Double Displacement Reaction

A double displacement reaction is a type of reaction where part of of one reactant is replaced by part of another reactant.

In double displacement reactions two ionic compounds switch cations.

The general pattern of a double displacement reaction is:

AB + CD → AD + CB

Double displacement reactions are driven by some force: either the formation of a precipitate, formation of a gas, or formation of water.

Formation of Precipitate

The formation of a solid precipitate can also pull a reaction forward. A precipitate forms because the combination of a positive and a negative ion forms a compound that is insoluble in water.

For example:                2 KI (aq) + Pb(NO3)2 (aq) → PbI2 (s) + 2 KNO3 (aq)

Pb2+ and I- ions combine to form a compound that is insoluble in water.

To predict if a precipitate will form you need to know which substances are soluble in water and which are insoluble. For this you must refer to the solubility rules (chart).


   Eg. Pb(NO3)2 (aq) + NaCl (aq)
     from their names; lead (II) nitrate reacts with sodium chloride →
       we know that if they switch partners than lead (II) chloride and
       sodium nitrate will be produced

    we know that sodium nitrate will be soluble because all nitrate and all
    group 1 containing compounds are soluble

    looking at chloride on the solubility table, it is show that an
     insolube compound is formed with Ag+, Pb+2, Hg2+2, Cu+, and Tl+
     PbCl2 will be insoluble

   Therefore; Pb(NO3)2 (aq) + NaCl (aq) → PbCl2 (s) + 2 NaCl (aq)
 
     Eg. Ca(C2H3O2)2 (aq) + Na2S (aq)
       from their names; calcium acetate reacts with sodium sulfide →
       we know that if they switch partners than calcium sulfide and
       sodium acetate will be produced

    we know that sodium acetate will be soluble all group 1 containing
    compounds are soluble

    we know that calcium sulfide will be soluble because sulfide forms a
    soluble compound with Ca+2

     Eg. Ca(C2H3O2)2 (aq) + Na2S (aq) → no visible rxn

 

Formation of a Gas

A double displacement reaction will likely occur if there is a gas formed as one of the products.

Example:
     Na2CO3 (aq) + 2 HCl (aq) → 2 NaCl (aq) + H2CO3 (aq)

          H2CO3 breaks down to H2O (l) + CO2 (g)      

     Na2CO3 (aq) + 2 HCl (aq) → 2 NaCl (aq) + H2O (l)+ CO2 (g)

The formation of CO2 pulls the reaction forward

Example:
     NH4Cl (aq) + NaOH (aq) → NaCl (aq) + NH4OH (aq)

               NH4OH(aq) breaks down into NH3 (g) + H2O

     NH4Cl (aq) + NaOH (aq) → NaCl (aq) + NH3 (g) + H2O

Reactions that Produce Water

These are a special type of double displacement reaction known as neutralization reactions. The reaction occurs between an acid and a base to form a salt and water.

For example

               HNO3 (aq) + KOH (aq) → KNO3 (aq) + H2O (l)

The formation of water pulls the reaction forward.