{"id":8357,"date":"2026-09-18T12:01:10","date_gmt":"2026-09-18T11:01:10","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-energetics\/standard-enthalpy-changes\/"},"modified":"2026-09-18T12:08:40","modified_gmt":"2026-09-18T11:08:40","slug":"standard-enthalpy-changes","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-energetics\/standard-enthalpy-changes\/","title":{"rendered":"Standard Enthalpy Changes"},"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      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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>3.1.4 Energetics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-energetics\/\">3.1.4 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-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\/aqa\/3-1-4-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\/aqa\/3-1-4-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\/aqa\/3-1-4-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\/aqa\/3-1-4-energetics\/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\/aqa\/3-1-2-amount-of-substance\/\">3.1.2 Amount of Substance<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-3-bonding\/\">3.1.3 Bonding<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-2-1-periodicity\/\">3.2.1 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    sidebar.querySelectorAll('.active, .active-main, .parent-active').forEach(function(item) {\r\n      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Changes<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the standard enthalpy changes of reaction, formation, combustion and neutralisation, with the equation that goes with each definition, for AQA A Level Chemistry.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 and 2 AQA<\/div>\n<div class=\"ols-badge\">3.1.4 Energetics<\/div>\n<div class=\"ols-badge\">7405\/1 and 7405\/2<\/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      <article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Why the Wording Matters<\/h2>\n<\/div>\n<p>Each <strong>standard enthalpy change<\/strong> is defined by exactly what is formed, burned or produced, and how much of it. Mark schemes award a mark for each part of a definition, so a definition that leaves out &#8220;one mole&#8221;, &#8220;standard states&#8221; or &#8220;completely&#8221; loses marks even when the chemistry is understood.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Enthalpy change<\/th><th>Symbol<\/th><th>Definition<\/th><th>Sign<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Reaction<\/strong><\/td><td>\u0394<sub>r<\/sub>H<sup>\u29b5<\/sup><\/td><td>the enthalpy change for the molar quantities in a stated equation, under standard conditions, with all substances in their standard states<\/td><td>varies<\/td><\/tr>\n<tr><td><strong>Formation<\/strong><\/td><td>\u0394<sub>f<\/sub>H<sup>\u29b5<\/sup><\/td><td>the enthalpy change when one mole of a compound is formed from its elements under standard conditions, all substances in their standard states<\/td><td>usually negative; zero for an element<\/td><\/tr>\n<tr><td><strong>Combustion<\/strong><\/td><td>\u0394<sub>c<\/sub>H<sup>\u29b5<\/sup><\/td><td>the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all substances in their standard states<\/td><td>always negative<\/td><\/tr>\n<tr><td><strong>Neutralisation<\/strong><\/td><td>\u0394<sub>neut<\/sub>H<sup>\u29b5<\/sup><\/td><td>the enthalpy change when one mole of water is formed in a reaction between an acid and a base under standard conditions<\/td><td>always negative<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Learn each definition word for word, then check your version has three parts: the quantity (one mole of what), the process, and standard conditions with standard states.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Enthalpy Change of Reaction<\/h2>\n<\/div>\n<p>The <strong>enthalpy change of reaction<\/strong> refers to the amounts written in a particular equation. If the equation is doubled, the enthalpy change doubles; if it is reversed, the sign changes. For example:<\/p>\n<p>N\u2082(g) + 3H\u2082(g) \u2192 2NH\u2083(g) &nbsp; \u0394<sub>r<\/sub>H<sup>\u29b5<\/sup> = \u221292 kJ mol\u207b\u00b9<\/p>\n<p>\u00bdN\u2082(g) + 1\u00bdH\u2082(g) \u2192 NH\u2083(g) &nbsp; \u0394<sub>r<\/sub>H<sup>\u29b5<\/sup> = \u221246 kJ mol\u207b\u00b9<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> kJ mol\u207b\u00b9 here means &#8220;per mole of the equation as written&#8221;, so always quote the equation beside the value.<\/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>Enthalpy Change of Formation<\/h2>\n<\/div>\n<p>The <strong>enthalpy change of formation<\/strong> is for one mole of a compound made from its <strong>elements in their standard states<\/strong>. The equation must show exactly one mole of product, which often means fractions in front of the elements:<\/p>\n<p>2C(s) + 3H\u2082(g) + \u00bdO\u2082(g) \u2192 C\u2082H\u2085OH(l) &nbsp; \u0394<sub>f<\/sub>H<sup>\u29b5<\/sup> = \u2212277 kJ mol\u207b\u00b9<\/p>\n<p>Because nothing changes when an element is &#8220;formed&#8221; from itself, the enthalpy change of formation of any element in its standard state is <strong>zero<\/strong>. Many formation reactions cannot be carried out directly, because the elements would react to give other products, so their values are found indirectly using Hess&#8217;s law.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Common mistake:<\/strong> Writing 2C + 3H\u2082 + O\u2082 \u2192 2C\u2082H\u2085OH. The definition fixes one mole of product, so halve it and use \u00bdO\u2082.<\/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>Enthalpy Change of Combustion<\/h2>\n<\/div>\n<p>The <strong>enthalpy change of combustion<\/strong> is for one mole of a substance <strong>completely<\/strong> burned in oxygen. Complete combustion of a hydrocarbon or alcohol gives carbon dioxide and water only; incomplete combustion also forms carbon monoxide or soot and releases less energy.<\/p>\n<p>C\u2083H\u2088(g) + 5O\u2082(g) \u2192 3CO\u2082(g) + 4H\u2082O(l) &nbsp; \u0394<sub>c<\/sub>H<sup>\u29b5<\/sup> = \u22122220 kJ mol\u207b\u00b9<\/p>\n<p>Combustion is always exothermic, so these values are always negative. The equation must show one mole of the fuel, which again may need a fractional amount of oxygen: C\u2082H\u2086(g) + 3\u00bdO\u2082(g) \u2192 2CO\u2082(g) + 3H\u2082O(l).<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> Water is H\u2082O(l) under standard conditions. Writing H\u2082O(g) changes the value.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Enthalpy Change of Neutralisation<\/h2>\n<\/div>\n<p>The <strong>enthalpy change of neutralisation<\/strong> is for the formation of one mole of water, H\u207a(aq) + OH\u207b(aq) \u2192 H\u2082O(l). For any strong acid with any strong base the value is about \u221257 kJ mol\u207b\u00b9, because the only reaction taking place is the same combination of hydrogen and hydroxide ions.<\/p>\n<p>A weak acid such as ethanoic acid gives a slightly less exothermic value, because some energy is used to complete its ionisation as the reaction proceeds. Sulfuric acid is diprotic, so one mole of H\u2082SO\u2084 forms two moles of water; the enthalpy change for the equation is doubled, but the enthalpy change of neutralisation is still quoted per mole of water.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam trap:<\/strong> The definition is per mole of water, not per mole of acid. With sulfuric acid, divide the energy by the moles of water formed.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Writing the Equation for a Definition<\/h2>\n<\/div>\n<p>Most questions on definitions ask for an equation as well as words. Work backwards from what the definition fixes.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Enthalpy change<\/th><th>Fix one mole of<\/th><th>Example equation<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Formation<\/strong><\/td><td>the compound formed<\/td><td>Na(s) + \u00bdCl\u2082(g) \u2192 NaCl(s)<\/td><\/tr>\n<tr><td><strong>Combustion<\/strong><\/td><td>the substance burned<\/td><td>CH\u2083OH(l) + 1\u00bdO\u2082(g) \u2192 CO\u2082(g) + 2H\u2082O(l)<\/td><\/tr>\n<tr><td><strong>Neutralisation<\/strong><\/td><td>water formed<\/td><td>\u00bdH\u2082SO\u2084(aq) + NaOH(aq) \u2192 \u00bdNa\u2082SO\u2084(aq) + H\u2082O(l)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-figure-card ols-figure-placeholder\">\n<div class=\"ols-figure-image\"><div class=\"ols-placeholder-box\">[IMAGE NEEDED: SHARED FIGURE, used on all five boards, draw once: summary card of the four standard enthalpy changes (reaction, formation, combustion, neutralisation), each with its one-mole quantity highlighted and an example equation]<\/div><\/div>\n<div class=\"ols-figure-caption\"><p>Each definition fixes one mole of something different; the highlighted species in each equation is the one that must be one mole.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> The standard enthalpy change of formation of ethanol is the enthalpy change when one mole of ethanol is formed from its elements in their standard states under standard conditions.<\/p>\n<\/div>\n<\/article>\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 the standard enthalpy change of combustion<\/h3><p>The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all substances in their standard states.<\/p>\n<h3>Write the equation for the enthalpy change of formation<\/h3><p>One mole of product, elements in their standard states, fractions allowed on the left.<\/p>\n<h3>State the enthalpy change of formation of oxygen<\/h3><p>Zero, because it is an element in its standard state.<\/p>\n<h3>Do not say<\/h3><p>&#8220;One mole of reactants&#8221;; &#8220;burned in air&#8221; instead of &#8220;completely burned in oxygen&#8221;; &#8220;per mole of acid&#8221; for neutralisation.<\/p>\n<\/article>\n<section class=\"ols-h5p-card\">\n<h2>Check Your Understanding<\/h2>\n<p>Check that you can write and recognise each definition for substances that are not used in the notes.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-550\" class=\"h5p-iframe\" data-content-id=\"550\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Energetics Drag: formation of propan-1-ol\"><\/iframe><\/div><\/div>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-551\" class=\"h5p-iframe\" data-content-id=\"551\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Energetics MCQ: combustion equation for butane\"><\/iframe><\/div><\/div>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-552\" class=\"h5p-iframe\" data-content-id=\"552\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Energetics MCQ: neutralisation per mole of water\"><\/iframe><\/div><\/div>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-553\" class=\"h5p-iframe\" data-content-id=\"553\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Energetics MCQ: zero enthalpy of formation\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the definitions AQA expects word for word.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why does the definition say &#8220;one mole&#8221;?<\/h3>\n<p>Each standard enthalpy change is fixed per mole of a particular species, so the equation has to show one mole of it, even if that needs fractions elsewhere.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the enthalpy change of formation of an element zero?<\/h3>\n<p>Forming an element in its standard state from itself involves no change, so no energy is transferred.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What is the difference between combustion in oxygen and burning in air?<\/h3>\n<p>The definition requires complete combustion in oxygen, which gives only carbon dioxide and water from a hydrocarbon. Burning in air can be incomplete, which releases less energy.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the enthalpy change of neutralisation about the same for all strong acids and bases?<\/h3>\n<p>The reaction in each case is H\u207a(aq) + OH\u207b(aq) \u2192 H\u2082O(l), so the energy released per mole of water is the same, about \u221257 kJ mol\u207b\u00b9.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Can an enthalpy change of formation be positive?<\/h3>\n<p>Yes. A few compounds, such as ethene and benzene, have positive values because they are less stable than their elements.<\/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\/aqa\/3-1-4-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\/aqa\/3-1-4-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\/aqa\/3-1-4-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\/aqa\/3-1-4-energetics\/bond-enthalpies\/\">Bond Enthalpies<\/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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