9.1 Introduction to Entropy

01Energy spreading out

Entropy (S) measures how dispersed energy and matter are. It increases when solids → liquids → gases, when more gas moles form, when things dissolve, and when temperature rises.

Predict ΔS sign by counting gas moles first.

02Notebook box

More gas moles on product side → ΔS > 0.

Worked example: 2H₂(g) + O₂(g) → 2H₂O(l): 3 gas → 0 gas → ΔS < 0.

Key takeaways

✦Gases have the most entropy.
✦Count gas moles.
✦Dissolving usually raises S.

Quick check

Did it stick?

1.ΔS for CaCO₃(s) → CaO(s) + CO₂(g)?

2.Highest entropy?

3.N₂ + 3H₂ → 2NH₃ has ΔS…