Answer
🧪 Entropy Change (ΔS) — Which Process Decreases Entropy?
🔍 Concept Recap
Entropy (ΔS) measures the degree of disorder in a system. A process has ΔS < 0 when the system becomes more ordered — such as:
- Gas → Liquid or Solid
- Multiple particles → fewer particles
- Formation of a solid from ions
❌ Options with ΔS > 0 (Entropy Increases)
Dry ice sublimes — CO₂(s) → CO₂(g) → Solid to gas transition ⟶ Entropy increases (ΔS > 0)
H₂O(l) → H₂O(g) — Liquid to gas transition ⟶ Entropy increases (ΔS > 0)
A cup of coffee is heated — Temperature increase adds energy → more molecular motion → ΔS > 0
4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) — Reactants: 9 moles of gas; Products: 10 moles of gas ⟶ ΔS > 0
✅ Correct Option with ΔS < 0
Pb²⁺(aq) + 2F⁻(aq) → PbF₂(s) — Two solvated ions become one solid precipitate ⟶ Entropy decreases (ΔS < 0)
This process results in the system becoming more ordered (solid formation from aqueous ions).
🧠 Final Answer:
Pb²⁺(aq) + 2F⁻(aq) → PbF₂(s)