Precipitation Reactions

Outline

• Precipitation Reactions
• Complex Ions
• Practice with Concepts
• Discussion Questions
• Optional Website Reading

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 K_a or K_b, or the solubility product value K_sp can tell us how the reaction will proceed.

To review:

• If Q <K, the reaction product concentrations are less than they will be at equilibrium and the reactant concentrations are greater than they will be at equilibrium, so the reaction will run forward.
• If Q = K, the reaction reactant and product concentrations are equal to what they will be at equilibrium, so the reaction "stops" at the macroscopic level, or (to look at it another way), the forward and reverse reactions are proceeding at identical rates.
• If Q > K, the reaction product concentrations are greater than they will be at equilibrium and the reactant concentrations are less than they will be at equilibrium, so the reaction will run in reverse.
• If Q <K_a for an acid dissociation reaction, the acid will continue to dissociate (donate H+ ions) until Q = K_a.
• If Q <K_b for a base reaction, the base will continue to fuse with available H+ ions until Q = K_b.
• If Q <K_sp, the reaction has not yet reached saturation, and any solid present will continue to dissolve until Q = K_sp and the solution is saturated. If Q > K_sp, ions present will precipitate to form solid, until the concentration of ions is reduced and Q = K_sp.

Complex Ions

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)

K_sp = 1.8 * 10^-10 Note that the small value for K_sp 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)

K_f = 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)

K_net = K_sp * K_f = (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

• How might you separate a precipitate consisting of a mixture of solid CuS and solid Cu(OH)₂?

Optional Readings

• Read more about complex ions on the ChemGuide Introduction to Complex Metal Ions page.

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