Double Replacement Reactions

In a double replacement reaction, two aqueous solutions react to form a solid precipitate.  Two sets of cations (positive ions) and anions (negative ions) switch places.  One solid precipitate forms, and the other ions do not "stay together."  They remain an aqueous solution.

Compound + Compound ---> Precipitate Compound + Aqueous Compound

Solubility charts can help you to predict if a precipitate forms.  Precipitates are classified as: insoluble (I) or slightly soluble (SS).

Examples:

1.  Which of the following are precipitates?  Use the Table of Solubilities

sodium acetate, aluminum hydroxide, magnesium carbonate

Right!  The ones in blue are solid precipitates because they are insoluble.  Solids are insoluble in water!

2.  CaCl_2(aq)  +   AgNO_3(aq)  --->  ???

• First, recognize that this is a double replacement reaction:  Compound + Compound

• Then, write out the equation in words:

calcium chloride + silver nitrate  --->  calcium nitrate  +  silver chloride

• Now use your ion chart to write correct formulas for each of the compounds.  Remember to "criss-cross" charges so that all of the compounds are neutral.

CaCl₂ + AgNO₃ --->  Ca(NO₃)₂ +  AgCl

• Finally, add coefficients to balance the equation:

CaCl₂ + 2AgNO₃ --->  Ca(NO₃)₂ +  2AgCl

• Which one of the products is the solid precipitate:  calcium nitrate or silver chloride?

Silver chloride is the precipitate:  The solubility chart shows it as "I" for insoluble solid.

CaCl_2(aq) + 2AgNO_3(aq) --->  Ca(NO₃)_2(aq) +  2AgCl_(s)

_{(The precipitate is marked with an (s).  All other compounds are aqueous solution (aq).)}

3.  SrBr_2(aq)  +   (NH₄)₂CO_3(aq)  --->  ???

• First, recognize that this is a double replacement reaction:  Compound + Compound

• Then, write out the equation in words:

strontium bromide + ammonium carbonate  ---> strontium carbonate  +  ammonium bromide

• Now use your ion chart to write correct formulas for each of the compounds.  Remember to "criss-cross" charges so that all of the compounds are neutral.

SrBr₂ + (NH₄)₂CO₃ --->  SrCO₃ +  NH₄Br

• Finally, add coefficients to balance the equation:

SrBr₂ + (NH₄)₂CO₃ --->  SrCO₃ +  2NH₄Br

• Which one of the products is the solid precipitate:  strontium carbonate or ammonium bromice?

Strontium carbonate is the precipitate:  The solubility chart shows it as "I" for insoluble solid.

SrBr_2(aq) + (NH₄)₂CO_3(aq) --->  SrCO_3(s) +  2NH₄Br_(aq)

_{(The precipitate is marked with an (s).  All other compounds are aqueous solution (aq).)}