{"id":10476,"date":"2026-09-26T11:30:28","date_gmt":"2026-09-26T10:30:28","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\/redox-classification\/"},"modified":"2026-10-04T23:02:37","modified_gmt":"2026-10-04T22:02:37","slug":"redox-classification","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\/redox-classification\/","title":{"rendered":"Redox Classification"},"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      --soft-green: #f0f7f1;\n      --gold: #c9973a;\n      --grey-text: 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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>Redox Classification<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Redox Classification<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to using oxidation numbers to decide whether a reaction is redox, disproportionation or neither, applied to s-block and p-block reactions, metals with acids and unfamiliar reactions given as 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      <article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Using Oxidation numbers to Classify Reactions<\/h2>\n<\/div>\n<p>Every reaction can be tested with one question: does the oxidation number of any element change? If it does, the reaction is <strong>redox<\/strong>; if nothing changes, it is not.<\/p>\n<p>Acid\u2013base reactions, precipitation reactions and thermal decompositions that only rearrange ions are not redox, even when they look dramatic.<\/p>\n<p>Combustion, displacement, the reactions of metals with acids and every reaction of an element are redox.<\/p>\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-redox-classification-flowchart.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-redox-classification-flowchart.png\" alt=\"Flowchart for deciding whether a reaction is redox, with four worked examples\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-redox-classification-flowchart.png\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The flowchart asks one question first, then names the element oxidised and the element reduced, then checks whether they are the same element.<\/p><\/div>\n<\/div>\n<p>The second use of the test is to sort redox reactions into ordinary redox, where two different elements change, and <strong>disproportionation<\/strong>, where one element goes both ways. The flowchart handles both.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Assign oxidation numbers to every element on both sides before deciding anything. A quick look at the equation is not enough: the reaction of chlorine with water looks like a simple addition until the numbers are written in.<\/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: Redox or Not?<\/h2>\n<p>Sort a set of equations you have not met on this page into redox and not redox by assigning oxidation numbers.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-860\" class=\"h5p-iframe\" data-content-id=\"860\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Summary: Redox or Not?\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Redox in the s-Block<\/h2>\n<\/div>\n<p>The Group 1 and Group 2 metals have low ionisation energies, so their reactions are all oxidations of the metal.<\/p>\n<p>When calcium reacts with water, Ca + 2H\u2082O \u2192 Ca(OH)\u2082 + H\u2082, calcium rises from 0 to +2 and is oxidised. Hydrogen in the water falls from +1 to 0 and is reduced; the oxygen stays at \u22122 throughout.<\/p>\n<p>Calcium is the reducing agent and water is the oxidising agent.<\/p>\n<p>When sodium burns in oxygen, 4Na + O\u2082 \u2192 2Na\u2082O, sodium rises from 0 to +1 and oxygen falls from 0 to \u22122. The same sentences describe magnesium burning, lithium in chlorine and barium in water: the metal is oxidised, the non-metal is reduced.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Calcium is oxidised because its oxidation number increases from 0 to +2; hydrogen is reduced because its oxidation number decreases from +1 to 0.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Redox in the p-Block<\/h2>\n<\/div>\n<p>The p-block non-metals are usually the oxidising agents.<\/p>\n<div class=\"ols-table-wrap\"><table class=\"ols-table\"><thead><tr><th>Reaction<\/th><th>Equation<\/th><th>Oxidation<\/th><th>Reduction<\/th><\/tr><\/thead><tbody><tr><td><strong>Chlorine displacing bromine from bromide ions<\/strong><\/td><td>Cl\u2082 + 2Br\u207b \u2192 2Cl\u207b + Br\u2082<\/td><td>oxidises bromine (\u22121 to 0)<\/td><td>reduces chlorine (0 to \u22121)<\/td><\/tr><tr><td><strong>Sulfur burning<\/strong><\/td><td>S + O\u2082 \u2192 SO\u2082<\/td><td>oxidises sulfur (0 to +4)<\/td><td>reduces oxygen.<\/td><\/tr><tr><td><strong>Carbon reducing iron(III) oxide in a blast furnace<\/strong><\/td><td>Fe\u2082O\u2083 + 3CO \u2192 2Fe + 3CO\u2082<\/td><td>oxidises carbon (+2 to +4)<\/td><td>reduces iron (+3 to 0).<\/td><\/tr><\/tbody><\/table><\/div>\n<p>The p-block also provides the reactions that are not redox. Sulfur dioxide dissolving in water to give sulfurous acid keeps sulfur at +4. Phosphorus(V) chloride hydrolysing keeps phosphorus at +5. In both, atoms are rearranged but no electrons move.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Reaction<\/th><th>Element up<\/th><th>Element down<\/th><th>Classification<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>2Al + 3Cl\u2082 \u2192 2AlCl\u2083<\/strong><\/td><td>Al, 0 to +3<\/td><td>Cl, 0 to \u22121<\/td><td>redox<\/td><\/tr>\n<tr><td><strong>SO\u2082 + H\u2082O \u2192 H\u2082SO\u2083<\/strong><\/td><td>none<\/td><td>none<\/td><td>not redox<\/td><\/tr>\n<tr><td><strong>2NO\u2082 + H\u2082O \u2192 HNO\u2083 + HNO\u2082<\/strong><\/td><td>N, +4 to +5<\/td><td>N, +4 to +3<\/td><td>disproportionation<\/td><\/tr>\n<tr><td><strong>Cl\u2082 + 2KI \u2192 2KCl + I\u2082<\/strong><\/td><td>I, \u22121 to 0<\/td><td>Cl, 0 to \u22121<\/td><td>redox<\/td><\/tr>\n<tr><td><strong>CaCO\u2083 \u2192 CaO + CO\u2082<\/strong><\/td><td>none<\/td><td>none<\/td><td>not redox<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Common mistake:<\/strong> Assuming that any reaction with oxygen in it is redox. Thermal decomposition of a carbonate moves oxygen atoms around without changing any oxidation number.<\/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 Reacting with Acids<\/h2>\n<\/div>\n<p>A metal reacting with a dilute acid to form a salt and hydrogen is a redox reaction in which the metal is oxidised and the hydrogen ions are reduced.<\/p>\n<p>For zinc in hydrochloric acid the full equation is Zn + 2HCl \u2192 ZnCl\u2082 + H\u2082 and the ionic equation is Zn + 2H\u207a \u2192 Zn\u00b2\u207a + H\u2082. The chloride ions are spectators and their oxidation number does not change.<\/p>\n<p>The half-equations make the electron transfer explicit: Zn \u2192 Zn\u00b2\u207a + 2e\u207b (oxidation) and 2H\u207a + 2e\u207b \u2192 H\u2082 (reduction). Magnesium with sulfuric acid, iron with hydrochloric acid and aluminium with any dilute acid follow the same pattern; only the charge on the metal ion, and so the number of electrons, changes.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Write the ionic equation with state symbols and be ready to say which species is the oxidising agent. In Zn + 2H\u207a \u2192 Zn\u00b2\u207a + H\u2082 the oxidising agent is H\u207a, because the hydrogen ions gain the electrons.<\/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: Metals, Acids and Half-Equations<\/h2>\n<p>Write the ionic equation and half-equations for a metal and acid combination not used on this page, and name the oxidising agent.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"861\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Unfamiliar Redox Reactions<\/h2>\n<\/div>\n<p>Exam questions often give an equation for a reaction that has never been taught and ask for the oxidation numbers, the species oxidised and reduced, and the oxidising and reducing agents. The method is always the same and does not need the chemistry to be recognised.<\/p>\n<ol>\n<li>Assign oxidation numbers to every element on both sides.<\/li>\n<li>Find the element whose value rises: it has been oxidised, and the species containing it is the reducing agent.<\/li>\n<li>Find the element whose value falls: it has been reduced, and the species containing it is the oxidising agent.<\/li>\n<li>Check that the total rise equals the total fall; if it does not, the equation is not balanced.<\/li>\n<\/ol>\n<p><strong>Worked example:<\/strong> 3Cu + 8HNO\u2083 \u2192 3Cu(NO\u2083)\u2082 + 2NO + 4H\u2082O.<\/p><p><strong>Step 1:<\/strong> Copper rises from 0 to +2 (oxidised, so copper is the reducing agent).<\/p><p><strong>Step 2:<\/strong> Nitrogen in HNO\u2083 is +5; in NO it is +2, so two of the eight nitrogen atoms have been reduced. Nitric acid is the oxidising agent.<\/p><p><strong>Answer:<\/strong> Total rise 3 \u00d7 2 = 6; total fall 2 \u00d7 3 = 6, so the equation balances.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Technique point:<\/strong> Not every atom of the oxidising agent is reduced. Six of the eight nitrate ions here are spectators that end up in copper(II) nitrate; only the two that form NO change.<\/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-unfamiliar.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-unfamiliar.jpg\" alt=\"Four-step method for an unfamiliar redox equation, worked for copper and nitric acid, with spectator nitrate ions shown.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t8x-unfamiliar.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Assign every oxidation number, find the element that rises and the one that falls, and check that the total rise equals the total fall.<\/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: An Unfamiliar Equation<\/h2>\n<p>Work through an equation for a reaction you have not studied and identify the oxidising and reducing agents.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-862\" class=\"h5p-iframe\" data-content-id=\"862\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Explain: An Unfamiliar Equation\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>State whether a reaction is redox and explain<\/h3><p>Yes or no, then the element and its change: &#8220;redox, because copper changes from 0 to +2 and nitrogen from +5 to +3&#8221;.<\/p>\n<h3>Identify the reducing agent<\/h3><p>The species containing the element that is oxidised, named as the whole species, for example &#8220;copper&#8221; or &#8220;iodide ions&#8221;.<\/p>\n<h3>Do not say<\/h3><p>&#8220;The reaction is redox because oxygen is involved&#8221;; &#8220;the reducing agent is reduced&#8221;; &#8220;nitrogen is reduced&#8221; without giving the values.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check how reactions are classified with oxidation numbers.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>How do I know if a reaction is redox?<\/h3>\n<p>Assign oxidation numbers to every element on both sides. If any value changes, the reaction is redox; if none changes, it is not.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Is neutralisation a redox reaction?<\/h3>\n<p>No. Hydrogen stays at +1, oxygen at \u22122 and the other elements unchanged, so no electrons are transferred.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What is the oxidising agent when magnesium reacts with acid?<\/h3>\n<p>The hydrogen ions, H\u207a, because they gain the electrons that magnesium loses and are reduced to hydrogen gas.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Does every atom of the oxidising agent get reduced?<\/h3>\n<p>Not always. In copper reacting with nitric acid only some of the nitrate ions are reduced to nitrogen monoxide; the rest end up unchanged in copper(II) nitrate.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I check an unfamiliar redox equation is balanced?<\/h3>\n<p>The total increase in oxidation number must equal the total decrease, and the charges on each side must be equal.<\/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\/electron-transfer-and-half-equations\/\">Electron Transfer and Half-Equations<\/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\/\">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\/redox-classification\/#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\/redox-classification\/\",\n      \"name\": \"Redox Classification | Topic 8A Redox Chemistry | Online Learning System\",\n      \"description\": \"Edexcel International A Level Chemistry revision notes on classifying reactions with oxidation numbers: redox or not, s-block and p-block examples, metals with acids, and 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