Question

Why decomposition reactions are called the opposite of combination reactions?

Answer 1

Combination reactions

Reactions in which two compounds react to form a single product are called combination reactions or synthesis reactions. These reactions usually involve evolution of heat and therefore are exothermic reactions. The reactants of a combination reaction may be elements or compounds.

The general form of a combination reaction is:

$A+B\to AB$

Decomposition reactions

Reactions in which a single compound breaks down into two or more products are called decomposition reactions. Generally, decomposition reactions are endothermic as it occurs by absorption of energy. The reactants of a decomposition reaction can only be compounds.

The general form of a decomposition reaction is:

$AB\to A+B$

Relationship between combination and decomposition reactions

From the general form of combination and decomposition reactions, it can be seen that decomposition reaction is the opposite of combination reactions. Combination reactions include formation of a compound from smaller reactants and decomposition reactions include breaking down of compounds into smaller products.

For example, Water (H₂O) is synthesized by the reaction of Hydrogen (H₂) with oxygen (O₂). The chemical equation for the combination reaction is:

$2H_2\left(g\right)+O_2\left(g\right)\to 2H_{2}O\left(l\right)$

(Hydrogen) (Oxygen) (Water)

Water undergoes electrolysis to form Hydrogen and Oxygen. The chemical equation for the decomposition reaction is:

$2H_{2}O\left(l\right)\to 2H_2\left(g\right)+O_2\left(g\right)$

(Water) (Hydrogen) (Oxygen)

Therefore, decomposition reaction is the opposite of combination reaction.