A decomposition reaction is a process in chemistry where a single, complex chemical compound breaks down into two or more simpler substances. This is analogous to a complex structure built from interlocking blocks breaking apart into its individual pieces. This breakdown is a specific chemical change where the bonds holding the original compound together are severed, resulting in new, less complex products.
The General Process of Decomposition
Every decomposition reaction is represented by the general chemical equation: AB → A + B. In this formula, ‘AB’ stands for the reactant, which is the initial compound. On the other side of the arrow, ‘A’ and ‘B’ represent the products, which can be either individual elements or smaller, less complex compounds.
For the chemical bonds within the reactant to be broken, an input of energy is almost always required. This energy overcomes the forces holding the atoms together, allowing them to rearrange into the new product formations. The specific source of this energy can vary, leading to different categories of decomposition reactions.
Types of Decomposition Reactions
Decomposition reactions are categorized by the energy source used to initiate the breakdown. The three main types are thermal, electrolytic, and photolytic, each distinguished by its unique energy source.
Thermal decomposition occurs when heat is the energy source that triggers the reaction. An industrial example is the heating of calcium carbonate (CaCO₃), or limestone. When subjected to high temperatures around 825°C, it breaks down into calcium oxide (CaO), also called quicklime, and carbon dioxide (CO₂) gas. The chemical equation for this process is CaCO₃(s) → CaO(s) + CO₂(g).
Electrolytic decomposition, or electrolysis, uses electrical energy to break down a compound. A well-known example is the electrolysis of water (H₂O). When a direct electric current is passed through water, it decomposes into its constituent elements: hydrogen gas (H₂) and oxygen gas (O₂). The balanced chemical equation for this reaction is 2H₂O(l) → 2H₂(g) + O₂(g).
Photolytic decomposition, or photolysis, occurs when light energy is the catalyst for the reaction. The breakdown of silver chloride (AgCl) upon exposure to sunlight is a prime example. When light strikes the silver chloride, it provides the energy to break the bonds, forming solid silver (Ag) and chlorine gas (Cl₂). This transformation, represented by 2AgCl(s) → 2Ag(s) + Cl₂(g), is responsible for the darkening of the compound and is a principle behind early photography.
Decomposition Reactions in Everyday Life
Decomposition reactions are at work in many familiar situations, such as with hydrogen peroxide (H₂O₂). The 3% solution found in brown bottles gradually breaks down into water (H₂O) and oxygen (O₂). This process is why it fizzes when applied to a cut; an enzyme in blood speeds up this decomposition, and the resulting bubbles are oxygen gas being released.
A more rapid example occurs during the deployment of a vehicle’s airbag. This safety feature relies on the fast decomposition of a compound called sodium azide (NaN₃). Upon impact, a sensor sends an electrical signal that heats the sodium azide, causing it to instantly break down into sodium metal (Na) and a large volume of nitrogen gas (N₂). This production of gas inflates the airbag in a fraction of a second.
Baking also relies on decomposition. When baking soda, or sodium bicarbonate (NaHCO₃), is used as a leavening agent, it undergoes thermal decomposition when heated. At temperatures above 80°C (176°F), it breaks down into sodium carbonate (Na₂CO₃), water (H₂O), and carbon dioxide (CO₂) gas. The release of this carbon dioxide gas creates bubbles within the batter, causing it to rise and giving baked goods their light, airy texture.