{"id":8353,"date":"2026-09-18T12:01:01","date_gmt":"2026-09-18T11:01:01","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-8-energetics-i\/bond-enthalpies\/"},"modified":"2026-09-26T10:29:57","modified_gmt":"2026-09-26T09:29:57","slug":"bond-enthalpies","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-8-energetics-i\/bond-enthalpies\/","title":{"rendered":"Bond Enthalpies"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-nature-of-covalent-bonding-9ch0-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      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px 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.ols-figure-placeholder .ols-figure-caption p { margin: 10px 0 0; font-size: 14px; color: #667085; text-align: center; }\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 Energetics I<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-8-energetics-i\/\">Topic 8 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-8-energetics-i\/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\/topic-8-energetics-i\/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\/topic-8-energetics-i\/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\/topic-8-energetics-i\/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\/topic-8-energetics-i\/bond-enthalpies\/\">Bond Enthalpies<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-8-enthalpy-change-via-hess-law\/\">Core Practical 8<\/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\/topic-2-bonding-and-structure\/\">Topic 2 Bonding and Structure<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/\">Topic 5 Formulae, Equations and Amounts<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-6-organic-chemistry-i\/\">Topic 6 Organic Chemistry I<\/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 = 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/\">Edexcel<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-8-energetics-i\/\">Topic 8 Energetics I<\/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 A Level Chemistry.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Unit: Paper 1<\/div>\n<div class=\"ols-badge\">Topic 8: Energetics I<\/div>\n<div class=\"ols-badge\">9CH0\/01<\/div>\n<\/div>\n\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:<br><span>Dr. Mohammed Al-Fatah<\/span>\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-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/average-bond-enthalpies-of-common-covalent-bonds.jpg\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" 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\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-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<section class=\"ols-dch-001\" id=\"olsDch001\">\n<style>\n@import url('https:\/\/fonts.googleapis.com\/css2?family=Poppins:wght@300;400;500;600&display=swap');\n\n.ols-dch-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  color:#1C244B; background:#ffffff;\n  border:1px solid rgba(28,36,75,0.14);\n  border-radius:28px;\n  box-shadow:0 18px 45px rgba(28,36,75,0.10);\n  padding:44px; margin:26px auto; max-width:980px;\n  box-sizing:border-box; -webkit-font-smoothing:antialiased;\n}\n.ols-dch-001 *{box-sizing:border-box;}\n\n.ols-dch-001 .dch-head{margin:0 0 22px;}\n.ols-dch-001 h2.dch-title{\n  font-size:26px; 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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\">Animation<\/span>\n    <button type=\"button\" class=\"dch-pill\" id=\"dchPlay\" aria-pressed=\"false\"><i class=\"dot\"><\/i>Play all<\/button>\n    <button type=\"button\" class=\"dch-pill\" id=\"dchReset\">Reset<\/button>\n  <\/div>\n\n  <div class=\"dch-row\">\n    <span class=\"dch-rowlab\">View<\/span>\n    <div class=\"dch-seg\" role=\"group\" aria-label=\"Camera view\">\n      <button type=\"button\" data-v=\"front\" aria-pressed=\"false\">Front<\/button>\n      <button type=\"button\" data-v=\"side\" aria-pressed=\"false\">Side<\/button>\n      <button type=\"button\" data-v=\"top\" aria-pressed=\"false\">Top<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"dch-row\">\n    <span class=\"dch-rowlab\">Show<\/span>\n    <button type=\"button\" class=\"dch-pill\" id=\"dchTogLab\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Bond labels<\/button>\n    <button type=\"button\" class=\"dch-pill\" id=\"dchTogBars\" aria-pressed=\"true\"><i class=\"dot\"><\/i>Energy bars<\/button>\n  <\/div>\n\n  <div class=\"dch-row\">\n    <span class=\"dch-rowlab\">Motion<\/span>\n    <button type=\"button\" class=\"dch-pill\" id=\"dchSpin\" aria-pressed=\"false\"><i class=\"dot\"><\/i>Rotation<\/button>\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=20260926\"><\/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: the unchanged bonds cancel, so the two routes give the same answer and the shorter one saves a great deal of time. Data books differ slightly in the values they quote, so always use the values printed in the question.<\/p>\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<\/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>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-course-cta-energetics\" id=\"olsCourseCtaEnergeticsDom001\" data-ols-soon=\"1\">\n  <style>\n    .ols-course-cta-energetics,\n    .ols-course-cta-energetics * {\n      box-sizing: border-box;\n    }\n\n    .ols-course-cta-energetics {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --body-text: #1f2937;\n      --grey-text: #667085;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.12);\n      --inner-shadow: 0 10px 26px rgba(28, 36, 75, 0.08);\n\n      width: 100%;\n      margin: 30px 0 24px;\n      font-family: Poppins, 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solid transparent;\n      border-bottom: 18px solid transparent;\n      border-left: 28px solid currentColor;\n    }\n\n    .ols-course-cta-energetics .ols-animation-start-text {\n      max-width: 540px;\n      margin: 0;\n      color: #ffffff;\n      font-size: clamp(20px, 3vw, 32px);\n      line-height: 1.2;\n      font-weight: 800;\n      text-shadow: 0 8px 20px rgba(0, 0, 0, 0.28);\n    }\n\n    .ols-course-cta-energetics .ols-animation-pause-button {\n      position: absolute;\n      left: 16px;\n      bottom: 16px;\n      z-index: 7;\n      display: none;\n      align-items: center;\n      justify-content: center;\n      min-width: 92px;\n      padding: 10px 16px;\n      border: 0;\n      border-radius: 999px;\n      background: rgba(28, 36, 75, 0.92);\n      color: #ffffff;\n      font-family: Poppins, Arial, sans-serif;\n      font-size: 14px;\n      line-height: 1.2;\n      font-weight: 800;\n      cursor: pointer;\n      box-shadow: 0 12px 28px rgba(28, 36, 75, 0.24);\n      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stretch;\n      }\n\n      .ols-course-cta-energetics .ols-course-button-top {\n        width: 100%;\n      }\n\n      .ols-course-cta-energetics .ols-course-cta-stats,\n      .ols-course-cta-energetics .ols-course-cta-features {\n        grid-template-columns: 1fr;\n      }\n\n      .ols-course-cta-energetics .ols-course-animation-wrap {\n        padding: 0;\n        border-radius: 0;\n      }\n\n      .ols-course-cta-energetics .ols-animation-frame-shell {\n        height: 420px;\n        border-radius: 18px;\n      }\n\n      .ols-course-cta-energetics .ols-animation-play-circle {\n        width: 76px;\n        height: 76px;\n      }\n\n      .ols-course-cta-energetics .ols-animation-play-icon {\n        border-top-width: 14px;\n        border-bottom-width: 14px;\n        border-left-width: 22px;\n      }\n\n      .ols-course-cta-energetics .ols-animation-pause-button {\n        left: 12px;\n        bottom: 12px;\n        min-width: 84px;\n        padding: 9px 14px;\n        font-size: 13px;\n      }\n\n      .ols-course-cta-energetics .ols-course-button {\n        width: 100%;\n      }\n    }\n  <\/style>\n\n  <div class=\"ols-course-cta-card\">\n\n    <div class=\"ols-course-cta-top\">\n\n      <a class=\"ols-course-cta-image-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/product\/energetics-i-topic-8-edexcel\/\" aria-label=\"Open the Edexcel A Level Chemistry Topic 8 Energetics I course page\">\n\n        <div class=\"ols-course-cta-image\">\n          <img decoding=\"async\"\n            src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/edexcel-a-level-chemistry-topic-8-energetics-i-interactive-course-banner.jpg\"\n            alt=\"Edexcel A Level Chemistry Topic 8 Energetics I interactive course banner\"\n          >\n        <\/div>\n\n      <\/a>\n\n      <div class=\"ols-course-cta-content\">\n\n        <div class=\"ols-course-cta-header-row\">\n\n          <div class=\"ols-course-cta-kicker\">\n            Paper 1 | 9CH0\/01 | Topic 8 Energetics I Course\n          <\/div>\n\n          <a class=\"ols-course-button ols-course-button-top\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/product\/energetics-i-topic-8-edexcel\/\">\n            View Course\n          <\/a>\n\n        <\/div>\n\n        <h2>\n          Master Energetics I for Edexcel A Level Chemistry\n        <\/h2>\n\n      <\/div>\n\n    <\/div>\n\n    <div class=\"ols-course-cta-intro-stats\">\n\n      <p class=\"ols-course-cta-intro\">\n        Continue from these free revision notes into the full Topic 8 Energetics I course, with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a specification assignment with a personalised progress report.\n      <\/p>\n\n      <div class=\"ols-course-cta-stats\">\n\n        <div class=\"ols-course-stat\">\n          <span>Guided learning<\/span>\n          <strong>Coming soon<\/strong>\n        <\/div>\n\n        <div class=\"ols-course-stat\">\n          <span>Video lessons<\/span>\n          <strong>Coming soon<\/strong>\n        <\/div>\n\n        <div class=\"ols-course-stat\">\n          <span>MCQ practice<\/span>\n          <strong>Coming soon<\/strong>\n        <\/div>\n\n        <div class=\"ols-course-stat\">\n          <span>SAQ practice<\/span>\n          <strong>Coming soon<\/strong>\n        <\/div>\n\n      <\/div>\n\n    <\/div>\n\n    <div class=\"ols-course-cta-features\">\n\n      <div class=\"ols-course-feature\">\n        <h3>Guided video teaching<\/h3>\n        <p>\n          Learn the chemistry and exam technique through structured video lessons with worked examples and walkthroughs.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-course-feature\">\n        <h3>Instant MCQ feedback<\/h3>\n        <p>\n          Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-course-feature\">\n        <h3>Teacher-marked SAQs<\/h3>\n        <p>\n          Submit written exam responses and receive chemistry specialist feedback with improvement guidance.\n        <\/p>\n      <\/div>\n\n      <div class=\"ols-course-feature\">\n        <h3>Progress tracking<\/h3>\n        <p>\n          Identify strengths and weaknesses across the full Topic 8 specification with targeted reporting.\n        <\/p>\n      <\/div>\n\n    <\/div>\n\n    <div class=\"ols-course-animation-wrap\">\n\n      <h3 class=\"ols-course-animation-title\">\n        See how the course works\n      <\/h3>\n\n      <div class=\"ols-animation-frame-shell\">\n        <iframe\n          id=\"olsCourseAnimationFrame001\"\n          title=\"OLS course preview animation\"\n          loading=\"lazy\"\n          referrerpolicy=\"no-referrer\">\n        <\/iframe>\n\n        <button\n          class=\"ols-animation-start-overlay\"\n          id=\"olsCourseAnimationStart001\"\n          type=\"button\"\n          aria-label=\"Play OLS course preview animation\">\n          <span class=\"ols-animation-start-content\">\n            <span class=\"ols-animation-play-circle\" aria-hidden=\"true\">\n              <span class=\"ols-animation-play-icon\"><\/span>\n            <\/span>\n            <span class=\"ols-animation-start-text\">\n              Play course preview animation\n            <\/span>\n          <\/span>\n        <\/button>\n\n        <button\n          class=\"ols-animation-pause-button\"\n          id=\"olsCourseAnimationPause001\"\n          type=\"button\"\n          aria-label=\"Pause course preview animation\">\n          Pause\n        <\/button>\n      <\/div>\n\n      <p class=\"ols-course-video-status\">\n        Click play to start the course preview animation.\n      <\/p>\n\n    <\/div>\n\n    <div class=\"ols-course-cta-bottom\">\n\n      <a class=\"ols-course-button\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/product\/energetics-i-topic-8-edexcel\/\">\n        View Energetics I Topic 8 Course\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n\n  <template id=\"olsCourseAnimationTemplate001\">\n<!DOCTYPE html>\n<html lang=\"en\">\n<head>\n<meta charset=\"UTF-8\">\n<meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0\">\n<title>OLS Combined Promo Animation<\/title>\n\n<style>\n*{box-sizing:border-box;}\n\nhtml.ols-animation-paused *,\nhtml.ols-animation-paused *::before,\nhtml.ols-animation-paused *::after{\n  animation-play-state:paused!important;\n}\n\n:root{\n  --ols-video-screen-w:1180;\n  --ols-video-screen-h:664;\n  --ols-video-screen-scale:1;\n  --ols-mcq-screen-w:1360;\n  --ols-mcq-screen-h:1130;\n  --ols-mcq-screen-scale:1;\n  --ols-saq-screen-w:1360;\n  --ols-saq-screen-h:965;\n  --ols-saq-screen-scale:1;\n}\n\nhtml,\nbody{\n  width:100%;\n  height:100%;\n}\n\nbody{\n  margin:0;\n  overflow:hidden;\n  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id=\"mcqWrongOption\" class=\"mcq-option-row\">\n                <span class=\"mcq-radio\"><\/span>\n                <span class=\"mcq-radio-ripple\"><\/span>\n                <span class=\"mcq-option-label\">sodium chloride<\/span>\n\n                <span id=\"mcqSpecificFeedback\" class=\"mcq-specific-feedback\">\n                  <span id=\"mcqCrossIcon\" class=\"mcq-cross-icon\">\u00d7<\/span>\n                  <span class=\"mcq-specific-feedback-text\">\n                    <span id=\"mcqSpecificText\"><\/span><span id=\"mcqSpecificCursor\" class=\"cursor mcq-specific-cursor\" style=\"display:none;\"><\/span>\n                  <\/span>\n                <\/span>\n              <\/div>\n\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">potassium chloride<\/span><\/div>\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">sodium fluoride<\/span><\/div>\n              <div class=\"mcq-option-row\"><span class=\"mcq-radio\"><\/span><span class=\"mcq-radio-ripple\"><\/span><span class=\"mcq-option-label\">potassium fluoride<\/span><\/div>\n            <\/div>\n\n            <button id=\"mcqSubmitButton\" class=\"mcq-submit-button\">Submit answer<\/button>\n          <\/section>\n\n          <section id=\"mcqGeneralFeedback\" class=\"mcq-general-feedback\">\n            <span id=\"mcqGeneralText\"><\/span><span id=\"mcqGeneralCursor\" class=\"cursor mcq-general-cursor\" style=\"display:none;\"><\/span>\n          <\/section>\n        <\/main>\n      <\/div>\n    <\/div>\n  <\/section>\n\n  <section id=\"sceneSaq\" class=\"ols-scene\">\n    <div class=\"ols-saq-stage\">\n      <div id=\"saqIntro\" class=\"ols-saq-intro\">\n        <div class=\"ols-saq-intro-title-wrap\">\n          <h1 class=\"ols-saq-intro-title\">\n            <span id=\"saqIntroTitleText\"><\/span><span id=\"saqIntroTitleCursor\" class=\"ols-title-cursor\"><\/span>\n          <\/h1>\n        <\/div>\n      <\/div>\n\n      <div class=\"ols-saq-scale-wrap\">\n        <main id=\"saqScreen\" class=\"ols-saq-screen\">\n          <section class=\"saq-question-area\">\n            <div class=\"saq-question-text\">\n              <strong>(ii)<\/strong>&nbsp;&nbsp; Melting temperature depends on the strength of metallic bonding.<br>\n              Explain why the metallic bonding in magnesium is much stronger than that in sodium.\n            <\/div>\n\n            <div class=\"saq-student-box\">\n              <span id=\"saqStudentText\"><\/span><span id=\"saqStudentCursor\" class=\"cursor\"><\/span>\n            <\/div>\n          <\/section>\n\n          <section id=\"saqTeacherFeedback\" class=\"saq-teacher-feedback\">\n            <span id=\"saqTeacherText\"><\/span><span id=\"saqTeacherCursor\" class=\"cursor saq-teacher-cursor\" 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NaF.\";\nconst mcqGeneralFeedbackText=\n\"Your answer is incorrect.\\n\\n\"+\n\"To determine which compound has the strongest ionic bonding, recall the key principle:\\n\\n\"+\n\"\u2022 Ionic bond strength increases when ionic radii are small, because the ions can get closer together.\\n\"+\n\"\u2022 Smaller ions \u2192 stronger electrostatic attraction \u2192 stronger ionic lattice.\\n\"+\n\"\u2022 Na\u207a (0.102 nm) is smaller than K\u207a (0.138 nm).\\n\"+\n\"\u2022 F\u207b (0.133 nm) is smaller than Cl\u207b (0.180 nm).\\n\\n\"+\n\"Therefore, the strongest ionic bonding occurs in the compound formed by the smallest cation and the smallest anion \u2192 sodium fluoride (NaF).\\n\\n\"+\n\"The correct answer is: sodium fluoride\";\n\nconst mcqIntroScreen=document.getElementById(\"mcqIntroScreen\");\nconst mcqIntroTitleText=document.getElementById(\"mcqIntroTitleText\");\nconst mcqIntroTitleCursor=document.getElementById(\"mcqIntroTitleCursor\");\nconst 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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 Energetics 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\/topic-8-energetics-i\/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\/topic-8-energetics-i\/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\/topic-8-energetics-i\/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\/topic-8-energetics-i\/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\/core-practicals\/cp-8-enthalpy-change-via-hess-law\/\">CP8 Enthalpy Change via Hess&#8217;s Law<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright and author footprint:<\/strong> This OLS revision page was written for Online Learning System by <strong>Dr. Mohammed Al-Fatah<\/strong>. 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