Question

Why is the decomposition reaction called the opposite of a combination reaction? Give examples for each.

Answer 1

Combination reaction:

• A combination reaction is described as a reaction in which two or more reactants come together to create a single product.
• The combination reaction's general reaction equation is,

$\mathrm{A}+\mathrm{B}\to \mathrm{AB}$

• Example of Combination reaction,

$\underset{\mathrm{Hydrogen}}{\underbrace{{\mathrm{H}}_2\left(\mathrm{g}\right)}}+\underset{\mathrm{Chlorine}}{\underbrace{{\mathrm{Cl}}_2\left(\mathrm{g}\right)}}\to \underset{\mathrm{Hydrochloric}\mathrm{Acid}}{\underbrace{\mathrm{HCl}\left(\mathrm{g}\right)}}$

• In the above example, the Hydrogen gas reacts with Chlorine to form Hydrochloric acid by following a combination reaction.

Decomposition reaction:

• A chemical reaction that results in the breakdown of one reactant into two or more products is known as a decomposition reaction.
• Below is a general outline of a decomposition reaction.

$\mathrm{AB}\to \mathrm{A}+\mathrm{B}$

• Example of Decomposition reaction,

$\underset{\mathrm{Calcium}\mathrm{Carbonate}}{\underbrace{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}}\to \underset{\mathrm{Calcium}\mathrm{Oxide}}{\underbrace{\mathrm{CaO}\left(\mathrm{s}\right)}}+\underset{\mathrm{Carbon}\mathrm{Dioxide}}{\underbrace{{\mathrm{CO}}_2\left(\mathrm{g}\right)}}$

• In the above example, Calcium carbonate decomposes to form calcium oxide and carbon dioxide as the product resulting in the generation of two products from one reactant.
• Therefore, the decomposition process is defined as the breaking of bigger molecules into two or smaller molecules, whereas the combination reaction is considered to be the reaction of two or more molecules to generate a larger molecule.

Hence, this illustrates how the decomposition process is the combination reaction's opposite.