common ion effect
- Related Topics:
- chemical equilibrium
- Le Chatelier’s principle
What is the common ion effect in chemistry?
How does the common ion effect relate to Le Chatelier’s principle?
What are some applications of the common ion effect?
common ion effect, in chemistry, phenomenon in which the addition of an ion that is already present in a solution reduces the solubility of a weak electrolyte or suppresses the ionization of a weak acid or base. It is a direct application of Le Chatelier’s principle, an underlying concept of chemical equilibrium, and serves a key role in buffer systems, pH control, and salt precipitation.
According to Le Chatelier’s principle, when a system at equilibrium is disturbed by adding more of a reactant, the system will shift to counteract the change. Thus, when added to a solution, a salt or an electrolyte that shares an ion in common with a dissolved weak acid, weak base, or slightly soluble salt induces a shift toward the reactants, resulting in reduced dissociation. This can result in decreased solubility of a slightly soluble salt or, in the case of a weak acid or weak base, decreased ionization.
Examples
Solubility reduction involving the sparingly soluble salt AgCl (silver chloride). AgCl dissolves in water as AgCl(s) ⇌ Ag+(aq) + Cl−(aq). If NaCl (sodium chloride) is added to a solution of AgCl, Cl− ions are released. The additional Cl− ions from NaCl cause the equilibrium to shift left, reducing the solubility of AgCl and causing more of it to precipitate.
Suppression of weak acid ionization involving CH3COOH (acetic acid). In the presence of CH3COONa (sodium acetate), acetic acid dissociates to release CH3COO− ions. The added CH3COO− ions shift the equilibrium left, reducing the ionization of acetic acid and making the solution less acidic.
Suppression of weak base ionization involving NH3 (ammonia). When NH4Cl (ammonium chloride) is added to a solution of ammonia, NH4Cl releases NH4+ ions, shifting the equilibrium left and reducing NH3 ionization, thereby lowering the pH of the solution.
Applications
The common ion effect has various applications. For example, in precipitation reactions, it is used to selectively precipitate ions in qualitative analysis. It can also be used to maintain a stable pH in biological and chemical buffer systems. In industrial chemistry, the common ion effect can be used to control solubility and reaction rates in pharmaceutical and chemical production. It is also used in the purification of salts, where it can control ion concentrations to reduce unwanted impurities.
