Double Replacement Reactions

            Double replacement (or displacement) reactions take place between two ionic compounds that are dissolved in water.  The cation of one compound replaces the cation in the other compound to produce two new compounds.  For the reaction to occur, a cation and anion present in the water solutions of the reactants must be removed from solution.  This can happen in one of three ways:

1)   One of the reactants is insoluble in water and drops to the bottom of the solution (called precipitate). You must check your solubility chart to see if an insoluble product is formed.

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

The two salts; silver nitrate and sodium chloride, are soluble in water so they actually dissociate and float around as free ions in the water solution. (aq) means aqueous (dissociates in water) and (s) means solid (the precipitate).

 

2)   The hydrogen ion from one reactant unites with the hydroxide ion from the other reactant  to form water. 

NaOH (aq)   +   HCl (aq)   à   NaCl (aq)   +   H₂O (l)

3)  A gas is formed that escapes from the reaction flask.

            NH₄ (aq)   +   NaOH (aq)   à   NH₃ (g)   +   H₂O (l)   +   NaCl (aq)

 Remember:  a double replacement reaction occurs only if the reaction produces                                                        
                      a solid, a gas, or water.

   

Some chemist prefer to classify reactions differently.  They would call the reaction in #1 above a precipitation reaction (and not use the double replacement terms). Other types are given here and on the handout with synthesis, decomposition, and single replacement.

4)  acid-base reactions – The acid and base start out as the reactants and swap ions to give a salt (either soluble or insoluble) and water as products.

                        HCl (aq)   +   NaOH (aq)   à   H₂O (l)   +   NaCl (aq)

                         H₃PO₄ (aq)   +   3NaOH (aq)   à   3H₂O (l)   +   Na₃PO₄ (aq)

 5)  combustion reactions  -  These are reactions between a compound an O₂.  What you get for products depends on what you start out with.

            a) if carbon is a reactant, you’ll get CO₂ as a product

            b) if hydrogen is a reactant, you’ll get H₂O as a product

            c) if a metal is a reactant, you’ll get a metal oxide as a product           

6) gas forming reactions  -  These are reactions involving carbonates (CO₃^2-) and
      sulfites (SO₃^2-)

            using baking soda (sodium bicarbonate) and vinegar (acetic acid) as an example:

            NaHCO₃ (aq)   +   2 HC₂H₃O₂ (aq)   à   H₂O (l)  +  CO₂ (g)   +   2NaC₂H₃O₂ (aq)

            When you are dealing with sulfite, it’s pretty much the same reactions, 
            but you get SO₂ (g)    instead of CO₂ (g) as a product.