23.3.2—Predict/explain the sign of the entropy

Entropy increases on melting, boiling, dissolving and heating, and decreases for the reverse changes. A reaction forming more gaseous molecules usually has positive ΔS; forming fewer has negative ΔS.

Use the strongest structural change first: gas count often dominates, while phase and temperature changes provide additional evidence. The sign prediction is qualitative unless data are supplied.

H₂O(s) → H₂O(l) gives ΔS>0; 2SO₂(g)+O₂(g) → 2SO₃(g) gives a negative gas-count contribution because three gas molecules become two.

A positive entropy change does not by itself prove spontaneity; Gibbs free energy also depends on enthalpy and temperature.