One common type of reaction that occurs in aqueous solution is the precipitation reaction, which results in the formation of an insoluble product, or precipitate. A precipitate is an insoluble solid that separates from the solution. Precipitation reactions usually involve ionic compounds. For example, when an aqueous solution of lead(II) nitrate [Pb(NO3)2] is added to an aqueous solution of potassium iodide (KI), a yellow precipitate of lead iodide (PbI2) is formed:
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
Potassium nitrate remains in solution. Figure 1 shows this reaction in progress. The preceding reaction is an example of a metathesis reaction (also called a double displacement reaction), a reaction that involves the exchange of parts between two compounds. (In this case, the compounds exchange the NO3- and I- ions.) As we will see, the precipitation reactions discussed in this chapter are examples of metathesis reactions.
Figure 1 Formation of yellow PbI2 precipitate as a solution of Pb(NO3)2 is added to a solution of KI.
How can we predict whether a precipitate will form when a compound is added to a solution or when two solutions are mixed? It depends on the solubility of the solute, which is defned as the maximum amount of solute that will dissolve in a given quantity of solvent at a specif c temperature. Chemists refer to substances as soluble, slightly soluble, or insoluble in a qualitative sense. A substance is said to be soluble if a fair amount of it visibly dissolves when added to water. If not, the substance is described as slightly soluble or insoluble. All ionic compounds are strong electrolytes, but they are not equally soluble. Table 1 classif es a number of common ionic compounds as soluble or insoluble. Keep in mind, however, that even insoluble compounds dissolve to a certain extent. Figure 2 shows several precipitates.
Table 1 Solubility Rules for Common Ionic Compounds in Water at 25˚C
Molecular Equations, Ionic Equations, and Net Ionic Equations
The equation describing the precipitation of lead iodide on page 100 is called a molecular equation because the formulas of the compounds are written as though all species existed as molecules or whole units. A molecular equation is useful because it identifes the reagents (that is, lead nitrate and potassium iodide). If we wanted to bring about this reaction in the laboratory, we would use the molecular equation. However, a molecular equation does not describe in detail what actually is happening in solution.
As pointed out earlier, when ionic compounds dissolve in water, they break apart into their component cations and anions. To be more realistic, the equations should show the dissociation of dissolved ionic compounds into ions. Therefore, returning to the reaction between potassium iodide and lead nitrate, we would write