Intermediate Topic 358 of 1102 Thermodynamics & Energetics
Entropy
A measure of disorder or the number of microstates available to a system.
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In-depth explanation of Entropy
Entropy, symbolized S, is a measure of the dispersal of energy and matter among the available microscopic arrangements of a system. The more ways energy can be distributed among the particles, the larger the entropy, and its unit is joules per kelvin. Gases have higher entropy than liquids, and liquids higher than solids, because gas molecules have far more translational and rotational states. Entropy also rises with temperature and with the number of particles present. Entropy changes help determine whether a process is spontaneous, because the second law requires the total entropy of the universe to increase in any spontaneous process.
Going deeper: Entropy
Key ideas: Entropy rises with temperature, volume, number of particles and molecular complexity, and with phase change from solid to liquid to gas. Predict the sign of delta S by counting moles of gas and noting dissolution or mixing. The unit is J per kelvin per mole.
Common mistakes and connections: Equating entropy with disorder is a helpful but imperfect picture, since the real quantity counts accessible energy states. Beginners also assume entropy of a system can never decrease, but it can if the surroundings gain more. The total entropy of the universe is what must rise, a point developed in the second law.
Visualization of Entropy
Particles distribute among available arrangements; the most probable, most dispersed distribution dominates. Entropy measures how many such arrangements exist. The interactive lesson shows this as a live animation or chart.
Worked example and use case
When ice melts to liquid water at 273 K, the entropy of the water increases because its molecules gain positional freedom.
Frequently asked questions about Entropy
What is Entropy?
A measure of disorder or the number of microstates available to a system.
Is Entropy a beginner, intermediate or advanced topic?
Entropy is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Entropy?
Study these earlier topics in Thermodynamics & Energetics first: Enthalpy of Atomization, Born-Haber Energetics, Spontaneous Process.
What comes after Entropy?
The next topics in Thermodynamics & Energetics are Boltzmann Entropy Formula and Second Law of Thermodynamics.
Can I practise Entropy with quizzes?
Yes. The interactive lesson includes a self-check question on Entropy, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Entropy fits in Thermodynamics & Energetics
Entropy is topic 30 of 91 in Thermodynamics & Energetics.