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Chemistry

Chemistry 17: 5-7

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Precipitation Reactions

Outline

Precipitation Reactions

Remember that we can calculate the reaction quotient Q at anytime during a reaction. Comparing the result to the equilibrium constant K, ionization constants Ka or Kb, or the solubility product value Ksp can tell us how the reaction will proceed.

To review:

Complex Ions

complexIons1

Complex ions are metal ions in solutions that form ion-dipole bonds with other molecules or ligands. All ligands have active lone pairs of electrons that can form dipole bonds with positively-charged metal ions. Water molecules, ammonia, and chloride ions are common ligands.

In the diagram, the metal aluminum cation forms a complex ion system with 6 water molecules acting as the ligands. Other examples of complex ions include pigments such as chlorophyll and beta carotene.

Compounds that do not normally dissolve in water or acid may dissolve in solutions where they can form complex ions. Silver chloride (AgCl) is an example of a solid that doesn't dissolve in water easily but will dissolve in ammonia. The dissolution reaction equation for silver chloride is

AgCl (s) ↔ Ag+ (aq) + Cl- (aq)

Ksp = 1.8 * 10-10 Note that the small value for Ksp shows that this substance dissolves very little in water.

However, silver ions form a complex with ammonia:

Ag+ (aq) + 2 NH3 (aq) ↔ [Ag(NH3)2]+ (aq)

Kf = 1.1 * 107

The net result is a much greater dissociation of silver chloride than in water:

AgCl (s) + 2 NH3 (aq) ↔ Cl- (aq) + [Ag(NH3)2]+ (aq)

Knet = Ksp * Kf = (1.8 * 10-10) * (1.1 * 107) = 2.0 * 10-3

By selecting solvents where metal ions will form complex ions, chemists can get a much greater yield of ions from an "insoluble" solid than possible with non-complex ion-forming other solvents.

Discussion Questions

Optional Readings