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Oxidation and Reduction
Edwin Ogie Library — Oxidation & Reduction (Redox)
Clear, school‑friendly notes on redox reactions: definitions, oxidation numbers, balancing redox equations, tests and nomenclature.
Page 1 — What is a Redox Reaction?
A redox (oxidation–reduction) reaction is any chemical change where electrons are transferred between species. One species loses electrons (oxidation) while another gains electrons (reduction).
- Redox can be described by addition/removal of oxygen or hydrogen, by electron transfer, or by a change in oxidation numbers.
- Every redox reaction has an oxidizing agent (accepts electrons) and a reducing agent (donates electrons).
Page 2 — Oxidation (Simple Definitions)
Oxidation is when a substance loses electrons. Other easy ways to spot oxidation are when a substance gains oxygen or loses hydrogen.
- Electron view: Loss of electrons (e⁻). Example: Zn → Zn²⁺ + 2e⁻ (Zn is oxidized).
- Oxygen/hydrogen view: Addition of oxygen or removal of hydrogen counts as oxidation.
Page 3 — Reduction (Simple Definitions)
Reduction is when a substance gains electrons. It can also be seen as loss of oxygen or gain of hydrogen.
- Electron view: Gain of electrons (e⁻). Example: Cu²⁺ + 2e⁻ → Cu (copper is reduced).
- Oxygen/hydrogen view: Removal of oxygen or addition of hydrogen counts as reduction.
Page 4 — Oxidation Numbers (How to Assign)
Oxidation numbers help us track electron transfer. Use these simple rules:
- Free elements have oxidation number 0 (e.g., H₂, O₂, Fe).
- For simple ions, the oxidation number equals the charge (Na⁺ → +1, Cl⁻ → -1).
- Oxygen is usually -2 (except in peroxides where it is -1).
- Hydrogen is usually +1 (except in metal hydrides where it is -1).
- The sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion it equals the ion charge.
Page 5 — Balancing Redox Equations (Simple Steps)
- Write the two half-reactions (oxidation & reduction).
- Balance atoms other than O and H.
- Balance O by adding H₂O; balance H by adding H⁺ (in acidic) or H₂O/OH⁻ (in basic).
- Balance charge by adding electrons (e⁻).
- Multiply half-reactions to equalize electrons and add them together.
- Cancel species that appear on both sides and simplify.
Page 6 — Balancing in Acidic and Basic Media
For reactions in acidic solution use H⁺ and H₂O to balance oxygen and hydrogen. For basic solutions, after balancing in acid, neutralize H⁺ with OH⁻ to form water.
Page 7 — Identifying Oxidizing and Reducing Agents
The oxidizing agent is the species reduced (gains electrons). The reducing agent is the species oxidized (loses electrons).
- Example: In 2Ag⁺ + Cu → 2Ag + Cu²⁺, Ag⁺ is reduced so it is the oxidizing agent; Cu is oxidized and is the reducing agent.
Page 8 — Tests for Oxidizing & Reducing Agents
- KMnO₄ test: Purple KMnO₄ is decolorized by reducing agents in acidic medium.
- Iodine-starch test: Iodine gives a blue color with starch; reducing agents remove iodine, fading the color.
- Dilute HNO₃ test: Strong oxidizing agents may produce brown NO₂ gas.
Page 9 — IUPAC Naming Using Oxidation Numbers
When a metal has variable oxidation states, indicate the state in Roman numerals in brackets after the metal name.
- Fe₂O₃ → iron(III) oxide (each Fe is +3).
- CuO → copper(II) oxide (Cu is +2).
Page 10 — Worked Examples (Short)
Balance: Fe²⁺ + MnO₄⁻ → Fe³⁺ + Mn²⁺ in acidic medium.\p>
Balanced: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O (electrons balanced: 5e⁻).
Identify oxidizing and reducing agents: Cu + 2Ag⁺ → Cu²⁺ + 2Ag
Ag⁺ is reduced (oxidizing agent); Cu is oxidized (reducing agent).
Edwin Ogie Library — Oxidation & Reduction (Redox)
Clear, school‑friendly notes on redox reactions with a timed practice quiz.
Practice: 20‑Question JAMB CBT Quiz (Timed)
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