How does a combination reaction differ from a decomposition reaction

What is Decomposition Reaction?

Decomposition reaction is another important reaction in the category of redox reactions. It is basically the opposite of combination reaction. A decomposition reaction is a reaction where reactant compound is broken down into products. Here, half-reactions occur at the same time as the oxidation reaction and the reduction reaction. But unlike in combination reaction, the reactant for both half reactions is the same in decomposition reactions. Decomposition reaction results in several products.

In the electrolysis of water, when direct current is passed through water, water molecules are decomposed to give oxygen and hydrogen gases. Here, the oxidation number is increased in oxygen atom and is decreased in hydrogen atom. Hence, it is a redox reaction and a decomposition reaction due to the breakdown of water molecules into oxygen and hydrogen gases.

What is a Combination Reaction?

A combination reaction, also called as synthesis reaction, is a reaction in which reactant compounds are combined to form a different compound as the product. In other words, the reaction of simple molecules results in a complex molecule. Some or all the bonds between atoms of that particular compound are broken down; at the same time, atoms will combine to form the new compound, which is the product.  In decomposition reactions, the same reactant acts as the starting material for both half reactions. Unlike in decomposition reactions, the half reactions in combination reactions have different reactants to begin with. Combination reaction results in a single product. Following is a common example that can be given as combustion reactions.

For example, when Aluminium(Al) is placed in liquid bromide (Br2) a combination reaction occurs and produces Aluminium bromide(AlBr3).  Here, the oxidation number is increased in Al and decreased in Br. Therefore, it is a redox reaction and is a combination reaction since two reactants have reacted to give one particular product.