Which of the following reactions would be expected to have a positive entropy change , D S > 0?
I. 3 O₂ (g) →2 O₃ (g)
II. CaCO₃ (s) →CaO (s) + CO₂ (g)
III. C₃H₈ (g) + 5 O₂ (g) →3 CO₂ (g) + 4 H₂O (g)

Question

Quizplus · Accepted Answer

For a reaction to have a positive entropy change, there must be an increase in the number of moles of gas, or a dispersal of particles or energy. 
In option I, the reactants have 3 moles of gas while the products have 2 moles of gas, so the entropy change is negative, ΔS < 0. 
In option II, the reactants have 1 solid and 1 gas while the products have 1 solid and 1 gas, so the entropy change is likely to be small or slightly negative, ΔS ≈ 0 or ΔS slightly < 0. 
In option III, the reactants have 6 moles of gas while the products have 7 moles of gas, so the entropy change is positive, ΔS > 0. Therefore, the correct answer is E (II and III).

Which of the Following Reactions Would Be Expected to Have