{"id":10474,"date":"2026-09-26T11:30:23","date_gmt":"2026-09-26T10:30:23","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/electron-transfer-and-half-equations\/"},"modified":"2026-10-04T23:02:34","modified_gmt":"2026-10-04T22:02:34","slug":"electron-transfer-and-half-equations","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/electron-transfer-and-half-equations\/","title":{"rendered":"Electron Transfer and Half-Equations"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-redox-groups-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      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border-radius: 18px;\n      padding: 18px 20px;\n      box-shadow: var(--inner-shadow);\n    }\n.ols-faq-item h3 {\n      margin: 0 0 8px;\n      font-size: 20px;\n      line-height: 1.3;\n      color: var(--navy);\n    }\n.ols-faq-item p {\n      margin: 0;\n      font-size: 16px;\n      line-height: 1.65;\n      color: var(--body-text);\n    }\n.ols-faq-card,\n      .ols-quicksnap-card,\n      .ols-attribution-card {\n        padding: 24px 18px;\n        border-radius: 22px;\n      }\n  <\/style>\n\n    <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Topic 8 Redox &amp; Groups 1, 2 and 7<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/\">Topic 8 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/\">Topic 8A Redox Chemistry<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/oxidation-numbers\/\">Oxidation Numbers<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/electron-transfer-and-half-equations\/\">Electron Transfer and Half-Equations<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/disproportionation\/\">Disproportionation<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/redox-classification\/\">Redox Classification<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/trends-down-groups-1-and-2\/\">Trends Down Groups 1 and 2<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/reactions-of-the-group-1-and-2-elements\/\">Reactions of the Group 1 and 2 Elements<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/oxides-hydroxides-and-carbonates\/\">Oxides, Hydroxides and Carbonates<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/solubility-trends-and-the-sulfate-test\/\">Solubility Trends and the Sulfate Test<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/thermal-stability-and-flame-colours\/\">Thermal Stability and Flame Colours<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/tests-for-ions\/\">Tests for Ions<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/titrations-and-measurement-uncertainty\/\">Titrations and Measurement Uncertainty<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-3-concentration-of-hcl-by-titration\/\">Core Practical 3<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-4-preparation-of-a-standard-solution\/\">Core Practical 4<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/\">Topic 8C Group 7<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/trends-in-the-halogens\/\">Trends in the Halogens<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/halogens-as-oxidising-agents\/\">Halogens as Oxidising Agents<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/disproportionation-reactions-of-chlorine\/\">Disproportionation Reactions of Chlorine<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8c-group-7\/halide-ions-reducing-agents-and-tests\/\">Halide Ions: Reducing Agents and Tests<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Topics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Topic 6 Energetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Topic 7 Intermolecular Forces<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-2-atomic-structure-and-the-periodic-table\/\">Topic 2 Atomic Structure &amp; The Periodic Table<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/\">Core Practicals<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/\">Topic 8 Redox Chemistry and Groups 1, 2 and 7<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/\">Topic 8A Redox Chemistry<\/a> \/\n<span>Electron Transfer and Half-Equations<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Electron Transfer and Half-Equations<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to oxidation and reduction as electron transfer and as changes in oxidation number, oxidising and reducing agents, why metals form positive and non-metals negative ions, and how to write and combine ionic half-equations.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 2: WCH12\/01<\/div>\n<div class=\"ols-badge\">Topic 8A Redox Chemistry<\/div>\n<\/div>\n        <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by: Dr. Mohammed Al-Fatah\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/p>\n\n      <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: OIL RIG<\/h2>\n<p>Three quick questions on oxidation and reduction as gain and loss of oxygen and of electrons.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"854\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Oxidation and Reduction as Electron Transfer<\/h2>\n<\/div>\n<p>At GCSE, oxidation meant gaining oxygen. At A Level the definition is widened so that it covers every reaction: <strong>oxidation<\/strong> is the loss of electrons and <strong>reduction<\/strong> is the gain of electrons.<\/p>\n<div class=\"ols-key-box\"><p><strong>Remember:<\/strong> The memory line OIL RIG (Oxidation Is Loss, Reduction Is Gain) is still the fastest way to keep the two the right way round.<\/p><\/div>\n<p>Electrons cannot be lost into thin air, so oxidation and reduction always happen together. When magnesium burns in chlorine, each magnesium atom loses two electrons and each chlorine atom gains one. The reaction is a <strong>redox reaction<\/strong>: one species is oxidised while another is reduced.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Oxidation is the loss of electrons; reduction is the gain of electrons. A redox reaction is one in which both happen.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Oxidation and Reduction as Changes in Oxidation number<\/h2>\n<\/div>\n<p>Because an oxidation number counts the electrons an atom controls, electron transfer shows up as a change in oxidation number. An atom that is <strong>oxidised loses electrons<\/strong>, so its oxidation number <strong>increases<\/strong>. An atom that is <strong>reduced gains electrons<\/strong>, so its oxidation number <strong>decreases<\/strong>.<\/p>\n<p>This second definition is more useful in practice, because it works even when no ions are formed. In the reaction of hydrogen with chlorine, H\u2082 + Cl\u2082 \u2192 2HCl, no electron is transferred outright, but hydrogen goes from 0 to +1 and chlorine from 0 to \u22121. Hydrogen has been oxidised and chlorine reduced.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Process<\/th><th>Electrons<\/th><th>oxidation number<\/th><th>Example<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Oxidation<\/strong><\/td><td>lost<\/td><td>increases<\/td><td>Mg \u2192 Mg\u00b2\u207a + 2e\u207b (0 to +2)<\/td><\/tr>\n<tr><td><strong>Reduction<\/strong><\/td><td>gained<\/td><td>decreases<\/td><td>Cl\u2082 + 2e\u207b \u2192 2Cl\u207b (0 to \u22121)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-electron-transfer-zinc-copper.png\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-electron-transfer-zinc-copper.png\" alt=\"Zinc in copper(II) sulfate: oxidation and reduction half-equations and the combined ionic equation\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-electron-transfer-zinc-copper.png\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Zinc is oxidised (0 to +2) and copper is reduced (+2 to 0); the two electrons lost by zinc are the two gained by each copper ion.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> State both halves of the change: &#8220;magnesium is oxidised because its oxidation number increases from 0 to +2&#8221; scores; &#8220;magnesium is oxidised&#8221; alone often does not.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Oxidised or Reduced?<\/h2>\n<p>Decide, from the change in oxidation number, which element is oxidised and which is reduced in reactions you have not met on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-855\" class=\"h5p-iframe\" data-content-id=\"855\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Quick Choice: Oxidised or Reduced?\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Oxidising Agents and Reducing Agents<\/h2>\n<\/div>\n<p>An <strong>oxidising agent<\/strong> oxidises another species, so it must take electrons from it. The oxidising agent is therefore itself <strong>reduced<\/strong>. A <strong>reducing agent<\/strong> gives electrons to another species and is itself <strong>oxidised<\/strong>. The names describe what the substance does to something else, which is why students so often reverse them.<\/p>\n<p>In the reaction Zn + Cu\u00b2\u207a \u2192 Zn\u00b2\u207a + Cu, zinc loses electrons, so zinc is the reducing agent, and copper(II) ions gain electrons, so Cu\u00b2\u207a is the oxidising agent.<\/p>\n<p>Common oxidising agents include oxygen, chlorine, acidified potassium manganate(VII) and acidified potassium dichromate(VI).<\/p>\n<p>Common reducing agents include reactive metals, hydrogen, carbon monoxide and iodide ions.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Memory line:<\/strong> The oxidising agent gets reduced; the reducing agent gets oxidised. An agent always does the opposite of what happens to it.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Metals and Non-Metals Forming Ions<\/h2>\n<\/div>\n<p>Metals have few outer electrons that are weakly held, so a metal atom loses them to form a <strong>positive ion<\/strong>. Its oxidation number rises from 0 to a positive value, which is oxidation.<\/p>\n<p>Non-metals have nearly full outer shells and attract electrons strongly, so a non-metal atom gains electrons to form a <strong>negative ion<\/strong>. Its oxidation number falls from 0 to a negative value, which is reduction.<\/p>\n<p>In sodium reacting with sulfur, 2Na + S \u2192 Na\u2082S, sodium is oxidised (0 to +1) and sulfur is reduced (0 to \u22122). The general pattern, metal oxidised and non-metal reduced, holds for every reaction between a metal and a non-metal.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Metal atoms are reducing agents because they give electrons away; non-metal atoms such as the halogens are oxidising agents because they take electrons.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Writing Ionic Half-Equations<\/h2>\n<\/div>\n<p>A <strong>half-equation<\/strong> shows one half of a redox reaction, either the oxidation or the reduction, with the electrons written in explicitly. Electrons appear on the right of an oxidation half-equation (they are lost) and on the left of a reduction half-equation (they are gained).<\/p>\n<ol>\n<li>Write the species before and after the change: Fe\u00b2\u207a \u2192 Fe\u00b3\u207a.<\/li>\n<li>Balance the atoms. Here the iron is already balanced.<\/li>\n<li>Balance the charge by adding electrons to the more positive side: Fe\u00b2\u207a \u2192 Fe\u00b3\u207a + e\u207b.<\/li>\n<\/ol>\n<p>Half-equations in acidic solution often need H\u207a and H\u2082O to balance oxygen and hydrogen. Take the reduction of manganate(VII) ions: MnO\u2084\u207b \u2192 Mn\u00b2\u207a.<\/p>\n<p><strong>Step 1:<\/strong> Add 4H\u2082O on the right to balance the four oxygens.<\/p>\n<p><strong>Step 2:<\/strong> Add 8H\u207a on the left to balance the hydrogens.<\/p>\n<p><strong>Step 3:<\/strong> The charge is now +7 on the left and +2 on the right, so add 5e\u207b on the left.<\/p>\n<p><strong>Answer:<\/strong> <strong>MnO\u2084\u207b + 8H\u207a + 5e\u207b \u2192 Mn\u00b2\u207a + 4H\u2082O<\/strong><\/p>\n<div class=\"ols-key-box\">\n<p><strong>Technique point:<\/strong> Balance atoms first, charge last. The number of electrons must equal the total change in oxidation number: manganese falls from +7 to +2, which is five electrons gained.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Building a Half-Equation<\/h2>\n<p>Put the steps of writing a half-equation in order, then complete a half-equation for an ion not used above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"856\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Combining Half-Equations<\/h2>\n<\/div>\n<p>To build the full ionic equation, the electrons lost in the oxidation half-equation must equal the electrons gained in the reduction half-equation. Multiply one or both half-equations until the electrons match, add them, and cancel the electrons and anything else that appears on both sides.<\/p>\n<p><strong>Worked example:<\/strong> acidified manganate(VII) ions oxidising iron(II) ions.<\/p>\n<p>Oxidation: Fe\u00b2\u207a \u2192 Fe\u00b3\u207a + e\u207b &nbsp;(\u00d7 5)<\/p>\n<p>Reduction: MnO\u2084\u207b + 8H\u207a + 5e\u207b \u2192 Mn\u00b2\u207a + 4H\u2082O<\/p>\n<p>Adding gives <strong>MnO\u2084\u207b + 8H\u207a + 5Fe\u00b2\u207a \u2192 Mn\u00b2\u207a + 4H\u2082O + 5Fe\u00b3\u207a<\/strong>. Check: the charge on the left is \u22121 + 8 + 10 = +17; on the right +2 + 15 = +17.<\/p>\n<p>Oxidation numbers give a second way to balance a redox equation. The total increase in oxidation number must equal the total decrease.<\/p>\n<p>Iron rises by 1 per ion and manganese falls by 5 per ion, so five iron(II) ions are needed for each manganate(VII) ion, which is the same ratio the electrons gave.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> After adding, check three things: atoms balance, charges balance, and no electrons are left in the equation. A leftover e\u207b means the multiplying step was missed.<\/p>\n<\/div>\n<div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-halfcombine.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-halfcombine.jpg\" alt=\"Four-step flow combining the iron(II) and manganate(VII) half-equations, with a charge check of +17 on each side.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-halfcombine.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Multiply the half-equations until the electrons lost equal the electrons gained, add them, cancel the electrons and check that the charges balance.<\/p><\/div>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Combining Half-Equations<\/h2>\n<p>Combine two half-equations for a redox reaction not shown on this page, then check the charge balance.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-857\" class=\"h5p-iframe\" data-content-id=\"857\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups MCQ: Combining Half-Equations\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">7<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Define oxidation and reduction<\/h3><p>Oxidation is loss of electrons or an increase in oxidation number; reduction is gain of electrons or a decrease in oxidation number.<\/p>\n<h3>Identify the oxidising agent<\/h3><p>The species that is reduced (gains electrons, oxidation number falls). Name the species, for example &#8220;Cu\u00b2\u207a&#8221;, not the element alone.<\/p>\n<h3>Write a half-equation in acid<\/h3><p>Balance the main atoms, add H\u2082O for oxygen, H\u207a for hydrogen, then electrons for charge.<\/p>\n<h3>Do not say<\/h3><p>&#8220;The oxidising agent is oxidised&#8221;; &#8220;oxidation is gain of oxygen&#8221; as the only definition; &#8220;electrons on both sides&#8221; in a finished ionic equation.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the electron-transfer ideas and half-equations.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>What is the quickest way to remember oxidation and reduction?<\/h3>\n<p>OIL RIG: oxidation is loss of electrons, reduction is gain. In terms of oxidation numbers, oxidation is an increase and reduction is a decrease.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the oxidising agent the one that is reduced?<\/h3>\n<p>An oxidising agent takes electrons from something else. Gaining those electrons is reduction, so the oxidising agent is reduced while it does its job.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Where do the electrons go in a half-equation?<\/h3>\n<p>On the right for oxidation, because they are lost, and on the left for reduction, because they are gained. Add enough to balance the charge.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I balance a half-equation in acid?<\/h3>\n<p>Balance the main element, add water to balance oxygen, add hydrogen ions to balance hydrogen, then add electrons to balance the charge.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I combine two half-equations?<\/h3>\n<p>Multiply them so the electrons lost equal the electrons gained, add them, and cancel the electrons and anything that appears on both sides.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 8A Redox Chemistry Pages<\/h2>\n<p>Use these pages to connect the ideas across redox chemistry and the rest of Topic 8.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/oxidation-numbers\/\">Oxidation Numbers<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/disproportionation\/\">Disproportionation<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/redox-classification\/\">Redox Classification<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/\">Topic 8A Redox Chemistry Overview<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\n      <\/section>\n    <\/main>\n  \n\n<script type=\"application\/ld+json\">\n{\n  \"@context\": \"https:\/\/schema.org\",\n  \"@graph\": [\n    {\n      \"@type\": \"WebPage\",\n      \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/electron-transfer-and-half-equations\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8a-redox-chemistry\/electron-transfer-and-half-equations\/\",\n      \"name\": \"Electron Transfer and Half-Equations | Topic 8A Redox Chemistry | Online Learning System\",\n      \"description\": \"Edexcel International A Level Chemistry revision notes on redox as electron transfer: oxidising and reducing agents, 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