Double-Replacement Reactions
A double-replacement reaction is a reaction in which the positive and negative ions of two
ionic compounds exchange places to form two new compounds. The general form of a doublereplacement (also called double-displacement) reaction is:
AB+CD→AD+BC
In this reaction, AA and CC are positively-charged cations, while BB and DD are negativelycharged anions. Double-replacement reactions generally occur between substances in
aqueous solution. In order for a reaction to occur, one of the products is usually a solid
precipitate, a gas, or a molecular compound such as water.
Formation of a Precipitate
A precipitate forms in a double-replacement reaction when the cations from one of the
reactants combine with the anions from the other reactant to form an insoluble ionic compound.
When aqueous solutions of potassium iodide and lead (II) nitrate are mixed, the following
reaction occurs.
2KI(aq)+Pb(NO3)2(aq)→2KNO3(aq)+PbI2(s)
There are very strong attractive forces that occur between Pb+2Pb2+ and I-1 ions and the result
is a brilliant yellow precipitate (see figure below). The other product of the reaction, potassium
nitrate, remains soluble.
Figure 11.9.111.9.1: Formation of lead iodide precipitate.
Formation of a Gas
Some double-replacement reactions produce a gaseous product which then bubbles out of the
solution and escapes into the air. When solutions of sodium sulfide and hydrochloric acid are
mixed, the products of the reaction are aqueous sodium chloride and hydrogen sulfide gas.
Na2S(aq)+2HCl(aq)→2NaCl(aq)+H2S(g)
Formation of a Molecular Compound
Another kind of double-replacement reaction is one that produces a molecular compound as
one of its products. Many examples in this category are reactions that produce water. When
aqueous hydrochloric acid is reacted with aqueous sodium hydroxide, the products are aqueous
sodium chloride and water.
HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
EXAMPLE 11.9.1
Write a complete and balanced chemical equation for the following double-replacement
reactions. One product is indicated as a guide.
a. NaCN(aq)+HBr(aq) → (hydrogen cyanide gas is formed)
b. (NH4)2SO4(aq)+Ba(NO3)2(aq) → (a precipitate of barium sulfate forms)
Solution:
Step 1: Plan the problem.
In A, the production of a gas drives the reaction. In B, the production of a precipitate drives
the reaction. In both cases, use the ionic charges of both reactants to construct the correct
formulas of the products.
Step 2: Solve.
A. The cations of both reactants are +1 charged ions, while the anions are −1 charged ions.
After exchanging partners, the balanced equation is:
NaCN(aq)+HBr(aq)→NaBr(aq)+HCN(g)
B. Ammonium ion and nitrate ion are 1+ and 1− respectively, while barium and sulfate
are 2+ and 2−. This must be taken into account when exchanging partners and writing the
new formulas. Then, the equation is balanced.
(NH4)2SO4(aq)+Ba(NO3)2(aq) → 2NH4NO3(aq)+BaSO4(s)
Step 3: Think about your result.
Both are double replacement reactions. All formulas are correct and the equations are balanced.
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Occasionally, a reaction will produce both a gas and a molecular compound. The reaction of a
sodium carbonate solution with hydrochloric acid produces aqueous sodium chloride, carbon
dioxide gas, and water.
Na2CO3(aq)+2HCl(aq) → 2NaCl(aq)+CO2(g)+H2O(l)
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Double Displacement Reaction Examples
The reaction between silver nitrate and sodium chloride is a
double displacement reaction. The silver trades its nitrite ion
for the sodium's chloride ion, causing the sodium to pick up
the nitrate anion.
AgNO3 + NaCl → AgCl + NaNO3
Here's another example:
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2 NaCl(aq)
A double displacement reaction is a type
of reaction where two reactants exchange ions
to form two new compounds. Double
displacement reactions typically result in the
formation of a product that is a precipitate.
The reaction occurs most
often between ionic
compounds, although
technically the bonds formed
between the chemical species
may be either ionic or
covalent in nature. Acids or
bases also participate in
double displacement
reactions. The bonds formed
in the product
compounds are the same type
of bonds as seen in the
reactant molecules. Usually,
the solvent for this type of
reaction is water.
Types of Double Displacement Reactions
Double displacement reactions may be classified into several
categories, including counter-ion exchange, alkylation,
neutralization, acid-carbonate reactions, aqueous metathesis
with precipitation (precipitation reactions), and aqueous
metathesis with double decomposition (double
decomposition reactions). The two types most commonly
encountered in chemistry classes are precipitation reactions
and neutralization reactions.
A precipitation reaction occurs between two
aqueous ionic compounds to form a new insoluble
ionic compound. Here's an example reaction,
between lead(II) nitrate and potassium iodide to
form (soluble) potassium nitrate and (insoluble)
lead iodide.
Pb(NO3)2(aq) + 2 KI(aq) → 2 KNO3(aq) + PbI2(s)
The lead iodide forms what is called the precipitate,
while the solvent (water) and soluble reactants and
products are termed the supernate or supernatant.
Formation of a precipitate drives the reaction in a
forward direction, as the product leaves the solution.
Neutralization reactions are double displacement
reactions between acids and bases. When the solvent is
water, a neutralization reaction typically produces
an ionic compound--a salt. This type of reaction
proceeds in the forward direction if at least one of the
reactants is a strong acid or a strong base. The reaction
between vinegar and baking soda in the classic baking
soda volcano is an example of a neutralization reaction.
This particular reaction then proceeds to release a gas (carbon dioxide), which is responsible
for the fizz of the reaction. The initial neutralization reaction is:
NaHCO3 + CH3COOH(aq) → H2CO3 + NaCH3COO
You'll notice the cations exchanged anions, but the way the compounds are written, it's a bit
trickier to notice the anion swap. The key to identifying the reaction as double displacement is
to look at the atoms of the anions and compare them on both sides of the reaction.
One example is when SODIUM
CHLORIDE and SILVER NITRATE
forms SODIUM NITRATE and SILVER
CHLORIDE