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Fugacity

In thermodynamics, the fugacity is a state function of matter at fixed temperature. 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 is a good approximation, fugacity is nearly equal to pressure. The ratio $\phi = f/P \,$ between fugacity $f\,$ and pressure $P\,$ is called the fugacity coefficient. For an ideal gas, $\phi = 1 \,$ .

For a given temperature $T\,$ , the fugacity $f\,$ is defined by the following differential relation:

$d \ln f = {dG \over RT} = {{V dP} \over RT} \,$

where $G\,$ is the Gibbs free energy, $R\,$ is the molar gas constant, and $V\,$ is the fluid's volume. Thus, for any two mutually-isothermal physical states, represented by subscripts 1 and 2, the ratio of the two fugacities is as follows:

$f_2 / f_1 = exp \left ({1 \over RT} \int_{G_1}^{G_2} dG \right) = exp \left ({1 \over RT} \int_{P_1}^{P_2} V\,dP \right) \,$

Categories: Thermodynamics

Last updated: 06-01-2005 19:14:15

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10-26-2009 08:16:03
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Fugacity