Fugacity

In thermodynamics, the fugacity is a state function of any isothermal system. The fugacity, which has units of pressure, represents the tendency of a fluid to escape or expand isothermally. For gases at low pressures where the ideal gas law holds, fugacity is equal to pressure. The ratio  between fugacity  and pressure  is called the fugacity coefficient; therefore,  for an ideal gas.

The fugacity  is defined at fixed temperature  by the following differential relation:

where  is the Gibbs free energy,  is the molar gas constant, and  is the fluid's volume.  For any two isothermal physical states, represented by subscripts 1 and 2, the ratio of the two fugacities is as follows:

Category:Thermodynamics