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: CaSO4(s) Ca+2 (aq) + SO4 -2 (aq)

Adding NaSO4 increases SO4 ion, rxn shifts left, by precipitating CaSO4.

EX: AgCl Ag+ (aq) + Cl- (aq)

Adding NaCl increases Cl ions, rxn shifts left, allowing AgCl to precipitate out.