JAMB Chemistry 2019 Objective — Question 8

Question 8 of 40 from the Joint Admissions and Matriculation Board (JAMB) Chemistry 2019 Objective paper, with the correct answer and a full explanation.

In which of the following is the change in entropy positive?

A. H₂(g) + F₂(g) → 2HF(g)
B. 2NO₂(g) → N₂O₄(g)
C. H₂O(l) → H₂O(g)Correct
D. N₂(g) + 3H₂(g) → 2NH₃(g)

Explanation

Entropy increases in the order solid → liquid → gas. A liquid changing to a gas (H₂O(l)→H₂O(g)) increases disorder, giving ΔS = +ve. The other reactions either decrease the number of gas moles or keep the state constant, giving ΔS = −ve or near zero.

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