DECOMPOSITION REACTIONS—IN A NUTSHELL
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In decomposition reactions, one substance breaks down, producing at least
two new substances.
Decomposition reactions may be exothermic (energy-releasing) or
endothermic (energy-absorbing). Exothermic reactions might initially require
a small input of energy to start the reaction.
The energy required to make a decomposition reaction occur is a measure of
the strength of the bonding within the reactant. If the bonding is strong, the
compound is very stable and therefore hard to break down.
These reactions help you to determine if a substance is an element or a
compound. Elements cannot be decomposed.
The energy for a decomposition reaction can be provided by heat, light or
electricity.
Thermal decomposition (decomposition by heat)
 Ionic carbonates are decomposed by heat into the corresponding oxides and
carbon dioxide. The temperature required for decomposition depends upon the
activity of the metal; the more active the metal, the more stable the carbonate.
Thus, the carbonates of sodium and potassium are stable at the highest
temperatures of a Bunsen burner flame, whereas the carbonates of silver and
copper are easily decomposed.
 Many other compounds can be decomposed using heat, for example mercury(I)
oxide and organic molecules such as sugars.
Decomposition by light
 Photons of electromagnetic radiation can cause decomposition. For example, UV
light can split chlorofluorocarbons (CFCs) in the atmosphere.
 If a compound is labelled ‘light sensitive’, that means that light energy can break
it down.
 Many silver salts (especially silver halides) are light sensitive, a property used in
the photographic industry. UV light is particularly effective at decomposing these
ionic compounds. The silver halides break down to give their constituent
elements.
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Electrolysis (decomposition by electricity)
 Electrical energy is used to force a non-spontaneous reaction to occur.
 ANODE: site of oxidation (loss of electrons)
 CATHODE: site of reduction (gain of electrons)
 ANODE POLARITY: positive
 CATHODE POLARITY: negative
 Conversion: Electrical energy  chemical energy
 At one electrode, electrons are produced; at the other, electrons are consumed.
The overall process is called a REDOX reaction.
Decomposition examples
Reaction
Endothermic or
exothermic
CaCO3(s)  CaO(s) Endothermic
+ CO2(g)
Type of
decomposition
Thermal
2NaN3(s)  2Na(s)
+ 3N2(g)
Reaction started
with heat
Exothermic
How is it useful?
CaO is used in
glass-making and to
change pH of soil
Airbags in cars
2H2O(l)  2H2(g) + Endothermic
O2(g)
Electrolysis
H2 and O2 can be
used in fuel cells
2NaHCO3(s) 
Endothermic
Na2CO3(s) + CO2(g)
+ H2O(g)
2AgBr(s)  2Ag(s) Endothermic
+ Br2(g)
Thermal
Makes cakes rise
(baking soda)
Light
2NI3(s)  N2(g) +
3I2(g)
Can be initiated by
heat, vibration etc
Photographic film
(along with AgCl
and AgI)
It isn’t…this is
touch powder..fun
but not useful
Exothermic
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DEMONSTRATION – DECOMPOSITION OF WATER
Note ALL observations.
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