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The Nernst heat theorem

المؤلف:  Peter Atkins، Julio de Paula

المصدر:  ATKINS PHYSICAL CHEMISTRY

الجزء والصفحة:  ص92-93

2025-11-06

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The Nernst heat theorem

The experimental observation that turns out to be consistent with the view that the entropy of a regular array of molecules is zero at T=0 is summarized by the Nernst heat theorem:

The entropy change accompanying any physical or chemical transformation approaches zero as the temperature approaches zero: ∆S→0 as T→0 provided all the substances involved are perfectly crystalline.

It follows from the Nernst theorem that, if we arbitrarily ascribe the value zero to the entropies of elements in their perfect crystalline form at T=0, then all perfect crystalline compounds also have zero entropy at T=0 (because the change in entropy that accompanies the formation of the compounds, like the entropy of all transformations at that temperature, is zero). This conclusion is summarized by the Third Law of thermodynamics:

The entropy of all perfect crystalline substances is zero at T=0. As far as thermodynamics is concerned, choosing this common value as zero is then a matter of convenience. The molecular interpretation of entropy, however, justifies the value S=0 at T=0.