Question

Which of the following reactions has the largest value of ∆S◦ ?

1. K(s) + O₂(g) → KO₂(s) 

2. BaCl₂· 2 H₂O(s) → BaCl₂(s) + 2 H₂O(g)

3. N₂(g) + 3 H₂(g) → 2 NH₃(g)

4. NH₃(g) + HCl(g) → NH₄Cl(s)

5. 2 H₂(l) + O₂(l) → 2 H₂O(g) 

Answer 1

Correct option (2) BaCl₂·2 H₂O(s) → BaCl₂(s) + 2 H₂O(g)

Explanation:

We can predict the sign and magnitude of ∆S by noting the relative order of entropy: solids (lowest) < liquids < solutions < gases (highest) and the number of moles of each type. For the reactions given we have NH₃(g) + HCl(g) → NH₄Cl(s 2 mol gas → 1 mol solid; 

∆S < 0 2 H₂(l) + O₂(l) → 2 H₂O(g)

3 mol liquid → 2 mol gas; ∆S > 0

N₂(g) + 3 H₂(g) → 2 NH₃(g) 4 mol gas → 2 mol gas; ∆S < 0

K(s) + O₂(g) → KO₂(s) 1 mol solid + 1 mol gas → 1 mol solid; ∆S < 0

BaCl₂ · 2 H₂O(s) → BaCl₂(s) + 2 H₂O(g)

1 mol solid → 1 mol solid + 1 mol gas; ∆S > 0

The greatest increase in S would be for the reaction

BaCl₂· 2 H₂O(s) → BaCl₂(s) + 2 H₂O(g)