The Nernst heat theorem was formulated by Walther Nernst

Walther Nernst

Walther Hermann Nernst FRS was a German physical chemist and physicist who is known for his theories behind the calculation of chemical affinity as embodied in the third law of thermodynamics, for which he won the 1920 Nobel Prize in chemistry...

 early in the twentieth century and was used in the development of the third law of thermodynamics

Third law of thermodynamics

The third law of thermodynamics is a statistical law of nature regarding entropy:For other materials, the residual entropy is not necessarily zero, although it is always zero for a perfect crystal in which there is only one possible ground state.-History:...

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The theorem

The Nernst heat theorem says that as absolute zero is approached, the entropy change ΔS for a chemical or physical transformation approaches 0. This can be expressed mathematically as follow



The above equation is a modern statement of the theorem. Nernst often used a form that avoided the concept of entropy.

Another way of looking at the theorem is to start with the definition of the Gibbs free energy (G), G = H - TS, where H stands for enthalpy. For a change from reactants to products at constant temperature and pressure the equation becomes .

In the limit of T = 0 the equation reduces to just ΔG = ΔH, as illustrated in the figure shown here, which is supported by experimental data. However, it is known from thermodynamics

Gibbs-Helmholtz equation

The Gibbs–Helmholtz equation is a thermodynamic equation useful for calculating changes in the Gibbs energy of a system as a function of temperature...

 that the slope of the ΔG curve is -ΔS. Since the slope shown here reaches the horizontal limit of 0 as T → 0 then the implication is that ΔS → 0, which is the Nernst heat theorem.

The significance of the Nernst heat theorem is that it was later used by Max Planck

Max Planck

Max Karl Ernst Ludwig Planck, ForMemRS, was a German physicist who actualized the quantum physics, initiating a revolution in natural science and philosophy. He is regarded as the founder of the quantum theory, for which he received the Nobel Prize in Physics in 1918.-Life and career:Planck came...

 to give the third law of thermodynamics

Third law of thermodynamics

The third law of thermodynamics is a statistical law of nature regarding entropy:For other materials, the residual entropy is not necessarily zero, although it is always zero for a perfect crystal in which there is only one possible ground state.-History:...

, which is that the entropy of all pure, perfectly crystalline homogeneous materials is 0 at absolute zero

Absolute zero

Absolute zero is the theoretical temperature at which entropy reaches its minimum value. The laws of thermodynamics state that absolute zero cannot be reached using only thermodynamic means....

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