Nernst heat theorem
Definition and meaning of Nernst heat theorem in chemistry.
The Nernst heat theorem states that the entropy change of a system in a chemical reaction approaches zero as the absolute temperature approaches absolute zero (0 K). Formulated by Walther Nernst in 1906, this principle forms the foundation for the Third Law of Thermodynamics.
In more detail
The theorem directly implies that the heat capacity of substances approaches zero as temperature approaches absolute zero. It provides the critical insight that absolute entropy values can be assigned to substances, unlike enthalpy, which requires an arbitrary reference state. This breakthrough allows chemists to calculate entropy changes for reactions and predict their spontaneity without needing to choose a reference point. The Nernst theorem is essential to constructing accurate thermodynamic tables used in chemistry today.
Key facts
| Field | Physical Chemistry |
|---|---|
| Discoverer | Walther Nernst (1906) |
| Relationship | Foundation for the Third Law of Thermodynamics |
| Key implication | Enables calculation of absolute entropy values for substances |
For a crystalline solid being cooled toward absolute zero, entropy decreases toward zero according to the Nernst theorem. This principle is applied to calculate standard absolute entropies (at 298 K) for elements and compounds, enabling prediction of whether reactions are thermodynamically favorable.
Frequently asked questions
How does the Nernst heat theorem relate to the Third Law of Thermodynamics?
The Nernst theorem is the precursor to the Third Law. While the theorem addresses entropy changes in reactions near absolute zero, the Third Law states more broadly that the entropy of a perfect crystal equals zero at absolute zero.
Why is the Nernst theorem important in practice?
It provides the thermodynamic foundation for assigning absolute entropy values to substances, allowing chemists to calculate entropy changes for any reaction and determine spontaneity without arbitrary reference states.