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Precipitation Reactions

Precipitate - an insoluble solid formed by a reaction in solution.

Consider the following molecular equation:

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

The net ionic equation of the above molecular equation is:

Ag+(aq) + Cl-(aq) AgCl(s)

In order to know which ions will combine to form precipitates, we have the solubility rules

Solubility Rules

  1. All salts containing NH4+ and group IA cations (Li+, Na+, K+, Rb+, Cs+, ) are soluble.
  2. All salts containing NO3-, C2H3O2-, HClO3-, and ClO4- are soluble.
  3. All salts containing Cl-, Br-, and I- are soluble, except those with Ag+, Hg22+, and Pb2+
  4. All salts containing SO42- are soluble, except PbSO4, BaSO4, HgSO4, CaSO4, and AgSO4.
  5. Most salts containing O2-, OH-, PO43-, CO32-, and S2- are insoluble, except those containing NH4+ and group IA cations.

Determine the net ionic equation for the following reaction:

Ba(NO3)2 + Na2SO4 BaSO4 + 2NaNO3

Rule number 4 tells us that BaSO4 is insoluble. So we can write the complete ionic equation as

Ba2+(aq)+2 NO3-(aq) +2 Na+(aq)+SO42-(aq) BaSO4(s)+2 Na+(aq)+NO3-(aq)

Removing the spectator ions leaves us with the net ionic equation:

Ba2+(aq) + SO42-(aq) BaSO4(s)

Homework from Chemisty, The Central Science, 10th Ed.

4.19, 4.21, 4.23, 4.25, 4.27, 4.39, 4.41, 4.43, 4.45, 4.47, 4.49, 4.51, 4.53, 4.55, 4.57