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

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

Answer: Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions 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: AB + Energy ⟶ A + B

2H₂O(l) + Electrolysis ⟶ 2H₂(g) + O₂(g)

Combination Reaction: A + B ⟶ AB + Energy

2H₂(g) + O₂(g)  ⟶  2H₂O(l) + Energy