Decomposition Reaction
A decomposition reaction is a type of chemical reaction in which a single compound breaks down into two or more simpler substances, following the general pattern AB → A + B. Decomposition reactions are the reverse of synthesis reactions and typically require an input of energy such as heat (thermal decomposition), light (photodecomposition), or electricity (electrolysis) to break the chemical bonds of the original compound. They play vital roles in industrial chemistry, such as the decomposition of limestone to produce quicklime, and in biological systems, such as the breakdown of hydrogen peroxide by the enzyme catalase.
Key Formula
AB → A + B (one compound breaks into two or more simpler substances)
LaTeX: AB \rightarrow A + B
| Symbol | Meaning | Unit |
|---|---|---|
| AB | Single reactant compound being broken down | dimensionless |
| A | First simpler product | dimensionless |
| B | Second simpler product | dimensionless |
Worked Example
Problem
Write the balanced equation for the thermal decomposition of calcium carbonate (limestone).
Solution
Step 1: Identify the reactant — calcium carbonate (CaCO₃), which decomposes on strong heating. Step 2: Identify the products — calcium oxide (CaO) and carbon dioxide gas (CO₂). Step 3: Write the equation: CaCO₃ → CaO + CO₂ Step 4: Count atoms: Ca: 1=1 ✓; C: 1=1 ✓; O: 3 = 1+2 = 3 ✓ Step 5: The equation is already balanced.
Answer
CaCO₃ → CaO + CO₂ (balanced decomposition reaction)
Types of Decomposition Reactions with Examples
| Type | Energy Input | Equation | Product(s) |
|---|---|---|---|
| Thermal decomposition | Heat | CaCO₃ → CaO + CO₂ | Calcium oxide + carbon dioxide |
| Electrolysis | Electricity | 2 H₂O → 2 H₂ + O₂ | Hydrogen + oxygen gases |
| Photodecomposition | Light | 2 AgBr → 2 Ag + Br₂ | Silver metal + bromine |
| Catalytic decomposition | Catalyst | 2 H₂O₂ → 2 H₂O + O₂ | Water + oxygen gas |
| Explosive decomposition | Shock/heat | 2 KClO₃ → 2 KCl + 3 O₂ | Potassium chloride + oxygen |
Interactive Tools
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Related Terms
Synthesis Reaction
A synthesis reaction (also called a combination reaction) is a type of chemical reaction in which two or more reactants combine to form a single, more complex product, following the general pattern A + B → AB. Synthesis reactions are fundamental in both nature and industry — for example, the formation of water from hydrogen and oxygen, or the industrial synthesis of ammonia by the Haber process. They are the opposite of decomposition reactions and often release energy in the form of heat or light.
Chemical Reaction
A chemical reaction is a process in which one or more substances (reactants) are transformed into one or more different substances (products) through the breaking and forming of chemical bonds. Chemical reactions involve changes in the arrangement of atoms, resulting in new materials with different properties from the original substances. They are fundamental to all biological, industrial, and environmental processes, from cellular respiration to the manufacture of medicines.
Oxidation-Reduction Reaction
An oxidation-reduction reaction (redox reaction) is a chemical reaction involving the simultaneous transfer of electrons between two species: the substance that loses electrons is oxidised (its oxidation state increases), and the substance that gains electrons is reduced (its oxidation state decreases). Redox reactions are among the most important in chemistry and biology — they underpin cellular respiration, photosynthesis, corrosion, electroplating, and the operation of batteries and fuel cells. The key mnemonic "OIL RIG" (Oxidation Is Loss, Reduction Is Gain) helps students remember the direction of electron transfer.
From Latin "decomponere" meaning "to take apart", combining "de-" (down, away) and "componere" (to put together). The term highlights the reverse nature of synthesis — instead of building up, decomposition tears apart chemical structures.