{"id":8345,"date":"2026-09-18T12:00:43","date_gmt":"2026-09-18T11:00:43","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/bond-enthalpies\/"},"modified":"2026-10-04T23:02:19","modified_gmt":"2026-10-04T22:02:19","slug":"bond-enthalpies","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/bond-enthalpies\/","title":{"rendered":"Bond Enthalpies"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-energetics-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: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 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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 6 Energetics<\/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 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/enthalpy-changes-and-standard-conditions\/\">Enthalpy Changes and Standard Conditions<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/standard-enthalpy-changes\/\">Standard Enthalpy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/calorimetry-measuring-enthalpy-changes\/\">Calorimetry: Measuring Enthalpy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/hess-law-and-enthalpy-cycles\/\">Hess's Law and Enthalpy Cycles<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/bond-enthalpies\/\">Bond Enthalpies<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-2-enthalpy-change-via-hess-law\/\">Core Practical 2<\/a>\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-3-bonding-structure\/\">Bonding &amp; Structure<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Alkenes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Intermolecular Forces<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8a-redox-chemistry\/\">Redox Chemistry<\/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    sidebar.querySelectorAll('.active, 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Topic 6 Energetics<\/a> \/\n<span>Bond Enthalpies<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Bond Enthalpies<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to bond enthalpies: what mean bond enthalpy means, why reactions are exothermic or endothermic, calculating \u0394H from bond enthalpies, and why the answers differ from Hess&#8217;s law values, for Edexcel International A Level Chemistry.<\/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 6: Energetics<\/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: Energy In and Energy Out<\/h2>\n<p>Before you start, check the GCSE rule this page puts numbers on.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"753\"><\/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>What Bond Enthalpy Means<\/h2>\n<\/div>\n<p>The <strong>bond enthalpy<\/strong> is the energy needed to break one mole of a given covalent bond in molecules in the <strong>gaseous state<\/strong>. Breaking a bond always needs energy, so bond enthalpies are always positive; making the same bond releases the same amount of energy.<\/p>\n<p>For a diatomic molecule such as H\u2082 the value is exact. For bonds that appear in many compounds, such as C\u2013H, the value quoted is a <strong>mean bond enthalpy<\/strong>: the average energy needed to break that type of bond, taken over a range of compounds.<\/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\/average-bond-enthalpies-of-common-covalent-bonds.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\/average-bond-enthalpies-of-common-covalent-bonds.jpg\" alt=\"Bar chart of average bond enthalpies for common covalent bonds in kJ per mol\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/average-bond-enthalpies-of-common-covalent-bonds.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Bond enthalpies for common bonds: multiple bonds are stronger than single bonds between the same atoms, and N\u2261N is the strongest shown.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Mean bond enthalpy is the energy needed to break one mole of a given type of bond in gaseous molecules, averaged over a range of compounds.<\/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>Quick Check: Exact or Mean?<\/h2>\n<p>Drag the words into place to decide which of two quoted values is exact.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-562\" class=\"h5p-iframe\" data-content-id=\"562\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Bond Enthalpies Drag: Exact Values and Mean Values\"><\/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>Why Reactions Are Exothermic or Endothermic<\/h2>\n<\/div>\n<p>Every reaction breaks some bonds and makes others. <strong>Bond breaking is endothermic<\/strong> and <strong>bond making is exothermic<\/strong>. The overall enthalpy change depends on which is larger:<\/p>\n<ul>\n<li>If more energy is released making the bonds in the products than is absorbed breaking the bonds in the reactants, the reaction is <strong>exothermic<\/strong>.<\/li>\n<li>If more energy is absorbed breaking bonds than is released making them, the reaction is <strong>endothermic<\/strong>.<\/li>\n<\/ul>\n<!-- 3D card: methane-combustion (26 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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border-radius:22px;}\n  .ols-dch-001 h2.dch-title{font-size:20.5px;}\n  .ols-dch-001 p.dch-sub{font-size:13.5px;}\n  .ols-dch-001 .dch-stage{height:420px;}\n  .ols-dch-001 .dch-rowlab{min-width:100%;}\n  .ols-dch-001 .dch-seg button{padding:9px 13px; font-size:13px;}\n  .ols-dch-001 .dch-lab{font-size:10.5px; padding:4px 8px;}\n  .ols-dch-001 .dch-caption{font-size:10.5px; padding:5px 9px; max-width:64%;}\n  .ols-dch-001 .dch-badge{font-size:10.5px; padding:4px 9px;}\n  .ols-dch-001 .dch-badge.b-shape{font-size:12px;}\n  .ols-dch-001 .dch-badges{max-width:70%;}\n  .ols-dch-001 .dch-info{padding:18px;}\n  .ols-dch-001 .dch-stack{top:auto; bottom:auto; top:48px; max-width:72%;}\n  .ols-dch-001 .r-line{font-size:10.5px;}\n  .ols-dch-001 .r-line span{display:block; margin-left:0; font-size:9.5px;}\n  .ols-dch-001 .dch-read{padding:5px 9px;}\n  .ols-dch-001 .dch-panel{left:auto; right:14px; transform:none; font-size:11px; padding:6px 10px;}\n}\n\/* on a phone the caption needs the full top row, so the drag hint goes *\/\n@media (max-width:480px){\n  .ols-dch-001 .dch-hint{display:none;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-dch-001 .dch-seg button,.ols-dch-001 .dch-pill{transition:none;}\n}\n\n.ols-cc-dch-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  margin:10px auto 0; text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-dch-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-dch-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"dch-head\">\n  <h2 class=\"dch-title\">Bonds broken and made when methane burns<\/h2>\n  <p class=\"dch-sub\">Break every bond in CH<sub>4<\/sub> and 2O<sub>2<\/sub>, form the bonds in CO<sub>2<\/sub> and 2H<sub>2<\/sub>O, and add up the bond enthalpies to find the enthalpy change of combustion.<\/p>\n<\/div>\n\n<div class=\"dch-stage\" id=\"dchStage\">\n  <canvas class=\"dch-canvas\" id=\"dchCanvas\"><\/canvas>\n  <div class=\"dch-overlay\" id=\"dchOverlay\"><\/div>\n  <div class=\"dch-caption\" id=\"dchCaption\"><\/div>\n  <div class=\"dch-hint\" id=\"dchHint\">Drag to rotate<\/div>\n  <div class=\"dch-stack\"><div class=\"dch-read hide\" id=\"dchRead\"><\/div><\/div>\n  <div class=\"dch-panel hide\" id=\"dchPanel\">&Delta;H = +2648 &minus; 3466 = <span class=\"neg\">&minus;818 kJ mol<sup>&minus;1<\/sup><\/span><\/div>\n  <div class=\"dch-badges\" id=\"dchBadges\"><\/div>\n<\/div>\n\n<div class=\"dch-controls\">\n  <div class=\"dch-row\">\n    <span class=\"dch-rowlab\">Step<\/span>\n    <div class=\"dch-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\">Reactants<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\">Break bonds<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Make bonds<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\">&Delta;H<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"dch-row\">\n    <span class=\"dch-rowlab\">View<\/span>\n    <button type=\"button\" class=\"dch-pill\" id=\"dchResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"dch-info\">\n  <h3 id=\"dchInfoTitle\"><\/h3>\n  <p id=\"dchInfoText\"><\/p>\n  <div class=\"dch-facts\" id=\"dchFacts\"><\/div>\n  <div class=\"dch-key\">\n    <span><i style=\"background:#484e5c\"><\/i>Carbon<\/span>\n    <span><i style=\"background:#eef0f5\"><\/i>Hydrogen<\/span>\n    <span><i style=\"background:#cd342c\"><\/i>Oxygen<\/span>\n    <span><i style=\"background:#8c92a0\"><\/i>Bond<\/span>\n    <span><i style=\"background:#e83c34\"><\/i>Bond breaking, energy in<\/span>\n    <span><i style=\"background:#38b258\"><\/i>Bond forming, energy out<\/span>\n    <span><i style=\"background:#c9961c\"><\/i>Bond angle, &Delta;H<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-dch-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/methane-combustion.js?v=20261003e\"><\/script>\n\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> \u0394H = energy absorbed breaking bonds \u2212 energy released making bonds. A negative answer means the products are held by stronger bonds overall.<\/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>Quick Check: Reading the Sign<\/h2>\n<p>In each round, choose the one statement that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-754\" class=\"h5p-iframe\" data-content-id=\"754\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Bond Enthalpies Summary: What a Negative Value Really Tells You\"><\/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>Calculating \u0394H from Bond Enthalpies<\/h2>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key equation:<\/strong> \u0394H = \u03a3(bond enthalpies of bonds broken) \u2212 \u03a3(bond enthalpies of bonds made)<\/p>\n<\/div>\n<p><strong>Worked example.<\/strong> Calculate the enthalpy change for the combustion of methane, CH\u2084(g) + 2O\u2082(g) \u2192 CO\u2082(g) + 2H\u2082O(g), using C\u2013H 413, O=O 498, C=O 805 and O\u2013H 464 kJ mol\u207b\u00b9.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Bonds<\/th><th>Number<\/th><th>Energy \/ kJ<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Broken: C\u2013H<\/strong><\/td><td>4<\/td><td>4 \u00d7 413 = 1652<\/td><\/tr>\n<tr><td><strong>Broken: O=O<\/strong><\/td><td>2<\/td><td>2 \u00d7 498 = 996<\/td><\/tr>\n<tr><td><strong>Total broken<\/strong><\/td><td><\/td><td>+2648<\/td><\/tr>\n<tr><td><strong>Made: C=O<\/strong><\/td><td>2<\/td><td>2 \u00d7 805 = 1610<\/td><\/tr>\n<tr><td><strong>Made: O\u2013H<\/strong><\/td><td>4<\/td><td>4 \u00d7 464 = 1856<\/td><\/tr>\n<tr><td><strong>Total made<\/strong><\/td><td><\/td><td>3466<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>\u0394H = 2648 \u2212 3466 = <strong>\u2212818 kJ mol\u207b\u00b9<\/strong><\/p>\n<p>Counting every bond always works. Where a bond is <strong>unchanged<\/strong> on both sides you may leave it out and count only the bonds that break and form.<\/p><p>The unchanged bonds cancel, so the two routes give the same answer, and the shorter one saves a great deal of time.<\/p><div class=\"ols-key-box\"><p><strong>Exam tip:<\/strong> Data books differ slightly in the values they quote, so always use the values printed in the question.<\/p><\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Draw out the displayed formulae first and count every bond. Most lost marks come from missing a bond, not from the arithmetic.<\/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>Quick Check: Calculate an Enthalpy Change<\/h2>\n<p>Work the calculation out on paper with the data in the question, then choose your answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-563\" class=\"h5p-iframe\" data-content-id=\"563\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Bond Enthalpies MCQ: Adding Hydrogen Bromide to Ethene\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Why Bond Enthalpy Values Differ from Hess Values<\/h2>\n<\/div>\n<p>The data book enthalpy change of combustion of methane is \u2212890 kJ mol\u207b\u00b9, but the bond enthalpy calculation gives \u2212818 kJ mol\u207b\u00b9. There are two reasons:<\/p>\n<ol>\n<li><strong>Mean values are used.<\/strong> The C\u2013H value is an average over many compounds, not the exact value for methane, and the same is true of C=O and O\u2013H.<\/li>\n<li><strong>Everything is assumed to be gaseous.<\/strong> Bond enthalpies apply to gaseous molecules, but standard enthalpy changes of combustion form liquid water. Condensing the water releases extra energy that the calculation leaves out. Put plainly, this calculation gives the value for H\u2082O(g), while the data book enthalpy change of combustion is quoted for H\u2082O(l).<\/li>\n<\/ol>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Values calculated from bond enthalpies are less accurate than those from Hess&#8217;s law because bond enthalpies are averages over many compounds and apply only to substances in the gaseous state.<\/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\/t17x-s4bondvshess.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\/t17x-s4bondvshess.jpg\" alt=\"Methane combustion: data book \u2212890 against bond enthalpy \u2212818 kJ mol\u207b\u00b9, explained by mean values and gaseous water.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t17x-s4bondvshess.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The combustion of methane from data book values and from bond enthalpies, and the two reasons the answers differ.<\/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>Quick Check: Explain the Difference<\/h2>\n<p>Write a short explanation, then compare it with the mark points and the model answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-565\" class=\"h5p-iframe\" data-content-id=\"565\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Bond Enthalpies Explain: Why the Calculated Value Is Not the Data Book Value\"><\/iframe><\/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>Finding a Mean Bond Enthalpy from \u0394H<\/h2>\n<\/div>\n<p>The same equation can be rearranged to find one unknown bond value when \u0394H and the others are known.<\/p>\n<p><strong>Worked example.<\/strong> H\u2082(g) + Cl\u2082(g) \u2192 2HCl(g), \u0394H = \u2212184 kJ mol\u207b\u00b9. Using H\u2013H 436 and Cl\u2013Cl 243 kJ mol\u207b\u00b9, find the H\u2013Cl value.<\/p>\n<ol>\n<li>\u0394H = (436 + 243) \u2212 2E(H\u2013Cl)<\/li>\n<li>\u2212184 = 679 \u2212 2E(H\u2013Cl)<\/li>\n<li>2E(H\u2013Cl) = 863, so E(H\u2013Cl) = <strong>432 kJ mol\u207b\u00b9<\/strong> (3 significant figures)<\/li>\n<\/ol>\n<p><strong>With the unknown on the left.<\/strong> H\u2082(g) + I\u2082(g) \u2192 2HI(g), \u0394H = \u221211 kJ mol\u207b\u00b9, using H\u2013H 436 and H\u2013I 299 kJ mol\u207b\u00b9. Bonds broken are 436 + E(I\u2013I); bonds made are 2 \u00d7 299 = 598. So \u221211 = 436 + E(I\u2013I) \u2212 598, giving E(I\u2013I) = <strong>151 kJ mol\u207b\u00b9<\/strong>.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> Two moles of HCl form, so two H\u2013Cl bonds are made. Divide by the number of bonds of the unknown type at the end.<\/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>Quick Check: Find the Unknown Bond<\/h2>\n<p>Rearrange the key equation to find the missing value, then choose your answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-564\" class=\"h5p-iframe\" data-content-id=\"564\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Bond Enthalpies MCQ: Finding the N-N Bond Enthalpy in Hydrazine\"><\/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>Bond Enthalpy and Reactivity<\/h2>\n<\/div>\n<p>Bond enthalpy data give an indication of how a molecule will react. The <strong>weakest bond<\/strong> in a molecule is usually the first to break, and a bond with a low enthalpy is easier to break.<\/p><p>Reactions that need strong bonds to break tend to be slow at room temperature because few collisions have enough energy.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Observation<\/th><th>Bond enthalpy explanation<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Iodoalkanes are hydrolysed faster than chloroalkanes<\/strong><\/td><td>C\u2013I (238) is much weaker than C\u2013Cl (346), so it breaks more easily<\/td><\/tr>\n<tr><td><strong>Nitrogen is very unreactive<\/strong><\/td><td>N\u2261N (945) is one of the strongest bonds, so reactions of N\u2082 are slow at room temperature<\/td><\/tr>\n<tr><td><strong>Chlorine radicals form when Cl\u2082 absorbs UV light<\/strong><\/td><td>Cl\u2013Cl (243) is weak enough for UV light to break it<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Bond enthalpy is only a guide: the rate of a reaction also depends on bond polarity and on the mechanism.<\/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>Quick Check: Predicting from the Data<\/h2>\n<p>Answer each one quickly, using only the values given.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-755\" class=\"h5p-iframe\" data-content-id=\"755\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Bond Enthalpies Quick Fire: Which Bond Breaks First\"><\/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 mean bond enthalpy<\/h3><p>The energy needed to break one mole of a given type of bond in gaseous molecules, averaged over a range of compounds.<\/p>\n<h3>Calculate \u0394H from bond enthalpies<\/h3><p>Bonds broken minus bonds made; count every bond from the displayed formulae.<\/p>\n<h3>Explain why the answer differs from the data book value<\/h3><p>Bond enthalpies are averages, and the calculation assumes all species are gases.<\/p>\n<h3>Do not say<\/h3><p>&#8220;Bond making needs energy&#8221;; &#8220;bond enthalpies are exact for every molecule&#8221;.<\/p>\n<\/article>\n\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the bond enthalpies ideas and calculations.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why are bond enthalpies always positive?<\/h3>\n<p>They are defined for breaking bonds, and breaking a bond always needs energy.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I calculate \u0394H from bond values?<\/h3>\n<p>Add up the bond enthalpies of all the bonds broken, add up those of all the bonds made, and subtract: bonds broken minus bonds made.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is my answer different from the data book value?<\/h3>\n<p>The bond enthalpies are averages over many compounds, and the method assumes every substance is a gas, whereas standard values may involve liquids.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What makes a reaction exothermic in terms of bonds?<\/h3>\n<p>More energy is released making the bonds in the products than is absorbed breaking the bonds in the reactants.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Are any bond values exact?<\/h3>\n<p>Yes, for diatomic molecules such as H\u2082 or Cl\u2082, where there is only one bond in one environment. Bonds found in many compounds, such as C\u2013H, have average values.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 6 Pages<\/h2>\n<p>Use these pages to connect enthalpy definitions, measurement and calculation methods across the topic.<\/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-6-energetics\/enthalpy-changes-and-standard-conditions\/\">Enthalpy Changes and Standard Conditions<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/standard-enthalpy-changes\/\">Standard Enthalpy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/calorimetry-measuring-enthalpy-changes\/\">Calorimetry: Measuring Enthalpy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/hess-law-and-enthalpy-cycles\/\">Hess&#8217;s Law and Enthalpy Cycles<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/cp-2-enthalpy-change-via-hess-law\/\">CP2 Enthalpy Change via Hess&#8217;s Law<\/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-6-energetics\/bond-enthalpies\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/bond-enthalpies\/\",\n      \"name\": \"Bond Enthalpies | Topic 6 Energetics | Online Learning System\",\n      \"description\": \"Bond enthalpies for Edexcel International A Level Chemistry: mean bond enthalpy, bond breaking and making, calculating enthalpy changes and their limitations.\",\n      \"isPartOf\": {\n        \"@id\": \"https:\/\/www.onlinelearningsystem.net\/#website\"\n      },\n      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bond enthalpy means, why reactions are exothermic or endothermic, calculating \u0394H from bond enthalpies, and why the answers differ from Hess&#8217;s law values, for Edexcel International A Level 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