{"id":11273,"date":"2026-09-27T14:51:29","date_gmt":"2026-09-27T13:51:29","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp2-measurement-of-an-enthalpy-change\/"},"modified":"2026-10-03T08:38:11","modified_gmt":"2026-10-03T07:38:11","slug":"rp2-measurement-of-an-enthalpy-change","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp2-measurement-of-an-enthalpy-change\/","title":{"rendered":"Required Practical 2: Measurement of an Enthalpy Change"},"content":{"rendered":"<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n\n<section class=\"ols-revision-page ols-core-practical-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      --soft-green: #f0f7f1;\n      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      --inner-shadow: 0 10px 26px rgba(28, 36, 75, 0.08);\n      font-family: Poppins, Arial, sans-serif;\n      color: var(--navy);\n      background: #ffffff;\n    }\n\n    .ols-revision-page * { box-sizing: border-box; }\n    .ols-main { min-width: 0; max-width: 970px; }\n    .ols-breadcrumbs { display: flex; flex-wrap: wrap; gap: 8px; margin: 10px 0 22px; font-size: 15px; color: var(--grey-text); }\n    .ols-breadcrumbs a, .ols-breadcrumbs span { color: var(--grey-text); text-decoration: none; font-weight: 600; }\n    .ols-breadcrumbs a:hover { color: var(--blue); text-decoration: underline; text-underline-offset: 3px; }\n    .ols-title-card, .ols-note-card, .ols-h5p-card, .ols-related-card, .ols-faq-card { background: #ffffff; border: 1px solid var(--border); border-radius: 28px; box-shadow: var(--shadow); padding: 34px; margin-bottom: 24px; overflow: hidden; }\n    .ols-title-card { background: linear-gradient(135deg, #ffffff 0%, #f8fbff 100%); }\n    .ols-title-card h1 { margin: 0 0 18px; font-size: clamp(36px, 5vw, 58px); line-height: 1.08; font-weight: 800; letter-spacing: -0.04em; color: #111827; }\n    .ols-page-intro { font-size: 19px; line-height: 1.7; font-weight: 300; color: var(--body-text); margin: 0 0 22px; }\n    .ols-badges { display: flex; flex-direction: column; align-items: flex-start; gap: 12px; margin-bottom: 22px; }\n    .ols-badge { display: inline-flex; align-items: center; padding: 9px 14px; border-radius: 999px; background: #ffffff; border: 1px solid var(--border); font-size: 15px; font-weight: 600; color: var(--navy); }\n    .ols-author { display: flex; align-items: center; gap: 20px; padding: 28px; border-radius: 28px; background: linear-gradient(135deg, #ffffff 0%, #f8fbff 100%); border: 1px solid rgba(28,36,75,0.12); box-shadow: 0 18px 45px rgba(28,36,75,0.10); margin-top: 22px; }\n    .ols-author-avatar-img { width: 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    .ols-rule-item h3 { margin: 0 0 8px; font-size: 20px; line-height: 1.25; color: var(--navy); }\n    .ols-rule-item p { margin: 0; font-size: 15px; line-height: 1.6; }\n\n    .ols-figure-card { margin: 26px auto 4px; border: 1px solid var(--border); border-radius: 26px; overflow: hidden; background: #ffffff; box-shadow: var(--inner-shadow); max-width: 100%; }\n    .ols-figure-card.medium { max-width: 820px; }\n    .ols-figure-card.compact { max-width: 700px; }\n    .ols-figure-image { width: 100%; min-height: 260px; display: flex; align-items: center; justify-content: center; background: #ffffff; padding: 20px; }\n    .ols-figure-image img { max-width: 100%; height: auto; display: block; }\n    .ols-figure-caption { padding: 18px 24px 22px; background: linear-gradient(135deg, #ffffff, #f8fbff); border-top: 1px solid var(--border); }\n    .ols-figure-caption p { margin: 0; color: #5f6b85; font-size: clamp(15px, 1.3vw, 17px); line-height: 1.6; font-weight: 300; font-style: italic; }\n    .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    .ols-related-item { border: 1px solid var(--border); border-radius: 18px; padding: 18px; background: #ffffff; color: var(--navy); text-decoration: none; font-weight: 600; line-height: 1.5; transition: transform 0.2s ease, box-shadow 0.2s ease; }\n    .ols-related-item:hover { transform: translateY(-2px); box-shadow: 0 10px 22px rgba(28, 36, 75, 0.08); }\n\n    .ols-course-cta-covalent {\n      width: 100%;\n      margin: 30px 0 24px;\n      font-family: Poppins, Arial, sans-serif;\n    }\n    .ols-course-cta-covalent, .ols-course-cta-covalent * { box-sizing: border-box; }\n    .ols-course-cta-card {\n      overflow: hidden;\n      border-radius: 30px;\n      border: 1px solid var(--border);\n      background: radial-gradient(circle at top left, rgba(37, 99, 235, 0.16), transparent 34%), linear-gradient(135deg, #ffffff 0%, #f8fbff 100%);\n      box-shadow: var(--shadow);\n      padding: 30px;\n    }\n    .ols-course-cta-top {\n      display: grid;\n      grid-template-columns: 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; 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}\n      .ols-course-cta-card { padding: 20px; border-radius: 24px; }\n      .ols-course-cta-header-row { align-items: stretch; }\n      .ols-course-button-top, .ols-course-button { width: 100%; }\n    }\n  \n    .ols-figure-placeholder .ols-placeholder-box { border: 2px dashed #c9973a; background: #fffaf0; border-radius: 16px; padding: 26px 22px; font-weight: 700; color: #7a4b12; text-align: center; line-height: 1.6; }\n    .ols-figure-placeholder .ols-figure-image { background: transparent; }\n    .ols-figure-placeholder { background: #ffffff; border: 1px solid rgba(28, 36, 75, 0.12); 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\n    \/* core practical pages (Sep 2026) *\/\n    .ols-note-card h3 { margin: 22px 0 8px; font-size: 18px; line-height: 1.25; font-weight: 500; color: var(--navy); }\n    .ols-table td { overflow-wrap: anywhere; }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>AQA A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Required Practicals<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/molar-volume-of-a-gas\/\">Gas volumes: Molar Volume of a Gas<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp2-measurement-of-an-enthalpy-change\/\">RP2: Enthalpy Change via Hess's Law<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-acid-base-titration\/\">RP1: Concentration of HCl by Titration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-making-a-volumetric-solution\/\">RP1: Preparation of a Standard Solution<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rates-of-hydrolysis-of-halogenoalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/preparing-an-organic-liquid-chlorination-of-2-methylpropan-2-ol\/\">Organic prep: Chlorination of 2-methylpropan-2-ol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp5-distillation-of-a-product-oxidation-of-ethanol\/\">RP5: Oxidation of an Alcohol<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp4-rp6-identifying-inorganic-ions-and-organic-functional-groups\/\">RP4 and RP6: Analysis of Inorganic and Organic Unknowns<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp3-effect-of-temperature-on-rate-of-reaction\/\">RP3: Rates of Reaction<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/gravimetric-analysis-water-of-crystallisation\/\">Gravimetric: Gravimetric and Composition Analysis<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Useful Links<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/\">AQA Chemistry<\/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>\ndocument.addEventListener('DOMContentLoaded', function () {\n  var currentPath = window.location.pathname.replace(\/\\\/$\/, '');\n  var sidebarLinks = document.querySelectorAll('.ols-sidebar .ols-topic-list a');\n  sidebarLinks.forEach(function (link) {\n    var linkPath = new URL(link.href, window.location.origin).pathname.replace(\/\\\/$\/, '');\n    if (linkPath === currentPath) { link.closest('li').classList.add('active'); } else { link.closest('li').classList.remove('active'); }\n  });\n});\n<\/script>\n\n  <main class=\"ols-main\">\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\/aqa\/\">AQA<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/\">Required Practicals<\/a> \/\n<span>Required Practical 2: Measurement of an Enthalpy Change<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Required Practical 2: Measurement of an Enthalpy Change<\/h1>\n        <p class=\"ols-page-intro\">AQA A Level Chemistry revision notes for Required Practical 2: finding the enthalpy change of decomposition of potassium hydrogencarbonate by measuring the reactions of K\u2082CO\u2083 and KHCO\u2083 with hydrochloric acid in a polystyrene cup, then combining them with Hess&#8217;s law. Method with reasons, q = mc\u0394T, moles, the excess check, the cycle, uncertainties and the direction of every error.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1, 2 and 3<\/div>\n<div class=\"ols-badge\">AQA<\/div>\n<div class=\"ols-badge\">Required Practical 2<\/div>\n<div class=\"ols-badge\">7405<\/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\">Chemistry specialist revision notes for AQA A Level Chemistry.<\/p>\n\n      <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: Energy Changes<\/h2>\n<p>Four quick questions on exothermic and endothermic changes, the polystyrene cup and the units of energy from GCSE.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"984\"><\/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 This Practical Is Testing<\/h2>\n<\/div>\n<p>The aim of Required Practical 2 is to find the <strong>enthalpy change of thermal decomposition<\/strong> of potassium hydrogencarbonate, a reaction that cannot be measured directly:<\/p>\n<p style=\"text-align:center\">2KHCO\u2083(s) \u2192 K\u2082CO\u2083(s) + CO\u2082(g) + H\u2082O(l)      \u0394H\u2083 (the target)<\/p>\n<p>Direct measurement fails for two reasons that both earn marks. First, the solid must be <strong>heated continuously<\/strong> to decompose it, so the energy supplied by the Bunsen cannot be separated from the energy absorbed by the reaction.<\/p><p>Second, the reaction is solid to solid and gas: there is <strong>no solution<\/strong> into which a thermometer can be placed to record a temperature change.<\/p><p>Instead, two related reactions that do take place in solution are measured with a thermometer in a polystyrene cup, and <strong>Hess&#8217;s law<\/strong> is used to combine them.<\/p><p>Water is written as H\u2082O(l) because both measured reactions produce liquid water at room temperature, so the value obtained is for the equation with H\u2082O(l).<\/p>\n<p>The written papers (at least 15% of the marks test practical skills) and the practical endorsement.<\/p><p>For this practical that means: the method with reasons, q = mc\u0394T with the assumptions stated, moles and \u0394H with the correct sign, the excess check, the Hess cycle, percentage uncertainties and the direction of the error caused by heat loss.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Measure two reactions you can do in a cup; calculate the one you cannot. Hess&#8217;s law says the enthalpy change is the same whichever route is taken from reactants to products, provided the conditions are the same.<\/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 Two Reactions Being Measured<\/h2>\n<\/div>\n<p>Both solids react with the <strong>same acid<\/strong>: 30.0 cm\u00b3 of 2.00 mol dm\u207b\u00b3 hydrochloric acid in a polystyrene cup.<\/p><p>Using the same volume and concentration of acid in both runs is deliberate: the acid appears on both sides of the Hess cycle and cancels out. The temperature change is measured after adding the solid.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Reaction<\/th><th>Equation<\/th><th>Observation and sign<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Reaction 1: potassium carbonate, \u0394H\u2081<\/strong><\/td><td>K\u2082CO\u2083(s) + 2HCl(aq) \u2192 2KCl(aq) + CO\u2082(g) + H\u2082O(l)<\/td><td>The temperature <strong>rises<\/strong>: exothermic, \u0394H\u2081 negative. Effervescence as CO\u2082 is given off.<\/td><\/tr>\n<tr><td><strong>Reaction 2: potassium hydrogencarbonate, \u0394H\u2082<\/strong><\/td><td>KHCO\u2083(s) + HCl(aq) \u2192 KCl(aq) + CO\u2082(g) + H\u2082O(l)<\/td><td>The temperature <strong>falls<\/strong>: endothermic, \u0394H\u2082 positive. Effervescence as CO\u2082 is given off.<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Notice the mole ratios. One mole of K\u2082CO\u2083 needs two moles of HCl; one mole of KHCO\u2083 needs one mole of HCl. The target equation contains <strong>two moles of KHCO\u2083<\/strong>, so \u0394H\u2082 will have to be doubled when the cycle is built.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Sign trap:<\/strong> A temperature rise gives a negative \u0394H; a temperature fall gives a positive \u0394H. Students calculate the size of q correctly and then lose the mark by giving the wrong sign.<\/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: Exothermic or Endothermic?<\/h2>\n<p>Decide the sign of \u0394H for temperature changes in reactions that are not the ones on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-985\" class=\"h5p-iframe\" data-content-id=\"985\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hess&#039;s Law Enthalpy Change Quick Choice: Exothermic or Endothermic?\"><\/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>Safety and Apparatus<\/h2>\n<\/div>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Apparatus<\/th><th>What it is for<\/th><th>Precision<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Polystyrene cup in a 250 cm\u00b3 beaker<\/strong><\/td><td>The reaction vessel; polystyrene is a poor conductor of heat and has a very small heat capacity, so almost all the energy change stays in the solution. The beaker stops the light cup tipping over.<\/td><td><\/td><\/tr>\n<tr><td><strong>Lid with a hole for the thermometer<\/strong><\/td><td>Reduces heat loss (or gain) through the open top and stops spray from the effervescence escaping.<\/td><td><\/td><\/tr>\n<tr><td><strong>Burette (or 25 cm\u00b3 pipette plus a 5 cm\u00b3 measure)<\/strong><\/td><td>Delivers exactly 30.0 cm\u00b3 of acid. A burette is chosen over a measuring cylinder because its uncertainty is smaller and the volume becomes the mass in q = mc\u0394T.<\/td><td>\u00b10.05 cm\u00b3 per reading<\/td><\/tr>\n<tr><td><strong>Thermometer, 0 to 50 \u00b0C<\/strong><\/td><td>Records the start temperature and the highest or lowest temperature reached. The bulb must stay fully in the liquid.<\/td><td>\u00b10.1 \u00b0C per reading (0.1 \u00b0C divisions) or \u00b10.5 \u00b0C (1 \u00b0C divisions)<\/td><\/tr>\n<tr><td><strong>Test tube and 2 d.p. balance<\/strong><\/td><td>The solid is weighed in the tube, tipped into the acid, and the tube is reweighed so the mass added is found by difference.<\/td><td>\u00b10.01 g per reading<\/td><\/tr>\n<tr><td><strong>Stirring rod (or the thermometer itself)<\/strong><\/td><td>Mixes the solid into the acid so the whole solution reaches the same temperature quickly.<\/td><td><\/td><\/tr>\n<tr><td><strong>Stopwatch<\/strong><\/td><td>Times the readings every 30 s so that a temperature-time graph can be plotted and extrapolated.<\/td><td><\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Hazards and precautions:<\/strong> 2.00 mol dm\u207b\u00b3 hydrochloric acid is an <strong>irritant<\/strong>: wear eye protection and wipe up spills.<\/p><p>Potassium carbonate is an irritant to the eyes and skin: avoid raising dust and wash hands after use.<\/p><p>Both reactions give off CO\u2082 rapidly, so add the solid steadily enough that the mixture does not froth over the rim of the cup, and keep the cup in the beaker for stability.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Method: Step by Step<\/h2>\n<\/div>\n<p>Every step has a reason, and the reason is what the mark scheme rewards. The method below is for reaction 1; reaction 2 repeats it with about 3.5 g of KHCO\u2083 and a fresh 30.0 cm\u00b3 of the same acid.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Step<\/th><th>What you do<\/th><th>Why<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>Weigh a test tube containing about 3 g of K\u2082CO\u2083 on a 2 d.p. balance and record the mass.<\/td><td>About 3 g is enough for a measurable \u0394T while the acid stays in excess. The exact mass does not matter because it is found later by difference.<\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>Use a burette to run 30.0 cm\u00b3 of 2.00 mol dm\u207b\u00b3 HCl into a polystyrene cup standing in a beaker.<\/td><td>The burette has a small uncertainty (\u00b10.05 cm\u00b3 per reading) and the 30.0 cm\u00b3 becomes the 30.0 g of solution in q = mc\u0394T. The beaker supports the cup.<\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>Put the thermometer in and record the temperature every 30 s for 3 minutes (or 4 minutes) until it is steady.<\/td><td>The readings before addition are extrapolated forward to the time of addition and give the true start temperature.<\/td><\/tr>\n<tr><td><strong>4<\/strong><\/td><td>At the next 30 s mark (the time of addition) tip all the solid in at once, stir, replace the lid, and continue recording every 30 s for a further 8 to 10 minutes without stirring stops.<\/td><td>Adding at a known time lets the cooling line be extrapolated back to that moment; the lid and stirring limit heat exchange and keep the solution uniform.<\/td><\/tr>\n<tr><td><strong>5<\/strong><\/td><td>Reweigh the emptied test tube and record the mass.<\/td><td>Weighing by difference: some solid always sticks to the tube, so the mass added is (mass before \u2212 mass after), not the nominal 3 g.<\/td><\/tr>\n<tr><td><strong>6<\/strong><\/td><td>Repeat steps 1 to 5 with about 3.5 g of KHCO\u2083 in a fresh 30.0 cm\u00b3 portion of the same acid; this time record the readings and extrapolate to the minimum.<\/td><td>Reaction 2 is endothermic so the temperature falls; using the same acid means it cancels in the Hess cycle.<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Why a polystyrene cup?:<\/strong> Polystyrene is a better insulator than glass, so less heat is transferred to or from the surroundings, and its own heat capacity is so small that the heat it absorbs can be ignored.<\/p><p>Standing the cup in a beaker stops it tipping over and is worth stating.<\/p>\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\/core-practical-hess-apparatus.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\/core-practical-hess-apparatus.jpg\" alt=\"Calorimetry apparatus (cup in beaker, thermometer, lid, burette) and weighing by difference on a balance\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The apparatus, and weighing by difference: the mass actually added is the difference between the two balance readings.<\/p><\/div>\n<\/div>\n<\/article>\n<!-- 3D card: calorimetry-hess (27 Sep 2026) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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width:100%; height:100%; margin:0; border-radius:0; border:0; padding:16px 20px; display:flex; flex-direction:column; overflow:auto;}\n.ols-cal-001:fullscreen .cal-head{display:none;}\n.ols-cal-001:fullscreen .cal-stage{flex:1 1 auto; height:auto; min-height:320px;}\n.ols-cal-001:-webkit-full-screen{max-width:none; width:100%; height:100%; margin:0; border-radius:0; border:0; padding:16px 20px; display:flex; flex-direction:column; overflow:auto;}\n.ols-cal-001:-webkit-full-screen .cal-head{display:none;}\n.ols-cal-001:-webkit-full-screen .cal-stage{flex:1 1 auto; height:auto; min-height:320px;}\n\n.ols-cc-cal-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-cal-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-cal-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"cal-head\">\n  <h2 class=\"cal-title\">Calorimetry Bench: K\u2082CO\u2083 and KHCO\u2083 with HCl<\/h2>\n  <p class=\"cal-sub\">Measure the temperature change when each potassium salt reacts with hydrochloric acid, then combine the two results in a Hess cycle to find the enthalpy change for the decomposition of potassium hydrogencarbonate.<\/p>\n<\/div>\n\n<div class=\"cal-stage\" id=\"calStage\" tabindex=\"0\" aria-label=\"Film of the practical. Space plays or pauses, the left and right arrow keys skip 10 seconds.\">\n  <canvas class=\"cal-canvas\" id=\"calCanvas\" aria-hidden=\"true\"><\/canvas>\n  <div class=\"cal-overlay\" id=\"calOverlay\"><\/div>\n  <div class=\"cal-subt\" id=\"calSubt\"><\/div>\n  <div class=\"cal-hint\" id=\"calHint\" hidden>Paused: drag to look around<\/div>\n  <button type=\"button\" class=\"cal-poster\" id=\"calPoster\" aria-label=\"Play the film\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M7 4.5v15l12.5-7.5z\"\/><\/svg><span>Replay<\/span><\/button>\n<\/div>\n\n<div class=\"cal-player\" id=\"calPlayer\" role=\"group\" aria-label=\"Film controls\">\n  <button type=\"button\" class=\"cal-pb\" id=\"calPlay\" aria-label=\"Play\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M7 4.5v15l12.5-7.5z\"\/><\/svg><\/button>\n  <button type=\"button\" class=\"cal-pb\" id=\"calBack\" aria-label=\"Back 10 seconds\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M12 4V1.5L7.5 5 12 8.5V6a7 7 0 1 1-7 7H3a9 9 0 1 0 9-9z\"\/><text x=\"12.2\" y=\"16.2\" text-anchor=\"middle\">10<\/text><\/svg><\/button>\n  <button type=\"button\" class=\"cal-pb\" id=\"calFwd\" aria-label=\"Forward 10 seconds\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M12 4V1.5L16.5 5 12 8.5V6a7 7 0 1 0 7 7h2a9 9 0 1 1-9-9z\"\/><text x=\"11.8\" y=\"16.2\" text-anchor=\"middle\">10<\/text><\/svg><\/button>\n  <button type=\"button\" class=\"cal-pb\" id=\"calRestart\" aria-label=\"Restart\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M5.5 5h2.2v14H5.5zM9.2 12l9.3 6.8V5.2z\"\/><\/svg><\/button>\n  <div class=\"cal-prog\" id=\"calProg\" role=\"slider\" tabindex=\"0\" aria-label=\"Seek\" aria-valuemin=\"0\" aria-valuemax=\"197\" aria-valuenow=\"0\" aria-valuetext=\"0:00\">\n    <div class=\"cal-track\"><div class=\"cal-fill\" id=\"calFill\"><\/div><\/div>\n    <div class=\"cal-thumb\" id=\"calThumb\"><\/div>\n  <\/div>\n  <span class=\"cal-time\" id=\"calTime\">0:00 \/ 3:17<\/span>\n  <button type=\"button\" class=\"cal-pb cal-speed\" id=\"calSpeed\" aria-label=\"Playback speed 1\u00d7\">1\u00d7<\/button>\n  <button type=\"button\" class=\"cal-pb\" id=\"calFs\" aria-label=\"Full screen\"><svg viewBox=\"0 0 24 24\" aria-hidden=\"true\"><path d=\"M4 9V4h5v2H6v3zm11-5h5v5h-2V6h-3zM4 15h2v3h3v2H4zm14 3v-3h2v5h-5v-2z\"\/><\/svg><\/button>\n<\/div>\n<div class=\"cal-chips\" id=\"calChips\" role=\"group\" aria-label=\"Chapters\"><\/div>\n<\/section>\n\n<p class=\"ols-cc-cal-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/calorimetry-hess.js?v=20260928b\"><\/script>\n\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Order the Method<\/h2>\n<p>Put the steps of the calorimetry method for a different carbonate into the correct order.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-986\" class=\"h5p-iframe\" data-content-id=\"986\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hess&#039;s Law Enthalpy Change Order: Method for a Carbonate and Acid\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Measuring the Temperature Change Accurately<\/h2>\n<\/div>\n<p>Heat exchange with the surroundings starts the moment the solid is added, so the highest (or lowest) temperature recorded is never the true final temperature.<\/p><p>While the reaction is still finishing, heat is already leaving (or entering) the cup. The <strong>extrapolation method<\/strong> corrects for this and is the method expected in Papers 1, 2 and 3.<\/p>\n<p>The correction is a <strong>temperature-time graph<\/strong>. Readings are taken every 30 s for 3 minutes before the solid is added, and for 8 to 10 minutes afterwards.<\/p><p>A straight line is drawn through the readings <strong>after the peak<\/strong> (never through the rising part of the curve) and extrapolated back to the time of addition; the pre-addition readings are extrapolated forward to the same time.<\/p><p>\u0394T is the <strong>vertical gap<\/strong> between the two lines at the time of addition. For an endothermic reaction the extrapolated minimum is lower than the recorded minimum, so \u0394T is again larger in magnitude than the uncorrected value.<\/p>\n<p>A results table for the K\u2082CO\u2083 run, with the solid added at 3.5 min, shows how the corrected value is read:<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Time \/ min<\/th><th>0.0<\/th><th>0.5<\/th><th>1.0<\/th><th>1.5<\/th><th>2.0<\/th><th>2.5<\/th><th>3.0<\/th><th>3.5<\/th><th>4.0<\/th><th>4.5<\/th><th>5.0<\/th><th>5.5<\/th><th>6.0<\/th><th>6.5<\/th><th>7.0<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Temperature \/ \u00b0C<\/strong><\/td><td>23.2<\/td><td>23.2<\/td><td>23.2<\/td><td>23.2<\/td><td>23.2<\/td><td>23.2<\/td><td>23.2<\/td><td>add<\/td><td>26.8<\/td><td>27.0<\/td><td>26.9<\/td><td>26.8<\/td><td>26.7<\/td><td>26.6<\/td><td>26.5<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The line through the readings from 4.5 min onwards falls by 0.1 \u00b0C every 30 s; extended back to 3.5 min it reaches <strong>27.2 \u00b0C<\/strong>.<\/p>\n<p>The corrected \u0394T is 27.2 \u2212 23.2 = +4.0 \u00b0C, slightly larger than the 3.8 \u00b0C from the highest reading alone.<\/p>\n<p>Give every column a heading with its unit and every reading the same number of decimal places: both are credited.<\/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\/core-practical-hess-extrap.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\/core-practical-hess-extrap.jpg\" alt=\"Temperature-time graph with the cooling line extrapolated back to the time of mixing and \u0394T read at that time\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Why the cooling line is extrapolated back to the time of addition, and how \u0394T is read as the vertical gap between the two lines at that time. Model data, not the results below.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Plot temperature against time, draw a line of best fit through the readings after the maximum and extrapolate it back to the time of addition; \u0394T is the difference between this value and the start temperature at that time.&#8221;<\/p><p>Name the line, the direction and where \u0394T is read.<\/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>Sample Results<\/h2>\n<\/div>\n<p>The same test tube (empty mass 23.04 g) was used for both solids. Temperatures are the start temperature and the extrapolated maximum or minimum; masses are read from a 2 d.p. balance.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Measurement<\/th><th>K\u2082CO\u2083 run (reaction 1)<\/th><th>KHCO\u2083 run (reaction 2)<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Mass of test tube with solid \/ g<\/strong><\/td><td>25.12<\/td><td>26.37<\/td><\/tr>\n<tr><td><strong>Mass of test tube after emptying \/ g<\/strong><\/td><td>23.04<\/td><td>23.04<\/td><\/tr>\n<tr><td><strong>Mass of solid added \/ g<\/strong><\/td><td>2.08<\/td><td>3.33<\/td><\/tr>\n<tr><td><strong>Volume of 2.00 mol dm\u207b\u00b3 HCl \/ cm\u00b3<\/strong><\/td><td>30.0<\/td><td>30.0<\/td><\/tr>\n<tr><td><strong>Start temperature \/ \u00b0C<\/strong><\/td><td>23.2<\/td><td>23.1<\/td><\/tr>\n<tr><td><strong>Maximum temperature \/ \u00b0C<\/strong><\/td><td>27.2<\/td><td><\/td><\/tr>\n<tr><td><strong>Minimum temperature \/ \u00b0C<\/strong><\/td><td><\/td><td>19.2<\/td><\/tr>\n<tr><td><strong>Temperature change \u0394T \/ \u00b0C<\/strong><\/td><td>+4.0<\/td><td>\u22123.9<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Masses are to 4 significant figures and temperatures to 3, so \u0394T is known to 2 significant figures and a final \u0394H quoted to 3 significant figures is the most that can be justified.<\/p><p>The sign of \u0394T tells you whether the reaction is exothermic or endothermic; q = mc\u0394T is then worked with the <strong>magnitude<\/strong> of \u0394T and the sign is put on \u0394H by inspection: rise \u2192 negative, fall \u2192 positive.<\/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\/core-practical-hess-tempgraph.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\/core-practical-hess-tempgraph.jpg\" alt=\"Temperature against time for both runs on one axis: rise to 27.2 \u00b0C for K\u2082CO\u2083 and fall to 19.2 \u00b0C for KHCO\u2083, start 23.2 \u00b0C\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Where the table values come from: both runs on one temperature-time axis, the K\u2082CO\u2083 run rising to 27.2 \u00b0C and the KHCO\u2083 run falling to 19.2 \u00b0C after the solid is added.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Record the mass by difference, not the nominal mass, and record both the start and the extreme temperature so that \u0394T can be checked. A table with headings, units and consistent decimal places earns marks on its own.<\/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: Completing a Results Table<\/h2>\n<p>Fill in the mass by difference, \u0394T and the sign of \u0394H for a fresh set of balance and thermometer readings.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-987\" class=\"h5p-iframe\" data-content-id=\"987\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hess&#039;s Law Enthalpy Change Fill In: Completing a Results Table\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">7<\/div>\n<h2>The Core Equation: q = mc\u0394T<\/h2>\n<\/div>\n<p>The energy transferred to or from the solution is calculated with <strong>q = mc\u0394T<\/strong>.<\/p>\n<p>Here q is the energy change in joules, m the mass of solution in grams, c the specific heat capacity in J g\u207b\u00b9 K\u207b\u00b9 and \u0394T the temperature change in K or \u00b0C.<\/p>\n<p>A change of 1 \u00b0C is a change of 1 K, so the two are interchangeable here.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Symbol<\/th><th>Meaning<\/th><th>In this practical<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>q<\/strong><\/td><td>Energy change of the solution \/ J<\/td><td>Calculated; for 2.08 g or 3.33 g of solid, not for a mole<\/td><\/tr>\n<tr><td><strong>m<\/strong><\/td><td>Mass of solution \/ g<\/td><td>30.0 cm\u00b3 of acid taken as 30.0 g: the density is assumed to be 1.00 g cm\u207b\u00b3 and the mass of the solid is ignored<\/td><\/tr>\n<tr><td><strong>c<\/strong><\/td><td>Specific heat capacity \/ J g\u207b\u00b9 K\u207b\u00b9<\/td><td>4.18 J g\u207b\u00b9 K\u207b\u00b9: the solution is assumed to have the same c as water<\/td><\/tr>\n<tr><td><strong>\u0394T<\/strong><\/td><td>Temperature change \/ \u00b0C or K<\/td><td>Highest or lowest temperature minus start temperature; magnitude used in the calculation<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Three <strong>assumptions<\/strong> are built in and are asked for by name: the density of the solution is 1.00 g cm\u207b\u00b3; the specific heat capacity of the solution equals that of water; and the heat capacity of the cup and thermometer is negligible.<\/p><p>State all three when asked why the calculated value is approximate.<\/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\/wave-29.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\/wave-29.jpg\" alt=\"Summary of the equation q = mc\u0394T with the meaning of each symbol, the insulated cup method and the main limitations of simple calorimetry\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The equation, the meaning of each symbol, the insulated-cup method and its limitations in one figure. Its diagram shows the version in which two solutions are mixed; the method on this page adds a solid to the acid.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> c = 4.18 J g\u207b\u00b9 K\u207b\u00b9 is the value given in the exam. If a question quotes 4.2, use 4.2 and round to 2 significant figures; never mix the two in one calculation.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">8<\/div>\n<h2>Worked Calculation: From \u0394T to \u0394H<\/h2>\n<\/div>\n<p>Three steps, each with units, for each reaction. Convert joules to kilojoules before dividing by moles so that \u0394H comes out in kJ mol\u207b\u00b9.<\/p>\n<h3>Reaction 1: K\u2082CO\u2083 (exothermic)<\/h3>\n<p>q = m \u00d7 c \u00d7 \u0394T = 30.0 g \u00d7 4.18 J g\u207b\u00b9 K\u207b\u00b9 \u00d7 4.0 K = <strong>501.6 J<\/strong> = 0.5016 kJ (energy given out by 2.08 g of K\u2082CO\u2083)<\/p>\n<p>Mr(K\u2082CO\u2083) = (2 \u00d7 39.1) + 12.0 + (3 \u00d7 16.0) = 138.2, so n = 2.08 g \u00f7 138.2 g mol\u207b\u00b9 = 0.01505 mol<\/p>\n<p>\u0394H\u2081 = \u22120.5016 kJ \u00f7 0.01505 mol = <strong>\u221233.3 kJ mol\u207b\u00b9<\/strong> (negative because the temperature rose)<\/p>\n<h3>Reaction 2: KHCO\u2083 (endothermic)<\/h3>\n<p>q = 30.0 g \u00d7 4.18 J g\u207b\u00b9 K\u207b\u00b9 \u00d7 3.9 K = <strong>489.1 J<\/strong> = 0.4891 kJ (energy taken in by 3.33 g of KHCO\u2083)<\/p>\n<p>Mr(KHCO\u2083) = 39.1 + 1.0 + 12.0 + (3 \u00d7 16.0) = 100.1, so n = 3.33 g \u00f7 100.1 g mol\u207b\u00b9 = 0.03327 mol<\/p>\n<p>\u0394H\u2082 = +0.4891 kJ \u00f7 0.03327 mol = <strong>+14.7 kJ mol\u207b\u00b9<\/strong> (positive because the temperature fell)<\/p>\n<p>Keep the moles to 4 significant figures inside the calculation and round only the final \u0394H. Rounding n to 0.0151 mol first and then writing \u221233.3 gives working that does not check (0.5016 \u00f7 0.0151 = 33.2), and examiners do check.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Important:<\/strong> \u0394H\u2081 is for one mole of K\u2082CO\u2083 reacting and \u0394H\u2082 for one mole of KHCO\u2083 reacting, exactly as the equations are written. The Hess cycle has to respect that ratio.<\/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: Full Calculation<\/h2>\n<p>Work through q, moles, \u0394H with its sign and the excess check for sodium hydrogencarbonate, then flip each card to compare your working.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-988\" class=\"h5p-iframe\" data-content-id=\"988\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hess&#039;s Law Enthalpy Change Work It Out: Full Calculation for Sodium Hydrogencarbonate\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">9<\/div>\n<h2>Checking That the Acid Is in Excess<\/h2>\n<\/div>\n<p>The acid must be in <strong>excess<\/strong> so that the solid is the limiting reagent: q is then the energy change for the moles of solid added, and dividing by those moles gives \u0394H per mole of solid.<\/p><p>The concentration and volume of acid are chosen so that even the largest mass of solid that might be added reacts completely.<\/p>\n<p>n(HCl) available = 2.00 mol dm\u207b\u00b3 \u00d7 30.0 cm\u00b3 \u00f7 1000 = <strong>0.0600 mol<\/strong><\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Solid<\/th><th>Moles of solid<\/th><th>HCl needed<\/th><th>HCl available<\/th><th>Excess?<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>K\u2082CO\u2083 (1 : 2)<\/strong><\/td><td>2.08 \u00f7 138.2 = 0.01505 mol<\/td><td>2 \u00d7 0.01505 = 0.0301 mol<\/td><td>0.0600 mol<\/td><td>Yes: 0.0600 > 0.0301<\/td><\/tr>\n<tr><td><strong>KHCO\u2083 (1 : 1)<\/strong><\/td><td>3.33 \u00f7 100.1 = 0.03327 mol<\/td><td>1 \u00d7 0.03327 = 0.0333 mol<\/td><td>0.0600 mol<\/td><td>Yes: 0.0600 > 0.0333 (about 1.8 times)<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The KHCO\u2083 run is the closer of the two, which is why the method says &#8220;about 3.5 g&#8221; and not more: with 6 g of KHCO\u2083 the acid would run out and the moles of solid would no longer limit the reaction.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Compare the moles: &#8220;0.0600 mol of HCl is available and only 0.0333 mol is needed, so the acid is in excess and the moles of solid limit the reaction.&#8221; Show both numbers.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">10<\/div>\n<h2>Using Hess&#8217;s Law to Find the Target \u0394H<\/h2>\n<\/div>\n<p>Hess&#8217;s law: the enthalpy change of a reaction is <strong>independent of the route<\/strong> taken. Draw the cycle rather than listing equations: examiners ask for it.<\/p><p>Across the top is the target, 2KHCO\u2083(s) \u2192 K\u2082CO\u2083(s) + CO\u2082(g) + H\u2082O(l), with 2HCl(aq) added to <strong>both<\/strong> sides so that each side can react with the same acid.<\/p><p>At the bottom are the common products, 2KCl(aq) + 2CO\u2082(g) + 2H\u2082O(l): one CO\u2082 and one H\u2082O come from reaction 1 and the other pair from the target itself.<\/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\/core-practical-hess-cycle.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\/core-practical-hess-cycle.jpg\" alt=\"Triangular Hess cycle for 2KHCO\u2083(s) \u2192 K\u2082CO\u2083(s) + CO\u2082(g) + H\u2082O(l) with 2\u0394H\u2082 = +29.4, \u0394H\u2081 = \u221233.3 and \u0394H\u2083 = +62.7 kJ mol\u207b\u00b9\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The Hess cycle. The left arrow is two lots of reaction 2 (2\u0394H\u2082), the right arrow is reaction 1 (\u0394H\u2081); going round the cycle, \u0394H\u2083 + \u0394H\u2081 = 2\u0394H\u2082.<\/p><\/div>\n<\/div>\n<p>Route 1 (left arrow): 2KHCO\u2083(s) + 2HCl(aq) \u2192 2KCl(aq) + 2CO\u2082(g) + 2H\u2082O(l), enthalpy change <strong>2\u0394H\u2082<\/strong> because two moles of KHCO\u2083 react.<\/p>\n<p>Route 2 (across then down): the target \u0394H\u2083 followed by reaction 1, K\u2082CO\u2083(s) + 2HCl(aq) \u2192 2KCl(aq) + CO\u2082(g) + H\u2082O(l), enthalpy change <strong>\u0394H\u2081<\/strong>.<\/p>\n<p>Equating the routes: \u0394H\u2083 + \u0394H\u2081 = 2\u0394H\u2082, so<\/p>\n<p>\u0394H\u2083 = 2\u0394H\u2082 \u2212 \u0394H\u2081 = 2(+14.7) \u2212 (\u221233.3) = +29.4 + 33.3 = <strong>+62.7 kJ mol\u207b\u00b9<\/strong><\/p>\n<p>The decomposition is endothermic, as expected for a reaction that needs heating.<\/p><p>Say what &#8220;per mole&#8221; means here: +62.7 kJ mol\u207b\u00b9 is <strong>per mole of the equation as written<\/strong>, that is per 2 mol of KHCO\u2083 or per mole of K\u2082CO\u2083 formed.<\/p><div class=\"ols-key-box\"><p><strong>Exam tip:<\/strong> If a question asks for the enthalpy change per mole of KHCO\u2083 decomposed, the answer is +62.7 \u00f7 2 = <strong>+31.4 kJ mol\u207b\u00b9<\/strong>. This is a frequent trap.<\/p><\/div>\n<p><strong>Comparison with the accepted value.<\/strong> From standard enthalpies of formation the value for the equation with H\u2082O(l) is about +96 kJ mol\u207b\u00b9, with \u0394H\u2081 about \u221233 kJ mol\u207b\u00b9 and \u0394H\u2082 about +32 kJ mol\u207b\u00b9.<\/p><p>The measured \u0394H\u2081 (\u221233.3) agrees within its uncertainty, but the measured \u0394H\u2082 (+14.7) is much less endothermic than the true value because the cold solution <strong>gained heat<\/strong> from the surroundings, so its temperature fall was too small.<\/p><p>Since \u0394H\u2082 is doubled in the cycle, that shortfall is doubled in \u0394H\u2083, which is why the experimental value is well below +96 kJ mol\u207b\u00b9.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;\u0394H\u2083 = 2\u0394H\u2082 \u2212 \u0394H\u2081: reaction 2 is doubled because the target contains 2 mol of KHCO\u2083, and reaction 1 is reversed because K\u2082CO\u2083 is a product in the target.&#8221; Give the reason for the 2 and the reason for the minus.<\/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: Building the Hess Cycle<\/h2>\n<p>Build the cycle for the sodium salts from two new measured values and avoid the per-mole trap.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-989\" class=\"h5p-iframe\" data-content-id=\"989\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hess&#039;s Law Enthalpy Change Drag: Building the Hess Cycle\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">11<\/div>\n<h2>Variant: Enthalpy Change of Hydration of Magnesium Sulfate<\/h2>\n<\/div>\n<p>The same Hess&#8217;s law idea is used for a target that cannot be measured directly for a different reason: anhydrous magnesium sulfate cannot be made to take up exactly seven moles of water in a cup. The target is<\/p>\n<p style=\"text-align:center\">MgSO\u2084(s) + 7H\u2082O(l) \u2192 MgSO\u2084\u00b77H\u2082O(s)      \u0394H (hydration)<\/p>\n<p>Both the anhydrous and the hydrated salt dissolve in water to give the <strong>same solution<\/strong>, MgSO\u2084(aq), so their two enthalpy changes of solution form the two other sides of the cycle.<\/p><p>The anhydrous salt dissolves exothermically (the temperature rises); the hydrated salt dissolves endothermically (the temperature falls).<\/p><p>\u0394H(hydration) = \u0394H\u2081(anhydrous) \u2212 \u0394H\u2082(hydrated).<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Step<\/th><th>What you do<\/th><th>Why<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>Weigh about 3.0 g of anhydrous MgSO\u2084 (0.025 mol) by difference; run 50.0 cm\u00b3 of deionised water into a polystyrene cup and record its temperature every 30 s for 3 min.<\/td><td>Anhydrous MgSO\u2084 absorbs water from the air, so weigh it quickly from a sealed container; the water readings give a steady baseline.<\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>At 3.5 min add the solid, stir until dissolved, put the lid on and record every 30 s to 12 min. Plot the graph and extrapolate to 3.5 min.<\/td><td>Dissolving takes longer than the carbonate reactions, so extrapolation is essential here.<\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>Repeat with about 6.2 g of MgSO\u2084\u00b77H\u2082O (also 0.025 mol) in 50.0 cm\u00b3 of water.<\/td><td>Equal moles give the same final solution; both q values are worked per mole of MgSO\u2084.<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Sample results:<\/p><div class=\"ols-table-wrap\"><table class=\"ols-table\"><thead><tr><th>Quantity<\/th><th>Anhydrous<\/th><th>Hydrated<\/th><\/tr><\/thead><tbody><tr><td><strong>Mass<\/strong><\/td><td>3.01 g<\/td><td>6.16 g<\/td><\/tr><tr><td><strong>Mr<\/strong><\/td><td>120.4<\/td><td>246.4<\/td><\/tr><tr><td><strong>n<\/strong><\/td><td>0.0250 mol<\/td><td>0.0250 mol<\/td><\/tr><tr><td><strong>\u0394T<\/strong><\/td><td>+10.6 \u00b0C<\/td><td>\u22121.7 \u00b0C<\/td><\/tr><tr><td><strong>q<\/strong><\/td><td>50.0 \u00d7 4.18 \u00d7 10.6 = 2215 J<\/td><td>50.0 \u00d7 4.18 \u00d7 1.7 = 355 J<\/td><\/tr><tr><td><strong>\u0394H<\/strong><\/td><td>\u0394H\u2081 = \u22122.215 \u00f7 0.0250 = <strong>\u221288.6 kJ mol\u207b\u00b9<\/strong><\/td><td>\u0394H\u2082 = +0.355 \u00f7 0.0250 = <strong>+14.2 kJ mol\u207b\u00b9<\/strong><\/td><\/tr><\/tbody><\/table><\/div><p>So \u0394H(hydration) = \u221288.6 \u2212 (+14.2) = <strong>\u2212103 kJ mol\u207b\u00b9<\/strong>, close to the accepted value of about \u2212104 kJ mol\u207b\u00b9.<\/p><p>The mass of water is taken as 50.0 g in both runs; strictly the hydrated salt adds 7 \u00d7 0.0250 = 0.175 mol of water, which is ignored.<\/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\/Using-Hesss-law-to-find-enthalpy-changes-v2.webp\" 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\/Using-Hesss-law-to-find-enthalpy-changes-v2.webp\" alt=\"Hess cycle for the hydration of anhydrous copper(II) sulfate found from the enthalpy changes of solution of the anhydrous and hydrated salts\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The same cycle drawn for copper(II) sulfate, CuSO\u2084(s) + 5H\u2082O(l) \u2192 CuSO\u2084\u00b75H\u2082O(s), from the two enthalpy changes of solution, with the extrapolation method used to find each \u0394T.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> The two solutions must be the same: equal moles of salt in equal volumes of water.<\/p><p>The small \u0394T for the hydrated salt (about \u22121.7 \u00b0C) carries the largest percentage uncertainty, so it is the value examiners ask you to improve, for example by using more solid in less water.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">12<\/div>\n<h2>Other Calorimetry Calculations You Must Recognise<\/h2>\n<\/div>\n<p>The same three-step pattern (q = mc\u0394T, moles of the reactant not in excess, q \u00f7 n with the sign) covers every calorimetry question. What changes is the mass in q = mc\u0394T and what &#8220;per mole&#8221; refers to.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Type of reaction<\/th><th>Mass m in q = mc\u0394T<\/th><th>Divide q by<\/th><th>Sign<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Solid added to excess solution (this practical, displacement)<\/strong><\/td><td>The volume of solution in g<\/td><td>Moles of the solid<\/td><td>Rise \u2192 negative; fall \u2192 positive<\/td><\/tr>\n<tr><td><strong>Neutralisation, acid + alkali<\/strong><\/td><td>The total volume of both solutions in g<\/td><td>Moles of water formed (equal to the moles of acid or alkali if neither is in excess)<\/td><td>Negative, about \u221257 kJ mol\u207b\u00b9 for strong acid and strong alkali<\/td><\/tr>\n<tr><td><strong>Combustion of a liquid fuel<\/strong><\/td><td>The water in the calorimeter, not the fuel<\/td><td>Moles of fuel burned (from the mass lost by the burner)<\/td><td>Negative; large heat losses make it far less negative than the data-book value<\/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\/wave-30.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\/wave-30.jpg\" alt=\"Worked calorimetry example for zinc reacting with 25.0 cm\u00b3 of 0.20 mol dm\u207b\u00b3 copper(II) sulfate solution giving an enthalpy change of \u2212146 kJ mol\u207b\u00b9\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Displacement: 25.0 cm\u00b3 of 0.20 mol dm\u207b\u00b3 copper(II) sulfate with excess zinc; the mass is the 25.0 g of solution and q is divided by the 0.0050 mol of Cu\u00b2\u207a, the reactant not in excess.<\/p><\/div>\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\/06\/Calculating-enthalpy-change-of-neutralisation.webp\" 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\/06\/Calculating-enthalpy-change-of-neutralisation.webp\" alt=\"Worked example of the enthalpy change of neutralisation of hydrochloric acid by sodium hydroxide using the total mass of both solutions\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Neutralisation: the mass is the total 50.0 g of both solutions and the result is per mole of water formed, here the same as per mole of HCl.<\/p><\/div>\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\/skel-04-enthalpy-combustion-calc.svg\" 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\/skel-04-enthalpy-combustion-calc.svg\" alt=\"Worked calculation of the enthalpy change of combustion of propan-1-ol, CH\u2083CH\u2082CH\u2082OH, from calorimetry data\" width=\"1508\" height=\"1192\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Combustion: the mass heated is the 150 g of water, the moles are those of propan-1-ol burned, and the answer is much less negative than the accepted \u22122021 kJ mol\u207b\u00b9 because of heat loss and incomplete combustion.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam strategy:<\/strong> Identify the target enthalpy change first, then decide which measured equation is reversed, which is multiplied and what is being heated. Only then pick up the calculator.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">13<\/div>\n<h2>Errors, Uncertainty and Improvements<\/h2>\n<\/div>\n<p>Questions ask for the source of an error, its <strong>direction<\/strong> (is the result too large or too small?) and a specific improvement. Vague answers (&#8220;human error&#8221;, &#8220;use better equipment&#8221;) score nothing.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Source of error<\/th><th>Effect on the result<\/th><th>Improvement<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Heat lost to the surroundings during the exothermic reaction<\/strong><\/td><td>\u0394T too small, so q too small and \u0394H\u2081 <strong>less negative<\/strong> than the true value<\/td><td>Lid on the cup, cup of low heat capacity standing in a beaker, plot temperature against time and extrapolate to the time of addition<\/td><\/tr>\n<tr><td><strong>Heat gained from the surroundings during the endothermic reaction<\/strong><\/td><td>\u0394T too small in magnitude, so \u0394H\u2082 <strong>less positive<\/strong> than the true value; the error is doubled in \u0394H\u2083 = 2\u0394H\u2082 \u2212 \u0394H\u2081<\/td><td>The same lid, insulation and extrapolation; add the solid in one portion so the reaction is over quickly<\/td><\/tr>\n<tr><td><strong>Specific heat capacity of the solution taken as that of water<\/strong><\/td><td>A small systematic error either way; acceptable for dilute solutions<\/td><td>Accepted approximation: state it as an assumption rather than proposing to change it<\/td><\/tr>\n<tr><td><strong>Heat absorbed by the cup and thermometer ignored<\/strong><\/td><td>Slightly reduces the measured \u0394T<\/td><td>Polystyrene has a very small heat capacity, so the effect is small: this is why the cup is chosen<\/td><\/tr>\n<tr><td><strong>Solid not all transferred, or nominal 3 g used instead of the mass by difference<\/strong><\/td><td>Moles too high, so \u0394H too small in magnitude<\/td><td>Weigh the tube before and after; use the difference<\/td><\/tr>\n<tr><td><strong>K\u2082CO\u2083 has absorbed water from the air (it is deliquescent)<\/strong><\/td><td>The mass weighed includes water, so the moles calculated are too high and \u0394H\u2081 too small in magnitude<\/td><td>Keep the solid in a sealed container and weigh quickly<\/td><\/tr>\n<tr><td><strong>CO\u2082 escaping carries some heat; spray lost during effervescence<\/strong><\/td><td>Small loss of energy and mass from the cup<\/td><td>Lid with a small hole for the thermometer; add the solid steadily<\/td><\/tr>\n<tr><td><strong>Thermometer bulb not fully in the liquid, or read before the reading is steady<\/strong><\/td><td>Random error in \u0394T in either direction<\/td><td>Keep the bulb immersed and stir; wait for a steady start temperature<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<h3>Percentage uncertainty in the K\u2082CO\u2083 run<\/h3>\n<p>Every quantity found from two readings carries twice the reading uncertainty.<\/p><p>Temperature: \u00b10.1 \u00b0C on each of two readings gives \u00b10.2 \u00b0C on \u0394T, so (0.2 \u00f7 4.0) \u00d7 100 = <strong>5.0 %<\/strong>.<\/p><p>Mass: \u00b10.01 g on each of two readings gives \u00b10.02 g on 2.08 g, so (0.02 \u00f7 2.08) \u00d7 100 = <strong>1.0 %<\/strong>.<\/p><p>Volume: \u00b10.05 cm\u00b3 on each burette reading gives \u00b10.1 cm\u00b3 on 30.0 cm\u00b3, so (0.1 \u00f7 30.0) \u00d7 100 = <strong>0.3 %<\/strong>.<\/p><p>Total about 6.3 %, which on \u0394H\u2081 = \u221233.3 kJ mol\u207b\u00b9 is \u00b12.1 kJ mol\u207b\u00b9. For the KHCO\u2083 run the temperature term is (0.2 \u00f7 3.9) \u00d7 100 = 5.1 % and the mass term 0.6 %.<\/p>\n<p>The temperature change is by far the largest contributor, so the worthwhile improvements attack \u0394T.<\/p><p>Use a <strong>larger mass of solid<\/strong> or a <strong>smaller volume of acid<\/strong> to give a bigger temperature change (keeping the acid in excess).<\/p><p>Or use a thermometer reading to 0.1 \u00b0C if a 1 \u00b0C thermometer was used (\u00b10.5 \u00b0C per reading would make the \u0394T term 25 %). Improving the balance changes almost nothing.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Percentage uncertainty = (2 \u00d7 0.1 \u00f7 4.0) \u00d7 100 = 5.0 %, the largest of the three, so the thermometer limits the accuracy; a larger \u0394T would reduce it.&#8221; Two readings, the arithmetic, the comparison and the improvement.<\/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: Uncertainty and Error Direction<\/h2>\n<p>Percentage uncertainties on a new data set, which instrument to improve first, and which way heat loss pushes each value.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"990\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">14<\/div>\n<h2>Common Mistakes<\/h2>\n<\/div>\n<ul>\n<li><strong>Wrong sign<\/strong>: calculating q correctly and then giving an exothermic reaction a positive \u0394H. Rise \u2192 negative; fall \u2192 positive.<\/li>\n<li><strong>Forgetting the 2<\/strong>: writing \u0394H\u2083 = \u0394H\u2082 \u2212 \u0394H\u2081 when the target contains two moles of KHCO\u2083.<\/li>\n<li><strong>Wrong &#8220;per mole&#8221;<\/strong>: quoting +62.7 kJ mol\u207b\u00b9 as the value per mole of KHCO\u2083 (it is +31.4).<\/li>\n<li><strong>Dividing joules by moles<\/strong> and forgetting to convert to kJ, giving an answer 1000 times too large.<\/li>\n<li><strong>Using the mass of solid<\/strong> as m in q = mc\u0394T, or adding it to the 30.0 g. The mass is the solution being heated.<\/li>\n<li><strong>Nominal mass<\/strong>: using &#8220;3.00 g&#8221; instead of the mass found by difference.<\/li>\n<li><strong>Cooling line through the rising points<\/strong> on a temperature-time graph, or reading \u0394T anywhere other than the time of addition.<\/li>\n<li><strong>&#8220;Human error&#8221; or &#8220;use better equipment&#8221;<\/strong> as an evaluation. Name the error, its direction and a specific improvement.<\/li>\n<li><strong>One reading, one uncertainty<\/strong>: forgetting that \u0394T and a mass by difference each come from two readings.<\/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: Assumptions and Final Checks<\/h2>\n<p>Pick the accurate statement about each assumption behind the calculation.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-991\" class=\"h5p-iframe\" data-content-id=\"991\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hess&#039;s Law Enthalpy Change Summary: Assumptions and Final Checks\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">15<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;The acid is in excess (0.0600 mol available, 0.0333 mol needed) so the solid limits the reaction.&#8221;<\/p><p>&#8220;q = 30.0 \u00d7 4.18 \u00d7 4.0 = 501.6 J, assuming a density of 1.00 g cm\u207b\u00b3 and the specific heat capacity of water.&#8221;<\/p><p>&#8220;\u0394H\u2083 = 2\u0394H\u2082 \u2212 \u0394H\u2081 because the target contains 2 mol of KHCO\u2083 and reaction 1 is reversed.&#8221;<\/p><p>&#8220;Heat gained from the surroundings makes \u0394H\u2082 less positive, and the error is doubled in the cycle.&#8221;<\/p><p>&#8220;Percentage uncertainty in \u0394T = (0.2 \u00f7 4.0) \u00d7 100 = 5.0 %.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;The reaction is exothermic so \u0394H is positive.&#8221; &#8220;The mass is 2.08 g&#8221; (in q = mc\u0394T). &#8220;Heat loss makes the value more negative.&#8221;<\/p><p>&#8220;Use a more accurate specific heat capacity&#8221; or &#8220;calibrate the calorimeter&#8221; as improvements.<\/p><p>&#8220;Extrapolate&#8221; when describing a method that records the highest temperature, or &#8220;highest temperature&#8221; when the question asked for the graphical method.<\/p>\n<h3>Watch for<\/h3><p>State symbols on every equation. Units on every line of working: g, J g\u207b\u00b9 K\u207b\u00b9, K, J, kJ, mol, kJ mol\u207b\u00b9. Significant figures: 3 in the final \u0394H.<\/p><p>Whether a question wants \u0394H per mole of KHCO\u2083 or per mole of equation. Whether the cooling line has been drawn only through the readings after the peak. Whether both readings have been counted in each percentage uncertainty.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>The questions students ask most about calorimetry, enthalpy calculations and Hess&#8217;s law for this practical.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why can the decomposition of potassium hydrogencarbonate not be measured directly?<\/h3>\n<p>The solid has to be heated continuously to decompose, so the energy supplied by the Bunsen cannot be separated from the energy absorbed by the reaction.<\/p>\n<p>There is no solution into which a thermometer can be placed to measure a temperature change. Two reactions that do happen in solution are measured instead and combined with Hess&#8217;s law.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is a polystyrene cup used, and why is it stood in a beaker?<\/h3>\n<p>Polystyrene is a poor conductor with a very small heat capacity, so almost all of the energy change stays in the solution and very little is absorbed by the cup or lost to the surroundings. The beaker simply supports the light cup so it cannot tip over.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why must the hydrochloric acid be in excess?<\/h3>\n<p>So that the solid is the limiting reagent. The energy change q then belongs to the moles of solid added, and dividing q by those moles gives \u0394H per mole of solid. With 30.0 cm\u00b3 of 2.00 mol dm\u207b\u00b3 acid there is 0.0600 mol of HCl against a maximum of about 0.033 mol needed.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is reaction 2 doubled and reaction 1 reversed in the Hess calculation?<\/h3>\n<p>The target equation contains 2 mol of KHCO\u2083 but reaction 2 is written for 1 mol, so \u0394H\u2082 is multiplied by 2. K\u2082CO\u2083 is a reactant in reaction 1 but a product in the target, so reaction 1 is reversed and the sign of \u0394H\u2081 changes. That gives \u0394H\u2083 = 2\u0394H\u2082 \u2212 \u0394H\u2081.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the experimental value so much smaller than the data-book value?<\/h3>\n<p>Mainly because the endothermic KHCO\u2083 reaction gains heat from the surroundings while the solution is cold, so its temperature fall and \u0394H\u2082 are too small; \u0394H\u2082 is doubled in the cycle, so the shortfall is doubled in \u0394H\u2083.<\/p><p>Heat exchange with the surroundings always makes the magnitude of a measured enthalpy change too small.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What is the single most common calculation mistake?<\/h3>\n<p>The sign. A temperature rise means the reaction is exothermic and \u0394H is negative; a temperature fall means endothermic and \u0394H is positive. Work q with the magnitude of \u0394T, then put the sign on by inspection.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Required Practicals Pages<\/h2>\n<p>Use these pages to connect the practical techniques with each other and with 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\/required-practicals\/molar-volume-of-a-gas\/\">Gas volumes: Molar Volume of a Gas<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-acid-base-titration\/\">RP1: Concentration of HCl by Titration<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp1-making-a-volumetric-solution\/\">RP1: Preparation of a Standard Solution<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rates-of-hydrolysis-of-halogenoalkanes\/\">Hydrolysis rates: Hydrolysis of Halogenoalkanes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/preparing-an-organic-liquid-chlorination-of-2-methylpropan-2-ol\/\">Organic prep: Chlorination of 2-methylpropan-2-ol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp5-distillation-of-a-product-oxidation-of-ethanol\/\">RP5: Oxidation of an Alcohol<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp4-rp6-identifying-inorganic-ions-and-organic-functional-groups\/\">RP4 and RP6: Analysis of Inorganic and Organic Unknowns<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/\">Required Practicals Overview<\/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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carbocation stability, with errors, improvements and exam points.\"\n          }\n        },\n        {\n          \"@type\": \"ListItem\",\n          \"position\": 5,\n          \"item\": {\n            \"@type\": \"Course\",\n            \"name\": \"Preparing an Organic Liquid: Chlorination of 2-methylpropan-2-ol\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/preparing-an-organic-liquid-chlorination-of-2-methylpropan-2-ol\/\",\n            \"description\": \"AQA A Level Chemistry revision notes on the chlorination of 2-methylpropan-2-ol (Practical Skills: Organic Preparation): S\u20991 mechanism via the tertiary carbocation, separating funnel technique, hydrogencarbonate wash, drying, simple distillation at 50\u201352 \u00b0C, silver nitrate test with nitric acid, worked percentage yield and evaluation.\"\n          }\n        },\n        {\n          \"@type\": \"ListItem\",\n          \"position\": 6,\n          \"item\": {\n            \"@type\": \"Course\",\n            \"name\": \"Required Practical 5: Distillation of a Product (Oxidation of Ethanol)\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/rp5-distillation-of-a-product-oxidation-of-ethanol\/\",\n            \"description\": \"AQA A Level Chemistry revision notes on Required Practical 5: oxidation of ethanol to ethanal by distillation and to ethanoic acid by reflux, with apparatus, boiling points, equations, product tests and exam points.\"\n          }\n        },\n        {\n          \"@type\": \"ListItem\",\n          \"position\": 7,\n          \"item\": {\n            \"@type\": \"Course\",\n            \"name\": \"Required Practicals 4 and 6: Identifying Ions and Functional Groups\",\n            \"url\": 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