| Themes > Science > Physics > Thermodynamics > Laws of Thermodynamics Basic Significance > Primarily Condensed Phases > Phase Stability |
The stability of a phase is determined by the Gibbs free energy, +G: +G = +H _ T +S +H = enthalpy +S = entropy T = absolute temperature Any system is said to be in equilibrium if there are no unbalanced forces within the system, i.e., +G = 0. If the free energy is less than zero, a phase can spontaneously transform to another phase; e.g., solid to liquid. This type of transformation is said to be "thermodynamically favorable." If the free energy is greater than zero, a phase transformation will not occur spontaneously. Keep in mind that the free energy condition is a thermodynamic one only; it does not describe the rate at which transformations from one phase to another may take place, even if they are thermodynamically favorable. Phase transformations are rate-dependent, and as such are discussed in the appropriate chapters on kinetics. |
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