UNIT: 7.12 In-depth Notes on Common Ion Effect

Definition of Common Ion Effect

• The common ion effect refers to the decrease in solubility of a salt when another source of one of its ions is added to the solution.
• When either the cation or anion of the salt is already present in the solution, less of the solid salt will dissolve compared to dissolving in pure water.

Solubility and the Common Ion Effect

• The effect is rooted in Le Chatelier's principle, which states that a system at equilibrium will respond to changes in such a way that it counteracts the change.
• If a common ion is introduced, it shifts the equilibrium to favor the solid form of the salt, thereby reducing its solubility.

Example of Lead Chromate

• Lead Chromate (PbCrO₄) is a compound that dissociates into lead ions (Pb²⁺) and chromate ions (CrO₄²⁻).
• In pure water, lead chromate has a limited solubility.
• If dissolved in a solution containing lithium chromate (Li₂CrO₄), which shares the common chromate ion, less lead chromate will dissolve, leading to more solid remaining in the solution.

Conceptual Problem

• Calcium Fluoride (CaF₂) in fluoridated tap water:
  • Solubility in pure deionized water: $8.58 imes 10^{-2}$ grams per liter.
  • Adding fluoride ions through fluoridated water would lower its solubility due to the common ion effect, as the reaction shifts towards solid formation.

Ksp and Calculations

• Solubility Product Constant (Ksp) is essential for calculations related to solubility in the presence of common ions.
• Example: Dissolving calcium phosphate (Ca₃(PO₄)₂) in a $0.2$ M sodium phosphate solution alters the expected solubility.

Dissolution Reaction and ICE Table

• Calcium phosphate dissociates into $3Ca^{2+} + 2PO_4^{3-}$.
• When setting up the ICE table, consider initial concentrations:
  • $Ca^{2+}$ starts at $0$,
  • $PO_4^{3-}$ starts at $0.2$ M due to the sodium phosphate.

Ksp Expression

• For the dissociation: $K<em>{sp} = [Ca^{2+}]^3[PO</em>4^{3-}]^2$
• Substitute expressions obtained from the ICE table into the Ksp equation to solve for solubility (s).
• Example: $[Ca^{2+}] = 3s$ and $[PO_4^{3-}] = 0.2 + 2s$.

Example Calculation for Calcium Phosphate

• Calculated solubility resulted in:
  • $s = 2.29 imes 10^{-11}$ moles per liter after solving Ksp equation.
• A significant outcome is that solubility decreases when compared to dissolving in pure water (where s would be larger).

Application in a Problem Scenario

• For magnesium fluoride (MgF₂) in different environments, solubility calculations demonstrate the impact of additional fluorine ions on the dissolution balance:
  • Part A: Write the dissolution equation for MgF₂.
  • Part B: Calculate its solubility in pure water.
  • Part C: Calculate solubility after adding sodium fluoride, finding a lower solubility due to common ion effect.
  • Final calculated solubility in sodium fluoride was $3.6 imes 10^{-5}$, significantly lower than in pure water.

Conclusion

• The common ion effect plays a crucial role in predicting solubility changes when ionic substances are present. Understanding these principles can aid in solution preparation, chemical reactions, and analysis in various fields of chemistry.