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.
Key Formula
Oxidation: A → Aⁿ⁺ + ne⁻ | Reduction: Bⁿ⁺ + ne⁻ → B (electrons lost by one species are gained by another)
LaTeX: \text{Oxidation: } A \rightarrow A^{n+} + ne^{-} \quad \text{Reduction: } B^{n+} + ne^{-} \rightarrow B
| Symbol | Meaning | Unit |
|---|---|---|
| A | Species being oxidised (electron donor, reducing agent) | dimensionless |
| B | Species being reduced (electron acceptor, oxidising agent) | dimensionless |
| n | Number of electrons transferred | dimensionless |
| e⁻ | Electron | dimensionless |
Worked Example
Problem
In the reaction: Zn + CuSO₄ → ZnSO₄ + Cu, identify which species is oxidised and which is reduced.
Solution
Step 1: Write the ionic equation: Zn + Cu²⁺ + SO₄²⁻ → Zn²⁺ + SO₄²⁻ + Cu Step 2: Identify the net ionic equation: Zn + Cu²⁺ → Zn²⁺ + Cu Step 3: Determine oxidation state changes: Zn goes from 0 → +2 (loses 2 electrons) → Zn is OXIDISED Cu goes from +2 → 0 (gains 2 electrons) → Cu is REDUCED Step 4: Write the half-reactions: Oxidation half: Zn → Zn²⁺ + 2e⁻ Reduction half: Cu²⁺ + 2e⁻ → Cu Step 5: Verify electron balance: 2e⁻ lost = 2e⁻ gained ✓
Answer
Zinc (Zn) is oxidised; Copper ion (Cu²⁺) is reduced. Zn is the reducing agent; Cu²⁺ is the oxidising agent.
Key Terms in Oxidation-Reduction (Redox) Reactions
| Term | Definition | Oxidation State Change | Example |
|---|---|---|---|
| Oxidation | Loss of electrons | Increases (+) | Zn → Zn²⁺ + 2e⁻ |
| Reduction | Gain of electrons | Decreases (−) | Cu²⁺ + 2e⁻ → Cu |
| Reducing agent | Species that gets oxidised (gives electrons) | Increases | Zn in Zn/Cu²⁺ reaction |
| Oxidising agent | Species that gets reduced (accepts electrons) | Decreases | Cu²⁺ in Zn/Cu²⁺ reaction |
| Half-reaction | Equation showing only oxidation or reduction | Shows e⁻ explicitly | Fe → Fe³⁺ + 3e⁻ |
| Redox couple | Paired oxidised/reduced forms of a species | Both states shown | Fe²⁺/Fe³⁺ |
Interactive Tools
Khan Academy – Redox Reactions
Comprehensive video series on oxidation states, half-reactions, and redox balancing
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Combustion Reaction
A combustion reaction is a rapid chemical reaction between a fuel and an oxidant (usually oxygen) that produces heat and light, most commonly forming carbon dioxide and water when the fuel contains carbon and hydrogen. Complete combustion occurs when sufficient oxygen is available and produces only CO₂ and H₂O, while incomplete combustion in limited oxygen produces carbon monoxide (CO) or soot (unburned carbon). Combustion reactions are the basis of energy production in engines, power plants, and heating systems, and are central to understanding air pollution and climate change.
Single Replacement Reaction
A single replacement reaction (also called a single displacement reaction) is a type of chemical reaction in which one element displaces another element from a compound, following the general pattern A + BC → AC + B. The reaction occurs when the displacing element is more reactive (higher on the activity series) than the element it replaces; if the displacing element is less reactive, no reaction occurs. Single replacement reactions are widely used in metallurgy and electrochemistry — for example, extracting metals from their ores or in galvanic cells.
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 originally referred to combination with oxygen (from "oxide" + "-ation"), coined by Antoine Lavoisier circa 1780. Reduction came from the metallurgical term "to reduce an ore" meaning to extract pure metal by removing oxygen. The broader electron-transfer definition was established by Wilhelm Ostwald in the early 20th century.