Fugacity facts for kids

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

The fugacity $f\,$ is defined at fixed temperature $T\,$ 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. 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 ({1 \over RT} \int_{G_1}^{G_2} dG \right) = exp \left ({1 \over RT} \int_{P_1}^{P_2} V\,dP \right) \,

Fugacity facts for kids

See also