Alternative Approach - The Clausis Inequality

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Note: If you are not familar with derivatives you may wish to skip this section.


The above development of the second law is based on intuition. It is a good approach to understanding the law, but it does not allow us to make any calculations.

So we will look at another approach. It puts the second law in the form of a mathematical postulate.[1][nb 1]

Second Law Postulate

The postulate has four parts:

  1. There exists an extensive, additive property called ENTROPY.
  2. The entropy can be divided into two parts:
    1. An external part due to heat transfer with the surroundings
    2. An internal part due to irreversibilities within the system
  3. The internal part is zero for reversible changes and positive for irreversible changes
  4. The external part (dSe) is related to heat by


where T is always positive

Clausis Inequality

The entropy is therefore the sum of the internal and external parts: [math]dS=dS_e+dS_i[/math] where dSe is given above and since dSi is either zero or positive according to the number 3 in the postulate.



This is called the CLAUSIS INEQUALITY.

The Clausis inequality is the probably the most important equation in thermodynamics.


  1. A postulate is a statement which is regarded as true


  1. Adapted from Reid, Charles E. (1990) "Chemical Thermodynamics" Singapore:McGraw-Hill