Generally, when we read some papers in the literature, especially for the Redlich-Kister form of binary liquid mixtures, we encounter different vocabularies for the excess properties. We can cite: excess property; mixture's property; property deviation; property change, etc. Some researchers use different names for thermodynamic or non thermodynamic properties, ...
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I should like to add that you have to compare the real mixture with an ideal mixture at the same pressure, temperature, and composition. An ideal mixture is characterized by Deltamix Vm = 0, Deltamix Hm = 0, and Deltamix Sm = -R Sum(xi ln xi).
Consequently, the excess enthalpy is equal to the “heat of mixing”.
Excess properties must not be confused with residual properties—differences between properties of a real fluid (liquid or gas) and those of an ideal gas at the same temperature, composition, and molar volume. Unfortunately, this mistake is seen frequently in modern literature.
Be careful, Orkid. Excess quantities are with respect to an ideal solution and residual quantities with respect to the ideal gas. Now, an ideal gas is an ideal solution but the inverse is not true.
Besides, as Ulrich also mentions, the temperature, pressure and composition have to be identical.
Let me then add to the answer of Farooq Sher that indeed for the energies, the excesses are additive. However, for the activities they become multiplicative: mole fraction becomes activity which is mole fraction multiplied by activity coefficient. The latter is related to the excess partial Gibbs energy. The terminology then becomes involved, because in the case where the ideal solution is the reference system the terms are activity and activity coefficient whereas in the case of the ideal gas reference the terms are fugacity and fugacity coefficient.
It's a simply request:
Please, help me how to purchase an ancient article from ACS / Industrial & Engineering Chemistry):
Otto Redlich, A. T. Kister, Algebraic Representation of Thermodynamic Properties and the Classification of Solutions. Ind. Eng. Chem., 1948, 40 (2), pp 345–348
DOI: 10.1021/ie50458a036
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Thank you in advance.
I should like to add to Farooq's answer that, for excess properties, the difference is taken between the real and the ideal solution at the same pressure, temperature, and composition.
Residual properties are differences between a real mixture and an ideal gas at the same density, temperature, and composition.
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