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The Nernst heat theorem was formulated by
Walther Nernst Walther Hermann Nernst (; 25 June 1864 – 18 November 1941) was a German physical chemist known for his work in thermodynamics, physical chemistry, electrochemistry, and solid-state physics. His formulation of the Nernst heat theorem helped ...
early in the twentieth century and was used in the development of the
third law of thermodynamics The third law of thermodynamics states that the entropy of a closed system at thermodynamic equilibrium approaches a constant value when its temperature approaches absolute zero. This constant value cannot depend on any other parameters characte ...
.


The theorem

The Nernst heat theorem says that as absolute zero is approached, the entropy change Δ''S'' for a chemical or physical transformation approaches 0. This can be expressed mathematically as follows: : \lim_ \Delta S = 0
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 In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of Work (thermodynamics), work, other than Work (thermodynamics)#Pressure–v ...
(''G''), G = H-TS, where ''H'' stands for
enthalpy Enthalpy () is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant extern ...
. For a change from reactants to products at constant temperature and pressure the equation becomes \Delta G = \Delta H - T\Delta S. In the limit of ''T'' = 0 the equation reduces to just Δ''G'' = Δ''H'', as illustrated in the figure shown here, which is supported by experimental data. – The labels on the figure have been modified. The original labels were A and Q, instead of ΔG and ΔH, respectively. However, it is known from
thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...
that the slope of the Δ''G'' curve is −Δ''S''. Since the slope shown here reaches the horizontal limit of 0 as ''T'' → 0 then the implication is that Δ''S'' → 0, which is the Nernst heat theorem. The significance of the Nernst heat theorem is that it was later used by
Max Planck Max Karl Ernst Ludwig Planck (; ; 23 April 1858 – 4 October 1947) was a German Theoretical physics, theoretical physicist whose discovery of energy quantum, quanta won him the Nobel Prize in Physics in 1918. Planck made many substantial con ...
to give the
third law of thermodynamics The third law of thermodynamics states that the entropy of a closed system at thermodynamic equilibrium approaches a constant value when its temperature approaches absolute zero. This constant value cannot depend on any other parameters characte ...
, which is that the entropy of all pure, perfectly crystalline homogeneous materials in complete internal equilibrium is 0 at
absolute zero Absolute zero is the lowest possible temperature, a state at which a system's internal energy, and in ideal cases entropy, reach their minimum values. The absolute zero is defined as 0 K on the Kelvin scale, equivalent to −273.15 ° ...
.


See also

*
Theodore William Richards Theodore William Richards (January 31, 1868 – April 2, 1928) was an American physical chemist and the first American scientist to receive the Nobel Prize in Chemistry, earning the award "in recognition of his exact determinations of the atomi ...
*
Entropy Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...


References and notes


Further reading

* {{cite book , last = Denbigh , first = Kenneth , title = The Principles of Chemical Equilibrium , publisher = Cambridge University Press , edition = 3 , year = 1971 – See especially pages 421 – 424


External links


Nernst heat theorem
Thermochemistry Walther Nernst de:Nernst-Theorem