Precipitation reactions

Not all ionic compounds are soluble in water. If PbSO₄ is added to water, none of the lead(II)sulfate will dissolve. The interaction between Pb²⁺ and SO₄^2- is simply stronger than the attraction of the water to either the Pb²⁺ or SO₄^2- (Both enthalpy and entropy play a roll. We will discuss entropy and lattice energy second semester. We will discuss enthalpy and bond energy later this semester).

Simple Rules for the Solubility of Salts in Water

1. Most nitrate (NO₃^-) salts are soluble.
2. Most salts containing the alkali metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Rb⁺) and the ammonium ion (NH₄⁺) are soluble.
3. Most chloride, bromide and iodide salts are soluble. Notable exceptions are salts containing the ions Ag⁺, Pb²⁺, and Hg₂²⁺.
4. Most sulfate salts are soluble. Notable exceptions are BaSO₄, PbSO₄, HgSO₄, and CaSO₄.
5. Most hydroxide salts are only slightly soluble. The important soluble hydroxides are NaOH and KOH. The compounds Ba(OH)₂, Sr(OH)₂, and Ca(OH)₂ are marginally soluble.
6. Most sulfide (S^-2), carbonate (CO₃^2-), chromate (CrO₄^2-), and phosphate (PO₄^-3) are only slightly soluble.

If two solutions are mixed together it is possible that two ions could combine to form an insoluble ionic complex.

A solution of silver nitrate is combined with a solution of sodium chloride. The resulting solution contains Na⁺, Ag⁺, Cl^-, and NO₃^-, but AgCl is not soluble in water. Since Ag⁺ is now in solution with Cl^- the two will combine to form AgCl, and the AgCl will precipitate from solution.

The reaction can be described in a number of ways

1. A "molecular " equation could be used:
2.   A "complete ionic " equation could be used:

   

3. A "net ionic " equation could be used.

   Net ionic equations are found by writing the full equation and then eliminating the spectator ions; spectator ions are ions that do not participate in the reaction. In the example above Na⁺(aq) and NO₃^- are present as both products and reactants. Since Na⁺(aq) and NO₃^- do not participate in the reaction they are called spectator ions.

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