REACTIONS OF METALS
REACTIONS WITH OXYGEN
The reactivity of metals can be determined by observing their reactivity with oxygen in air and pure
oxygen.
Li
Na
K
Mg
Ca
Fe
Cu
OBSERVATIONS
EQUATION
SOLUBLE
Ignites when heated in air
and burns more vigorously
in oxygen. White flame.
White fumes form.
Ignites when heated in air
and burns more vigorously
in oxygen. Yellow flame.
White powder forms.
Ignites when heated in air
and burns more vigorously
in oxygen. Violet flame.
White powder forms.
Ignites when heated in air
and burns vigorously in
oxygen. Bright white flame.
White powder forms.
4Li + O2  2Li2O
Yes
Does not ignite when
heated in air but burns
vigorously in oxygen.
Brick red flame. White
powder forms.
Does not ignite in air but
burns with a yellow glow in
oxygen giving of yellow
sparks. A black mass of
iron oxide is formed.
Reacts very slowly with a
dull glow. A black layer
forms on the surface of the
metal.
2Ca + O2  2CaO
COLOUR
ACID/
OF LITMUS BASE
SOLUTION
blue
Base
Forms LiOH
4Na + O2  2Na2O
Yes
blue
Base
blue
Base
blue
Base
blue
Base
3Fe + 2O2  Fe3O4
Slightly
soluble.
Forms a
suspension
of Mg(OH)2.
Slightly
soluble.
Forms a
suspension
of Ca(OH)2.
Insoluble
no effect
-
2Cu + O2  2CuO
Insoluble
no effect
-
Forms NaOH
4K + O2  2K2O
Yes
Forms KOH
2Mg + O2  2MgO
Li, Na, K and Mg ignited readily in air and burnt vigorously in pure oxygen.
Ca, Fe and Cu did not ignite in air, Ca and Fe ignited in pure oxygen and copper did not ignite in
pure oxygen forming a black layer of copper oxide on the surface of the metal.
THERMAL DECOMPOSITION OF METAL OXIDES
Thermal decomposition reactions are reactions in which substances decompose when heated.
The use of thermal decomposition reactions to determine the reactivity of metals is based on the
principle that a metal that reacts strongly with oxygen to form an oxide will not liberate its oxygen
from the oxide easily when heated.
Test for oxygen: A glowing splinter bursts into flame.
OXIDE
OBSERVATION
EQUATION
CuO
Does not liberate oxygen.
-
MgO
Does not liberate oxygen.
-
PbO2
HgO
The brown colour of
lead(IV) oxide changes to
the yellow colour of
lead(II) oxide. Small
quantities of oxygen are
given off.
Droplets of mercury form
at the cool mouth of the
test tube. Oxygen easily
liberated.
2PbO2 (brown)  2PbO (yellow) + O2
2HgO  2Hg + O2
Since Cu and Mg did not give up their oxygen, they are the most reactive of the four elements.
Pb and Hg are less reactive with Hg being the least reactive because it gave up its oxygen easily.
REACTIONS WITH WATER
The reactivity of metals can also be determined by observing their reactions with water. Metals
react with water to produce hydrogen. The rate at which hydrogen is produced indicates the
reactivity of the metal.
Test for hydrogen: A small explosion occurs when a match is held at the mouth of the test tube.
Alkali metals are stored under paraffin to prevent oxidation.
Reactions with cold water:
Li
Na
K
Ca
Mg
Fe
Cu
OBSERVATIONS
Floats, does not melt,
moves around the surface
and gives off hydrogen gas.
Floats, melts, moves
around the surface and
gives off hydrogen gas.
Floats, melts, moves
around the surface and
gives off hydrogen gas that
ignites and burns with a
blue flame.
Calcium sinks and bubbles
of hydrogen are given off.
The solution becomes
cloudy due to the Ca(OH)2
produced that is slightly
soluble in water.
No reaction
No reaction
No reaction
EQUATION
2Li + 2H2O  2LiOH + H2
2Na + 2H2O  2NaOH + H2
2K + 2H2O  2KOH + H2
Ca + 2H2O  Ca(OH)2 + H2
-
The metals that did not react with cold water are then placed in warm water:
Mg
Fe
Cu
OBSERVATIONS
Magnesium sinks and
bubbles of hydrogen are
given off. The solution
becomes cloudy due to the
Mg(OH)2 produced that is
slightly soluble in water.
No reaction
No reaction
EQUATION
Mg + 2H2O  Mg(OH)2 + H2
-
The metals that did not react with warm water are then reacted with steam:
Fe
Cu
OBSERVATIONS
Glows brightly giving off
hydrogen. Black iron(IV)
oxide is formed.
No reaction
EQUATION
3Fe + 4H2O  Fe3O4 + 2H2
-
REACTIONS OF METALS WITH METAL IONS
In the above reactions, metals have been in competition with hydrogen for the possession of
oxygen in water. Since all the metals except copper reacted with the oxygen in water, it can be
concluded that hydrogen is more reactive than copper.
The same principle can be applied to reactions between metals and metal ions.
A metal can displace from solution any other metal ion that is lower in the reactivity series, except
when the metal reacts with water. Metals cannot displace their own ions.
The following table shows the displacement of metal ions from solution by metals:
Mg
Zn
Fe
Cu
MgSO4 (Mg2+)
x
x
x
x
ZnSO4 (Zn2+)

x
x
x
FeSO4 (Fe2+)


x
x
CuSO4 (Cu2+)



x
The following reactions occurred in the above table:
Mg + ZnSO4  MgSO4 + Zn
Mg + FeSO4  MgSO4 + Fe
Mg + CuSO4  MgSO4 + Cu
Zn + FeSO4  ZnSO4 + Fe
Zn + CuSO4  ZnSO4 + Cu
Fe + CuSO4  FeSO4 + Cu
The above metals can be arranged in order of increasing reactivity as follows:
Cu
Fe
Zn
Mg
REACTIVITY SERIES
From observations of reactions between metals and oxygen, thermal decomposition of oxides,
reactions between metals and water, and the displacement of metal ions from solution, metals can
be placed in order of increasing reactivity in the reactivity series.
K
Na
Li
Mg
Ca
Zn
Fe
Pb
(H)
Cu
Hg
Ag