Third law of thermodynamics

Definition

The third law of thermodynamics states that as the temperature of a perfect crystal approaches absolute zero, the entropy approaches a minimum value, typically taken to be zero. This law highlights the relationship between temperature and entropy, emphasizing that at absolute zero, a perfectly ordered state is achieved where the system exhibits no randomness or disorder.

5 Must Know Facts For Your Next Test

1. At absolute zero, the entropy of a perfect crystalline substance is theoretically zero because there is no disorder.
2. The third law is crucial for calculating absolute entropies of substances by providing a reference point.
3. Real systems approach absolute zero but never reach it due to quantum mechanical effects and practical limitations.
4. The third law has implications for understanding low-temperature physics and chemistry, particularly in cryogenics.
5. This law also helps explain phenomena such as the behavior of superconductors and superfluids near absolute zero.

Review Questions

• How does the third law of thermodynamics relate to the concept of entropy in physical systems?
  • The third law of thermodynamics defines how the entropy of a perfect crystal behaves as it approaches absolute zero. As the temperature decreases, the disorder within the system reduces, leading to lower entropy. In this context, at absolute zero, a perfectly ordered state is attained, where entropy reaches its minimum value, illustrating a direct relationship between temperature and entropy.
• Discuss the significance of absolute zero in relation to the third law of thermodynamics and its practical implications in cryogenics.
  • Absolute zero is fundamental to the third law of thermodynamics because it represents the point where all molecular motion ceases and entropy reaches its lowest theoretical value. In practical terms, achieving temperatures close to absolute zero allows researchers to study quantum phenomena and material properties that only emerge under these conditions. In cryogenics, this understanding is applied to develop techniques for preserving biological samples and enhancing superconductivity.
• Evaluate how the third law of thermodynamics impacts our understanding of free energy changes in chemical reactions at low temperatures.
  • The third law of thermodynamics impacts our understanding of free energy changes by providing insight into how entropy behaves at low temperatures. As temperature approaches absolute zero, changes in free energy become increasingly dominated by entropic contributions. Understanding this relationship allows chemists to predict reaction spontaneity and equilibrium positions more accurately under conditions where thermal motion is minimal. This knowledge is critical in fields like cryochemistry, where reactions are studied at very low temperatures.

Related terms

Entropy: A measure of the disorder or randomness in a system, often interpreted as the number of ways a system can be arranged.

Absolute Zero: The lowest possible temperature, 0 Kelvin, where all molecular motion stops and a system reaches its minimum energy state.

Free Energy: The energy in a physical system that can be converted to do work; often represented by Gibbs free energy or Helmholtz free energy in thermodynamic calculations.