A chemical reaction transforms one or more substances into different ones by rearranging atoms and forming or breaking chemical bonds. Decomposition is a fundamental category of these transformations, representing the chemical process of breaking things apart. This contrasts with a synthesis reaction, where multiple simpler substances combine to form a complex compound.
Defining the Structure of Decomposition Reactions
A decomposition reaction is defined by a single starting material, known as the reactant. This complex substance then breaks down into two or more simpler substances, which are the products of the reaction. The chemical complexity of the initial material is reduced as its constituent atoms reorganize themselves into less complicated molecules or even individual elements. The process can be represented by the general chemical formula A → B + C. The defining characteristic remains having only one compound on the reactant side of the equation.
The Energy Input Required for Breakdown
Decomposition reactions require an input of energy because chemical bonds must be broken to initiate the process. This bond-breaking step is intrinsically energy-absorbing, which means most decomposition reactions are classified as endothermic. The energy supplied must overcome the stability the atoms gained when the bonds were originally formed.
The necessary energy can be delivered in several different forms, classifying the reaction type. Thermal decomposition uses heat to break the bonds, such as heating limestone to produce lime. Electrolytic decomposition, or electrolysis, uses electrical energy to drive the breakdown. Photodecomposition is triggered by light energy, which can break bonds in light-sensitive compounds.
However, some compounds, like hydrogen peroxide, are thermodynamically unstable, meaning their decomposition releases energy overall, making them exothermic. Even these unstable reactions require an initial energy ‘push,’ known as activation energy, to get started.
Common Examples of Decomposition Reactions
A classic example illustrating the need for electrical energy is the electrolysis of water. When an electric current is passed through water, the single reactant, water (H2O), decomposes to produce hydrogen gas (H2) and oxygen gas (O2). This process is used to generate pure hydrogen, a potential clean fuel source.
Hydrogen peroxide (H2O2) is an example of exothermic decomposition often found in medicine cabinets. This compound naturally breaks down into water and oxygen gas over time, which is why it is stored in dark bottles. A catalyst, such as the enzyme catalase found in blood, or exposure to light significantly speeds up the reaction. The release of oxygen gas causes the bubbling seen when applied to a cut.
Baking soda, or sodium bicarbonate (NaHCO3), provides an example of thermal decomposition. This single compound decomposes when heated above 80°C. Heating causes it to break down into three products: sodium carbonate (Na2CO3), water, and carbon dioxide gas (CO2). The carbon dioxide gas is responsible for the rising action in baked goods.