{"id":12494,"date":"2026-10-03T08:33:13","date_gmt":"2026-10-03T07:33:13","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/lattice-energy-and-born-haber-cycles\/"},"modified":"2026-10-03T10:00:41","modified_gmt":"2026-10-03T09:00:41","slug":"lattice-energy-and-born-haber-cycles","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/lattice-energy-and-born-haber-cycles\/","title":{"rendered":"Lattice Enthalpy and Born\u2013Haber Cycles"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-nature-of-covalent-bonding-9ch0-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      --soft-green: #f0f7f1;\n      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: 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92px; height: 92px; border-radius: 50%; object-fit: cover; object-position: center; flex-shrink: 0; border: 4px solid #ffffff; box-shadow: 0 14px 32px rgba(28,36,75,0.18), 0 0 0 1px rgba(28,36,75,0.10); transition: transform 0.25s ease, box-shadow 0.25s ease; }\n    .ols-author:hover .ols-author-avatar-img { transform: scale(1.04); box-shadow: 0 20px 42px rgba(28,36,75,0.22), 0 0 0 1px rgba(28,36,75,0.10); }\n    .ols-author-content { min-width: 0; }\n    .ols-author-title { margin: 0 0 4px; font-size: clamp(24px,3vw,34px); line-height: 1.15; font-weight: 700; color: var(--navy); letter-spacing: -0.03em; }\n    .ols-author-description { margin: 0; font-size: 17px; line-height: 1.7; font-weight: 300; color: var(--grey-text); }\n    .ols-linkedin-pill { display: inline-flex; align-items: center; justify-content: center; gap: 10px; margin-top: 16px; padding: 11px 18px; border-radius: 999px; background: linear-gradient(135deg,#0A66C2,#004182); color: #ffffff; text-decoration: none; font-size: 14px; line-height: 1; font-weight: 700; letter-spacing: 0.01em; box-shadow: 0 10px 22px rgba(10,102,194,0.24); position: relative; overflow: hidden; transition: transform 0.22s ease, box-shadow 0.22s ease; }\n    .ols-linkedin-pill::before { content: \"\"; position: absolute; top: 0; left: -120%; width: 100%; height: 100%; background: linear-gradient(120deg, rgba(255,255,255,0) 0%, rgba(255,255,255,0.28) 50%, rgba(255,255,255,0) 100%); transition: left 0.7s ease; }\n    .ols-linkedin-pill:hover { transform: translateY(-3px) scale(1.03); box-shadow: 0 16px 34px rgba(10,102,194,0.34), 0 0 0 6px rgba(10,102,194,0.10); color: #ffffff; }\n    .ols-linkedin-pill:hover::before { left: 120%; }\n    .ols-linkedin-icon { width: 18px; height: 18px; display: block; flex-shrink: 0; }\n\n    .ols-note-card.soft { background: linear-gradient(135deg, #ffffff 0%, var(--soft-red) 100%); }\n    .ols-note-card.purple { background: linear-gradient(135deg, #ffffff 0%, var(--soft-purple) 100%); }\n    .ols-note-card.orange { background: linear-gradient(135deg, #ffffff 0%, var(--soft-orange) 100%); }\n    .ols-note-title { display: flex; align-items: center; gap: 14px; margin-bottom: 20px; }\n    .ols-note-icon { width: 52px; height: 52px; border-radius: 16px; background: var(--navy); color: #ffffff; display: grid; place-items: center; font-size: 24px; font-weight: 800; flex-shrink: 0; }\n    .ols-note-title h2, .ols-h5p-card h2, .ols-related-card h2, .ols-faq-card h2 { margin: 0; font-size: clamp(28px, 3.5vw, 42px); line-height: 1.15; font-weight: 800; color: #111827; letter-spacing: -0.03em; }\n    .ols-h5p-card h2, .ols-related-card h2, .ols-faq-card h2 { margin-bottom: 14px; }\n    .ols-note-card p, .ols-note-card li { font-size: clamp(15px, 1.25vw, 17px); line-height: 1.65; font-weight: 300; color: var(--body-text); }\n    .ols-h5p-card p, .ols-related-card p, .ols-faq-card p { font-size: clamp(15px, 1.3vw, 18px); line-height: 1.65; font-weight: 300; color: var(--body-text); }\n    .ols-note-card strong, .ols-h5p-card strong, .ols-related-card strong, .ols-faq-card strong { font-weight: 700; color: var(--navy); }\n    .ols-note-card ul, .ols-note-card ol { margin: 0; padding-left: 22px; }\n\n    .ols-key-box { margin-top: 20px; background: var(--soft-green); border: 1px solid rgba(63, 143, 70, 0.25); border-radius: 20px; padding: 18px 20px; }\n    .ols-key-box p { margin: 0; font-weight: 400; color: var(--navy); }\n    .ols-definition-box { background: linear-gradient(135deg, #ffffff, var(--soft-purple)); border: 2px dashed rgba(122, 62, 157, 0.35); border-radius: 24px; padding: 22px 24px; margin-top: 22px; }\n    .ols-definition-box p { margin: 0; color: var(--navy); font-weight: 400; }\n\n    .ols-rule-list { display: grid; gap: 14px; margin-top: 18px; }\n    .ols-rule-item { background: #ffffff; border: 1px solid var(--border); border-left: 5px solid var(--blue); border-radius: 18px; padding: 18px; box-shadow: var(--inner-shadow); }\n  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 .ols-zoom-card, .ols-zoom-card * { box-sizing: border-box; }\n    .ols-zoom-card { display: block !important; margin: 26px auto 34px !important; border: 1px solid rgba(28, 36, 75, 0.14) !important; border-radius: 26px !important; background: #ffffff !important; box-shadow: 0 18px 45px rgba(28, 36, 75, 0.10) !important; overflow: visible !important; position: relative !important; z-index: 1 !important; isolation: isolate !important; }\n    .ols-zoom-card.medium { max-width: 820px; }\n    .ols-zoom-card.compact { max-width: 700px; }\n    .ols-zoom-card.wide { max-width: 940px; }\n    .ols-zoom-card.slim { max-width: 600px; }\n    .ols-zoom-card:hover { z-index: 50 !important; }\n    .ols-zoom-card-image { display: flex !important; align-items: center !important; justify-content: center !important; width: 100% !important; min-height: 240px !important; padding: 20px !important; background: #ffffff !important; overflow: visible !important; border-top-left-radius: 26px !important; 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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minmax(260px, 0.9fr) minmax(0, 1.1fr);\n      gap: 28px;\n      align-items: center;\n      margin-bottom: 24px;\n    }\n    .ols-course-cta-image-link { display: block; text-decoration: none; border-radius: 24px; }\n    .ols-course-cta-image {\n      width: 100%;\n      border-radius: 24px;\n      overflow: hidden;\n      border: 1px solid var(--border);\n      background: #ffffff;\n      box-shadow: var(--inner-shadow);\n      transition: transform 0.35s ease, box-shadow 0.35s ease;\n    }\n    .ols-course-cta-image img { width: 100%; height: auto; display: block; transition: transform 0.45s ease; }\n    .ols-course-cta-image-link:hover .ols-course-cta-image { transform: translateY(-8px) scale(1.015); box-shadow: 0 26px 60px rgba(28, 36, 75, 0.18), 0 0 0 1px rgba(37, 99, 235, 0.12); }\n    .ols-course-cta-image-link:hover .ols-course-cta-image img { transform: scale(1.03); }\n    .ols-course-cta-header-row { display: flex; flex-wrap: wrap; align-items: center; justify-content: 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, #f8fbff);\n      border: 1px solid rgba(28, 36, 75, 0.14);\n      border-radius: 28px;\n      box-shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      padding: 34px;\n      margin-bottom: 24px;\n      overflow: hidden;\n      font-family: Poppins, Arial, sans-serif;\n      box-sizing: border-box;\n    }\n    .ols-attribution-card, .ols-attribution-card * { box-sizing: border-box; }\n    .ols-attribution-card p { margin: 0; font-size: 14px; line-height: 1.65; font-weight: 300; color: #667085; }\n    .ols-attribution-card strong { font-weight: 700; color: #1C244B; }\n\n    @media (max-width: 1050px) {\n      .ols-main { max-width: none; }\n      .ols-related-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }\n      .ols-course-cta-top { grid-template-columns: 1fr; }\n      .ols-course-cta-image-link { max-width: 520px; margin: 0 auto; }\n    }\n\n    @media (max-width: 760px) {\n      .ols-title-card, .ols-note-card, .ols-h5p-card, .ols-related-card, .ols-faq-card, .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>3.1.8 Thermodynamics<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/\">3.1.8 Thermodynamics Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/lattice-energy-and-born-haber-cycles\/\">Lattice Enthalpy and Born\u2013Haber Cycles<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/lattice-energy-trends-and-covalent-character\/\">Lattice Enthalpy Trends and Covalent Character<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-8-thermodynamics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-4-energetics\/\">3.1.4 Energetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/\">3.1.9 Rate Equations<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-2-2-group-2-the-alkaline-earth-metals\/\">3.2.2 Group 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/\">Required 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, 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Cycles<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Lattice Enthalpy and Born\u2013Haber Cycles<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to lattice enthalpy, the enthalpy changes of atomisation and electron affinity, the two definitions of lattice enthalpy (formation and dissociation), how to construct a Born\u2013Haber cycle for sodium chloride and magnesium chloride, and how to calculate lattice enthalpy or any other missing step from the cycle.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 AQA<\/div>\n<div class=\"ols-badge\">3.1.8 Thermodynamics<\/div>\n<div class=\"ols-badge\">7405\/1<\/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: Hess\u2019s Law and Ionic Bonding<\/h2>\n<p>Three quick questions on what you already know: Hess\u2019s law, the standard enthalpy change of formation, and what holds the ions together in a giant ionic lattice.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1110\"><\/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>What Lattice enthalpy Measures<\/h2>\n<\/div>\n<p>When oppositely charged gaseous ions come together into a giant lattice, a great deal of energy is released. The <strong>lattice enthalpy<\/strong> measures how much.<\/p>\n<ul>\n<li>For sodium chloride, Na\u207a(g) + Cl\u207b(g) \u2192 NaCl(s) releases 787 kJ per mole of solid formed.<\/li>\n<li>So the lattice enthalpy of sodium chloride is \u2212787 kJ mol\u207b\u00b9.<\/li>\n<li>It is always exothermic: the lattice is lower in energy than the separated ions.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Lattice enthalpy: the energy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions. It is always negative (exothermic).<\/p>\n<\/div>\n<p>The lattice enthalpy is a direct <strong>measure of ionic bond strength<\/strong>: the more exothermic it is, the stronger the attraction between the ions.<\/p>\n<p>It cannot be measured directly, because gaseous ions cannot be combined in a calorimeter. It is found from quantities that can be measured, linked by Hess\u2019s law (from <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-energetics\/\">3.1.4 Energetics<\/a>) in a Born\u2013Haber cycle.<\/p>\n<p>The same quantity is defined in two directions, with the same size and opposite signs.<\/p>\n<ul>\n<li><strong>Enthalpy of lattice formation<\/strong>: the enthalpy change when one mole of the ionic solid is formed from its gaseous ions. Negative: \u2212787 kJ mol\u207b\u00b9 for sodium chloride.<\/li>\n<li><strong>Enthalpy of lattice dissociation<\/strong>: the enthalpy change when one mole of the ionic solid is separated into its gaseous ions. Positive: +787 kJ mol\u207b\u00b9.<\/li>\n<\/ul>\n<p>Questions give either one, so read the definition in the data. A positive value is a dissociation value, and its sign must be reversed before it goes into a formation cycle.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Lattice formation: &#8220;one mole of solid ionic lattice formed from its gaseous ions&#8221;. Lattice dissociation: &#8220;one mole of solid ionic lattice broken into its gaseous ions&#8221;. Same size, opposite sign.<\/p>\n<\/div>\n<!-- 3D card: nacl-lattice (3 Oct 2026, reused) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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color:#5a6180;\n}\n.ols-nac-001 .nac-key i{\n  display:inline-block; width:13px; height:13px; border-radius:3px;\n  margin-right:7px; vertical-align:-2px; border:1px solid rgba(28,36,75,0.18);\n}\n\n@media (max-width:760px){\n  .ols-nac-001{padding:26px; border-radius:22px;}\n  .ols-nac-001 h2.nac-title{font-size:20.5px;}\n  .ols-nac-001 p.nac-sub{font-size:13.5px;}\n  .ols-nac-001 .nac-stage{height:420px;}\n  .ols-nac-001 .nac-rowlab{min-width:100%;}\n  .ols-nac-001 .nac-seg button{padding:9px 13px; font-size:13px;}\n  .ols-nac-001 .nac-lab{font-size:10.5px; padding:4px 8px;}\n  .ols-nac-001 .nac-caption{font-size:10.5px; padding:5px 9px; max-width:64%;}\n  .ols-nac-001 .nac-badge{font-size:10.5px; padding:4px 9px;}\n  .ols-nac-001 .nac-badge.b-shape{font-size:12px;}\n  .ols-nac-001 .nac-badges{max-width:70%;}\n  .ols-nac-001 .nac-info{padding:18px;}\n}\n\/* on a phone the caption needs the full top row, so the drag hint goes *\/\n@media (max-width:480px){\n  .ols-nac-001 .nac-hint{display:none;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-nac-001 .nac-seg button,.ols-nac-001 .nac-pill{transition:none;}\n}\n\n.ols-cc-nac-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-nac-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-nac-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"nac-head\">\n  <h2 class=\"nac-title\">The Sodium Chloride Giant Ionic Lattice<\/h2>\n  <p class=\"nac-sub\">Build sodium chloride from one Na<sup>+<\/sup> and its six Cl<sup>&minus;<\/sup> neighbours up to the unit cell and the giant lattice, then see how the structure explains its properties.<\/p>\n<\/div>\n\n<div class=\"nac-stage\" id=\"nacStage\">\n  <canvas class=\"nac-canvas\" id=\"nacCanvas\"><\/canvas>\n  <div class=\"nac-overlay\" id=\"nacOverlay\"><\/div>\n  <div class=\"nac-caption\" id=\"nacCaption\"><\/div>\n  <div class=\"nac-hint\" id=\"nacHint\">Drag to rotate<\/div>\n  <div class=\"nac-badges\" id=\"nacBadges\"><\/div>\n<\/div>\n\n<div class=\"nac-controls\">\n  <div class=\"nac-row\">\n    <span class=\"nac-rowlab\">Step<\/span>\n    <div class=\"nac-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\">One ion<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\">Unit cell<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Giant lattice<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\">Melting<\/button>\n      <button type=\"button\" data-step=\"5\" aria-pressed=\"false\">Brittle<\/button>\n      <button type=\"button\" data-step=\"6\" aria-pressed=\"false\">Dissolving<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"nac-row\">\n    <span class=\"nac-rowlab\">View<\/span>\n    <button type=\"button\" class=\"nac-pill\" id=\"nacResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"nac-info\">\n  <h3 id=\"nacInfoTitle\"><\/h3>\n  <p id=\"nacInfoText\"><\/p>\n  <div class=\"nac-facts\" id=\"nacFacts\"><\/div>\n  <div class=\"nac-key\">\n    <span><i style=\"background:#965ac8\"><\/i>Na<sup>+<\/sup><\/span>\n    <span><i style=\"background:#3aa048\"><\/i>Cl<sup>&minus;<\/sup><\/span>\n    <span><i style=\"background:#ffffff; border:1.5px dashed rgba(212,168,20,0.95)\"><\/i>Electrostatic attraction<\/span>\n    <span><i style=\"background:#ffffff; border:1.5px solid rgba(28,36,75,0.75)\"><\/i>Unit cell<\/span>\n    <span><i style=\"background:#cd342c\"><\/i>O of water<\/span>\n    <span><i style=\"background:#eef0f5\"><\/i>H of water<\/span>\n    <span><i style=\"background:#d63a3a\"><\/i>Repulsion<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-nac-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/nacl-lattice.js?v=20261003c\"><\/script>\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 Enthalpy Changes in the Cycle<\/h2>\n<\/div>\n<p>A Born\u2013Haber cycle turns the elements into gaseous ions one step at a time, and each step has a defined enthalpy change. Every definition refers to <strong>one mole<\/strong> of something, and the exact wording matters.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Enthalpy change<\/th><th>Definition<\/th><th>Equation (sodium chloride)<\/th><th>Value \/ kJ mol\u207b\u00b9<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Enthalpy of formation, \u0394fH<\/strong><\/td><td>one mole of compound formed from its elements in their standard states<\/td><td>Na(s) + \u00bdCl\u2082(g) \u2192 NaCl(s)<\/td><td>\u2212411<\/td><\/tr>\n<tr><td><strong>Enthalpy of atomisation, \u0394atH<\/strong><\/td><td>one mole of gaseous atoms formed from the element in its standard state<\/td><td>Na(s) \u2192 Na(g); \u00bdCl\u2082(g) \u2192 Cl(g)<\/td><td>+107 (Na); +122 (Cl)<\/td><\/tr>\n<tr><td><strong>First ionisation energy, IE\u2081<\/strong><\/td><td>one mole of gaseous atoms each loses one electron to form gaseous 1+ ions<\/td><td>Na(g) \u2192 Na\u207a(g) + e\u207b<\/td><td>+496<\/td><\/tr>\n<tr><td><strong>Second ionisation energy, IE\u2082<\/strong><\/td><td>one mole of gaseous 1+ ions each loses one electron to form gaseous 2+ ions<\/td><td>Mg\u207a(g) \u2192 Mg\u00b2\u207a(g) + e\u207b<\/td><td>+1451 (Mg)<\/td><\/tr>\n<tr><td><strong>First electron affinity, EA\u2081<\/strong><\/td><td>one mole of gaseous atoms each gains one electron to form gaseous 1\u2212 ions<\/td><td>Cl(g) + e\u207b \u2192 Cl\u207b(g)<\/td><td>\u2212349<\/td><\/tr>\n<tr><td><strong>Second electron affinity, EA\u2082<\/strong><\/td><td>one mole of gaseous 1\u2212 ions each gains one electron to form gaseous 2\u2212 ions<\/td><td>O\u207b(g) + e\u207b \u2192 O\u00b2\u207b(g)<\/td><td>+798 (O)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Three of these need care.<\/p>\n<ul>\n<li><strong>Atomisation<\/strong> makes one mole of atoms, not molecules, and it is always endothermic. For chlorine it is \u00bdCl\u2082(g) \u2192 Cl(g).<\/li>\n<li>So \u0394atH(Cl) is half the Cl\u2013Cl bond enthalpy: \u00bd \u00d7 244 = +122 kJ mol\u207b\u00b9.<\/li>\n<li>The <strong>first electron affinity<\/strong> is exothermic for the halogens and oxygen: the nucleus attracts the incoming electron.<\/li>\n<li>The <strong>second electron affinity<\/strong> is always endothermic: the electron is pushed onto an ion that is already negative, so repulsion must be overcome.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Ionisation energies and electron affinities are always for gaseous atoms or ions, and always per mole of electrons removed or gained. Write the state symbols in every equation.<\/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: Definitions and Equations<\/h2>\n<p>Match each enthalpy change to its definition and its equation with state symbols, and decide which steps are endothermic, for compounds other than the ones on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1111\" class=\"h5p-iframe\" data-content-id=\"1111\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Drag: Naming the Born\u2013Haber Steps\"><\/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>Building the Born\u2013Haber Cycle for Sodium Chloride<\/h2>\n<\/div>\n<p>The cycle is an <strong>energy level diagram<\/strong>. Start with the elements in their standard states, Na(s) + \u00bdCl\u2082(g), on a horizontal line.<\/p>\n<p>Build upwards through the endothermic steps and downwards through the exothermic ones, ending at the solid.<\/p>\n<ol>\n<li><strong>Atomise the sodium:<\/strong> Na(s) \u2192 Na(g), \u0394atH = +107 kJ mol\u207b\u00b9, up.<\/li>\n<li><strong>Ionise the sodium:<\/strong> Na(g) \u2192 Na\u207a(g) + e\u207b, IE\u2081 = +496, up.<\/li>\n<li><strong>Atomise the chlorine:<\/strong> \u00bdCl\u2082(g) \u2192 Cl(g), \u0394atH = +122, up.<\/li>\n<li><strong>Add the electron to chlorine:<\/strong> Cl(g) + e\u207b \u2192 Cl\u207b(g), EA\u2081 = \u2212349, down to the gaseous ions Na\u207a(g) + Cl\u207b(g).<\/li>\n<li><strong>Form the lattice:<\/strong> Na\u207a(g) + Cl\u207b(g) \u2192 NaCl(s), the lattice enthalpy, a long arrow down to the solid.<\/li>\n<\/ol>\n<p>The direct route from elements to solid is the enthalpy of formation, \u0394fH = \u2212411, a shorter arrow down from the starting line.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-06-bornhaber.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-t12-06-bornhaber.jpg\" alt=\"Born\u2013Haber cycle for sodium chloride drawn as an energy level diagram with every step labelled and its value, and the lattice energy worked out\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The Born\u2013Haber cycle for sodium chloride as an energy level diagram: four steps up and down to the gaseous ions, then the lattice enthalpy down to the solid, with the enthalpy of formation as the direct route.<\/p><\/div>\n<\/div>\n<p>Hess\u2019s law says the two routes from elements to solid have the same total enthalpy change:<\/p>\n<p style=\"text-align:center\"><strong>\u0394fH = \u0394atH(Na) + IE\u2081(Na) + \u0394atH(Cl) + EA\u2081(Cl) + LE<\/strong><\/p>\n<p><strong>Step 1:<\/strong> rearrange for the lattice enthalpy, written LE.<\/p>\n<p style=\"text-align:center\">LE = \u0394fH \u2212 [\u0394atH(Na) + IE\u2081(Na) + \u0394atH(Cl) + EA\u2081(Cl)]<\/p>\n<p><strong>Step 2:<\/strong> substitute. Every value goes in with its own sign.<\/p>\n<p style=\"text-align:center\">LE = \u2212411 \u2212 (107 + 496 + 122 \u2212 349)<\/p>\n<p><strong>Step 3:<\/strong> add up the bracket, then subtract it.<\/p>\n<p style=\"text-align:center\">LE = \u2212411 \u2212 376<\/p>\n<p><strong>Answer:<\/strong> LE = <strong>\u2212787 kJ mol\u207b\u00b9<\/strong><\/p>\n<div class=\"ols-key-box\">\n<p><strong>Worked example:<\/strong> LE = \u0394fH \u2212 (sum of the steps from the elements to the gaseous ions) = \u2212411 \u2212 (+376) = \u2212787 kJ mol\u207b\u00b9. A lattice forming must be exothermic, so a positive answer means an arithmetic slip.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Magnesium Chloride: Two Chlorines and a 2+ Ion<\/h2>\n<\/div>\n<p>The cycle for MgCl\u2082 has the same shape with two changes.<\/p>\n<ul>\n<li>The formula has <strong>two chloride ions<\/strong>, so the chlorine steps are doubled.<\/li>\n<li>2 \u00d7 \u0394atH(Cl) = 2 \u00d7 122 = +244 and 2 \u00d7 EA\u2081(Cl) = 2 \u00d7 (\u2212349) = \u2212698.<\/li>\n<li>The cation is Mg\u00b2\u207a, so <strong>both ionisation energies<\/strong> are needed, one after the other: IE\u2081 = +738 and IE\u2082 = +1451.<\/li>\n<\/ul>\n<p>With \u0394atH(Mg) = +148 and \u0394fH(MgCl\u2082) = \u2212641:<\/p>\n<p style=\"text-align:center\"><strong>LE = \u2212641 \u2212 (148 + 738 + 1451 + 244 \u2212 698) = \u2212641 \u2212 1883 = \u22122524 kJ mol\u207b\u00b9<\/strong><\/p>\n<p>The lattice enthalpy is more than three times that of sodium chloride. The 2+ ion attracts each chloride far more strongly, and there are twice as many of them.<\/p>\n<p>That is why the large second ionisation energy is worth paying, and magnesium forms MgCl\u2082 rather than MgCl.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-07-mgcl2.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-t12-07-mgcl2.jpg\" alt=\"Born\u2013Haber cycle for magnesium chloride as an energy level diagram showing the two ionisation energies, the dou\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The Born\u2013Haber cycle for magnesium chloride: two ionisation energies, doubled chlorine steps, and a lattice enthalpy of \u22122524 kJ mol\u207b\u00b9.<\/p><\/div>\n<\/div>\n<p>For an oxide such as MgO the anion is O\u00b2\u207b, so the cycle needs <strong>both electron affinities<\/strong> of oxygen.<\/p>\n<ul>\n<li>EA\u2081 = \u2212141 and EA\u2082 = +798, a total of +657 kJ mol\u207b\u00b9.<\/li>\n<li>The O\u00b2\u207b ion exists in the solid only because the lattice enthalpy of the oxide (\u22123791 kJ mol\u207b\u00b9 for MgO) repays that cost many times over.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Multiply every step by the number of moles of that atom or ion in the formula, and include every ionisation energy or electron affinity up to the charge on the ion.<\/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: Constructing and Reading a Cycle<\/h2>\n<p>Put the steps of a Born\u2013Haber cycle in order, decide which arrows point up, and read a value from a completed cycle for a compound that is not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1112\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Finding a Different Unknown<\/h2>\n<\/div>\n<p>Any step in the cycle can be the unknown. If the lattice enthalpy is known, a question may ask for the <strong>electron affinity<\/strong>, the <strong>enthalpy of formation<\/strong> or an <strong>ionisation energy<\/strong> instead.<\/p>\n<p>The method is the same every time.<\/p>\n<ol>\n<li>Write the Hess equation for the cycle.<\/li>\n<li>Put in every known value with its sign.<\/li>\n<li>Rearrange for the unknown.<\/li>\n<\/ol>\n<p><strong>Example 1:<\/strong> for sodium chloride, find EA\u2081(Cl) given LE = \u2212787.<\/p>\n<p style=\"text-align:center\">EA\u2081 = \u0394fH \u2212 \u0394atH(Na) \u2212 IE\u2081 \u2212 \u0394atH(Cl) \u2212 LE<\/p>\n<p style=\"text-align:center\">EA\u2081 = \u2212411 \u2212 107 \u2212 496 \u2212 122 \u2212 (\u2212787) = <strong>\u2212349 kJ mol\u207b\u00b9<\/strong><\/p>\n<p>Subtracting a negative lattice enthalpy adds 787.<\/p>\n<p><strong>Example 2:<\/strong> find \u0394fH given everything else. Add all the steps.<\/p>\n<p style=\"text-align:center\">\u0394fH = 107 + 496 + 122 \u2212 349 \u2212 787 = <strong>\u2212411 kJ mol\u207b\u00b9<\/strong><\/p>\n<p>Two rules keep the algebra honest.<\/p>\n<ul>\n<li>An arrow going <strong>up<\/strong> the diagram enters the equation positive and one going <strong>down<\/strong> enters negative.<\/li>\n<li>A route that runs against an arrow reverses that value\u2019s sign.<\/li>\n<\/ul>\n<p>If the data give an <strong>enthalpy of lattice dissociation<\/strong>, there are two safe ways to use it.<\/p>\n<ul>\n<li>Change its sign to get the formation value and use the cycle above.<\/li>\n<li>Or draw the dissociation arrow pointing up from the solid to the gaseous ions and write the Hess equation to match.<\/li>\n<\/ul>\n<p>An arrow the wrong way for the sign of the value is the most common lost mark on this topic.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Set the two routes equal, substitute with signs, rearrange, then check the sign of the answer: formation and lattice values negative, atomisation and ionisation positive, first electron affinity negative for a halogen.<\/p>\n<\/div>\n<\/article>\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>Say<\/h3>\n<ul>\n<li>&#8220;Lattice enthalpy: the energy change when one mole of an ionic solid is formed from its gaseous ions.&#8221;<\/li>\n<li>&#8220;Enthalpy of atomisation: one mole of gaseous atoms from the element in its standard state.&#8221;<\/li>\n<li>&#8220;The second electron affinity is endothermic because an electron is added to an ion that is already negative, so repulsion must be overcome.&#8221;<\/li>\n<\/ul>\n<h3>Do not say<\/h3>\n<ul>\n<li>&#8220;Atomisation of chlorine is Cl\u2082(g) \u2192 2Cl(g)&#8221; (that is two moles of atoms, twice \u0394atH).<\/li>\n<li>&#8220;Electron affinity is the energy needed to add an electron&#8221; (for a halogen it is released).<\/li>\n<li>&#8220;The lattice enthalpy of MgCl\u2082 includes one electron affinity&#8221; (two chlorides, two electron affinities).<\/li>\n<\/ul>\n<h3>Watch for<\/h3>\n<ul>\n<li>A lattice enthalpy quoted as a positive value: it is a dissociation and its arrow points up.<\/li>\n<li>A question that gives the bond enthalpy of Cl\u2082 rather than the atomisation enthalpy: halve it.<\/li>\n<li>A Group 2 halide or an oxide, where a step is doubled or a second ionisation energy or electron affinity is added.<\/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: Calculations With a Missing Step<\/h2>\n<p>Full Born\u2013Haber calculations for compounds not on this page: find the lattice enthalpy, then an electron affinity or an enthalpy of formation, keeping every sign right.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1113\"><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the lattice enthalpy definitions and the Born\u2013Haber cycle.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is the second electron affinity endothermic?<\/h3>\n<p>Because the second electron is being added to an ion that is already negative. The repulsion between the incoming electron and the O\u207b or S\u207b ion has to be overcome, so energy must be supplied. The first electron affinity of oxygen is exothermic, about \u2212141 kJ mol\u207b\u00b9, but the second is about +798 kJ mol\u207b\u00b9.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Which way does the lattice enthalpy arrow point?<\/h3>\n<p>It depends which definition the question uses. Lattice enthalpy of formation is for gaseous ions forming one mole of solid, so the arrow goes down and the value is negative. Lattice enthalpy of dissociation is the reverse, one mole of solid becoming gaseous ions, so the arrow goes up and the value is positive with the same size. State which one you are using.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why do I have to include the enthalpy of atomisation for the non-metal as well as the metal?<\/h3>\n<p>Because the cycle must start from the elements in their standard states and turn every one of them into separate gaseous atoms before ions can be made. For a chloride that means breaking half a mole of Cl\u2082 per mole of Cl\u207b, so \u0394atH for chlorine (about +121 kJ mol\u207b\u00b9) is needed once for KCl and twice for CaCl\u2082.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why do I sometimes have to double an ionisation energy or electron affinity?<\/h3>\n<p>Because the cycle is per mole of compound, and a formula such as CaBr\u2082 needs two moles of Br\u207b. Every quantity that refers to bromine, the atomisation and the first electron affinity, is multiplied by two, while the calcium terms, two successive ionisation energies, appear once each.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I avoid sign errors when calculating the missing value?<\/h3>\n<p>Write the cycle as a Hess route: going round one way must equal going round the other. Enthalpy of formation equals the sum of atomisation, ionisation and electron affinity terms plus the lattice term. Rearrange once for the unknown, substitute with every sign attached, and check the answer makes sense, for example that a lattice value is large and negative.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related 3.1.8 Thermodynamics Pages<\/h2>\n<p>Use these pages to connect the ideas across 3.1.8 Thermodynamics 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\/aqa\/3-1-8-thermodynamics\/lattice-energy-trends-and-covalent-character\/\">Lattice Enthalpy Trends and Covalent Character<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-8-thermodynamics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-8-thermodynamics\/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\/aqa\/3-1-8-thermodynamics\/\">3.1.8 Thermodynamics Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-energetics\/\">3.1.4 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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