Thermal
Decomposition
Reactions
Stable and Unstable Substances
• Stable in Chemistry means unreactive in
the conditions stated.
• Unstable means reactive in the conditions
stated.
Thermal Decomposition
• When 2 elements combine to form a
compound, the product requires heat
energy for the elements to be reformed.
• i.e. the compound is more stable than the
elements at room temperature.
• Many compounds undergo thermal
decomposition – the compound breaks up
into compounds of lower mass, or into its
elements.
Thermal Decomposition
• The thermal stability of a metal compound
is directly related to the chemical activity of
the metal.
– Sodium compounds are the most thermally
stable.
– Gold compounds do not exist because they
are thermally unstable.
Activity Series of Metals
• By observing the reaction of metals with
oxygen, acid and water, it is possible to
arrange the metals in an order of activity.
• This is the activity series.
Potassium > Sodium > Lithium > Calcium >
Magnesium > Aluminium > Zinc > Iron > Lead >
(hydrogen) > Copper > Silver > Gold
Metal Carbonates and Metal
Hydrogen Carbonates
• The conditions required for the
decomposition of metal carbonates and
metal hydrogen carbonates depend on the
chemical activity of the metal
– Sodium and potassium carbonate are
thermally stable and can not be decomposed
by heating.
Metal Carbonates and Metal
Hydrogen Carbonates
• Highly reactive metals form stable
compounds that require high energy to
undergo thermal decomposition.
– Calcium carbonate – the high reactivity of
calcium metal results in a stable compound
and thus high energy is needed to
decompose calcium carbonate.
Thermal Decomposition of
Copper Carbonate
• Copper is low in the activity series so,
copper carbonate is thermally
decomposed with low energy. The
equation is:
CuCO3(s)
CuO(s)
CO2(g)
Thermal Decomposition of
Copper Carbonate
• Copper carbonate is a green powder, on
gentle heating, it decomposes into copper
oxide, a black powder and carbon dioxide
gas.
Thermal Decomposition of Iron II
Hydroxide
Fe(OH)2(s)
Green solid
FeO(s)
Black solid
– changes
to rust red
on contact
with air.
H2O(g)
Thermal Decomposition of
Copper Hydroxide
Cu(OH)2(s)
Blue-Green
solid
CuO(s)
Black solid
H2O(g)