6.6
Types of Chemical Reactions:
Single and Double Displacement
Magnesium metal is one of the most combustible substances in your school’s
chemical storeroom. Most fires can be extinguished with a standard carbon
dioxide fire extinguisher. A magnesium fire is particularly dangerous
because spraying carbon dioxide onto it only makes it worse (Figure 1).
single Displacement Reactions
Figure 1 The beaker contains solid
carbon dioxide that quickly sublimates
to release carbon dioxide gas. This
gas then reacts vigorously with hot
magnesium metal.
single displacement reaction a reaction
in which an element displaces another
element in a compound, producing a new
compound and a new element
The word and chemical equations for the reaction of magnesium with carbon
dioxide are
magnesium + carbon dioxide → magnesium oxide + carbon
2 Mg(s) +
CO2(g)
→
2 MgO(s)
This reaction is an example of a single displacement reaction. In a
single displacement reaction, one element displaces or replaces an element
in a compound (Figure 2). The general pattern for this type of reaction is
A + BC → AC + B
A represents an element; BC represents a compound.
A
BC
AC
+
To see a dramatic video of the
reaction of magnesium and
carbon dioxide,
Go to NElsoN sCIENCE
LeaRning Tip
Non-Metals Place Second
Remember that the second element
in a chemical compound is almost
always a non-metal. That is why, in
Figure 2, element C is written second
in both compounds: BC and AC.
+ C(s)
B
+
Figure 2 In a single displacement reaction, one element, A, displaces element B in a compound,
BC. The new compound, AC, is one product. The displaced element, B, is the second product.
Notice how the chemical equation for the reaction of magnesium with
carbon dioxide is similar to this general pattern. In single displacement
reactions involving an ionic compound and a metal, it is always the positive
ion (cation) that is replaced in the compound.
Singledisplacementreactionsoftenoccurinaqueoussolution.Figure3
shows what happens when a coiled copper wire is placed into a solution of
silver nitrate:
copper + silver nitrate → copper(II) nitrate + silver
Cu(s) + 2 AgNO3(aq) → Cu(NO3)2(aq) + 2 Ag(s)
Figure 3 (a) A coil of copper wire is
placed into a solution of the silver nitrate.
(b) The fuzzy coating on the copper
wire is silver metal. The solution is blue
because of dissolved Cu2+(aq) ions.
240
Chapter 6 • Chemicals and Their Reactions
(a)
(b)
NEL
Single displacement reactions also occur when metals are placed into
acids. The chemical formula of any acid includes one or more hydrogen
atoms. (For example, the chemical formula for hydrochloric acid is
HCl(aq).) In these reactions, metal atoms displace the hydrogen atoms
in the compound. Figure 4 shows the reaction of zinc metal and
hydrochloric acid:
zinc + hydrochloric acid → hydrogen + zinc chloride
Zn(s) + 2 HCl(aq)
→ H2(g) + ZnCl2(aq)
Hydrogen, you will recall, forms diatomic molecules. That is why the
chemical formula for hydrogen is H2 rather than H.
Displacement Reactions in Mining
Metals rarely occur naturally as pure elements. Instead, they combine with
other elements to form rock deposits called ores. Nickel, for example, occurs
in rock as nickel sulfide. The processing of nickel ore is called smelting.
The first step in smelting is converting nickel sulfide into nickel oxide.
Nickel oxide is then burned with coke (carbon) to produce pure nickel and
poisonous carbon monoxide:
Figure 4 Zinc reacts in hydrochloric
acid. As zinc displaces the hydrogen
in the acid, bubbles of hydrogen gas
appear on the surface of the zinc.
C(s) + NiO(s) → Ni(s) + CO(g)
Note that the chemical equation for smelting fits the general pattern of
single displacement reactions.
The factory in which this process occurs is called a smelter. Another of
the products of processing nickel is sulfur dioxide. As you will learn in the
next chapter, sulfur dioxide emissions from nickel smelters are responsible
for some of the damage caused by acid precipitation.
RESEARCH THIS
WHen gOLd LOSeS iTS gLiTTeR
SKILLS: Defining the Issue, Researching, Communicating
The cyanide process is one of the most effective methods of
extracting gold from rock. However, it is controversial because
it uses sodium cyanide—a highly toxic substance. The used
cyanide must be collected, stored, and treated to keep it out
of the environment (Figure 5).
SKILLS HANDBOOK
4.A., 4.C.
Consider this scenario. A company wants to mine a newly
discovered gold deposit near a remote town in northern Ontario.
The president of the company has invited the following people to
discuss the project:
• the mayor of the town
• a representative from the Ministry of the Environment
• the leader of a First Nations group
• a member of a local non-governmental environmental group
1. Take the role of one of the four people at the meeting.
2. Research background information to support your role,
including the history of cyanide use in gold processing.
Figure 5 Water from gold mines is treated to remove the cyanide
before being released into the environment.
NEL
A. In your role, summarize your perspective on the
development of the mine. Present your perspective as an
opening argument. T/I C
6.6 Types of Chemical Reactions: Single and Double Displacement
241
Double Displacement Reactions
double displacement reaction a reaction
that occurs when elements in different
compounds displace each other or exchange
places, producing two new compounds
Double displacement reactions occur when two elements in different
compounds trade places (Figure 6). The general pattern for these reactions is:
AB + CD → AD + CB
The symbols A, B, C, and D represent atoms, single ions, or polyatomic ions.
LeaRning Tip
Polyatomic Ions
Treat polyatomic ions as a single unit
in a chemical equation. In a double
displacement reaction, a polyatomic
ion (such as the nitrate ion, NO3−) can
change places with an ion composed
of only one atom (such as chloride, Cl−).
DID YOU KNOW?
Heavy Metals Banned
Heavy metals such as lead, cadmium,
and mercury are very toxic. Solutions
containing cations of these metals are
also dangerous. For this reason, many
school boards have banned their use.
Even in schools where they are not
banned, they are generally only used
in very small quantities. After use, they
are collected in special containers so
that as little as possible is released
into the environment. What is your
school’s policy on heavy metals?
Figure 7 (a) When silver nitrate solution
is added to sodium chloride solution,
specks of silver chloride appear. (b)
When silver nitrate solution is added
to tap water, a faint haze appears. This
haze indicates that chloride ions are
present in the water—perhaps from
road salt.
AB
CD
AD
+
CB
+
Figure 6 In a double displacement reaction, the two non-metals, B and D, trade places.
Alternatively, you could think of it as the two metals, A and C, switching over.
Many double displacement reactions occur between two ionic compounds
in solution. For example, Figure 7 shows the reaction of a solution of silver
nitrate with a solution of sodium chloride. In this reaction, nitrate ions
and chloride ions trade places. The word and chemical equations for this
reaction are as follows:
silver nitrate + sodium chloride → silver chloride + sodium nitrate
NaCl(aq) →
AgCl(s) + NaNO3(aq)
AgNO3(aq) +
Notice that both the word and chemical equations fit the general pattern
for this reaction:
AB + CD → AD + CB
(a)
(b)
Forming a Precipitate
precipitate a solid formed from the
reaction of two solutions
242
Look closely at the chemical equation for the reaction in Figure 7. Note that the
reactants are both in solution (aq), and so is one of the products: sodium nitrate.
The other product, silver chloride, is a solid (s). Chemists have discovered,
through experimentation, that some ionic compounds do not dissolve in water.
If these insoluble compounds are formed during a reaction, they become visible
as a precipitate: tiny specks of solid material in the solution. The silver chloride
formed in Figure 7 is a precipitate.
Chapter 6 • Chemicals and Their Reactions
NEL
Not all double displacement reactions result in the formation of a
precipitate, but many do. Lead(II) nitrate and potassium iodide are both
soluble in water. When their solutions are mixed, a bright yellow precipitate
of lead(II) iodide appears (Figure 8).
AB + CD → AD + CB
lead(II) nitrate + potassium iodide → lead(II) iodide + potassium nitrate
Figure 8 The bright yellow precipitate
is insoluble lead(II) iodide. It is produced
when a solution containing Pb2+ ions is
mixed with a solution containing I− ions.
in
Pb(NO3)2(aq) +
2 KI(aq)
→
PbI2(s)
+
2 KNO3(aq)
Potassium iodide can be used to test for Pb2+ ions in water: a yellow
precipitate indicates that lead ions are present.
SUMMARY
•Inasingledisplacementreaction,anelementand
a compound react to produce a different element
and compound and have the general pattern
A + BC → AC + B.
•Sometimes,inareactionofaqueous
reactants, one of the products is insoluble.
This product, called a precipitate, appears
as a solid in the solution.
•Inadoubledisplacementreaction,two
compounds react to produce two different
compounds and have the general pattern
AB + CD → AD + CB.
CHECK
YOUR LeaRning
1. Compare single and double displacement reactions.
K/U
2. What types of reactants are likely to be involved in
(a) a single displacement reaction?
(b) a double displacement reaction? K/U
3. Classify the following word equations as representing
either single or double displacement reactions: K/U
(a) aluminum + iron(III) oxide → aluminum oxide + iron
(b) barium chloride + sodium sulfate →
barium sulfate + sodium chloride
(c) zinc + copper(II) sulfate → zinc sulfate + copper
(d) silver nitrate + sodium phosphate →
silver phosphate + sodium nitrate
(e) calcium + water → hydrogen + calcium hydroxide
7. The dark tarnish that sometimes forms on silver is silver
sulfide, Ag2S. A common home remedy for tarnish is
represented by the chemical equation
3 Ag2S(s) + 2 Al(s) → 6 Ag(s) + Al2S3(s) (Figure 9) K/U A
(a) Classify the reaction as a single displacement or a
double displacement reaction.
(b) Which method do you think is better for cleaning
silverware: the method described in (a) or scrubbing
and polishing? Why?
4. Rewrite the word equations in question 3 as balanced
chemical equations (without state symbols). K/U T/I
5. Consider the chemical equation
CuSO4(aq) + Fe(s) → FeSO4(aq) + Cu(s). K/U A
(a) Classify the reaction as a single displacement or a
double displacement reaction.
(b) Copper compounds such as copper(II) sulfate are toxic.
Before disposal, these compounds must be treated to
reduce their toxicity. Describe how to use steel wool
(which is made mostly of iron) to remove the Cu2+ ions
from an aqueous solution of copper(II) sulfate.
6. Firefighters suggest that the best way to put out a
magnesium fire is to pour sand or salt over it. Why is this
better than using a carbon dioxide fire extinguisher? A
NEL
Figure 9 To remove the tarnish from silverware, soak it
in a hot solution of baking soda in an aluminum pan.
8. The fuzzy silver coating in Figure 3 of this section is impure
silver. It can be converted back into silver nitrate by reacting
it with nitric acid, HNO3(aq), as shown:
Ag(s) + HNO3 (aq) → AgNO3(aq) + NO2(g) + H2O(l) K/U T/I A
(a) Balance the equation.
(b) What has to be done to the reaction mixture to recover
solid silver nitrate?
(c) Why must this process be done in a well-ventilated
area? (Hint: see Figure 1 in Section 5.10.)
6.6 Types of Chemical Reactions: Single and Double Displacement
243