Entropy And Spontaneity
For which of the following reactions is the entropy change (ΔS) expected to be most positive?
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About This Question
- Subject
- chemistry
- Chapter
- chemical thermodynamics
- Topic
- entropy and spontaneity
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
CaCO3(s)→CaO(s)+CO2(g)
- Principle: ΔS is most positive when the number of gaseous moles increases (gas has much higher entropy than solids or liquids).
- Analyse Each Reaction:
- CaCO₃(s) → CaO(s) + CO₂(g): Solid → Solid + Gas. Net change in gas moles = +1. ΔS is positive.
- N₂ + 3H₂ → 2NH₃: 4 mol gas → 2 mol gas. Net change = −2. ΔS is strongly negative.
- 2SO₂ + O₂ → 2SO₃: 3 mol gas → 2 mol gas. Net change = −1. ΔS is negative.
- H₂O(l) → H₂O(s): Liquid → Solid. Ordering increases. ΔS is negative.
- Conclusion: Only the decomposition of CaCO₃ produces a gaseous product from a solid reactant, giving the most positive ΔS.
- Additional Insight: Thermal decomposition reactions generally show positive entropy changes because a compact solid breaks into multiple simpler products including a gas.
This easy difficulty chemistry question is from the chapter chemical thermodynamics, covering the topic of entropy and spontaneity. It appeared in the 2025 exam.
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