Double-replacement reactions

The driving force behind double-replacement reactions is the formation of a stable product. Double replacement reactions have the form

AB + CD AD + CB

The ions in compounds AB and CD switch partners.

This type of reaction will take place in aqueous solution when one of the products is either:

1. An insoluble or nearly insoluble compound, a precipitate, or
2. A covalent compound, which includes a gas, and water.

To determine whether a double-replacement reaction will proceed, use the solubility rules table to determine whether the products formed would be an insoluble solid in aqueous solution (i.e. a precipitate), a gas or water.

Formation of a precipitate: Lead(II) nitrate reacts with potassium iodide to produce a precipitate of lead(II) iodide. The ions that remain in solutions are potassium ions and nitrate ions.

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

Formation of a gas: Hydrogen chloride gas is commercially prepared by reacting sulfuric acid, H₂SO₄ (or rewritten as HHSO₄), with sodium chloride, NaCl. This is a double replacement reaction and the driving force behind this reaction is the formation of HCl (g).

HHSO₄ (aq) + NaCl (aq) HCl (g) + NaHSO₄ (aq)

Example

When a solution of sodium chloride is brought into conact with a solution of silver nitrate, would a reaction occurs?

Point to consider:

• the possible products are sodium nitrate, NaNO₃, and silver chloride, AgCl. Is either of these compounds insoluble in aqueous solution?

The solubility rules tell us that:

1. All nitrate ions are soluble. So, we would expect that NaNO3 remains in solution.
2. There are three chlorides that are insoluble. They are AgCl, Hg₂Cl₂, and PbCl₂.

Since AgCl is insoluble in aqueous solution, we will have a reaction in which the driving force is to produce NaNO₃ (aq) (which stays in solution), and AgCl (s) (which precipitates out of solution).

The balanced chemical equation is:

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

External link

Precipitation reactions

Content suitability

BCIT courses: CHEM 0011