{"id":12474,"date":"2026-10-03T08:32:18","date_gmt":"2026-10-03T07:32:18","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/"},"modified":"2026-10-04T23:04:39","modified_gmt":"2026-10-04T22:04:39","slug":"feasibility-gibbs-energy-and-temperature","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/","title":{"rendered":"Feasibility and Temperature"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-kinetics-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: 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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 12 Entropy and 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-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility and Temperature<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/\">Lattice Energy Trends and Covalent Character<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\">Enthalpy of Solution and Hydration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\">Predicting Solubility<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Sections<\/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 Energetics (AS)<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-11-kinetics\/\">Topic 11 Kinetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/\">Core Practicals<\/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      item.classList.remove('active', 'active-main', 'parent-active');\r\n    });\r\n\r\n    sidebar.querySelectorAll('a[data-ols-disabled-parent=\"1\"], 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true;\r\n  }\r\n\r\n  if (!highlightActive()) {\r\n    document.addEventListener('DOMContentLoaded', highlightActive);\r\n  }\r\n})();\r\n<\/script>\r\n\n\n    <main class=\"ols-main\">\n\n      <!-- BREADCRUMBS - updated with full path and correct links -->\n      <nav class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Edexcel International<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics<\/a> \/\n<span>Feasibility and Temperature<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Feasibility and Temperature<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to what makes a reaction feasible: the balance between the enthalpy change and the entropy change, the total entropy change, how temperature changes the balance and how to calculate the temperature at which a reaction becomes feasible.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 4: WCH14\/01<\/div>\n<div class=\"ols-badge\">Topic 12: Entropy and 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      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>The Balance Between Enthalpy and Entropy<\/h2>\n<\/div>\n<p>Whether a reaction is feasible depends on two things:<\/p>\n<ul class=\"ols-list\">\n<li>the enthalpy change, which decides what happens to the entropy of the surroundings<\/li>\n<li>the entropy change of the system<\/li>\n<\/ul>\n<p>The table shows the four possible combinations of signs.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>\u0394H<\/th><th>\u0394S_system<\/th><th>\u0394S_total<\/th><th>Feasible?<\/th><th>Example<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>negative (exothermic)<\/strong><\/td><td>positive<\/td><td>always positive<\/td><td>at all temperatures<\/td><td>combustion of a hydrocarbon, which makes more gas<\/td><\/tr>\n<tr><td><strong>positive (endothermic)<\/strong><\/td><td>negative<\/td><td>always negative<\/td><td>never<\/td><td>the reverse of a combustion<\/td><\/tr>\n<tr><td><strong>negative (exothermic)<\/strong><\/td><td>negative<\/td><td>positive only at low T<\/td><td>below a certain temperature<\/td><td>water freezing, N\u2082 + 3H\u2082 \u2192 2NH\u2083<\/td><\/tr>\n<tr><td><strong>positive (endothermic)<\/strong><\/td><td>positive<\/td><td>positive only at high T<\/td><td>above a certain temperature<\/td><td>ice melting, CaCO\u2083 decomposing<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<ul class=\"ols-list\">\n<li>In two of the rows the two factors pull the same way, so the outcome is the same at every temperature.<\/li>\n<li>In the other two they pull against each other, and the <strong>temperature<\/strong> decides which wins.<\/li>\n<li>The reason is that the surroundings term \u2212\u0394H\/T shrinks as T rises, while \u0394S_system does not change much.<\/li>\n<\/ul>\n<h3>The two temperature-dependent rows<\/h3>\n<p>These are the interesting ones.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Reaction<\/th><th>Feasible when<\/th><th>Why<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Exothermic with a negative \u0394S_system<\/strong><\/td><td>cold<\/td><td>at low temperature the heat given out raises the entropy of the surroundings by a great deal<\/td><\/tr>\n<tr><td><strong>Endothermic with a positive \u0394S_system<\/strong><\/td><td>hot<\/td><td>at high temperature the heat it takes in costs the surroundings little entropy<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>This is why endothermic reactions can occur spontaneously at room temperature: their positive \u0394S_system outweighs the small negative \u0394S_surroundings.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Feasibility depends on the balance between \u0394H and \u0394S_system. When their effects oppose each other the temperature decides, because the influence of \u0394H on the surroundings falls as T rises.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>The Gibbs Equation<\/h2>\n<\/div>\n<p>The condition for feasibility, \u0394S_total > 0, can be rewritten in terms of energy.<\/p>\n<ol>\n<li>Start from \u0394S_total = \u0394S_system \u2212 \u0394H\/T.<\/li>\n<li>Multiply every term by \u2212T: \u2212T\u0394S_total = \u0394H \u2212 T\u0394S_system.<\/li>\n<li>The right-hand side is called the <strong>Gibbs energy change<\/strong>, \u0394G.<\/li>\n<\/ol>\n<p>\u0394G = \u0394H \u2212 T\u0394S_system<\/p>\n<p>Because \u0394G = \u2212T\u0394S_total and T is always positive, a positive \u0394S_total is exactly the same as a negative \u0394G.<\/p>\n<ul class=\"ols-list\">\n<li>A reaction is <strong>feasible when \u0394G is negative or zero<\/strong>.<\/li>\n<li>\u0394G = 0 marks the temperature at which it just becomes feasible.<\/li>\n<li>The term \u2212T\u0394S_system carries the entropy change of the system in energy units, so the equation compares the enthalpy change with the entropy change directly, on the same scale, kJ mol\u207b\u00b9.<\/li>\n<\/ul>\n<h3>Units: the source of most errors<\/h3>\n<p>\u0394H is in kJ mol\u207b\u00b9 and \u0394S_system in J K\u207b\u00b9 mol\u207b\u00b9, so <strong>divide the entropy by 1000<\/strong> before substituting, and use T in kelvin.<\/p>\n<h3>Worked example: nitrogen monoxide and oxygen at 298 K<\/h3>\n<p>For the oxidation of nitrogen monoxide, 2NO(g) + O\u2082(g) \u2192 2NO\u2082(g), \u0394H = \u2212114 kJ mol\u207b\u00b9 and \u0394S_system = \u2212146 J K\u207b\u00b9 mol\u207b\u00b9.<\/p>\n<p><strong>Step 1.<\/strong> Convert the entropy: \u0394S_system = \u2212146 J K\u207b\u00b9 mol\u207b\u00b9 = \u22120.146 kJ K\u207b\u00b9 mol\u207b\u00b9.<\/p>\n<p><strong>Step 2.<\/strong> Substitute: \u0394G = \u2212114 \u2212 (298 \u00d7 \u22120.146) = \u2212114 + 43.5.<\/p>\n<p><strong>Answer.<\/strong> \u0394G = <strong>\u221270.5 kJ mol\u207b\u00b9<\/strong>. \u0394G is negative, so the reaction is feasible at room temperature.<\/p>\n<ul class=\"ols-list\">\n<li>Nitrogen monoxide turns brown in air within seconds.<\/li>\n<li>The entropy of the system falls (three moles of gas become two), but the heat given out more than compensates.<\/li>\n<\/ul>\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\/entropy-t12-04-feasibility.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\/entropy-t12-04-feasibility.jpg\" alt=\"Graph of \u0394G against temperature as a straight line with gradient \u2212\u0394S and intercept \u0394H, the feasible region shaded below \u0394G = 0, beside a four-row ta\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-04-feasibility.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>\u0394G against temperature as a straight line with gradient \u2212\u0394S and intercept \u0394H, the feasible region below zero, beside the four sign combinations and when each is feasible.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Worked example:<\/strong> 2NO(g) + O\u2082(g) \u2192 2NO\u2082(g), \u0394H = \u2212114 kJ mol\u207b\u00b9, \u0394S_system = \u2212146 J K\u207b\u00b9 mol\u207b\u00b9. At 298 K: \u0394G = \u2212114 \u2212 (298 \u00d7 \u22120.146) = \u221270.5 kJ mol\u207b\u00b9. Negative, so feasible. Convert the entropy to kJ first.<\/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>Check: Sign Combinations and \u0394G<\/h2>\n<p>Decide when reactions are feasible from the signs of \u0394H and \u0394S, and calculate \u0394G at a given temperature, for reactions not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1105\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>How Temperature Changes Feasibility<\/h2>\n<\/div>\n<p>Over the range of temperatures met in a question, \u0394H and \u0394S_system hardly change. So \u0394G = \u0394H \u2212 T\u0394S_system is the equation of a <strong>straight line<\/strong> when \u0394G is plotted against T.<\/p>\n<ul class=\"ols-list\">\n<li>The <strong>intercept<\/strong> on the \u0394G axis (at T = 0) is \u0394H.<\/li>\n<li>The <strong>gradient<\/strong> is \u2212\u0394S_system.<\/li>\n<\/ul>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Sign of \u0394S_system<\/th><th>The line<\/th><th>Feasibility<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Positive<\/strong><\/td><td>slopes downwards<\/td><td>the reaction becomes feasible above a certain temperature<\/td><\/tr>\n<tr><td><strong>Negative<\/strong><\/td><td>slopes upwards<\/td><td>the reaction stops being feasible above a certain temperature<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Where the line crosses \u0394G = 0 the reaction is just feasible. Setting \u0394H \u2212 T\u0394S_system = 0 gives that temperature:<\/p>\n<p>T = \u0394H \u00f7 \u0394S_system<\/p>\n<p>Both quantities must be in the same units (both in kJ, or both in J).<\/p>\n<h3>Worked example: the nitrogen monoxide reaction<\/h3>\n<p><strong>Step 1.<\/strong> \u0394S_system is negative, so the line slopes upwards: the reaction becomes <strong>less<\/strong> feasible as the temperature rises.<\/p>\n<p><strong>Step 2.<\/strong> T = 114 \u00f7 0.146.<\/p>\n<p><strong>Answer.<\/strong> The reaction stops being feasible above <strong>781 K<\/strong>.<\/p>\n<h3>Worked example: calcium carbonate from page 2<\/h3>\n<p><strong>Step 1.<\/strong> Both \u0394H and \u0394S_system are positive, so the line slopes downwards: the reaction becomes feasible on heating.<\/p>\n<p><strong>Step 2.<\/strong> T = 178 \u00f7 0.1604.<\/p>\n<p><strong>Answer.<\/strong> The reaction becomes feasible above 1110 K, which is why a lime kiln is run at over 1100 K.<\/p>\n<div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-13-crossingtemp.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\/entropy-t12-13-crossingtemp.jpg\" alt=\"Poster with two straight lines of \u0394G against temperature, one rising from \u2212114 and crossing zero at 781 K and one falling from +178 and crossing zero at 1110 K, each marked with its feasible side, beside a three-step method box for T = \u0394H \u00f7 \u0394S with both quantities in the same units\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-13-crossingtemp.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Two \u0394G against temperature lines: one rising and crossing zero at 781 K, one falling and crossing zero at 1110 K, with the three steps for finding the temperature at which feasibility changes.<\/p><\/div>\n<\/div>\n<h3>Why temperature has this effect<\/h3>\n<ul class=\"ols-list\">\n<li>In terms of entropy, the same answer comes from setting \u0394S_total = 0: \u0394S_system = \u0394H\/T, so T = \u0394H \u00f7 \u0394S_system.<\/li>\n<li>Raising the temperature reduces the size of \u0394S_surroundings = \u2212\u0394H\/T, so the surroundings term matters less and the sign of \u0394S_system counts for more.<\/li>\n<li>A large positive \u0394S_system therefore favours reactions at high temperatures and a large negative \u0394S_system favours them at low temperatures, whatever the sign of \u0394H.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Worked example:<\/strong> For 2NO + O\u2082 \u2192 2NO\u2082, T = \u0394H \u00f7 \u0394S_system = \u2212114 \u00f7 \u22120.146 = 781 K. Below 781 K \u0394G is negative (feasible); above it \u0394G is positive. Quote the temperature and the side on which the reaction is feasible.<\/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>Check: The Temperature of Feasibility<\/h2>\n<p>Calculate the temperature at which a reaction becomes, or stops being, feasible, for reactions not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1106\"><\/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>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3>\n<ul class=\"ols-list\">\n<li>&#8220;The reaction is feasible when \u0394S_total is positive, which is the same as \u0394G being negative.&#8221;<\/li>\n<li>&#8220;\u0394G = \u0394H \u2212 T\u0394S_system; the reaction is feasible when \u0394G \u2264 0.&#8221;<\/li>\n<li>&#8220;T = \u0394H \u00f7 \u0394S_system = &#8230; K, so the reaction is feasible above this temperature.&#8221;<\/li>\n<li>&#8220;The gradient of the \u0394G against T graph is \u2212\u0394S_system and the intercept is \u0394H.&#8221;<\/li>\n<\/ul>\n<h3>Do not say<\/h3>\n<ul class=\"ols-list\">\n<li>&#8220;\u0394G = \u2212114 \u2212 298 \u00d7 \u2212146&#8221; (units mixed: convert \u0394S to kJ K\u207b\u00b9 mol\u207b\u00b9).<\/li>\n<li>&#8220;Exothermic reactions are always feasible&#8221; (not if \u0394S_system is negative and T is high).<\/li>\n<li>&#8220;T = 25 \u00b0C&#8221; in the equation (use 298 K).<\/li>\n<\/ul>\n<h3>Watch for<\/h3>\n<ul class=\"ols-list\">\n<li>Graph questions: read \u0394H from the intercept and \u0394S_system from minus the gradient, and identify the feasible range as the temperatures at which the line is below zero.<\/li>\n<li>Questions that give \u0394G at two temperatures and ask why it differs: the answer is the \u2212T\u0394S_system term.<\/li>\n<li>A question may say &#8220;explain why the reaction becomes feasible on heating&#8221;: it wants the sign of \u0394S_system and the growing size of the T\u0394S_system term.<\/li>\n<\/ul>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: \u0394G Against T and Mixed Calculations<\/h2>\n<p>Interpret \u0394G against temperature graphs and combine \u0394H, \u0394S_system and \u0394G in calculations for reactions not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1107\" class=\"h5p-iframe\" data-content-id=\"1107\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Summary: Reading a \u0394G Against T Graph\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the feasibility ideas.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Does a positive \u0394Stotal mean the reaction will definitely happen?<\/h3>\n<p>No. A positive total entropy change means the reaction is thermodynamically feasible, so it can happen, not that it will happen at a useful rate. Many reactions with a large positive \u0394Stotal, such as the combustion of methane at room temperature, do not go because the activation energy is too high.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What does T = \u0394H\/\u0394S actually tell me?<\/h3>\n<p>It is the temperature at which \u0394Stotal is exactly zero, so the reaction is on the point of becoming feasible. For an endothermic reaction with a positive \u0394S the reaction is feasible above that temperature; for an exothermic reaction with a negative \u0394S it is feasible below it. Remember that \u0394H and \u0394S must be in the same energy unit before you divide.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does feasibility change with temperature for some reactions but not others?<\/h3>\n<p>It only changes when \u0394Ssystem and \u0394Ssurroundings have opposite signs, because the surroundings term \u2212\u0394H\/T shrinks as T rises while the system term stays about the same. An exothermic reaction with a positive \u0394Ssystem is feasible at all temperatures; an endothermic one with a negative \u0394Ssystem is never feasible.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is a reaction with a negative \u0394Stotal not impossible?<\/h3>\n<p>Because a negative total entropy change only means that the equilibrium lies well over to the reactants, not that nothing forms at all. There will always be a small amount of product, and the reaction can be driven forward by removing a product or by changing the temperature so that \u0394Stotal becomes positive.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why do we assume \u0394H and \u0394S do not change with temperature?<\/h3>\n<p>Because both change only slightly with temperature compared with the size of the T in the T\u0394S term, so treating them as constant gives an answer close enough for exam work. Questions will tell you to make this assumption; it breaks down badly only if a substance changes state between the two temperatures.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 12 Entropy and Energetics Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 12 Entropy and Energetics and the rest of the course.<\/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-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/\">Lattice Energy Trends and Covalent Character<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\">Enthalpy of Solution and Hydration<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\">Predicting Solubility<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics Overview<\/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-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\",\n      \"name\": \"Feasibility and Temperature | Topic 12 Entropy and Energetics | Online Learning System\",\n      \"description\": \"Edexcel International A Level Chemistry revision notes on feasibility: \u0394Stotal = \u0394Ssystem + \u0394Ssurroundings, the effect of temperature and the temperature at which a 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