17-3: The Common-Ion Effect

Recall: Le Chatlier’s Principle: If a change is imposed on a system that is at equilibrium, then the reaction will shift to the side that minimizes the change. In solubility chemistry, this is called the common-ion effect:

Common-ion effect: a shift in equilibrium that occurs because the concentration of an ion that is part of the equilibrium is changed.

EX: CaSO 4 (s) Ca +2 (aq) + SO 4 -2 (aq) Adding NaSO 4 increases SO 4 ion, rxn shifts left, by precipitating CaSO 4.

EX: AgCl Ag+ (aq) + Cl - (aq) Adding NaCl increases Cl ions, rxn shifts left, allowing AgCl to precipitate out.