Question

Why are decomposition reactions called the opposite of combination reactions? Explain by giving one example (with equation) of each type of reaction.

Answer 1

Decompositon reaction are those in which a compound breaks down to form two or more substances. These reaction require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.
Decomposition reaction: $2{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{electrolysis}}{\to }2{\mathrm{H}}_2+{\mathrm{O}}_2$
Combination reaction: $2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+\mathrm{energy}$