Entropy and Spontaneity
Reactions favour more disorder and lower energy.
About this topic
Entropy (S) is a measure of disorder or the number of ways energy and matter can be arranged. The second law of thermodynamics says the entropy of the universe increases in any spontaneous process. Gibbs free energy combines enthalpy, entropy and temperature: ΔG = ΔH − TΔS, and a reaction is spontaneous when ΔG is negative.
Entropy increases when a solid melts or a liquid boils, when a gas expands, when a substance dissolves, and when a reaction produces more moles of gas. Exothermic reactions tend to be spontaneous because they release energy to the surroundings, and reactions that increase disorder are favoured too. When these factors conflict, temperature decides: ΔH positive and ΔS positive is spontaneous only at high temperature, while ΔH negative and ΔS negative is spontaneous only at low temperature.
Example: ice melts above 0 degrees Celsius even though melting is endothermic, because the entropy gain times T outweighs ΔH. A tidy room becomes messy easily but not the reverse; a drop of ink spreads in water but never gathers itself back. Spontaneous means it can proceed without continuous outside energy input, not that it is fast.
A misconception is that spontaneous equals instantaneous: diamond turning to graphite is spontaneous but very slow. Another is that entropy can never decrease locally; living things and freezing water reduce entropy locally by increasing it elsewhere.
Key ideas
- ΔG = ΔH − TΔS
- Negative ΔG is spontaneous
- Entropy of gases is highest
Quick check
Use cases: where Entropy and Spontaneity helps
- Homework: review the key idea behind Entropy and Spontaneity before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Entropy and Spontaneity without notes.
- Visual learning: play with the interactive lab to see how Entropy and Spontaneity changes when you adjust its inputs.
- Teaching a friend: explain Entropy and Spontaneity in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Entropy and Spontaneity?
Reactions favour more disorder and lower energy.
What will I learn in Entropy and Spontaneity?
ΔG = ΔH − TΔS. Negative ΔG is spontaneous. Entropy of gases is highest.
Is Entropy and Spontaneity part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Entropy and Spontaneity lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Entropy and Spontaneity?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.