JAMB Chemistry 2010 Paper

Concept:Entropy, $\Delta S$, measures the degree of disorder in a system. A positive entropy change occurs when a process produces more particles or more gaseous particles.Explanation:Thermal dissociation of ammonium chloride is represented as:$$\mathrm{NH_4Cl(s) \rightarrow NH_3(g) + HCl(g)}$$A single solid gives two gaseous products. This greatly increases disorder, so $\Delta S > 0$.An acid–base reaction combines ions into fewer particles, so it does not show a clear positive entropy change.Adding concentrated acid to water mainly involves hydration of ions, which increases order.In dissolving sodium metal in water, hydrated ions are formed, and hydration lowers the entropy of the water molecules.Therefore, the clear positive entropy change is from the thermal dissociation of ammonium chloride.Answer:A. Thermal dissociation of ammonium chloride