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 $\varphi =f/P$ between fugacity $f$ and pressure $P$ is called the fugacity coefficient; therefore, $\varphi =1$ for an ideal gas.

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

$d\ln f=\frac{dG}{RT}=\frac{VdP}{RT}$

where $G$ is the Gibbs free energy, $R$ is the molar gas constant, and $V$ 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:

$f_2/f_1=exp\left(\frac{1}{RT}\underset{G_1}{\overset{G_2}{\int }}dG\right)=exp\left(\frac{1}{RT}\underset{P_1}{\overset{P_2}{\int }}VdP\right)$

Template:Physics-stub