Displacement reactions (swopping partners)
Reaction Types: Single Replacement
During single replacement, one element replaces another element in a compound. There are two
different possibilities:
1. One cation (normally a metal) replaces another.
Its a bit like this:
AX
metal compound
+
Y
metal
+
YX
new metal compound
A
metal
Element Y has replaced A (in the compound AX) to form a new compound YX and the free
element A.
Remember that A and Y are both METALS [cations (positively-charged ions) in this example].
Some examples are:
Cu
copper
+
Fe
iron
+
Ca
calcium
+
Zn
zinc
+
2AgNO3
silver nitrate
Cu(NO3) 2
copper nitrate
2HOH
water
2HCl
hydrogen chloride
2Ag
silver
+
Cu
copper
+
H2
hydrogen
H2
hydrogen
+
+
Cu(NO3) 2
copper nitrate
Fe(NO3) 2
iron nitrate
Ca(OH) 2
calcium hydroxide
ZnCl2
zinc chloride
2. One non metal anion replaces another.
It's a bit like this:
A
non metal
+
XY non-metal
compound
XA
new non-metal compound
+
Y
non metal
Element A has replaced Y (in the compound XY) to form a new compound XA and the free
element Y.
Remember that A and Y are both non metals. [ anions (negatively-charged ions) in this example].
The only examples that need to be known are those involving the group 7 elements i.e. the
HALOGENS
Cl2
chlorine
+
2NaBr
sodium bromide
2NaCl
sodium chloride
+
Br2
bromine
Cl2
chlorine
Br2
bromine
+
2NaI
sodium iodide
2NaCl
sodium chloride
+
+
2NaI
sodium iodide
2NaBr
sodium bromide
+
I2
iodine
I2
iodine
In single replacement, one reactant is always an element. The other reactant will be a compound.
Here are several examples which are solved below:
O2
oxygen
K
potassium
+
2ZnS
zinc sulphide
2ZnO
zinc oxide
+
2S
sulphur
+
2HOH
water
+
H2
hydrogen
Fe
iron
+
2HCl
hydrogen chloride
+
H2
hydrogen
F2
fluorine
+
2NaI
sodium iodide
2KOH
potassium
hydroxide
FeCl2
iron(II)
chloride
2NaF
sodium
fluoride
+
I2
iodine
Reaction Types: Double Replacement
During double replacement, the cations (METALS) and anions (NON METALS) of two different
compounds switch places.
Simply:
AB
compound
+
XY
compound
+
AY
new compound
XB
new compound
A and X are the cations (postively-charged ions) in this example, with B and Y being the anions
(negatively-charged ions).
Look at the examples below:
2KOH
potassium
hydroxide
+
FeS
iron (II)
sulphide
+
2NaCl
sodium
chloride
+
AgNO3
silver nitrate
+
H2SO4
hydrogen
sulphate
2HCl
hydrogen
chloride
K2SO4
potassium
sulphate
FeCl2
iron(II)
chloride
+
HOH
hydrogen
hydroxide
+
H2SO4
hydrogen
sulphate
NaCl
sodium
chloride
Na2SO4
sodium
sulphate
AgCl
silver
chloride
+
H2S
hydrogen
sulphide
HCl
hydrogen
chloride
+
NaNO3
sodium nitrate
An added complication involves carbonates.
In the double replacement involving calcium carbonate and hydrochloric acid, one of the products
hydrogen carbonate decomposes into water and carbon dioxide.
+
2HCl
CaCO3
hydrogen
chloride
calcium carbonate
+
CaCl2
H2CO3
calcium chloride
hydrogen carbonate
Then
H2CO3
hydrogen carbonate
H2O
hydrogen oxide
+
CO2
carbon dioxide
Notice how, one of the two product compounds decomposes.
In double replacement, both reactants are compounds, each with a cation part and an anion part.
In the examples below complete the following:
1).
Ca(OH)2
+
HCl
+
2)
Al(NO3) 3
+
H2SO4
+
3)
Pb(NO3) 2
+
K2S
+
4)
Pb(NO3) 2
+
CuSO4
+
Practice Problems 2:
Write correct formulas for the products in these double replacement reactions.