Entropy
Start with the big picture
The Second Law relates spontaneity to the entropy change of the universe: a spontaneous process has a positive total change, a reversible process has zero, and a negative value is impossible. Entropy is a state function, so its change depends on the initial and final states rather than the path. The lesson’s topic structure connects this principle to core equations and examples, including phase changes, ideal-gas expansion, temperature dependence, and mixing. It also introduces the statistical interpretation of entropy, residual entropy, and the Third Law reference of a perfect crystal at 0 K. Finally, entropy links to surroundings, thermodynamic cycles, irreversibility, and Gibbs free energy as a predictor of spontaneity at constant temperature and pressure.
What you'll learn
- State the Second Law criteria for spontaneous, reversible, and impossible processes.
- Explain entropy as a state function and identify its SI units.
- Recognize entropy-change relations for selected physical processes and temperature changes.
- Describe how entropy connects to cycles, surroundings, irreversibility, and spontaneity.
- Distinguish thermodynamic and statistical interpretations of entropy.
Continue your study
Work through the complete notes and reinforce the topic with the study tools available in the full lesson.
