fugacities

Fugacity is a thermodynamic property that measures the "effective pressure" of a substance in a mixture. It is particularly useful for describing the behavior of non-ideal gases and is essentially a modified form of partial pressure that accounts for intermolecular forces and other deviations from ideal gas behavior. The concept of fugacity was introduced by G.N. Lewis in 1901.

For an ideal gas, fugacity is equal to its partial pressure. However, for real gases, intermolecular attractions and repulsions cause the fugacity to deviate from the partial pressure. Fugacity is always positive and increases with temperature and pressure. It is a measure of the escaping tendency of a substance from a particular phase.

The fugacity of a substance in a mixture is defined such that the chemical potential of that substance is equal to the chemical potential of the pure substance at its standard state plus the ideal gas constant times temperature times the natural logarithm of the fugacity divided by the standard state fugacity. This relationship allows for the calculation of equilibrium constants and phase behavior in complex systems. The fugacity coefficient, which is the ratio of fugacity to partial pressure, quantifies the deviation from ideal behavior.