Metathesis Reactions-
Predicting the Products of Precipitation Reactions

Metathesis reactions (or exchange reactions) are reactions in which the products are formed by the exchange of the ions present in the two reactants. Consider the following reaction as an example:

MgCl2 (aq) + 2 NaOH (aq) --> Mg(OH)2 (s) + 2 NaCl (aq)

Notice that the magnesium ion was originally paired with the chloride ion on the reactant side of the equation while the sodium ion was originally paired with the hydroxide ion. In the products, however, the magnesium ion and the sodium ion have exchanged "partners" (anions). On the product side, the magnesium is now paired with hydroxide ion forming an insoluble solid or precipitate, magnesium hydroxide. Also on the product side, the sodium ion is now paired with the chloride ion forming sodium chloride, a soluble strong electrolyte.

The previous equation is an example of one type of methathesis reaction commonly called a precipitation reaction. In a precipitation reaction, one of the products formed by the exchange of ions is a precipitate (an insoluble solid).

In order to predict the products and write a molecular equation describing a precipitation reactions, follow the following steps:

  1. Break each reactant apart into the cation and anion present. At this point, you can ignore the actual number of each that are present.
  2. Exchange anions.
  3. Combine each cation with its new anion and write the formula for the electrically neutral compound. Don't forget to balance the charges!
  4. Determine the physical state of the products using the solubility guidelines.
  5. If one of the products formed is an insoluble solid, write the unbalanced molecular equation by placing the formulas for the reactants (including physical states) on the left side of the arrow and the formulas for the products (including physical states) on the right side of the arrow.
  6. Balance the equation.
  7. If neither of the products formed is an insoluble solid, then no reaction occurred. Write the formulas (including physical states) for the reactants on the left side of the arrow. On the right side of the arrow write "No Reaction."

Example: Write the molecular equation for the reaction between an aqueous solution of silver nitrate and an aqueous solution of calcium chloride.

  • Break each reactant apart into the cation and anion present
Ag+
NO3-
Ca2+
Cl-
  • Exchange anions.
  • Pair Ag+ ion with Cl- ion.
    Pair
    Ca2+ ion with NO3- ion.

  • Combine each cation with its new anion and write the formula for the electrically neutral compound.
  • Combine Ag+ ion with Cl- ion to form AgCl.
    Combine
    Ca2+ ion with NO3- ion to form Ca(NO3)2.

  • Determine the physical state of the products using the solubility guidelines.
  • AgCl is an insoluble solid.
    Ca(NO3)2 is soluble in water.

  • If one of the products formed is an insoluble solid, write the unbalanced molecular equation by placing the formulas for the reactants (including physical states) on the left side of the arrow and the formulas for the products (including physical states) on the right side of the arrow.
  •  

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

 

  • Balance the equation.

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

 

Example: Write a balanced molecular equation for the reaction between aqueous solutions of ammonium phosphate [(NH4)3PO4] and sodium carbonate [Na2CO3].

  • Break each reactant apart into the cation and anion present.
NH4+ PO43-
Na+ CO32-
  • Exchange anions.

    Pair NH4+ ion with CO32- ion.
    Pair Na+ ion with PO43- ion.

  • Combine each cation with its new anion and write the formula for the electrically neutral compound.

    Combine NH4+ ion with CO32- ion to form (NH4)2CO3
    Combine Na+ ion with PO43- ion to form Na3PO4

  • Determine the physical state of the products using the solubility guidelines.

    (NH4)2CO3 is soluble in water.
    Na3PO4 is soluble in water.

  • If neither of the products formed is an insoluble solid, then no reaction occurred. Write the formulas (including physical states) for the reactants on the left side of the arrow. On the right side of the arrow write "No Reaction."

(NH4)3PO4 (aq) + Na2CO3 (aq) --> No reaction

 

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Practice Problems

Write balanced molecular equations for the reactions that occur when the following aqueous solutions are mixed together. If a precipitate does not form, write "No reaction."

  1. Na2SO4 (aq) + BaCl2 (aq) -->
  2. Ca(OH)2 (aq) + Na3PO4 (aq) -->
  3. CaSO4 (aq) + Na2S (aq) -->

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