the equation—enthalpy and entropy—are both solely dependent on the system. Because both enthalpy and entropy are state functions, a new state function can be defined as −TΔSuniv. This new term is called Gibbs free energy and is denoted by the letter G. The equation for ΔG leads to a new criterion for spontaneous reactions. The difference between the enthalpy change and the temperature or entropy change must be less than zero. ΔG is also known as chemical potential because it is similar to the mechanical potential energy of a system. Just as a ball will always roll downhill to lower its potential energy, a chemical reaction proceeds to lower its chemical potential. Thus, at constant temperature and pressure, if the free energy of the system decreases (that is, ΔG < 0), the reaction is spontaneous. Conversely, if the free energy of the system increases, ΔG > 0, and the reaction is not spontaneous. If ΔG = 0, then the reactants and products are in equilibrium.