Fugacity is a thermodynamic property used to describe the ‘escaping tendency’ of a substance from a phase.

It is especially useful when dealing with real gases, where deviations from ideal gas behaviour occur. Here’s a more detailed explanation:

Definition

Fugacity is defined as a corrected pressure that accounts for deviations from ideal gas behaviour. For an ideal gas, fugacity is equivalent to the pressure of the gas.

However, for real gases, fugacity accounts for intermolecular forces and volume occupied by gas molecules, thus providing a more accurate measure of a substance’s tendency to escape from the phase.

Mathematical Representation

Fugacity () can be expressed as:

where:

• is the fugacity coefficient, which measures the deviation from ideal behaviour.
• is the actual pressure of the gas.

For an ideal gas, equals 1, so fugacity equals the pressure. For real gases, is typically less than 1 due to intermolecular forces and volume effects.

Importance in Thermodynamics

Fugacity is crucial in various thermodynamic calculations, such as:

• Phase Equilibria: It helps in determining the conditions for phase transitions, like boiling and condensation, by equalising the fugacity of a substance in different phases.
• Chemical Equilibria: It is used to calculate the equilibrium constant for reactions involving gases, providing a more accurate description than using partial pressures.
• Mixing and Non-ideal Solutions: Fugacity is employed to describe the behaviour of components in non-ideal mixtures, allowing for more precise calculations in processes like distillation and extraction.

Relationship with Chemical Potential

Fugacity is directly related to the chemical potential () of a substance, which is a measure of its thermodynamic potential to undergo a phase change or a chemical reaction. The relationship is given by:

where:

• is the standard chemical potential.
• is the universal gas constant.
• is the absolute temperature.
• is the standard fugacity.

This equation indicates how fugacity influences the chemical potential and thus the equilibrium properties of a system.

Practical Use

In practice, fugacity is used in engineering and chemical industries to design and optimise processes involving gases, ensuring efficiency and safety.

Calculations involving fugacity provide more accurate results than those using ideal gas laws, making it an essential concept in advanced thermodynamics and chemical engineering.