# Third law of thermodynamics

The third law of thermodynamics, sometimes called Nernst's Theorem or Nernst's Postulate, relates the entropy and the temperature of a physical system.

The third law of thermodynamics states that absolute zero can not be achieved in a finite number of stages. The third law of thermodynamics can also be defined as that when reaching absolute zero, 0 degrees Kelvin, any process of a physical system stops and when reaching absolute zero the entropy reaches a minimum and constant value.

This principle states that the entropy of a system at the absolute zero temperature is a well-defined constant. This is because, at the absolute zero temperature, a system is in a basic state and entropy increases are achieved by degeneration from this basic state.

Nernst's theorem states that the entropy of a perfect crystal of any element at the absolute zero temperature is zero. However, this observation does not take into account that the real crystals have to be formed at temperatures above zero. Consequently, they will have defects that will not be eliminated when cooled down to absolute zero. As they are not perfect crystals, the information necessary to describe the existing defects will increase the entropy of the crystal.

## Theorems and statements of the third law of thermodynamics

Nernst's Terorema: A chemical reaction between pure crystalline phases that occurs at absolute zero does not produce any change in entropy.

Nernst-Simon Statement: The entropy change that results from any reversible isothermal transformation of a system tends to zero as the temperature approaches zero.

Planck statement: For T → 0, the entropy of any system in equilibrium approximates a constant that is independent of the other thermodynamic variables.

Absolute zero inaccessibility theorem: There is no process capable of reducing the temperature of a system to absolute zero in a finite number of steps.

4th Callen Postulate: The entropy of any system is annulled in the state for which

## History of the third law of thermodynamics

The third law was developed by chemist Walther Nernst during the years 1906-12, and that is why it is often referred to as Nernst's theorem or Nernst's postulate. The third law of thermodynamics states that the entropy of a system at absolute zero is a well-defined constant. This is because there is a zero temperature system in its ground state, so its entropy is determined only by the degeneracy of the ground state.

In 1912, Nernst declared the law thus: "It is impossible for a procedure to lead to the isotherm T = 0 in a finite number of steps".

An alternative version of the third law of thermodynamics as established by Gilbert N. Lewis and Merle Randall in 1923: "If the entropy of each element in some crystalline (perfect) state is taken as zero at absolute zero of temperature, each substance has a finite positive entropy; but at the absolute zero of the temperature, the entropy can become zero, and it is in the case of the perfect crystalline substances. "

This version indicates that not only Δ S will reach zero to 0 degrees Kelvin, but that S will also reach zero as long as the crystal has a ground state with only one configuration. Some crystals form defects that cause a residual entropy. This residual entropy disappears when the kinetic barriers for the transition to a ground state are overcome.

## References

Last review: April 12, 2018