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In which of the following reactions, standard entropy change (ΔS°) is positive and standard Gibb’s energy change (ΔG°) decreases sharply with increasing temperature?

1. C(graphite) + 1/2 O₂(g) → CO(g)
2. CO(g) + 1/2 O₂(g) → CO₂(g)
3. Mg(s) + 1/2 O₂(g) → MgO(s)
4. 1/2 C(graphite) + 1/2 O₂(g) → 1/2 CO₂(g)

Correct answer: C(graphite) + 1/2 O₂(g) → CO(g)

Solution

In option A, the reaction involves the formation of a gas (CO) from a solid (C) and a gas (O₂). This increases the randomness (entropy), making ΔS° positive. Additionally, as temperature increases, the negative TΔS° term in ΔG° becomes more significant, causing ΔG° to decrease sharply.

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