Excess properties are nonlinear functions of the composition, temperature and pressure. They usually need to be obtained from experiments. Also, they are normally used with liquid solutions or whenever deviations from a non ideal solution at the same composition, pressure and temperature need to be measured. These properties help to measure their deviation from ideal solutions behavior. (1) In few words, an excess property can be defined as the difference between the real property value of the solution and the value it would have if it was ideal at the same composition, pressure and temperature. Thus: |
ME = M- Mid
Where M represents the molar value of any themodynamic property (enthalpy, entropy, internal energy, etc) and ME represents the difference between the actual property value and the ideal value of the solution. (2)
The excess property has a relationship with the property change of mixing. An example could be the excess Gibbs free energy.
Through the behaviour of GE, SE and HE it is possible to analyze the non-ideality or real liquid solutions. The excess Gibbs energy is typically obtained from low pressure vapor-liquid equilibrium data. HE can be obtained by measuring isothermal enthalpy change. (3)
The excess property has a relationship with the property change of mixing. An example could be the excess Gibbs free energy.
Through the behaviour of GE, SE and HE it is possible to analyze the non-ideality or real liquid solutions. The excess Gibbs energy is typically obtained from low pressure vapor-liquid equilibrium data. HE can be obtained by measuring isothermal enthalpy change. (3)
Example of excess properties at 50°C for 6 binary liquid systems: (a) chloroform(1)/n-heptane(2); (b) acetone(1)/methanol(2); (c) acetone(1)/chloroform(2); (d) ethanol(1)/n-heptane(2); (e) ethanol(1)/chloroform(2); ( f) ethanol(1)/water(2).
(Source: H.C Van Ness and M. M. Abbott, Perry’s Chemical Engineer’s Handbook (7th ed.), McGraw Hill, 1997.)
Additional resources:
Paper: THERMODYNAMIC EXCESS PROPERTIES IN BINARY MIXTURES OF MOLECULES OF DIFFERENT SIZES AND SHAPES
http://gw-chimie.math.unibuc.ro/anunivch/2002-1/AUBCh2002XI13744.pdf
References:
1. Sandler S. Chemical and engineering thermodynamics. 1st ed. New York: Wiley; 1999.
(2) Smith JVan Ness H. Introduction to chemical engineering thermodynamics. 1st ed. New York: McGraw-Hill; 1987.
(3)H.C Van Ness and M. M. Abbott, Perry’s Chemical Engineer’s Handbook (7th ed.), McGraw Hill, 1997.
1. Sandler S. Chemical and engineering thermodynamics. 1st ed. New York: Wiley; 1999.
(2) Smith JVan Ness H. Introduction to chemical engineering thermodynamics. 1st ed. New York: McGraw-Hill; 1987.
(3)H.C Van Ness and M. M. Abbott, Perry’s Chemical Engineer’s Handbook (7th ed.), McGraw Hill, 1997.