Entropy And Spontaneity

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For which of the following reactions is the entropy change (ΔS) expected to be most positive?

RankGuru · Accepted Answer

1. **Principle**: ΔS is most positive when the number of gaseous moles increases (gas has much higher entropy than solids or liquids).
2. **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.
3. **Conclusion**: Only the decomposition of CaCO₃ produces a gaseous product from a solid reactant, giving the most positive ΔS.
4. **Additional Insight**: Thermal decomposition reactions generally show positive entropy changes because a compact solid breaks into multiple simpler products including a gas.

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Entropy And Spontaneity

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