Chapter 16: Thermodynamics

Third Law of Thermodynamics

As a system approaches absolute zero, the entropy of the system approaches a minimum value.

Absolute Zero

The lowest possible temperature (0 K or -273.15 °C) where particle motion ceases and no thermal energy remains in a system.

Key Concepts

• Entropy becomes constant as temperature approaches absolute zero.
• Absolute zero is unattainable in practice, as it would require infinite energy removal.
• Perfect crystals at 0 K have zero entropy if all atoms are in a perfectly ordered state.

Applications of the Third Law

• Low-Temperature Physics: Studies quantum phenomena at near-zero temperatures.
• Refrigeration: Cryogenic cooling systems rely on principles of entropy reduction.
• Material Science: Thermodynamic properties of materials are studied as they approach low temperatures.

Example 16-3: Entropy at Low Temperatures

If a system at 5 K has an entropy of 0.2 J/K, what would its entropy be at absolute zero according to the Third Law?

• Solution: At absolute zero, entropy approaches zero for a perfectly ordered crystal structure.
• Answer: 0 J/K for a perfect crystal at 0 K.

Limitations of the Third Law

• Applies only to perfect crystalline substances.
• Does not account for systems with residual entropy due to defects or disorder.

Questions for Students

1. State the Third Law of Thermodynamics and explain its significance.
2. Define absolute zero and describe what happens to entropy at this temperature.
3. List three applications of the Third Law and their practical importance.
4. Why is absolute zero considered unattainable?
5. Discuss the limitations of the Third Law for real-world substances.