In which of the following is the entropy change positive?

Correct answer: Thermal dissociation of ammonium chloride.

• Thermal dissociation of ammonium chloride: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). A solid producing two gases greatly increases disorder ⇒ ΔS > 0.
• Reaction between an acid and a base: H⁺(aq) + OH⁻(aq) → H₂O(l). Fewer, more ordered species in solution ⇒ typically ΔS < 0.
• Addition of concentrated acid to water: Strong hydration/ordering of ions outweighs mixing ⇒ usually ΔS ≤ 0.
• Dissolution of sodium metal in water: Although H₂(g) forms (which tends to raise entropy), competing hydration/ordering effects make this less clear-cut; the syllabus’ unambiguous positive-entropy example is the NH₄Cl dissociation above.