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Difficulty: Very hardSalt Hydrolysis and Solution Acidity/Alkalinity

When potassium sulfite (K2SO3K_2SO_3) is dissolved in distilled water at 25∘C25^\circ\text{C}, the solution turns red litmus paper blue. Which of the following net ionic equations correctly represents the hydrolysis reaction responsible for the basic nature of this solution?

  1. SO32−(aq)+H2O(l)⇌HSO3−(aq)+OH−(aq)SO_3^{2-}(aq) + H_2O(l) \rightleftharpoons HSO_3^-(aq) + OH^-(aq)Answer
  2. B
    K+(aq)+H2O(l)⇌KOH(aq)+H+(aq)K^+(aq) + H_2O(l) \rightleftharpoons KOH(aq) + H^+(aq)
  3. C
    SO32−(aq)+2H2O(l)⇌H2SO3(aq)+2OH−(aq)SO_3^{2-}(aq) + 2H_2O(l) \rightleftharpoons H_2SO_3(aq) + 2OH^-(aq)
  4. D
    2K+(aq)+SO32−(aq)+H2O(l)⇌2KOH(aq)+SO2(g)2K^+(aq) + SO_3^{2-}(aq) + H_2O(l) \rightleftharpoons 2KOH(aq) + SO_2(g)

Answer

The net ionic equation representing the hydrolysis of potassium sulfite is SO32−(aq)+H2O(l)⇌HSO3−(aq)+OH−(aq)SO_3^{2-}(aq) + H_2O(l) \rightleftharpoons HSO_3^-(aq) + OH^-(aq), which generates hydroxide ions and causes the solution to be alkaline.
Potassium sulfite (K2SO3K_2SO_3) dissociates in water into K+K^+ and SO32−SO_3^{2-} ions. Because KOHKOH is a strong base, the K+K^+ ion does not react with water. However, sulfurous acid (H2SO3H_2SO_3) is a weak acid, meaning its conjugate anion (SO32−SO_3^{2-}) acts as a Bronsted-Lowry base. It hydrolyzes by removing a proton from water to form HSO3−HSO_3^- and OH−OH^-. The excess hydroxide ions render the solution alkaline, turning red litmus paper blue.

Step-by-Step Solution

1
Identify the parent acid and parent base of the salt K2SO3K_2SO_3.
Potassium sulfite (K2SO3K_2SO_3) is formed from a strong base (KOHKOH) and a weak diprotic acid (H2SO3H_2SO_3).
Determining the strength of parent species dictates which ion undergoes hydrolysis in aqueous solution.
2
Determine which constituent ion undergoes hydrolysis in water.
Potassium ions (K+K^+) do not hydrolyze because they are cations of a strong base. Sulfite ions (SO32−SO_3^{2-}), being conjugate bases of a weak acid, undergo anion hydrolysis.
Conjugate bases of weak acids are sufficiently strong to abstract protons from water molecules.
3
Write the net ionic proton-transfer equation for anion hydrolysis.
SO32−(aq)+H2O(l)⇌HSO3−(aq)+OH−(aq)SO_3^{2-}(aq) + H_2O(l) \rightleftharpoons HSO_3^-(aq) + OH^-(aq)
The sulfite anion abstracts one proton (H+H^+) from water, yielding hydrogen sulfite (HSO3−HSO_3^-) and releasing a free hydroxide ion (OH−OH^-), which increases solution pH above 7.

Key Concept

Salt Hydrolysis of Weak Acid-Strong Base Salts
Estimated Time:1m 30s
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Topics

Salt Hydrolysis and Solution Acidity/Alkalinity

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