Fugacity

fugacity [fyü′gas·əd·ē]

(thermodynamics)

A function used as an analog of the partial pressure in applying thermodynamics to real systems; at a constant temperature it is proportional to the exponential of the ratio of the chemical potential of a constituent of a system divided by the product of the gas constant and the temperature, and it approaches the partial pressure as the total pressure of the gas approaches zero.

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Fugacity 

a quantity used to calculate the properties of real gases by means of the thermodynamic relationships developed for ideal gases (seeGASES). The concept of fugacity was introduced by G. Lewis in 1901.

The fugacity f of a given gas or a given component of a gas mixture is a function of pressure p and temperature T and of the concentration of each component of a mixture. When the fugacity is substituted for the partial pressure in the thermodynamic equations for an ideal gas, the equations are rendered valid for a real gas under the conditions considered. Although this technique is a formal mathematical procedure, it is productive because the use of more complex equations of state for real gases entails substantial computational difficulties and may not provide the necessary accuracy, since any equation of state for a real gas is valid only within a specific range of values of p and T.

The ratio flp is called the fugacity coefficient; for an ideal gas, it is obviously equal to unity under any conditions. Thus, the difference between the value of flp and unity characterizes the extent to which a gas departs from the ideal state. The role of the fugacity of a gas with respect to the partial pressure of the gas is similar to the role of the activity of a component of a solution with respect to the concentration of the component.

Since the fugacity of a substance that forms or is a constituent of a condensed phase is equal to the fugacity of the substance in the saturated vapor of the phase, the fugacity may also be regarded as a value that quantitatively characterizes—for a given p, T, and phase composition—the ability of the substance to leave the phase.

REFERENCES

See references under THERMODYNAMICS, CHEMICAL.

M. KH. KARAPET’IANTS