Precipitation Reactions in Aqueous Solution

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.

Solubility

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.

Figure 2 Appearance of several precipitates. From left to right: CdS, PbS, Ni(OH)₂, Al(OH)₃.

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

Pb²⁺(aq) + 2NO₃^- (aq) + 2K⁺(aq) + 2I^-(aq) → PbI₂(s) + 2K⁺(aq) + 2NO₃^- (aq)

The preceding equation is an example of an ionic equation, which shows dissolved species as free ions. To see whether a precipitate might form from this solution, we first combine the  cation and anion from different compounds; that is, PbI₂ and KNO₃. Referring to Table 1, we see that PbI₂ is an insoluble compound and KNO₃ is soluble. Therefore, the dissolved KNO₃ remains in solution as separate K⁺ and NO₃^- ions, which are called spectator ions, or ions that are not involved in the overall reaction. Because spectator ions appear on both sides of an equation, they can be eliminated from the ionic equation 

Finally, we end up with the net ionic equation, which shows only the species that actually take part in the reaction:

Pb²⁺(aq) + 2I^-(aq) → PbI₂(s)

Looking at another example, we find that when an aqueous solution of barium chloride (BaCl₂) is added to an aqueous solution of sodium sulfate (Na₂SO₄), a white precipitate is formed (Figure 3). Treating this as a metathesis reaction, the products are BaSO₄ and NaCl. From Table 1 we see that only BaSO₄ is insoluble.

Figure 3 Formation of BaSO₄ precipitate.

Therefore, we write the molecular equation as

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

The ionic equation for the reaction is Canceling the spectator ions (Na⁺ and Cl^-) on both sides of the equation gives us the net ionic equation

Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

The following four steps summarize the procedure for writing ionic and net ionic equations:

1.      Write a balanced molecular equation for the reaction, using the correct formulas for the reactant and product ionic compounds. Refer to Table 1 to decide which of the products is insoluble and therefore will appear as a precipitate.

2.      Write the ionic equation for the reaction. The compound that does not appear as the precipitate should be shown as free ions.

3.      Identify and cancel the spectator ions on both sides of the equation. Write the net ionic equation for the reaction.

4.      Check that the charges and number of atoms balance in the net ionic equation.

Reference

Chang, R and Overby, J. (2003). General Chemistry: The Essential Concept. 6th .ed. The McGraw-Hill Companies: New York.