{"id":8367,"date":"2026-09-18T12:01:28","date_gmt":"2026-09-18T11:01:28","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-2-1-enthalpy-changes\/bond-enthalpies\/"},"modified":"2026-09-26T10:30:00","modified_gmt":"2026-09-26T09:30:00","slug":"bond-enthalpies","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-2-1-enthalpy-changes\/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>3.2.1 Enthalpy Changes<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-2-1-enthalpy-changes\/\">3.2.1 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-2-1-enthalpy-changes\/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\/ocr-a\/3-2-1-enthalpy-changes\/standard-enthalpy-changes\/\">Standard Enthalpy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-2-1-enthalpy-changes\/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\/ocr-a\/3-2-1-enthalpy-changes\/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\/ocr-a\/3-2-1-enthalpy-changes\/bond-enthalpies\/\">Bond Enthalpies<\/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\/ocr-a\/2-1-3-amount-of-substance\/\">2.1.3 Amount of Substance<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/\">2.2.2 Bonding and Structure<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-1-periodicity\/\">3.1.1 Periodicity<\/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\/ocr-a\/3-2-1-enthalpy-changes\/\">3.2.1 Enthalpy Changes<\/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 average bond enthalpies: what average bond enthalpy means, why reactions are exothermic or endothermic, calculating \u0394H from average bond enthalpies, and why the answers differ from Hess&#8217;s law values, for OCR A A Level Chemistry.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1<\/div>\n<div class=\"ols-badge\">3.2.1: Enthalpy Changes<\/div>\n<div class=\"ols-badge\">H432\/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 average 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 an <strong>average 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>Average 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> Average 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 Average Bond Enthalpies<\/h2>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key equation:<\/strong> \u0394H = \u03a3(average bond enthalpies of bonds broken) \u2212 \u03a3(average 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> Average 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 average bond enthalpies are less accurate than those from Hess&#8217;s law because average 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 an Average 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 average 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 average 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>Average 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;average 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 average bond enthalpies ideas and calculations.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why are average 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 average 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 average 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 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\/ocr-a\/3-2-1-enthalpy-changes\/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\/ocr-a\/3-2-1-enthalpy-changes\/standard-enthalpy-changes\/\">Standard Enthalpy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-2-1-enthalpy-changes\/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\/ocr-a\/3-2-1-enthalpy-changes\/hess-law-and-enthalpy-cycles\/\">Hess&#8217;s Law and Enthalpy Cycles<\/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>. It is designed for A Level Chemistry revision and should not be copied or redistributed without permission.<\/p>\n      <\/section>\n\n      <div class=\"ols-image-lightbox\" id=\"olsImageLightboxNatureCovalentBonding9ch0\" aria-hidden=\"true\" role=\"dialog\" aria-modal=\"true\" aria-label=\"Expanded revision image\">\n        <div class=\"ols-image-lightbox-inner\">\n          <button class=\"ols-image-lightbox-close\" type=\"button\" aria-label=\"Close enlarged image\">\u00d7<\/button>\n          <img decoding=\"async\" class=\"ols-image-lightbox-img\" src=\"\" alt=\"\">\n        <\/div>\n      <\/div>\n\n      <script>\n        (function(){\n          var page = document.querySelector(\".ols-nature-of-covalent-bonding-9ch0-page\");\n          if (!page) { return; }\n          var lightbox = page.querySelector(\"#olsImageLightboxNatureCovalentBonding9ch0\");\n          if (!lightbox) { return; }\n          var lightboxImage = lightbox.querySelector(\".ols-image-lightbox-img\");\n    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