Unlike for real gases (pure or mixtures) the EOS based approach to calculation of thermodynamic properties of real liquid solutions have not proved very successful. However, as molar residual property is defined for real gases, for real liquid solutions one may formulate a different departure function called the molar excess property that quantify the deviation from ideal solution property. The mathematical formalism of excess properties is, therefore, analogous to that of the residual properties.
If M represents the molar (or unit-mass) value of any extensive thermodynamic property(e.g., V,U, H, S, G, etc.), then an excess property is defined as the difference between the actual property value of a solution and the value it would have as an ideal solution at the same temperature, pressure, and composition. Thus: |
