The Common Ion Effect: Impact on Solubility Equilibria

Expert reviewed • 22 November 2024 • 4 minute read

The addition of a common ion to a solution can significantly affect its equilibrium state and solubility. This principle, known as the common ion effect, is a direct application of Le Châtelier's principle and plays a crucial role in understanding solution equilibria.

Understanding the Common Ion Effect

The common ion effect occurs when an ionic compound containing an ion already present in an equilibrium system is added to that system. This addition shifts the equilibrium position according to Le Châtelier's principle, typically resulting in decreased solubility of the original compound.

Key Principles

  • When a common ion is added, the equilibrium shifts to reduce the effect of the added ion
  • The solubility product constant (KspK_{sp}) remains constant despite changes in ion concentrations
  • The solubility of the original compound decreases when a common ion is added

Examples in Practice

1. Barium Sulfate Equilibrium

Consider the equilibrium of barium sulfate:

BaSO4(s)⇌Ba2+(aq)+SO42−(aq)\text{BaSO}_4(s) \rightleftharpoons \text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq)

When sodium sulfate (Na2SO4\text{Na}_2\text{SO}_4) is added to this saturated solution:

  • The concentration of SO42−\text{SO}_4^{2-} increases
  • The equilibrium shifts left to counteract this change
  • More solid BaSO4\text{BaSO}_4 precipitates
  • The concentration of Ba2+\text{Ba}^{2+} decreases

2. Lead(II) Chromate System

For lead(II) chromate in equilibrium:

PbCrO4(s)⇌Pb2+(aq)+CrO42−(aq)\text{PbCrO}_4(s) \rightleftharpoons \text{Pb}^{2+}(aq) + \text{CrO}_4^{2-}(aq)

Ksp=[Pb2+][CrO42−]=2.3×10−13K_{sp} = [\text{Pb}^{2+}][\text{CrO}_4^{2-}] = 2.3 \times 10^{-13}

Adding sodium chromate (Na2CrO4\text{Na}_2\text{CrO}_4):

  • Increases CrO42−\text{CrO}_4^{2-} concentration
  • Shifts equilibrium left
  • Decreases Pb2+\text{Pb}^{2+} concentration
  • Reduces overall solubility

Summary

  • Adding a common ion decreases solubility
  • Removing a common ion increases solubility
  • The KspK_{sp} value remains constant regardless of common ion concentration changes

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