# Purpose of Residual property and Excess property in Thermodynamics

Can anybody tell me what are Residual property and Excess property in thermodynamics for?

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FredGarvin
Residual functions are used to calculate properties of substances at non ideal gas states. I have never heard of the Excess property myself. Perhaps someone else can answer that for you.

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stewartcs
Excess functions are introduced in Thermodynamics to modify simple models of substance behaviour (ideal gas model, ideal mixture model, ideal solution model), to account for deviations shown in real substances.
Source: http://imartinez.etsin.upm.es/bk3/c07/Excess functions.htm

This essentially means that it is a way to "adjust" your model so that it matches the natural phenomenon that you are modeling more closely.

It's like using a polytropic process with n=1.2 instead of 1.4 to allow for losses due to a gas having thermal losses/gains or doing some amount of work, etc.

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siddharth
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Can anybody tell me what are Residual property and Excess property in thermodynamics for?
A Residual property is a a measure of the deviation from the value that some property would have for an ideal gas at the same conditions.

For example, if M is the actual value of a thermodynamic property (like the Gibbs free nergy) for a nonideal gas, and Mig is the value the property will have if the gas was ideal at the same T & P, then the residual property MR is defined as,

MR = M - Mig

Excess properties are usually used with liquid solutions, or when we want to measure deviations from a nonideal solution. The excess property is similarly defined as

ME = M - Mid

ie, the difference between the value of a thermodynamic property of the actual solution and an ideal solution at the same T,P and composition.

If you want a reference text, try Chapter 11 of "Chemical Engineering Thermodynamics" by Smith, Van Ness and Abbot.