Redox & Electron Transfer
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
Definitions in Terms of Electrons (Extended) Oxidation The loss of electrons from a substance. Reduction The gain of electrons by a substance. Memory Aid - OIL RIG O xidation I s L oss (of electrons) R eduction I s G ain (of electrons) Example: Reaction of Magnesium with Oxygen 2Mg + O₂ → 2Mg²⁺O²⁻ Mg → Mg²⁺ + 2e⁻ (magnesium is oxidized - loses electrons) O₂ + 4e⁻ → 2O²⁻ (oxygen is reduced - gains electrons) Definitions in Terms of Oxidation Numbers (Extended) Oxidation An increase in oxidation number.
Reduction A decrease in oxidation number. Example Fe²⁺ → Fe³⁺ Oxidation number increases from +2 to +3, so iron is oxidized . Rules for Assigning Oxidation Numbers Elements in their natural state have oxidation number 0 (e.g., O₂, Fe, Cl₂) Oxygen in compounds is usually -2 (except in peroxides) Hydrogen in compounds is usually +1 Group 1 metals are always +1 Group 2 metals are always +2 The sum of oxidation numbers in a neutral compound is 0 The sum of oxidation numbers in an ion equals the charge on the ion Identifying Redox by Electron Transfer If electrons are transferred between substances, the reaction is a redox reaction: The substance that loses electrons is oxidized The substance that gains electrons is reduced Oxidizing and Reducing Agents Oxidizing Agent A substance that oxidizes another substance while being reduced itself.
It accepts electrons. Reducing Agent A substance that reduces another substance while being oxidized itself. It donates electrons. Example: Reaction of Zinc with Copper(II) Sulfate Zn + CuSO₄ → ZnSO₄ + Cu Zinc is the reducing agent (it is oxidized: Zn → Zn²⁺ + 2e⁻) Copper(II) ions are the oxidizing agent (they are reduced: Cu²⁺ + 2e⁻ → Cu) Remember The oxidizing agent gets reduced, and the reducing agent gets oxidized!
Tests for Redox Reactions (Extended) 1. Acidified Potassium Manganate(VII) - KMnO₄ Color Change Purple → Colorless KMnO₄ acts as an oxidizing agent. When it oxidizes another substance, it is reduced and loses its purple color.
Test Procedure Add acidified potassium manganate(VII) to a reducing agent (e.g., sulfite ions, iron(II) ions) Observation: The purple solution turns colorless This indicates a redox reaction has occurred 2.
Potassium Iodide - KI Color Change Colorless → Brown Iodide ions (I⁻) can be oxidized to iodine (I₂), which is brown in solution. Test Procedure Add an oxidizing agent (e.g., chlorine water, acidified KMnO₄) to potassium iodide solution Observation: The colorless solution turns brown due to formation of iodine Half equation: 2I⁻ → I₂ + 2e⁻ (iodide is oxidized) Test Reagent Color Change What It Shows For reducing agents Acidified KMnO₄ Purple → Colorless KMnO₄ is reduced (oxidizing agent) For oxidizing agents KI solution Colorless → Brown I⁻ is oxidized to I₂
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