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Advanced Placement Chemistry

Lesson Plan #7

1 class period

Precipitation Reactions

Reactions that result in the formation of an insoluble product are known as precipitation reactions. A precipitate is an insoluble solid formed by a reaction in solution.

Solubility Guidelines for Ionic Compounds

The solubility of a substance is the amount of that substance that can be dissolved in a given amount of solvent. Only 1.2 X 10-3 mol of PbI2 dissolves in a liter of water at 25¡ C. In our discussions any substance with solubility of less than 0.01 mol/L will be referred to as insoluble. [PbI2 has a very low solubility in water.] In those cases the attraction between the oppositely charged ions in the solid is too great for the water molecules to separate them to any significant extent, and the substance remains largely undissolved.

All common ionic compounds of the alkali metal ions (group 1A of the periodic table) and of the ammonium ion, NH4+, are soluble in water.

Solubility Guidelines for Common Ionic Compounds in Water

Soluble Compounds containing

Important Exceptions

NO3-

None

C2H3O2-

None

Cl-

Compounds of Ag+, Hg22+, and Pb2+

Br-

Compounds of Ag+, Hg22+, and Pb2+

I-

Compounds of Ag+, Hg22+, and Pb2+

SO42-

Compounds of Sr2+, Ba2+, Hg22+, and Pb2+

Insoluble Compounds containing

Important Exceptions

S2-

Compounds of NH4+, the alkali metal cations, and Ca2+, Sr2+, and Ba2+

CO32-

Compounds of NH4+, and the alkali metal cations

PO43-

Compounds of NH4+, and the alkali metal cations

OH-

Compounds of the alkali metal cations, and Ca2+, Sr2+, and Ba2+

Exchange (Metathesis) Reactions

Reactions in which positive ions and negative ions appear to exchange partners conform to the following general equation:

AX + BY ß AY + BX

Such reactions are known as exchange reactions or metathesis reactions. Precipitation reactions conform to this pattern.

Ionic Equations

Let's consider the precipitation reaction between Pb(NO3)2 and 2KI.

Pb(NO3)2(aq) + 2KI(aq) ß PbI2(s) + 2KNO3(aq)

An equation written in this fashion, showing the complete chemical formulas of the reactants and products, is called a molecular equation, because it shows the chemical formulas of the reactants and products without indicating their ionic character.

Now consider this reaction:

Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq) ß

PbI2(s) + 2K+(aq) + 2NO3-(aq)

An equation written in this form, with all soluble strong electrolytes shown as ions, is known as a complete ionic equation.

Notice that K+(aq) and NO3-(aq) appear on both sides of the equation. Ions that appear in identical forms among both the reactants and products of a complete ionic equation are called spectator ions. They are present but play no direct role in the reaction. When spectator ions are omitted from the equation, we are left with the net ionic equation.

Pb2+(aq) + 2I-(aq) ß PbI2(s)

The ability to write net ionic equations is an important one. The following steps summarize the procedure:

  1. Write a balanced molecular equation for the reaction.

  2. Rewrite the equation to show the ions that form in solutions when each soluble strong electrolyte dissociates or ionizes into its component ions. Only dissolved strong electrolytes are written in ionic form.

  3. Identify and cancel spectator ions that occur on both sides of the equation.