Common Ion Effect

Different ionic compounds can release the same ions into solution. This ultimately means that different ionic compounds can affect the equilibrium of other ionic compounds.

Common ion effect: The solubility of a salt can be reduced by the presence of another salt that has a common ion.

Example: if NaCl is dissolved and HCl_(aq) is added to the solution. This introduces chlorine ions and forces the equilibrium to the left due to Le Châtelier’s Principle, producing more NaCl_(s).

Practice: If KBr is added to a solution containing PbBr_2(aq), will more PbBr₂ solidify? Yes

If KBr is added to a solution containing NaCl, will more NaCl precipitate? No, no common ion

Example: How many moles of PbCl_2(s) will dissolve in 1L of 0.2M NaCl at SATP?

Common ion = Cl^-1

[Cl^-1]=[NaCl]=0.2M

Since there are no lead ions to start, Q=0 and the rxn shifts to the right

I     –           0          0.2

C    –         +x         +2x

E    –           x       0.2+2x