4.4 Double Replacement Reactions

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    Double Replacement Reactions
    (-a.k.a. Double Displacement, Exchange, or Metathesis Reactions)

    AB(aq) + CD(aq) → AD + CB

    Strong Electrolytes, Weak Electrolytes, Nonelectrolytes

    The following table summarizes how to distinguish between strong electrolytes, weak electrolytes and nonelectrolytes.

    Electrolytes dissolve in water to form ions making the solution electrically conductive.
    Strong Electrolytes
    dissociate into ions completely
    1. Soluble Ionic Compounds
    2. Strong Acids ((HCl, HBr, HI, HClO4 HClO3, H2SO4, HNO3)
    3. Strong Bases (Group I metal hydroxides, Ba(OH)2, Sr(OH)2, Ca(OH)2)
    Weak Electrolytes
    dissociate into ions only partially
    1. Weak Acids (ex. HF, HCN, CH3COOH, etc.)
    2. Weak Bases (ex. NH3, CH3NH2, etc.)
    don’t dissociate to form ions
    Molecular compounds (except acids/bases)
    ex. C6H12O6 and CH3OH

    Solubility Rules

    Soluble Compounds Exceptions Insoluble Compounds Exceptions
    Compounds containing Compounds containing
    NO3- None S2- NH4+, Li+, Na+, K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+
    CH3COO- (acetate) None CO32- NH4+, Li+, Na+, K+, Rb+, Cs+
    Cl-, Br-, I- Ag+, Hg22+, Pb2+ PO43- NH4+, Li+, Na+, K+, Rb+, Cs+
    SO42- Sr2+, Ba2+, Hg22+, Pb2+ OH- NH4+, Li+, Na+, K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+

    Label the following Ionic compounds as ‘soluble’ or ‘insoluble’:

    CuS                                        MnCl2                                        PbSO4

    AgNO3                                    K2S                                           Na2CO3

    Types of Double Replacement Reactions

    1. Precipitation Reactions

    In a precipitation reaction an insoluble, solid product is produced in a double replacement reaction.

    1. Precipitation Reactions
    Pb(NO3)2(aq) + 2NaI(aq) → Molecular Equation
    Ionic Equation
    Net Ionic Equation

    2. Acid-Base Neutralization Reactions

    Not all acid-base neutralization reactions qualify as double replacement reactions, but those that do involve an acid and base as reactants and often produce water and a salt (i.e. an ionic compound).

    2. Acid-Base Neutralization Reactions
    HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq) Molecular Equation
    Net Ionic Equation
    HF(aq) + NaOH → Molecular Equation
    Net Ionic Equation

    3. Gas-Forming Reactions

    First of all, most gas-forming reactions are not double replacement reactions.  It is just this type of reaction shown in the table below that you should be familiar with.  If the product of a double replacement reaction is predicted to be H2CO3 (carbonic acid), then you should replace it with H2O (l) and CO2 (g) instead as carbonic acid is unstable and decomposes to water and carbon dioxide.


    H2CO3 → H2O (l) + CO2 (g)


    3. Gas-Forming Reactions
    Na2CO3(aq) + 2HCl(aq) → H2O(l) + CO2(g) + 2NaCl(aq)