Question

Arrange the following reactions in order of increasing ΔS° value.
1. H₂(g) + F₂(g) → 2HF(g)
2. NH₄NO₃(s) → N₂O(g) + 2H₂O(l)
3. (NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + 4H₂O(l) + N₂(g)

	A.	1 < 2 < 3
	B.	2 < 1 < 3
	C.	3 < 1 < 2
	D.	1 < 3 < 2
	E.	2 < 3 < 1

Answer 1

Ans :- Option (A) i.e. 1 < 2 < 3 is the correct answer.

Explanation :-

Increase in entropy means in increase in degree randomness or disordered . The change in entropy increases when a solid converted to liquid or gas. Also change in entropy increases when number of moles of products greater than the reactants because ΔS° = S°_products - S°_reactants .

here, S°_products = Entropy of products

S°_reactants = Entropy of reactants

(1). H₂(g) + F₂(g) → 2HF(g)

2 moles of gases i.e. H₂(g) and F₂(g) giving 2 moles of HF gas.

(2). NH₄NO₃(s) → N₂O(g) + 2H₂O(l)

1 mole giving 3 moles conversion is from solid to liquid and gas

(3). (NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + 4H₂O(l) + N₂(g

1 mole giving 6 moles of product and it converts to liquid and gas too

Hence Option (A) is the correct answer.