Double Replacement

A double-replacement reaction is one in which two different compounds exchange positive ions and form two new compounds. The general form of a double replacement reaction is

$\mathrm{AB}+\mathrm{CD}\to \mathrm{AD}+\mathrm{CB}$

Notice that two replacements take place in this reaction. Not only is A replacing C, but C is also replacing A.

Solutions of lead(II) nitrate, Pb(NO₃)₂, and potassium iodide, KI, are both colorless. However, when these solutions are mixed, as shown in Figure 13, a yellow precipitate forms as a result of a double-replacement reaction. The equation for this reaction is

$\mathrm{Pb}({\mathrm{NO}}_3{)}_2 + 2\mathrm{Kl} \to {\mathrm{Pbl}}_{2 }+ 2{\mathrm{KNO}}_3$

The lead ions in Pb(NO₃)₂, trade places with the potassium ions in KI. The products are lead(II) iodide, PbI₂, which precipitates out of solution, and potassium nitrate, KNO₃, which remains in solution.

When geologists test the calcium carbonate content in a rock, they make use of the following double-replacement reaction.

${\mathrm{CaCO}}_{3 }+ 2\mathrm{HCL} \to {\mathrm{CaCl}}_2 + {\mathrm{H}}_2{\mathrm{CO}}_3$

One of the products of this reaction is calcium chloride, CaCl₂. The other product is carbonic acid, H₂CO₃, which decomposes into carbon dioxide gas and water.

${\mathrm{CaCO}}_{3 }+ 2\mathrm{HCL} \to {\mathrm{CaCl}}_2 + {\mathrm{H}}_2{\mathrm{CO}}_3$

Figure 13 When potassium iodide solution is poured into a solution of lead(II) nitrate, a double-replacement reaction takes place. Lead(II) iodide forms as a yellow precipitate.

Comparing and Contrasting How does a double-replacement reaction differ from a single-replacement reaction?

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