In which of the following is the entropy change positive?

Analyze the question: Entropy (S) measures disorder. The entropy change is positive (ΔS > 0) when a system becomes more disordered, for example when a solid turns into gases, or when the number of gas particles increases.

Concept: ΔS is positive when:

• a solid or liquid changes to a gas
• one substance breaks into several substances
• the number of moles of gas increases

Solution, option by option:

A. Thermal dissociation of ammonium chloride NH₄Cl(s) ⇌ NH₃(g) + HCl(g) One ordered solid becomes two gases. Disorder increases greatly, so ΔS is positive.

B. Acid-base reaction H⁺(aq) + OH⁻(aq) → H₂O(l) Several free ions combine into one liquid molecule, so disorder decreases. ΔS is negative, or at best small.

C. Adding concentrated acid to water This is mainly dilution and mixing, which is an exothermic process. The ions become hydrated and ordered with water molecules, so the change is not clearly an increase in disorder. It is not the best choice.

D. Dissolving sodium in water 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) This does release hydrogen gas, so ΔS could also be argued as positive. However, the metal also forms hydrated ions, and it is not the standard textbook example. Option A is the clearest case: a solid fully decomposes into gases.

Option D is arguable, but A is the cleanest and most definite example of a positive entropy change.

Answer: A

Exam tip: When asked about positive entropy, look for a solid or liquid turning into gas, or one reactant splitting into several products. Thermal decomposition and sublimation are classic examples.