Common Ion Effect
The common ion effect refers to the shift in the equilibrium position of a chemical reaction when an ion already present in the system is added. This phenomenon is a direct application of Le Chatelier’s Principle, which states that a system in equilibrium will adjust to minimize any disturbance. When a common ion is added, it alters the concentration of ions involved in the equilibrium, causing the system to shift either toward the reactants or products to restore balance. [1-4]
This principle is crucial for understanding how chemical equilibria respond to changes in concentration and forms the basis for many applications, such as the preparation of buffer solutions and the control of pH in industrial and biological processes.
Common Ion Effect on pH
Adding a common ion reduces the ionization of a weak acid or weak base compared to its ionization without the common ion. It inhibits the ionization by increasing the concentration of an ion that is a product of the equilibrium, thereby altering the pH of the solution. [1-6]
Weak Acid
An example of weak acid dissociation is that of acetic acid in water. Acetic acid (CH₃COOH) dissociates into acetate ions (CH₃COO⁻) and hydrogen ions (H⁺), resulting in an equilibrium between the reactants and the products.
CH₃COOH (aq) ⇌ H+ (aq) + CH₃COO− (aq)
On the other hand, sodium acetate (CH₃COONa) is an organic salt that dissolves completely in water as follows:
CH₃COONa (aq) → Na+ (aq) + CH₃COO− (aq)
The addition of sodium acetate increases the concentration of acetate ions in the solution. According to Le Chatelier’s principle, the acetate ions suppress the ionization of acetic acid, causing the equilibrium to shift to the left. It reduces the concentration of hydrogen ions (H⁺), making the solution less acidic and increasing the pH.
Weak Base
Ammonia (NH3) is a weak base and ionizes in aqueous solution as follows:
NH₃ (aq) + H₂O (l) ⇌ NH₄+ (aq) + OH− (aq)
On the other hand, ammonium chloride (NH₄Cl) is an inorganic acidic salt that dissociates as follows:
NH₄Cl (aq) → NH₄+ (aq) + Cl− (aq)
When ammonium chloride is dissolved in aqueous ammonia solution, the excess ammonium ions (NH₄⁺) from the former go on to react with the hydroxide ions (OH−), thereby suppressing the ionization of ammonia. This shift reduces the concentration of hydroxide ions (OH⁻), resulting in a decrease in pH, making the solution less basic.
Common Ion Effect on Solubility [1-6]
The common ion effect plays a significant role in the solubility of sparingly soluble salts. For instance, calcium fluoride (CaF₂) dissociates in water as follows:
CaF₂ (s) ⇌ Ca2+ (aq) + 2 F− (aq)
The equilibrium concentrations of the calcium ions (Ca²⁺) and fluoride ions (F⁻) in a saturated solution are governed by the solubility product constant (Kₛₚ).
On the other hand, sodium fluoride (NaF) is an inorganic salt that is completely soluble in water. It dissociates completely into sodium ions (Na⁺) and fluoride ions (F⁻), as shown:
NaF (aq) → Na+ (aq) + F− (aq)
Adding sodium fluoride to the calcium fluoride solution increases the concentration of fluoride ions. According to Le Chatelier’s Principle, the increase in the concentration of F⁻ causes the equilibrium to shift to the left. This shift results in the precipitation of calcium fluoride and reduces its solubility in the solution. This phenomenon is critical in processes like water treatment, where the common ion effect is used to precipitate out unwanted cations selectively.
