Question

Why is decomposition reaction called opposite of combination reaction? Write equations for these reactions.

Answer 1

1. Decomposition reaction

a. Definition

The reaction in which a single compound breaks down into two or more elements or compounds in presence of heat, light or electrical energy is called decomposition reaction.

b. Example:

Thermal decomposition of lead nitrate

$2\mathrm{Pb}\left(\mathrm{NO}{}_3){}_2(\mathrm{s}\right)\to 2\mathrm{PbO}\left(\mathrm{S}\right)+4\mathrm{NO}{}_2(\mathrm{g})+\mathrm{O}{}_2(\mathrm{g})$

(Lead nitrate) (Lead oxide) ( Nitrogen dioxide) (Oxygen)

Here lead(II) nitrate undergo thermal decomposition to give yellow colour lead (II) oxide, colourless gas of oxygen and brown colour nitrogen dioxide gas.

c. It is endothermic in nature.

2. Combination reaction

a. Definition:

Reaction in which two or more elements or compounds combine to form product.

b. Example:

Formation of calcium oxide by the combination elements calcium and oxygen.

$2\mathrm{Ca}\left(\mathrm{s}\right)+\mathrm{O}{}_2(\mathrm{g})\to 2\mathrm{CaO}(\mathrm{s})$

(Calcium) (Oxygen) (Calcium Oxide)

c. It is exothermic in nature.

Hence, we can realize from above examples that a decomposition reaction is opposite of combination reaction.