Why does adding sodium chloride to a saturated solution of lead chloride reduce the solubility of #PbCl_2#?

Question

Luna Chen · Accepted Answer

This is a standard problem in solubility equilibria; the solubility of #PbCl_2# depends on the concentration of ions FROM ALL SOURCES.

The solubility expression can be expressed as follows: #PbCl_2(s) rightleftharpoons Pb^(2+) + 2Cl^-#, is the reaction, AND #K_(sp) = [Pb^(2+)][Cl^-]^2# is the solubility expression; #sp# stands for solubility product, and #K_(sp)# has been measured for a variety of salts at various temperatures. The solubility expression depends on just the concentration of the lead and chloride ions; from where the ions come makes no difference. So if #[Cl^-]# is artificially raised (by adding sodium chloride), it logically follows that #[Pb^(2+)]# must be correspondingly reduced in order for the solubility expression to be obeyed. The only way for #[Pb^(2+)]# to be reduced is for more #PbCl_2# to precipitate. The same effect would pertain if I added lead nitrate, a soluble lead salt. #[Pb^(2+)]# would increase, and how would #[Cl^-]# evolve? Such a phenomenon is known as #"salting out"#. It pushes the solubility equilibrium to the left hand side as WRITTEN. IF you were trying to isolate a precious metal, i.e. gold or platinum or rhodium, you want to salt out the precious metal salt and leave little of its ions in solution. #"MORE ADVANCED TREATMENT,"# #"2nd year analytical chemistry:"# Given what I have said above, it might seem that I could reduce #[Pb^(2+)]# to any desired level, simply by ramping up the concentration of chloride/halide ion. At very high concentrations of halide, however, soluble complex ions of lead may occur, i.e. #[PbX_4]^(2-)#. For simple solubility equilibria, however, the formation of these complex ions may be ignored.

Cooper Adams · Answer

undefined Adding sodium chloride to a saturated solution of lead chloride reduces the solubility of PbCl2 because of the common ion effect. Sodium chloride provides chloride ions, which are also present in lead chloride. The increased concentration of chloride ions shifts the equilibrium of the dissolution reaction of lead chloride towards the solid phase, thereby decreasing its solubility.