Unit #8 CHEMICAL REACTIONS

8.4 - Types of Reactions

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

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.

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)
click here for a detailed look at this equation

The other driving force behind double replacement reactions is when one of the products is a precipitate.

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)
click here for a detailed look at this equation

Use the solubility rules to determine whether the products formed would be insoluble in aqueous solution.

The table of Solubility rules can be found by clicking on the a chemist's tools.
Predict a reaction:

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)

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Section 9.1
Double-Replacement Reactions ..p230


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