4.4 Double Replacement Reactions
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 | |
|---|---|
| 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.) | |
| Nonelectrolytes don’t dissociate to form ions | Molecular compounds (except acids/bases) ex. C6H12O6 and CH3OH |
Solubility Rules
| SOLUBILITY RULES FOR IONIC COMPOUNDS IN WATER | |||
|---|---|---|---|
| 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) |