The common ion effect is the change in the position of a chemical equilibrium when an ion already involved in that equilibrium is added from another source. So it commonly causes a weak electrolyte to ionize less or a sparingly soluble salt to become less soluble. Understanding the common ion effect is important for predicting precipitation, calculating pH, preparing buffers, and controlling chemical reactions.
Introduction to the Common Ion Effect
A common ion is an ion that appears in more than one substance within the same chemical system. As an example, if silver chloride is present in water, it establishes an equilibrium involving silver ions and chloride ions:
[ \text{AgCl(s)} \rightleftharpoons \text{Ag}^+(aq) + \text{Cl}^-(aq) ]
If sodium chloride is added, it supplies additional chloride ions:
[ \text{NaCl(aq)} \rightarrow \text{Na}^+(aq) + \text{Cl}^-(aq) ]
Chloride is therefore a common ion because it participates in both the sodium chloride and silver chloride systems. Practically speaking, adding extra chloride causes the silver chloride equilibrium to shift toward the solid. Which means more AgCl precipitates, and the concentration of dissolved silver ions decreases But it adds up..
This behavior is explained by Le Châtelier’s principle, which states that a system at equilibrium responds to a disturbance by shifting in a direction that reduces that disturbance.
How the Common Ion Effect Works
Chemical equilibrium is dynamic. Reactants continue forming products, while products simultaneously form reactants, but their concentrations remain constant when the forward and reverse reaction rates are equal Worth knowing..
When the concentration of one ion increases, the equilibrium must adjust so that the equilibrium constant remains valid. The value of the constant does not change simply because more ions were added. Instead, the concentrations
