Which of the following reactions will have a negative value of ∆S?

Question

(i) Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

(ii) 2 Na(s) + 2 H₂O(l) → 2 NaOH(aq) + H₂(g)

(iii) 2 C₂H₆(g) + 7 O₂(g) → 4 CO₂(g) + 6 H₂O(l)

answer is i and iii

Why is (iii) negative

Answer 1

∆S signifies the change in ENTROPY.

(i) Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s) ... ∆S is negative because it goes from aqueous to solid

(ii) 2 Na(s) + 2 H₂O(l) → 2 NaOH(aq) + H₂(g) ... ∆S is positive because it goes from s and l to a gas

(iii) 2 C₂H₆(g) + 7 O₂(g) → 4 CO₂(g) + 6 H₂O(l) ... ∆S is negative because it goes from 9 moles of gas to 4 moles of gas