{"id":8341,"date":"2026-09-18T12:00:36","date_gmt":"2026-09-18T11:00:36","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/enthalpy-changes-and-standard-conditions\/"},"modified":"2026-10-04T23:02:13","modified_gmt":"2026-10-04T22:02:13","slug":"enthalpy-changes-and-standard-conditions","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/enthalpy-changes-and-standard-conditions\/","title":{"rendered":"Enthalpy Changes and Standard Conditions"},"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      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20px;\n      box-shadow: var(--inner-shadow);\n    }\n.ols-faq-item h3 {\n      margin: 0 0 8px;\n      font-size: 20px;\n      line-height: 1.3;\n      color: var(--navy);\n    }\n.ols-faq-item p {\n      margin: 0;\n      font-size: 16px;\n      line-height: 1.65;\n      color: var(--body-text);\n    }\n.ols-faq-card,\n      .ols-quicksnap-card,\n      .ols-attribution-card {\n        padding: 24px 18px;\n        border-radius: 22px;\n      }\n  <\/style>\n\n    <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Topic 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>Enthalpy Changes and Standard Conditions<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Enthalpy Changes and Standard Conditions<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to enthalpy change, exothermic and endothermic reactions, enthalpy level diagrams, activation energy and standard conditions 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 Diagrams and Which Way the Energy Goes<\/h2>\n<p>Before you start, check the GCSE picture that this page turns into \u0394H.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"758\"><\/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 an Enthalpy Change Is<\/h2>\n<\/div>\n<p>An <strong>enthalpy change<\/strong>, \u0394H, is the heat energy transferred in a reaction measured at constant pressure.<\/p><p>It is measured in kJ mol\u207b\u00b9, so it always refers to the amounts shown in a stated equation or to one mole of a named substance. Scale the equation and the value scales with it.<\/p><div class=\"ols-table-wrap\"><table class=\"ols-table\"><thead><tr><th>Equation<\/th><th>\u0394H<\/th><\/tr><\/thead><tbody><tr><td><strong>CH\u2084(g) + 2O\u2082(g) \u2192 CO\u2082(g) + 2H\u2082O(l)<\/strong><\/td><td>\u2212890 kJ mol\u207b\u00b9<\/td><\/tr><tr><td><strong>2CH\u2084(g) + 4O\u2082(g) \u2192 2CO\u2082(g) + 4H\u2082O(l)<\/strong><\/td><td>\u22121780 kJ mol\u207b\u00b9 (the same reaction written for twice as much)<\/td><\/tr><\/tbody><\/table><\/div>\n<p>Chemists split the world into two parts. The <strong>system<\/strong> is the reacting chemicals; the <strong>surroundings<\/strong> are everything else, including the solvent, the container and the air. An enthalpy change describes energy moving between the two, which is why a thermometer placed in the surroundings can be used to measure it.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Enthalpy change, \u0394H, is the heat energy change measured at constant pressure.<\/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>Exothermic and Endothermic Reactions<\/h2>\n<\/div>\n<p>In an <strong>exothermic<\/strong> reaction energy is transferred from the system to the surroundings, so the surroundings get warmer and \u0394H is <strong>negative<\/strong>. The products have less enthalpy than the reactants. Combustion of fuels, neutralisation and respiration are all exothermic.<\/p>\n<p>In an <strong>endothermic<\/strong> reaction energy is transferred from the surroundings to the system, so the surroundings cool and \u0394H is <strong>positive<\/strong>. The products have more enthalpy than the reactants. Thermal decomposition of calcium carbonate, photosynthesis and dissolving ammonium nitrate in water are endothermic.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Feature<\/th><th>Exothermic<\/th><th>Endothermic<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Direction of energy transfer<\/strong><\/td><td>system to surroundings<\/td><td>surroundings to system<\/td><\/tr>\n<tr><td><strong>Sign of \u0394H<\/strong><\/td><td>negative<\/td><td>positive<\/td><\/tr>\n<tr><td><strong>Temperature of surroundings<\/strong><\/td><td>rises<\/td><td>falls<\/td><\/tr>\n<tr><td><strong>Enthalpy of products<\/strong><\/td><td>lower than reactants<\/td><td>higher than reactants<\/td><\/tr>\n<tr><td><strong>Example<\/strong><\/td><td>CH\u2084 + 2O\u2082 \u2192 CO\u2082 + 2H\u2082O<\/td><td>CaCO\u2083 \u2192 CaO + CO\u2082<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Common mistake:<\/strong> A temperature rise means the reaction is exothermic, not endothermic. The heat has left the chemicals and entered the solution, which is what the thermometer measures.<\/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-s3exoendo.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-s3exoendo.jpg\" alt=\"Two-column poster comparing exothermic and endothermic reactions by energy direction, sign of \u0394H, temperature and examples.\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/t17x-s3exoendo.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>In an exothermic reaction energy leaves the system, the surroundings warm up and \u0394H is negative; in an endothermic reaction the reverse is true.<\/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: Two Everyday Changes<\/h2>\n<p>Drag the words into place to complete the two accounts.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-546\" class=\"h5p-iframe\" data-content-id=\"546\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Enthalpy Changes Drag: Sherbet and a Camping Stove\"><\/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>Drawing Enthalpy Level Diagrams<\/h2>\n<\/div>\n<p>An <strong>enthalpy level diagram<\/strong> shows the enthalpy of the reactants and the products as two horizontal levels, with an arrow between them for \u0394H. The vertical axis is labelled enthalpy; the horizontal axis is the progress of the reaction.<\/p>\n<ol>\n<li>Draw the reactants level and the products level, and label each with its formula.<\/li>\n<li>For an exothermic reaction place the products <strong>below<\/strong> the reactants; for an endothermic reaction place them <strong>above<\/strong>.<\/li>\n<li>Draw a vertical arrow from the reactants level to the products level and label it with \u0394H and its sign, for example \u0394H = \u2212890 kJ mol\u207b\u00b9.<\/li>\n<\/ol>\n<div class=\"ols-figure-card\">\n<div class=\"ols-figure-image\"><img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/energetics-1.jpg\" alt=\"three panels: an exothermic reaction (products below, arrow down, \u0394H negative), an endothermic reaction (products above, arrow up, \u0394H positive) and a reaction profile curve with Ea and \u0394H marked\"><\/div>\n<div class=\"ols-figure-caption\"><p>The direction of the \u0394H arrow shows the sign: down for exothermic, up for endothermic.<\/p><\/div>\n<\/div>\n<!-- 3D card: exothermic-profile (26 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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bottom:auto; top:48px; max-width:72%;}\n  .ols-exo-001 .r-line{font-size:10.5px;}\n  .ols-exo-001 .r-line span{display:block; margin-left:0; font-size:9.5px;}\n  .ols-exo-001 .exo-read{padding:5px 9px;}\n  .ols-exo-001 .exo-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-exo-001 .exo-hint{display:none;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-exo-001 .exo-seg button,.ols-exo-001 .exo-pill{transition:none;}\n}\n\n.ols-cc-exo-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-exo-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-exo-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"exo-head\">\n  <h2 class=\"exo-title\">An Exothermic Reaction: Energy Profile<\/h2>\n  <p class=\"exo-sub\">Follow hydrogen burning in oxygen, 2H<sub>2<\/sub> + O<sub>2<\/sub> \u2192 2H<sub>2<\/sub>O, bond by bond, with a live energy profile showing why more energy comes out than goes in.<\/p>\n<\/div>\n\n<div class=\"exo-stage\" id=\"exoStage\">\n  <canvas class=\"exo-canvas\" id=\"exoCanvas\"><\/canvas>\n  <div class=\"exo-overlay\" id=\"exoOverlay\"><\/div>\n  <div class=\"exo-caption\" id=\"exoCaption\"><\/div>\n  <div class=\"exo-hint\" id=\"exoHint\">Drag to rotate<\/div>\n  <div class=\"exo-badges\" id=\"exoBadges\"><\/div>\n<\/div>\n\n<div class=\"exo-controls\">\n  <div class=\"exo-row\">\n    <span class=\"exo-rowlab\">Step<\/span>\n    <div class=\"exo-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\">Activation<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Bonds form<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\">Products<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\">Catalyst<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"exo-row\">\n    <span class=\"exo-rowlab\">View<\/span>\n    <button type=\"button\" class=\"exo-pill\" id=\"exoResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"exo-info\">\n  <h3 id=\"exoInfoTitle\"><\/h3>\n  <p id=\"exoInfoText\"><\/p>\n  <div class=\"exo-facts\" id=\"exoFacts\"><\/div>\n  <div class=\"exo-key\">\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:#e08a1e\"><\/i>Heat released<\/span>\n    <span><i style=\"background:#2563eb\"><\/i>Energy profile<\/span>\n    <span><i style=\"background:#ffffff; border:1.5px dashed #2f9e57\"><\/i>Catalysed route<\/span>\n    <span><i style=\"background:#c9961c\"><\/i>E<sub>a<\/sub>, &Delta;H, progress dot<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-exo-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/exothermic-profile.js?v=20261003e\"><\/script>\n\n<!-- 3D card: endothermic-profile (26 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-end-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-end-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"end-head\">\n  <h2 class=\"end-title\">An Endothermic Reaction: Energy Profile<\/h2>\n  <p class=\"end-sub\">Heat calcium carbonate until CO<sub>2<\/sub> leaves, CaCO<sub>3<\/sub> \u2192 CaO + CO<sub>2<\/sub>, with a live energy profile showing why the products end up higher in energy than the reactant.<\/p>\n<\/div>\n\n<div class=\"end-stage\" id=\"endStage\">\n  <canvas class=\"end-canvas\" id=\"endCanvas\"><\/canvas>\n  <div class=\"end-overlay\" id=\"endOverlay\"><\/div>\n  <div class=\"end-caption\" id=\"endCaption\"><\/div>\n  <div class=\"end-hint\" id=\"endHint\">Drag to rotate<\/div>\n  <div class=\"end-badges\" id=\"endBadges\"><\/div>\n<\/div>\n\n<div class=\"end-controls\">\n  <div class=\"end-row\">\n    <span class=\"end-rowlab\">Step<\/span>\n    <div class=\"end-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\">Reactant<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\">Heating<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Products form<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\">Products<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\">Compare<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"end-row\">\n    <span class=\"end-rowlab\">View<\/span>\n    <button type=\"button\" class=\"end-pill\" id=\"endResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"end-info\">\n  <h3 id=\"endInfoTitle\"><\/h3>\n  <p id=\"endInfoText\"><\/p>\n  <div class=\"end-facts\" id=\"endFacts\"><\/div>\n  <div class=\"end-key\">\n    <span><i style=\"background:#3aa048\"><\/i>Calcium ion, Ca<sup>2+<\/sup><\/span>\n    <span><i style=\"background:#484e5c\"><\/i>Carbon<\/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:#e08a1e\"><\/i>Energy released<\/span>\n    <span><i style=\"background:#f2a93b\"><\/i>Flame<\/span>\n    <span><i style=\"background:#2563eb\"><\/i>Energy profile<\/span>\n    <span><i style=\"background:#ffffff; border:1.5px dashed #8b92a8\"><\/i>Exothermic, for comparison<\/span>\n    <span><i style=\"background:#c9961c\"><\/i>E<sub>a<\/sub>, &Delta;H, progress dot<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-end-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/endothermic-profile.js?v=20261003e\"><\/script>\n\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Label both levels with formulae and state symbols, and make the arrow point from reactants to products. An arrow drawn the wrong way reverses the sign and loses the mark.<\/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: Levels and Arrows<\/h2>\n<p>Answer each one quickly, using the value or the observation given.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-759\" class=\"h5p-iframe\" data-content-id=\"759\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Enthalpy Changes Quick Fire: Above, Below and Which Way the Arrow Points\"><\/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>Activation Energy and the Reaction Profile<\/h2>\n<\/div>\n<p>The <strong>activation energy<\/strong>, E<sub>a<\/sub>, is the minimum energy that colliding particles need for a reaction to occur. It is the energy needed to start breaking bonds, so even very exothermic reactions such as the combustion of methane need a spark or flame to begin.<\/p>\n<p>An enthalpy level diagram shows only the enthalpy change. A <strong>reaction profile<\/strong> adds the curve over the top, so the activation energy can be read as the height of the peak above the reactants. Activation energy is developed fully in the kinetics topic.<\/p>\n<p>The same peak gives the activation energy for the <strong>reverse<\/strong> reaction, measured from the products level up to the top of the curve. For an exothermic reaction the reverse barrier is the larger of the two; for an endothermic reaction it is the smaller.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> \u0394H is fixed by the reactants and products; activation energy is the barrier between them. A catalyst lowers the barrier without changing \u0394H.<\/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: Read the Two Heights<\/h2>\n<p>Work out both values from the described profile, then choose your answer.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-547\" class=\"h5p-iframe\" data-content-id=\"547\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Enthalpy Changes MCQ: Reading a Profile You Have Not Seen\"><\/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>Standard Conditions<\/h2>\n<\/div>\n<p>Enthalpy changes depend on temperature, pressure and concentration, so values are compared under <strong>standard conditions<\/strong>:<\/p><div class=\"ols-table-wrap\"><table class=\"ols-table\"><thead><tr><th>Condition<\/th><th>Standard value<\/th><\/tr><\/thead><tbody><tr><td><strong>Pressure<\/strong><\/td><td>100 kPa<\/td><\/tr><tr><td><strong>Temperature<\/strong><\/td><td>a stated temperature (usually 298 K)<\/td><\/tr><tr><td><strong>Solutions<\/strong><\/td><td>a concentration of 1 mol dm\u207b\u00b3<\/td><\/tr><tr><td><strong>Every substance<\/strong><\/td><td>in its <strong>standard state<\/strong>, the physical state it adopts under those conditions<\/td><\/tr><\/tbody><\/table><\/div><p>The symbol \u29b5 shows that a value was measured, or corrected, to standard conditions.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Condition<\/th><th>Standard value<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Pressure<\/strong><\/td><td>100 kPa<\/td><\/tr>\n<tr><td><strong>Temperature<\/strong><\/td><td>298 K (25 \u00b0C) unless another temperature is stated<\/td><\/tr>\n<tr><td><strong>Solution concentration<\/strong><\/td><td>1 mol dm\u207b\u00b3<\/td><\/tr>\n<tr><td><strong>Physical state<\/strong><\/td><td>the normal state at 298 K and 100 kPa, for example H\u2082O(l), CO\u2082(g), C(s, graphite)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Standard states matter because changing a state involves an enthalpy change of its own.<\/p>\n<p>Forming water as a gas gives a less exothermic value than forming liquid water, because the energy that would be released on condensing is not released.<\/p>\n<p>That is about 44 kJ mol\u207b\u00b9 for every mole of water, so the two values can differ by a great deal.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> Standard pressure is 100 kPa. Temperature must be stated; 298 K is used unless a question says otherwise.<\/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: Standard States<\/h2>\n<p>Click every formula that shows the substance in its standard state.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-548\" class=\"h5p-iframe\" data-content-id=\"548\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Enthalpy Changes Mark the Words: Formulae Written in the Standard State\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Pick the Accurate Statement<\/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-549\" class=\"h5p-iframe\" data-content-id=\"549\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Enthalpy Changes Summary: Signs, States and the Standard Symbol\"><\/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 enthalpy change<\/h3><p>The heat energy change measured at constant pressure.<\/p>\n<h3>Explain why a reaction is exothermic from a temperature change<\/h3><p>The temperature of the surroundings rose, so energy was transferred from the chemicals to the surroundings and \u0394H is negative.<\/p>\n<h3>Draw an enthalpy level diagram<\/h3><p>Label the enthalpy axis, both levels with formulae, and an arrow from reactants to products labelled with \u0394H and its sign.<\/p>\n<h3>Do not say<\/h3><p>&#8220;Exothermic reactions absorb heat&#8221;; &#8220;\u0394H is positive because heat is given out&#8221;; &#8220;standard temperature is 273 K&#8221;.<\/p>\n<\/article>\n\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the enthalpy ideas that appear most often in Edexcel International A Level Chemistry questions.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>What is an enthalpy change?<\/h3>\n<p>It is the heat energy change measured at constant pressure, quoted in kJ mol\u207b\u00b9. It describes energy moving between the reacting chemicals and their surroundings.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I know if a reaction is exothermic?<\/h3>\n<p>The temperature of the surroundings rises, because energy has been transferred from the chemicals to the surroundings. \u0394H is negative and the products have less enthalpy than the reactants.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What does an enthalpy level diagram show?<\/h3>\n<p>The enthalpy of the reactants and products as two levels, with an arrow from reactants to products labelled with \u0394H. It does not show the activation energy; a reaction profile does.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What are standard conditions?<\/h3>\n<p>A pressure of 100 kPa, a stated temperature (usually 298 K), solutions at 1 mol dm\u207b\u00b3, and every substance in its standard state. The symbol \u29b5 marks a standard value.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Does a catalyst change \u0394H?<\/h3>\n<p>No. A catalyst lowers the activation energy, so the reaction is faster, but the enthalpies of the reactants and products are unchanged, so \u0394H stays the same.<\/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\/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\/topic-6-energetics\/bond-enthalpies\/\">Bond Enthalpies<\/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\/enthalpy-changes-and-standard-conditions\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/enthalpy-changes-and-standard-conditions\/\",\n      \"name\": \"Enthalpy Changes and Standard Conditions | Topic 6 Energetics | Online Learning System\",\n      \"description\": \"Enthalpy changes for Edexcel International A Level Chemistry: exothermic and endothermic reactions, enthalpy level diagrams, activation energy and standard conditions.\",\n      \"isPartOf\": {\n        \"@id\": 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