Chemistry 2010 JAMB Past Questions - Question 26: In which of the following is the entropy change positive?

Correct Answer

A

Explanation

Out of the options you provided, the entropy change (ΔS) is positive in **thermal dissociation of ammonium chloride**. Here's why:

**1. Thermal dissociation of ammonium chloride:**

- This reaction involves breaking down the solid ammonium chloride (NH?Cl) into ammonia gas (NH?) and hydrogen chloride gas (HCl):
    **NH?Cl (s) -> NH? (g) + HCl (g)**

- When a solid decomposes into gases, the molecules become more spread out and have more freedom of movement, increasing the disorder and thus, **entropy (ΔS is positive)**.

**2. Reaction between an acid and a base:**

- While this reaction does lead to some product species like water (H?O), which is a liquid, the overall change in the number of particles might not be significant enough to cause a positive ΔS. Moreover, neutralization reactions often involve ionic compounds which already have high disorder, minimizing the increase in entropy.

**3. Addition of concentrated acid to water:**

- Dissolving a concentrated acid in water involves mixing two liquids, which doesn't necessarily lead to a significant increase in disorder or number of particles. Depending on the acid and its concentration, the change in entropy might be positive, negative, or negligible.

**4. Dissolution of sodium metal in water:**

- This reaction is highly exothermic and releases a large amount of heat. While the sodium metal dissolves, it reacts with water to form hydrogen gas (H?), increasing the number of gas molecules and potentially contributing to a positive ΔS. However, the overall entropy change may still be negative due to the exothermic nature and the formation of ionic species in solution.

Therefore, considering the number of particles and their freedom of movement after the reaction, the only option with a definitively positive entropy change is the **thermal dissociation of ammonium chloride**.