Formulation Of Third Law: Definition Of Third Law Of Thermodynamics

The third law of thermodynamics has a controversial past and a number of formulations due to PLANCK, EINSTEIN and NERNST. Between the time period of 1906 to 1912, many formulations of the third law of thermodynamics have been presented by various scientists, but among them all, NERNST'S PRINCIPLE of un-attainability was the most accepted principle for the third law of thermodynamics. Thus, it is correct to say that this law was developed by the German chemist Walther Nernst between 1906 and 1912. 

Let us now look at the various formulations of the third law of thermodynamics proposed by Planck and Nernst.

NERNST HEAT THEOREM

Walther Nernst was the first to recognize the principle that underlies the third law. The third law of thermodynamics, sometimes called Nernst's theorem or Nernst's Postulate, relates to the entropy and temperature of a physical system.  

Walther Nernst’s first formulation of the third law of thermodynamics, called the heat theorem, was the subject of intense discussion. 

From published experimental results, he inferred the Nernst heat theorem as "It asserts that the entropy change for any reaction of pure crystalline substances goes to zero as the temperature goes to zero". 

MAX PLANCK’S FORMULATION

In 1911, The German physicist MAX PLANCK used Walther Nernst's heat theorem to define the third law of thermodynamics in terms of entropy and absolute zero. Max Planck suggested that "When the temperature of a pure substance approaches absolute zero, its entropy approaches zero; may hold for many crystalline substances, but it is not true in general". 

               ( T ➝ 0 then entropy, S ➝ 0 )

Further investigation then showed that Planck’s formulation fails for substances like carbon monoxide, in which the crystalline solid does not become perfectly ordered at the temperature goes to zero.

NERNST UNAVAILABILITY PRINCIPLE

The most accepted version of the third law of thermodynamics, NERNST'S the unavailability principle, states that "Any thermodynamics process can not reach the absolute zero temperature in a finite number of steps and within a finite time".

NERNST proposed that- "The entropy of a system at Absolute Zero Temperature would be a well defined constant instead of being 'Zero'. Entropy at Absolute Zero could be a non-zero constant due to the fact that a system may have degeneracy (having several ground state at same energy level)".

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