Entropy Introduction

Learning Objective 9.1.A Identify the sign and relative magnitude of the entropy change associated with chemical or physical processes.

Quick Notes

Entropy (S) is a measure of the disorder or dispersal of matter and energy in a system.
Entropy increases when:
- Matter becomes more spread out (e.g., melting, evaporation).
- Energy becomes more spread out (e.g., temperature increase).
- The number of gas particles increases in a reaction.
The entropy of the system can change in a reaction (ΔS).

If ΔS > 0 then increase in entropy (more disorder).
If ΔS < 0 then decrease in entropy (less disorder).

Full Notes

Note – Entropy has been covered in more detail here this page is just specifically for AP Chemistry.

What is Entropy?

Entropy (S) measures the number of possible microstates – the different ways the energy of a system can be arranged at the particle level – of a system. A greater number of arrangements or greater molecular disorder corresponds to higher entropy.

Entropy reflects both physical disorder (such as the spacing and motion of particles) and the distribution of energy within a system. During chemical or physical changes, the entropy of a system often changes (represented as ΔS) as particles rearrange and energy becomes more or less dispersed.

Note: Entropy is often described as a measure of "disorder" in a system. In this context, disorder refers to the number of possible microstates — the more ways the particles and energy can be arranged, the greater the disorder and the higher the entropy.

Entropy and Phase Changes

Entropy increases when particles gain more freedom of movement, meaning entropy change occurs when a substance changes state.

AP Chemistry diagram comparing entropy for solid, liquid, and gas; entropy increases from solid to liquid to gas.

Solid → Liquid: +ΔS (particles move more freely)
Liquid → Gas: +ΔS (much more disorder)
Gas → Liquid or Solid: −ΔS (movement becomes restricted)

For example When one mole’s worth of solid ice melts to liquid and then turns to gas, the entropy of the system increases.

AP Chemistry sequence showing H2O(s) to H2O(l) to H2O(g) with increasing entropy.

Entropy and Volume (for Gases)

At constant temperature, the volume of a gas impacts the entropy of the system.

AP Chemistry diagram showing that a gas at larger volume has higher entropy than at smaller volume.

Increasing volume means gas molecules have more space to move – entropy increases, +ΔS
Decreasing volume means gas molecules have less space, more order – entropy decreases -ΔS

Entropy and Number of Gas Molecules

Reactions that produce more gas particles typically increase entropy, while those that cause a decrease in gas decrease entropy.

General Rule: If gas moles increase, +ΔS; if gas moles decrease, −ΔS.

For Example:

AP Chemistry reaction Mg(s) + Cl2(g) → MgCl2(s) showing gas moles drop and total particles decrease, giving negative entropy change.

When solid magnesium reacts with chlorine gas, the total moles of particles decrease from two to one and, more specifically, the moles of gas decrease from one to zero, meaning a decrease in entropy (−ΔS).

Entropy and Temperature

According to the Kinetic Molecular Theory, KMT (see Topic 5.5), entropy increases with temperature because energy becomes more widely dispersed among particles.

As temperature increases, particles gain kinetic energy and move faster. This leads to a broader distribution of molecular energies (see Maxwell–Boltzmann distribution curves. The wider spread of energy amongst particles means the disorder (number of accessible microstates) increases, resulting in greater entropy (ΔS > 0).

AP Chemistry diagram illustrating that increasing temperature increases entropy due to greater molecular motion and energy dispersion.

Summary

Entropy increases with greater freedom of movement (phase change to gas), greater volume, more gas particles, and higher temperatures.
We can use ΔS to predict whether a process involves greater or lesser molecular disorder.