Chemical Thermodynamics: Excess Properties

Excess Properties in Thermodynamics

• Excess properties in thermodynamic properties like enthalpy, internal energy, heat capacity, etc. do not represent new thermodynamic properties.
• However, excess properties in entropy and entropy-related functions are different from property changes of mixing and represent new and useful quantities.

Definition of Excess Properties

• Excess properties indicate the deviations from ideal solution behavior and are easily related to activity coefficients.
• Excess properties can be positive or negative, depending on the deviation from ideality.

Partial Molar Excess Functions

• The definition of partial molar excess functions is analogous to that of partial molar thermodynamic properties.
• The partial molar excess property of component i is denoted by M.
• The partial molar excess function can be written as A M-EM.

Excess Property Change of Mixing

• The excess property change of mixing is defined as the difference between an actual property change of mixing and the property change of mixing calculated for an ideal solution.
• The excess property change of mixing is denoted by DME.
• DME is equal to the excess property ME.

Example of Excess Volume

• The excess volume of a solution is denoted by VE.
• VE is equal to the volume change of mixing DV, since ideal solution involves no volume change of mixing.
• The excess volume of a solution and the volume change of mixing are the same.