Question

(a) Define decomposition reaction and write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
[4 Marks]
(b) What is the difference between displacement and double displacement reaction? Write equations for these reactions.
[4 Marks]

Answer 1

(a)Decomposition reaction: These are the reactions in which a compound breaks down to form two or more elements or new compounds.
These reactions require a source of energy (heat, light or electrical) to proceed.
[1 Mark]
Types of decomposition reaction are as follows (based on source of energy):
In the presence of heat (Thermal decomposition reaction):
${\text{CaCO}}_3\text{(s)}\stackrel{\text{Heat}}{\to }\text{CaO(s)}+{\text{CO}}_2\text{(g)}$
[1 Mark]

In the presence of sunlight (Photolytic decomposition reaction):
$2\text{AgCl(s)}\stackrel{\text{Sunlight}}{\to }2\text{Ag(s)}+{\text{Cl}}_2\text{(g)}$
[1 Mark]
In the presence of electricity (Electrolytic decomposition reaction):
$2{\text{H}}_2\text{O(l)}\stackrel{\text{Electricity}}{\to }2{\text{H}}_2\text{(g)}+{\text{O}}_2\text{(g)}$
[1 Mark]

(b) In a displacement reaction, a more reactive element displaces a less reactive element from its compound or salt solution.
[1 Mark]

For example: ${\text{CuSO}}_4\text{(aq)}+\text{Zn(s)}\to {\text{ZnSO}}_4\text{(aq)}+\text{Cu(s)}$
[1 Mark]

In a double displacement reaction, two compounds react by exchanging their ions to form new compounds. It is typically identified by the formation of a product that is a precipitate.
[1 Mark]

For example: ${\text{Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{(aq)}\to {\text{BaSO}}_4\text{(s)}+2\text{NaCl(aq})$
[1 Mark]