9.1 Introduction to Entropy

What entropy measures

Entropy (S) is a measure of the number of ways energy and matter can be dispersed/distributed within a system — often described as a measure of disorder or the number of microstates. Higher entropy means energy and particles are spread over more possible arrangements. Entropy is a state function, so it depends only on the current state, not the path taken.

Predicting relative entropy

Entropy generally increases when: a solid melts to liquid or liquid vaporizes to gas, a substance dissolves into solution, the number of gas particles increases in a reaction, temperature increases, or volume increases for a gas. Gases have far more entropy than liquids, which have more than solids, because gas particles have many more possible positions/energies (microstates).

The Second Law connection

The Second Law of Thermodynamics states that the entropy of the universe always increases for any spontaneous process (ΔS_universe > 0). A process can decrease a system's entropy locally as long as the surroundings' entropy increases enough to compensate, keeping the total change positive.

Key terms

Entropy (S)

A measure of the number of ways energy and matter can be distributed (microstates) in a system; often linked to disorder.

Microstate

One possible arrangement of energy and particles corresponding to a given system state.

Second Law of Thermodynamics

The entropy of the universe always increases for a spontaneous process (ΔS_universe > 0).

State function

A property (like entropy) that depends only on the current state of a system, not the path taken to reach it.

Entropy trend: solid → liquid → gas

Entropy increases in this order because particles gain more freedom of motion and possible positions/energies.

Try an AP-style question

Which has the highest entropy?

1. They are equal
2. 1 mol H₂O(s)
3. 1 mol H₂O(g)
4. 1 mol H₂O(l)