{"id":12520,"date":"2026-10-03T08:34:05","date_gmt":"2026-10-03T07:34:05","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-23-chemical-energetics\/entropy-and-the-direction-of-change\/"},"modified":"2026-10-03T10:01:09","modified_gmt":"2026-10-03T09:01:09","slug":"entropy-and-the-direction-of-change","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-23-chemical-energetics\/entropy-and-the-direction-of-change\/","title":{"rendered":"Entropy and the Direction of Change"},"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      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border-top-right-radius: 26px !important; position: relative !important; z-index: 2 !important; }\n    .ols-zoom-card img.ols-zoomable-img { display: block !important; max-width: 100% !important; height: auto !important; border-radius: 18px !important; cursor: zoom-in !important; pointer-events: auto !important; user-select: none !important; -webkit-user-drag: none !important; transform: translateZ(0) scale(1) !important; transform-origin: center center !important; transition: transform 0.32s ease, box-shadow 0.32s ease, filter 0.32s ease !important; position: relative !important; z-index: 2 !important; }\n    @media (hover: hover) and (pointer: fine) { .ols-zoom-card img.ols-zoomable-img:hover { transform: translateZ(0) scale(1.35) !important; box-shadow: 0 28px 70px rgba(28, 36, 75, 0.34) !important; filter: saturate(1.02) contrast(1.01) !important; z-index: 100 !important; } }\n    .ols-zoom-card-caption { padding: 18px 22px 20px !important; background: linear-gradient(135deg, #ffffff, #f8fbff) !important; border-bottom-left-radius: 26px !important; border-bottom-right-radius: 26px !important; position: relative !important; z-index: 1 !important; }\n    .ols-zoom-card-caption p { margin: 0 !important; color: #5f6b85 !important; font-size: 15px !important; line-height: 1.65 !important; font-weight: 300 !important; font-style: italic !important; font-family: Poppins, Arial, sans-serif !important; }\n    .ols-image-lightbox { position: fixed; inset: 0; z-index: 999999; display: none; align-items: center; justify-content: center; padding: 34px; background: rgba(10, 15, 35, 0.86); backdrop-filter: blur(8px); -webkit-backdrop-filter: blur(8px); }\n    .ols-image-lightbox.is-open { display: flex; }\n    .ols-image-lightbox-inner { position: relative; width: min(96vw, 1500px); max-height: 92vh; display: flex; align-items: center; justify-content: center; }\n    .ols-image-lightbox-img { display: block; max-width: 100%; max-height: 92vh; height: auto; width: auto; border-radius: 22px; background: #ffffff; box-shadow: 0 32px 90px rgba(0, 0, 0, 0.45); object-fit: contain; }\n    .ols-image-lightbox-close { position: absolute; top: -18px; right: -18px; width: 46px; height: 46px; border: 0; border-radius: 50%; background: #ffffff; color: var(--navy); font-family: Poppins, Arial, sans-serif; font-size: 28px; line-height: 1; font-weight: 700; cursor: pointer; box-shadow: 0 16px 34px rgba(0, 0, 0, 0.28); display: flex; align-items: center; justify-content: center; transition: transform 0.2s ease, background 0.2s ease, color 0.2s ease; }\n    .ols-image-lightbox-close:hover { transform: scale(1.08); background: var(--blue); color: #ffffff; }\n    @media (max-width: 760px) { .ols-zoom-card { border-radius: 22px !important; overflow: hidden !important; } .ols-zoom-card-image { min-height: auto !important; padding: 12px !important; overflow: hidden !important; border-top-left-radius: 22px !important; border-top-right-radius: 22px !important; } .ols-zoom-card img.ols-zoomable-img, .ols-zoom-card img.ols-zoomable-img:hover { transform: none !important; box-shadow: none !important; } .ols-zoom-card-caption { border-bottom-left-radius: 22px !important; border-bottom-right-radius: 22px !important; } .ols-image-lightbox { padding: 16px; } .ols-image-lightbox-inner { width: 100%; max-height: 88vh; } .ols-image-lightbox-img { max-height: 88vh; border-radius: 16px; } .ols-image-lightbox-close { top: 10px; right: 10px; width: 42px; height: 42px; font-size: 26px; } }\n\n\n    .ols-table-wrap { overflow: hidden; border-radius: 22px; border: 1px solid var(--border); background: #ffffff; margin-top: 18px; }\n    .ols-table { width: 100%; border-collapse: collapse; table-layout: fixed; }\n    .ols-table th { background: var(--navy); color: #ffffff; padding: 16px; text-align: left; font-size: clamp(14px, 1.2vw, 17px); font-weight: 600; overflow-wrap: anywhere; }\n    .ols-table td { padding: 16px; border-bottom: 1px solid var(--border); font-size: clamp(14px, 1.15vw, 16px); line-height: 1.45; color: var(--body-text); vertical-align: top; overflow-wrap: anywhere; }\n    .ols-table tr:last-child td { border-bottom: none; }\n\n    .ols-h5p-card { background: linear-gradient(135deg, #ffffff 0%, #f7f0ff 100%); }\n    .ols-h5p-frame { margin-top: 22px; padding: 18px; border-radius: 24px; background: #ffffff; border: 1px solid var(--border); box-shadow: inset 0 0 0 1px rgba(28, 36, 75, 0.03); }\n\n    .ols-faq-list { display: grid; gap: 14px; margin-top: 18px; }\n    .ols-faq-item { background: #ffffff; border: 1px solid var(--border); border-radius: 18px; padding: 18px 20px; box-shadow: var(--inner-shadow); }\n    .ols-faq-item h3 { margin: 0 0 8px; font-size: 20px; line-height: 1.3; color: var(--navy); }\n    .ols-faq-item p { margin: 0; font-size: 16px; line-height: 1.65; color: var(--body-text); }\n\n    .ols-related-grid { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 16px; margin-top: 18px; }\n  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space-between; gap: 14px; margin-bottom: 18px; }\n    .ols-course-cta-kicker { display: inline-flex; align-items: center; padding: 8px 14px; border-radius: 999px; background: #ffffff; border: 1px solid rgba(37, 99, 235, 0.18); color: var(--blue); font-size: 14px; line-height: 1.2; font-weight: 700; box-shadow: var(--inner-shadow); }\n    .ols-course-cta-covalent h2 { margin: 0 0 14px; font-size: clamp(28px, 3.5vw, 42px); line-height: 1.15; font-weight: 800; letter-spacing: -0.03em; color: #111827; }\n    .ols-course-cta-intro { margin: 0 0 24px; color: var(--body-text); font-size: clamp(16px, 1.4vw, 18px); line-height: 1.7; font-weight: 300; }\n    .ols-course-cta-features { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 14px; margin: 0 0 30px; }\n    .ols-course-feature { background: rgba(255, 255, 255, 0.78); border: 1px solid var(--border); border-radius: 18px; padding: 16px 18px; }\n    .ols-course-feature h3 { margin: 0 0 6px; color: var(--navy); font-size: 18px; line-height: 1.3; font-weight: 800; }\n    .ols-course-feature p { margin: 0; color: var(--body-text); font-size: 15px; line-height: 1.6; font-weight: 300; }\n    .ols-course-cta-bottom { display: flex; justify-content: center; padding-top: 22px; border-top: 1px solid var(--border); }\n    .ols-course-button { display: inline-flex; align-items: center; justify-content: center; padding: 15px 28px; border-radius: 999px; background: var(--navy); color: #ffffff; text-decoration: none; font-size: 16px; line-height: 1.2; font-weight: 800; box-shadow: 0 12px 26px rgba(28, 36, 75, 0.18); transition: transform 0.2s ease, background 0.2s ease, box-shadow 0.2s ease; }\n    .ols-course-button:hover { transform: translateY(-2px); background: var(--blue); color: #ffffff; box-shadow: 0 16px 32px rgba(37, 99, 235, 0.24); }\n    .ols-course-button-top { flex-shrink: 0; padding: 12px 22px; font-size: 15px; }\n\n    .ols-attribution-card {\n      background: linear-gradient(135deg, #ffffff, 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.ols-attribution-card { padding: 24px 18px; border-radius: 22px; }\n      .ols-note-title { align-items: flex-start; }\n      .ols-related-grid, .ols-course-cta-features { grid-template-columns: 1fr; }\n      .ols-figure-card { border-radius: 22px; }\n      .ols-figure-image { min-height: 210px; padding: 14px; }\n      .ols-figure-caption { padding: 16px; }\n      .ols-table-wrap { border: none; background: transparent; }\n      .ols-table, .ols-table thead, .ols-table tbody, .ols-table th, .ols-table td, .ols-table tr { display: block; width: 100%; }\n      .ols-table { border-collapse: separate; border-spacing: 0; }\n      .ols-table thead { display: none; }\n      .ols-table tr { margin-bottom: 14px; border: 1px solid var(--border); border-radius: 18px; overflow: hidden; background: #ffffff; box-shadow: var(--inner-shadow); }\n      .ols-table td { border-bottom: 1px solid var(--border); padding: 14px 16px; overflow-wrap: anywhere; }\n      .ols-table td:last-child { border-bottom: 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border-radius: 22px; padding: 14px; margin: 18px 0 6px; }\n    .ols-figure-placeholder .ols-figure-caption p { margin: 10px 0 0; font-size: 14px; color: #667085; text-align: center; }\n\n    \/* inline checks (H5P re-flow, Sep 2026) *\/\n    .ols-h5p-card.ols-h5p-inline { padding: 26px 28px; border-left: 6px solid #7c3aed; }\n    .ols-h5p-card.ols-h5p-inline h2 { font-size: clamp(20px, 2.2vw, 27px); letter-spacing: -0.02em; }\n    .ols-h5p-card.ols-h5p-inline > p { margin: 8px 0 0; }\n    .ols-h5p-card.ols-h5p-inline .ols-h5p-frame { margin-top: 16px; padding: 14px; border-radius: 20px; }\n    .ols-h5p-kicker { display: inline-block; margin-bottom: 10px; padding: 5px 12px; border-radius: 999px; background: #ede9fe; color: #5b21b6; font-size: 12px; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; }\n    .ols-h5p-card.ols-h5p-recap { border-left-color: #c9973a; background: linear-gradient(135deg, #ffffff 0%, #fff8e8 100%); }\n    .ols-h5p-recap .ols-h5p-kicker { background: #fdf0d2; color: #8a5a00; }\n    @media (max-width: 760px) { .ols-h5p-card.ols-h5p-inline { padding: 20px 16px; } }\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 23 Chemical Energetics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-23-chemical-energetics\/\">Topic 23 Chemical Energetics Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility, Gibbs Energy and Temperature<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-23-chemical-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/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\/cie\/topic-5-chemical-energetics\/\">Topic 5 Chemical Energetics (AS)<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/\">Topic 26 Reaction Kinetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-10-group-2\/\">Topic 10 Group 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/practical-skills\/\">Practical Skills<\/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, 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-23-chemical-energetics\/\">Topic 23 Chemical Energetics<\/a> \/\n<span>Entropy and the Direction of Change<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Entropy and the Direction of Change<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to entropy: what it measures, why it rises with temperature and jumps at melting and boiling, why a perfect crystal at 0 K has zero entropy, why endothermic reactions can happen, and how to predict the sign of the entropy change for changes of state, dissolving and reactions that change the number of gas molecules.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">A Level<\/div>\n<div class=\"ols-badge\">Topic 23: Chemical Energetics<\/div>\n<div class=\"ols-badge\">9701 Paper 4<\/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>AS Recap: Enthalpy Changes<\/h2>\n<p>Three quick questions on what you already know: the sign of \u0394H for exothermic and endothermic changes, what standard conditions are, and how a reaction profile shows the enthalpy change.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1098\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Enthalpy Alone Is Not Enough<\/h2>\n<\/div>\n<p>At AS (<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-5-chemical-energetics\/\">Topic 5 Chemical Energetics<\/a>) you learned that exothermic reactions give out heat and endothermic reactions take it in. It is tempting to think that reactions happen because they release energy.<\/p>\n<p>Many do, but the rule fails too often to be the whole story. Every one of these changes happens on its own at room temperature, and every one absorbs heat:<\/p>\n<ul class=\"ols-list\">\n<li>Ice melts above 0 \u00b0C even though melting is <strong>endothermic<\/strong>.<\/li>\n<li>Ammonium nitrate dissolves in water and the solution goes cold.<\/li>\n<li>Sodium hydrogencarbonate fizzes with citric acid in a cold pack, taking in heat.<\/li>\n<li>Ethanoic acid reacts with ammonium carbonate, taking in heat.<\/li>\n<li>Stir solid barium hydroxide octahydrate, Ba(OH)\u2082\u00b78H\u2082O, with solid ammonium chloride and the flask gets cold enough to freeze to a wet block of wood.<\/li>\n<\/ul>\n<p>So a second factor must be at work, one that can outweigh an unfavourable enthalpy change. That factor is <strong>entropy<\/strong>.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Change<\/th><th>Why it happens<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Burning magnesium in air<\/strong><\/td><td>the enthalpy term wins<\/td><\/tr>\n<tr><td><strong>Dissolving ammonium nitrate<\/strong><\/td><td>the entropy term wins<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Endothermic changes can occur spontaneously at room temperature, so the enthalpy change alone does not decide whether a change happens. A second quantity, entropy, is needed.<\/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>What Entropy Is<\/h2>\n<\/div>\n<p>Entropy, symbol <strong>S<\/strong>, is the number of possible arrangements of the particles and their energy in a given system.<\/p>\n<p>The more ways there are of arranging the particles of a substance, and of sharing out the energy quanta among them, the higher its entropy.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>State<\/th><th>Particles<\/th><th>Number of arrangements<\/th><th>S of water \/ J K\u207b\u00b9 mol\u207b\u00b9 (about)<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Crystalline solid<\/strong><\/td><td>fixed in a lattice<\/td><td>few, so the entropy is low<\/td><td>48 (ice)<\/td><\/tr>\n<tr><td><strong>Liquid<\/strong><\/td><td>can move past one another<\/td><td>more<\/td><td>70 (liquid water)<\/td><\/tr>\n<tr><td><strong>Gas<\/strong><\/td><td>can be anywhere in the container<\/td><td>far greater<\/td><td>189 (steam)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>For every substance, <strong>S increases from solid to liquid to gas<\/strong>. The jump at boiling is much larger than the jump at melting.<\/p>\n<h3>Units, the zero of entropy and temperature<\/h3>\n<ul class=\"ols-list\">\n<li>Entropy is measured in <strong>J K\u207b\u00b9 mol\u207b\u00b9<\/strong>: joules, not kilojoules. This matters when it is later combined with enthalpy values.<\/li>\n<li>Standard entropies, S\u29b5, are quoted at 298 K and 100 kPa.<\/li>\n<li>Unlike enthalpy, entropy has a true zero. A <strong>perfect crystal at 0 K<\/strong> has zero entropy, because there is only one way to arrange its particles and no energy quanta to share.<\/li>\n<li>As a substance is warmed its particles vibrate, rotate and move more. The energy can be shared in more ways, so the <strong>entropy rises with temperature<\/strong>.<\/li>\n<li>It follows that elements do not have zero entropy, unlike their enthalpies of formation: every substance above 0 K has a positive standard entropy.<\/li>\n<\/ul>\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\/entropy-t12-01-dispersal.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\/entropy-t12-01-dispersal.jpg\" alt=\"Entropy poster: the same particles as a solid, liquid and gas with more ways of arranging them and their energy quanta, and a graph of entropy against temperature with jumps at melting and boiling and zero entropy at 0 K\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The same particles as a solid, liquid and gas with more ways of arranging them and their energy, and how entropy climbs with temperature, jumping at melting and boiling from zero at 0 K.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Entropy, S, is the number of possible arrangements of the particles and their energy in a given system. Units: J K\u207b\u00b9 mol\u207b\u00b9.<\/p>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> Entropy increases from solid to liquid to gas and rises with temperature; a perfect crystal at 0 K has zero entropy.<\/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: What Entropy Measures<\/h2>\n<p>Rank substances and states by entropy and say how entropy changes with temperature, for substances not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1099\" class=\"h5p-iframe\" data-content-id=\"1099\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Summary: What Entropy Measures\"><\/iframe><\/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>Why Entropy Changes<\/h2>\n<\/div>\n<p>The entropy change of a process, <strong>\u0394S<\/strong>, is positive if the products have more ways of arranging their particles and energy than the reactants, and negative if they have fewer.<\/p>\n<p>Three kinds of change come up again and again. In each, the sign follows from the states and the numbers of particles.<\/p>\n<h3>Changes of state<\/h3>\n<ul class=\"ols-list\">\n<li>Melting and boiling free the particles to move, so \u0394S is positive, and much larger for boiling than for melting.<\/li>\n<li>Freezing and condensing are the reverse, with negative \u0394S.<\/li>\n<li>Warming a substance without a change of state also raises its entropy, because the extra energy quanta can be shared in more ways.<\/li>\n<\/ul>\n<h3>Dissolving an ionic lattice<\/h3>\n<ul class=\"ols-list\">\n<li>When sodium chloride dissolves, an ordered crystal becomes separate Na\u207a and Cl\u207b ions free to move anywhere in the solution, so the entropy of the ions rises greatly.<\/li>\n<li>There is a competing effect: each ion holds a shell of water molecules in place around it, so the water becomes more ordered.<\/li>\n<li>For most salts of singly charged ions the freeing of the ions wins and \u0394S is positive, which is why so many endothermic salts dissolve.<\/li>\n<li>For small, highly charged ions the ordering of the water can make \u0394S negative.<\/li>\n<\/ul>\n<h3>Changes in gas moles<\/h3>\n<p>Gases have such high entropies that they dominate the count. A reaction that makes more moles of gas than it uses has a positive \u0394S; one that reduces the number of gas molecules has a negative \u0394S.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Reaction<\/th><th>Gas molecules<\/th><th>Sign of \u0394S<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>CaCO\u2083(s) \u2192 CaO(s) + CO\u2082(g)<\/strong><\/td><td>one solid becomes a solid and a gas<\/td><td>positive<\/td><\/tr>\n<tr><td><strong>N\u2082(g) + 3H\u2082(g) \u2192 2NH\u2083(g)<\/strong><\/td><td>four moles of gas become two<\/td><td>negative<\/td><\/tr>\n<tr><td><strong>2Mg(s) + O\u2082(g) \u2192 2MgO(s)<\/strong><\/td><td>a gas is used up and none is made<\/td><td>negative<\/td><\/tr>\n<tr><td><strong>H\u2082(g) + Cl\u2082(g) \u2192 2HCl(g)<\/strong><\/td><td>unchanged<\/td><td>small; the sign cannot be predicted without data<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\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\/entropy-t12-02-entropychanges.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\/entropy-t12-02-entropychanges.jpg\" alt=\"Three panels showing why entropy changes: ice melting and water\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Why entropy changes: changes of state, an ionic lattice breaking into hydrated ions (with the water becoming more ordered around them), and reactions that make or use up gas molecules, each with the sign of \u0394S.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;\u0394S is positive because a gas is produced from a solid, so the products have more ways of arranging the particles and their energy.&#8221; Count the moles of gas on each side first; only if they are equal look further.<\/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: Predicting the Sign of \u0394S<\/h2>\n<p>Predict and explain the sign of the entropy change for changes of state, dissolving and reactions not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1100\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>The Natural Direction of Change<\/h2>\n<\/div>\n<p>Open a bottle of perfume in a corner of a room and within minutes the smell is everywhere. The molecules never gather back into the bottle.<\/p>\n<p>There are enormously more arrangements with the molecules spread through the room than with them all in one corner, so spreading out is overwhelmingly the more probable outcome.<\/p>\n<p>The <strong>natural direction of change<\/strong> is the direction of <strong>increasing total entropy<\/strong>. Each of these increases the total number of arrangements, and none reverses itself:<\/p>\n<ul class=\"ols-list\">\n<li>a gas filling a room<\/li>\n<li>two gases mixing<\/li>\n<li>a hot object cooling to room temperature<\/li>\n<\/ul>\n<h3>Why the total matters<\/h3>\n<p>The word <strong>total<\/strong> is essential, because a change can lower the entropy of the substances involved and still happen.<\/p>\n<ul class=\"ols-list\">\n<li>Water freezes below 0 \u00b0C although ice has a lower entropy than liquid water.<\/li>\n<li>Magnesium burns although the reaction uses up a gas.<\/li>\n<\/ul>\n<p>In each case heat is given out to the <strong>surroundings<\/strong> and spreads among their particles, raising their entropy.<\/p>\n<ul class=\"ols-list\">\n<li>The reacting substances are the <strong>system<\/strong>; everything else is the surroundings.<\/li>\n<li>It is the sum that must be positive: \u0394S_total = \u0394S_system + \u0394S_surroundings.<\/li>\n<li>The colder the surroundings, the more a given amount of heat raises their entropy, which is why freezing happens below 0 \u00b0C but not above.<\/li>\n<\/ul>\n<p>The next two pages show how this idea is built into the Gibbs equation, which is the form you will use in calculations.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Changes happen in the direction that increases the total entropy of the system and its surroundings. Heat given out to the surroundings raises their entropy, so an exothermic reaction can be feasible even when the entropy of the system falls.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3>\n<ul class=\"ols-list\">\n<li>&#8220;Entropy is the number of possible arrangements of the particles and their energy in a given system.&#8221;<\/li>\n<li>&#8220;\u0394S is positive because the number of moles of gas increases.&#8221;<\/li>\n<li>&#8220;Endothermic reactions can be feasible because the increase in entropy outweighs the enthalpy change.&#8221;<\/li>\n<li>&#8220;A perfect crystal at 0 K has zero entropy.&#8221;<\/li>\n<\/ul>\n<h3>Do not say<\/h3>\n<ul class=\"ols-list\">\n<li>&#8220;Entropy is energy&#8221; (its units are J K\u207b\u00b9 mol\u207b\u00b9, not kJ mol\u207b\u00b9).<\/li>\n<li>&#8220;Elements have zero entropy&#8221; (they have zero enthalpy of formation, not zero entropy).<\/li>\n<li>&#8220;The reaction happens because it is exothermic&#8221; (some exothermic reactions do not happen, and some endothermic ones do).<\/li>\n<\/ul>\n<h3>Watch for<\/h3>\n<ul class=\"ols-list\">\n<li>Questions that ask you to explain the sign of \u0394S without data: talk about states and moles of gas, and mention the ordering of water molecules when a salt dissolves.<\/li>\n<li>Questions about the direction of change: the answer is always in terms of the <strong>total<\/strong> entropy increasing, not the entropy of the reactants alone.<\/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: The Direction of Change<\/h2>\n<p>Explain, in terms of entropy, why changes not described on this page happen or do not happen.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1101\" class=\"h5p-iframe\" data-content-id=\"1101\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Quick Choice: Direction of Change\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the entropy ideas that come up most often.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why can an endothermic reaction happen on its own?<\/h3>\n<p>Because enthalpy is not the only thing that matters. If the entropy increase in the reaction is large enough to outweigh the energy taken in from the surroundings, the change is still feasible. Dissolving ammonium nitrate and the reaction of barium hydroxide with ammonium chloride are cold to the touch but happen on their own, driven by the gain in entropy.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Is entropy the same as disorder?<\/h3>\n<p>Disorder is the everyday picture, but entropy is really a measure of the number of ways the particles and their energy quanta can be arranged. The more arrangements available, the higher the entropy. A gas has far more ways to spread its molecules and energy than a solid, which is why it looks more disordered.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the entropy of a gas so much bigger than that of a liquid or a solid?<\/h3>\n<p>Gas molecules move freely through the whole container, so there is an enormous number of positions and energies available to them. In a liquid the particles are close together but can still move past each other; in a solid they only vibrate about fixed positions. Boiling therefore causes a much bigger jump in entropy than melting.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Do elements have zero entropy?<\/h3>\n<p>No. Elements have zero enthalpy of formation by definition, but their standard entropies are positive, for example about 5.7 J K\u207b\u00b9 mol\u207b\u00b9 for graphite and 131 J K\u207b\u00b9 mol\u207b\u00b9 for hydrogen gas. Only a perfect crystal at 0 K has zero entropy, because there is then only one arrangement.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I predict the sign of \u0394S without any data?<\/h3>\n<p>Count the gas molecules on each side, because gases dominate. More moles of gas on the product side means \u0394S is positive; fewer means it is negative. If the gas count does not change, look at changes of state (solid to liquid to gas raises entropy) and dissolving a solid, which usually raises it.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 23 Chemical Energetics Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 23 Chemical 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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility, Gibbs Energy and Temperature<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-23-chemical-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\/cie\/topic-23-chemical-energetics\/\">Topic 23 Chemical Energetics Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-5-chemical-energetics\/\">Topic 5 Chemical Energetics<\/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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