Fugacity of pure substances | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

It has been shown in section 6.3 that the chemical potential provides a fundamental description of phase equilibria. As we shall further see in chapter 8, it also proves an effective tool for depicting chemical reaction equilibria. Nevertheless, its direct usage is restricted, as it is not easy to directly relate the chemical potential to thermodynamic properties amenable to easy experimental determination, such as the volumetric properties. The definition of a new function called fugacity, itself related to the chemical potential, helps bridge the gap.
The concept of fugacity is advanced based on the following thermodynamic relation for an ideal gas. For a single component closed system containing an ideal gas we have (from eqn. 5.7): At constant temperature, for a pure ideal gas ‘i’ the above equation reduces to: | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

(6.85) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

[Where, Gi(T) is the constant of integration] Utilizing the essential simplicity of eqn. 6.85 we apply it a real fluid but by replacing pressure with fugacity, since it is not valid for a real fluid: (At const T) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

Thus, ; Hence
| (6.86) |