Advanced Topic 361 of 1102 Thermodynamics & Energetics
Third Law of Thermodynamics
The entropy of a perfect crystal is zero at absolute zero.
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In-depth explanation of Third Law of Thermodynamics
The third law states that the entropy of a perfect crystal approaches zero as the absolute temperature approaches zero kelvin. At absolute zero a perfect crystal has only one possible microstate, so by the Boltzmann formula its entropy is zero. This law provides the reference point for absolute entropies, so the entropy of a substance at higher temperature can be calculated by integrating heat capacity divided by temperature from zero. Real crystals may retain small residual entropy because of disorder frozen in place, which is an important exception discussed under residual entropy. Absolute entropies are therefore measurable in principle, unlike internal energies.
Going deeper: Third Law of Thermodynamics
Key ideas: Because entropy has a true zero, absolute entropies can be tabulated, unlike enthalpies. Entropy at temperature T is obtained from the integral of Cp divided by T from zero to T, plus latent heat contributions at each phase transition. It also implies that absolute zero is unattainable in a finite number of steps.
Common mistakes and connections: Beware of assuming every substance has zero entropy at 0 K: only perfect crystals do. Glasses and crystals like carbon monoxide keep residual entropy. The law underlies standard molar entropy tables used in every reaction entropy calculation.
Visualization of Third Law of Thermodynamics
Entropy of a perfect crystal tends to zero as T approaches 0 K, then rises with temperature as more thermal motion becomes accessible. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The standard molar entropy of graphite at 298 K is about 5.7 J per mole per kelvin, obtained by integrating heat capacity data down from 0 K.
Frequently asked questions about Third Law of Thermodynamics
What is Third Law of Thermodynamics?
The entropy of a perfect crystal is zero at absolute zero.
Is Third Law of Thermodynamics a beginner, intermediate or advanced topic?
Third Law of Thermodynamics is a advanced topic in Thermodynamics & Energetics. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Third Law of Thermodynamics?
Study these earlier topics in Thermodynamics & Energetics first: Entropy, Boltzmann Entropy Formula, Second Law of Thermodynamics.
What comes after Third Law of Thermodynamics?
The next topics in Thermodynamics & Energetics are Standard Molar Entropy and Entropy Changes in Reactions.
Can I practise Third Law of Thermodynamics with quizzes?
Yes. The interactive lesson includes a self-check question on Third Law of Thermodynamics, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Third Law of Thermodynamics fits in Thermodynamics & Energetics
Third Law of Thermodynamics is topic 33 of 91 in Thermodynamics & Energetics.