When the ionic compound (NH4)2SO3 is dissolved in water, which species will react with water to a considerable extent? You can assume the compound is completely soluble in water.
anion
cation
both
neither
The Correct Answer and Explanation is :
Correct Answer: Cation (NH₄⁺)
Explanation:
When the ionic compound (NH₄)₂SO₃ (ammonium sulfite) dissolves in water, it dissociates completely into its constituent ions:
[
(NH_4)_2SO_3 (s) \rightarrow 2 NH_4^+ (aq) + SO_3^{2-} (aq)
]
Now, let’s analyze whether these ions react significantly with water.
1. Ammonium Ion (NH₄⁺)
The ammonium ion (NH₄⁺) is the conjugate acid of ammonia (NH₃), which is a weak base. In water, NH₄⁺ undergoes hydrolysis to a considerable extent:
[
NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+
]
This reaction releases hydronium ions (H₃O⁺), making the solution slightly acidic. Since this reaction is significant and contributes to the pH of the solution, the ammonium ion does react with water to a considerable extent.
2. Sulfite Ion (SO₃²⁻)
The sulfite ion (SO₃²⁻) is the conjugate base of sulfurous acid (H₂SO₃), which is a weak acid. It can undergo hydrolysis in water:
[
SO_3^{2-} + H_2O \rightleftharpoons HSO_3^- + OH^-
]
However, the extent of this reaction is less significant compared to NH₄⁺ because H₂SO₃ is a weak acid, and SO₃²⁻ is not highly reactive in water. Although it can slightly increase the pH, the effect is not as strong as NH₄⁺’s effect in making the solution acidic.
Conclusion:
Since NH₄⁺ hydrolyzes significantly, whereas SO₃²⁻ does so only weakly, the correct answer is cation (NH₄⁺).