Advanced Placement Chemistry
Lesson Plan #7
1 class period
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.
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.
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: