Lesson 4 of 5
Entropy and accessible arrangements
Predict changes from temperature, phase and particle numbers.
A-Level 9476 (2026-2027)
I can explain entropy.Syllabus
Syllabus 9476, 7(g). Explain entropy.
I can explain qualitative effects on system entropy.Syllabus
Syllabus 9476, 7(h). Explain qualitative effects on system entropy.
I can predict the sign of an entropy change.Syllabus
Syllabus 9476, 7(i). Predict the sign of an entropy change.
Entropy describes how widely energy and particles can be distributed
Think of the number of accessible microscopic arrangements, not simply visible mess.
Make a guess. You are not marked.
Ice melts to liquid water at 273 K. What is the sign of the entropy change of the water?
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Positive, as a liquid is more disordered
Molecules in a liquid can move and arrange themselves in far more ways than in the rigid ice lattice.
Entropy, S, measures the dispersal of energy among accessible microscopic arrangements. A state with more accessible arrangements has greater entropy. Qualitative predictions work best when a large change, such as gas production or a phase change, dominates; do not infer a precise value from appearance.
| Change | Reasoning | Typical sign for the system |
|---|---|---|
| Temperature rises | More energy levels and microscopic arrangements become accessible. | Positive |
| Solid melts; liquid vaporises | Particles gain greater freedom of position and movement, especially on becoming gas. | Positive |
| Gas is compressed at fixed temperature | The particles have less available volume and fewer positional arrangements. | Negative |
| More moles of gas are formed in a reaction | Usually greatly increases possible particle arrangements. | Positive, other contributions permitting |
| Gas is consumed to form a liquid or solid | A large loss of translational freedom usually dominates. | Negative |
Worked exampleCompare two reactions using their gaseous particles
Predict the signs of ΔS for CaCO3(s) → CaO(s) + CO2(g), and N2(g) + 3H2(g) → 2NH3(g).
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- Carbonate decomposition produces gas from solids; accessible arrangements increase strongly.
- Ammonia formation reduces the gas amount from four moles to two moles per equation.
- Count gaseous coefficients, not every coefficient regardless of phase.
Carbonate decomposition has positive ΔS; ammonia formation has negative ΔS under the usual conditions considered.
Dissolution often increases positional dispersal, but strongly hydrated ions can order nearby water molecules. The net entropy of dissolution is not guaranteed positive. If gas amounts are unchanged or competing changes are similar, supplied entropy data is stronger evidence than a simple counting rule. The syllabus does not require calculating reaction entropy from tables of absolute standard entropies.