Question

Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer 1

Decomposition reactions are those 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. Thus, they are called the exact opposite of combination reactions, in which two or more reactants combine to give a new product with the release of energy.

For example:
Decomposition reaction: ${\text{CaCO}}_3\text{(s)}\stackrel{\text{Heat}}{\to }\text{CaO (s)}+{\text{CO}}_2\text{(g)}$
Combination reaction: $\text{CaO(s)}+{\text{H}}_2\text{O(l)}\to {\text{Ca(OH)}}_2\text{(aq)}$