Third Law of Thermodynamics
Introduction
The Third Law of Thermodynamics is a fundamental principle of thermodynamics that states that the entropy of a perfect crystal at absolute zero temperature is zero. This law provides an absolute reference point for the determination of entropy. The entropy determined relative to this point is the absolute entropy.
Statement of the Third Law
The Third Law of Thermodynamics can be stated in two ways:
1. Nernst's Heat Theorem: As the temperature of a system approaches absolute zero, the change in entropy ΔS for a chemical reaction approaches zero.
2. Planck's Statement: The entropy of a perfect crystal at absolute zero temperature is zero.
These two statements are essentially equivalent, and they both express the idea that the entropy of a system at absolute zero temperature is a well-defined constant.
Implications of the Third Law
The Third Law of Thermodynamics has several important implications in physical chemistry and materials science:
1. It provides a basis for the definition of absolute entropy, which is a key concept in thermodynamics.
2. It implies that it is impossible to cool a system to absolute zero temperature.
3. It implies that all isotropic materials have the same thermal properties at absolute zero temperature.
4. It implies that the heat capacity of a solid at constant volume vanishes at absolute zero temperature.
5. It implies that the entropy change for any physical or chemical transformation approaches zero as the temperature approaches absolute zero.
Experimental Verification of the Third Law
The Third Law of Thermodynamics has been experimentally verified in a number of ways. For example, measurements of the heat capacities of solids at low temperatures have confirmed that they approach zero as the temperature approaches absolute zero. Similarly, measurements of the entropy changes for chemical reactions at low temperatures have confirmed that they approach zero as the temperature approaches absolute zero.
Applications of the Third Law
The Third Law of Thermodynamics has a wide range of applications in various fields of science and engineering. Some of the key applications are:
1. In physical chemistry, it is used to calculate the absolute entropy of substances.
2. In materials science, it is used to predict the thermal properties of materials at low temperatures.
3. In statistical mechanics, it is used to derive the Boltzmann distribution.
4. In quantum mechanics, it is used to derive the Fermi-Dirac statistics and the Bose-Einstein statistics.
5. In cryogenics, it is used to design refrigeration systems that can achieve temperatures close to absolute zero.
Limitations of the Third Law
While the Third Law of Thermodynamics is a fundamental principle of nature, it has certain limitations:
1. It is not applicable to non-crystalline solids (glasses), because they do not have a well-defined crystal structure at absolute zero temperature.
2. It is not applicable to systems with quantum mechanical ground state degeneracy, because they have a non-zero entropy at absolute zero temperature.
3. It is not applicable to systems with long-range order, because they have a non-zero entropy at absolute zero temperature.
4. It is not applicable to systems with topological order, because they have a non-zero entropy at absolute zero temperature.
Despite these limitations, the Third Law of Thermodynamics is a powerful tool for understanding the behavior of systems at low temperatures.
