Thermodynamics Directory  Heat Transfer Directory
Thermodynamic Properties Free Energy
Free Energy
is defined as the capacity of a system to perform work or produce heat. T he concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system minus the amount of energy that cannot be used to perform work. This unusable energy is given by the entropy of a system multiplied by the temperature of the system.
Like the internal energy, the free energy is a thermodynamic state function . Energy is a generalization of free energy.
Free energy is that portion of any firstlaw energy that is available to perform thermodynamic work ; i.e. , work mediated by thermal energy . Free energy is subject to irreversible loss in the course of such work. Since firstlaw energy is always conserved, it is evident that free energy is an expendable, secondlaw kind of energy that can perform work within finite amounts of time. Several free energy functions may be formulated based on system criteria. Free energy functions are Legendre transformations of the internal energy . For processes involving a system at constant pressure p and temperature T , the Gibbs free energy is the most useful because, in addition to subsuming any entropy change due merely to heat , it does the same for the p d V work needed to "make space for additional molecules" produced by various processes. (Hence its utility to solution  phase chemists, including biochemists.) The Helmholtz free energy has a special theoretical importance since it is proportional to the logarithm of the partition function for the canonical ensemble in statistical mechanics . (Hence its utility to physicists ; and to gas phase chemists and engineers, who do not want to ignore p d V work.)
The Gibbs free energy is given by G = H − TS , where H is the enthalpy . ( H = U + pV , where p is the pressure and V is the volume.)
