{"id":10585,"date":"2026-09-26T11:35:27","date_gmt":"2026-09-26T10:35:27","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/redox-classification\/"},"modified":"2026-09-26T12:43:53","modified_gmt":"2026-09-26T11:43:53","slug":"redox-classification","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/redox-classification\/","title":{"rendered":"Redox Classification"},"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: #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; 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: none; }\n      .ols-table td::before { content: attr(data-label); display: block; 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760px) { .ols-h5p-card.ols-h5p-inline { padding: 20px 16px; } }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Topic 6 Electrochemistry<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/\">Topic Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/oxidation-numbers\/\">Oxidation Numbers<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/electron-transfer-and-half-equations\/\">Electron Transfer and Half-Equations<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/disproportionation\/\">Disproportionation<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/redox-classification\/\">Redox Classification<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Sections<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-10-group-2\/\">Topic 10 Group 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-11-group-17\/\">Topic 11 Group 17<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-2-atoms-molecules-and-stoichiometry\/\">Topic 2 Atoms, Molecules and Stoichiometry<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-9-the-periodic-table-chemical-periodicity\/\">Topic 9 Chemical Periodicity<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-5-chemical-energetics\/\">Topic 5 Chemical Energetics<\/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   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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/\">Cambridge International (CIE)<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/\">Topic 6 Electrochemistry<\/a> \/\n<span>Redox Classification<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Redox Classification<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to using oxidation numbers to decide whether a reaction is redox, disproportionation or neither, applied to s-block and p-block reactions, metals with acids and unfamiliar reactions given as equations.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">AS Level<\/div>\n<div class=\"ols-badge\">Topic 6: Electrochemistry (6.1 Redox)<\/div>\n<div class=\"ols-badge\">9701 Papers 1 and 2<\/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      <article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Using Oxidation numbers to Classify Reactions<\/h2>\n<\/div>\n<p>Every reaction can be tested with one question: does the oxidation number of any element change? If it does, the reaction is <strong>redox<\/strong>; if nothing changes, it is not. Acid\u2013base reactions, precipitation reactions and thermal decompositions that only rearrange ions are not redox, even when they look dramatic. Combustion, displacement, the reactions of metals with acids and every reaction of an element are redox.<\/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\/redox-groups-redox-classification-flowchart.png\" 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\/redox-groups-redox-classification-flowchart.png\" alt=\"Flowchart for deciding whether a reaction is redox, with four worked examples\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The flowchart asks one question first, then names the element oxidised and the element reduced, then checks whether they are the same element.<\/p><\/div>\n<\/div>\n<p>The second use of the test is to sort redox reactions into ordinary redox, where two different elements change, and <strong>disproportionation<\/strong>, where one element goes both ways. The flowchart handles both.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Assign oxidation numbers to every element on both sides before deciding anything. A quick look at the equation is not enough: the reaction of chlorine with water looks like a simple addition until the numbers are written in.<\/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: Redox or Not?<\/h2>\n<p>Sort a set of equations you have not met on this page into redox and not redox by assigning oxidation numbers.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-860\" class=\"h5p-iframe\" data-content-id=\"860\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Summary: Redox or Not?\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Redox in the s-Block<\/h2>\n<\/div>\n<p>The Group 1 and Group 2 metals have low ionisation energies, so their reactions are all oxidations of the metal. When calcium reacts with water, Ca + 2H\u2082O \u2192 Ca(OH)\u2082 + H\u2082, calcium rises from 0 to +2 and is oxidised. Hydrogen in the water falls from +1 to 0 and is reduced; the oxygen stays at \u22122 throughout. Calcium is the reducing agent and water is the oxidising agent.<\/p>\n<p>When sodium burns in oxygen, 4Na + O\u2082 \u2192 2Na\u2082O, sodium rises from 0 to +1 and oxygen falls from 0 to \u22122. The same sentences describe magnesium burning, lithium in chlorine and barium in water: the metal is oxidised, the non-metal is reduced.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Calcium is oxidised because its oxidation number increases from 0 to +2; hydrogen is reduced because its oxidation number decreases from +1 to 0.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Redox in the p-Block<\/h2>\n<\/div>\n<p>The p-block non-metals are usually the oxidising agents. Chlorine displacing bromine from bromide ions, Cl\u2082 + 2Br\u207b \u2192 2Cl\u207b + Br\u2082, reduces chlorine (0 to \u22121) and oxidises bromine (\u22121 to 0). Sulfur burning, S + O\u2082 \u2192 SO\u2082, oxidises sulfur (0 to +4) and reduces oxygen. Carbon reducing iron(III) oxide in a blast furnace, Fe\u2082O\u2083 + 3CO \u2192 2Fe + 3CO\u2082, oxidises carbon (+2 to +4) and reduces iron (+3 to 0).<\/p>\n<p>The p-block also provides the reactions that are not redox. Sulfur dioxide dissolving in water to give sulfurous acid keeps sulfur at +4. Phosphorus(V) chloride hydrolysing keeps phosphorus at +5. In both, atoms are rearranged but no electrons move.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Reaction<\/th><th>Element up<\/th><th>Element down<\/th><th>Classification<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>2Al + 3Cl\u2082 \u2192 2AlCl\u2083<\/strong><\/td><td>Al, 0 to +3<\/td><td>Cl, 0 to \u22121<\/td><td>redox<\/td><\/tr>\n<tr><td><strong>SO\u2082 + H\u2082O \u2192 H\u2082SO\u2083<\/strong><\/td><td>none<\/td><td>none<\/td><td>not redox<\/td><\/tr>\n<tr><td><strong>2NO\u2082 + H\u2082O \u2192 HNO\u2083 + HNO\u2082<\/strong><\/td><td>N, +4 to +5<\/td><td>N, +4 to +3<\/td><td>disproportionation<\/td><\/tr>\n<tr><td><strong>Cl\u2082 + 2KI \u2192 2KCl + I\u2082<\/strong><\/td><td>I, \u22121 to 0<\/td><td>Cl, 0 to \u22121<\/td><td>redox<\/td><\/tr>\n<tr><td><strong>CaCO\u2083 \u2192 CaO + CO\u2082<\/strong><\/td><td>none<\/td><td>none<\/td><td>not redox<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Common mistake:<\/strong> Assuming that any reaction with oxygen in it is redox. Thermal decomposition of a carbonate moves oxygen atoms around without changing any oxidation number.<\/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>Metals Reacting with Acids<\/h2>\n<\/div>\n<p>A metal reacting with a dilute acid to form a salt and hydrogen is a redox reaction in which the metal is oxidised and the hydrogen ions are reduced. For zinc in hydrochloric acid the full equation is Zn + 2HCl \u2192 ZnCl\u2082 + H\u2082 and the ionic equation is Zn + 2H\u207a \u2192 Zn\u00b2\u207a + H\u2082. The chloride ions are spectators and their oxidation number does not change.<\/p>\n<p>The half-equations make the electron transfer explicit: Zn \u2192 Zn\u00b2\u207a + 2e\u207b (oxidation) and 2H\u207a + 2e\u207b \u2192 H\u2082 (reduction). Magnesium with sulfuric acid, iron with hydrochloric acid and aluminium with any dilute acid follow the same pattern; only the charge on the metal ion, and so the number of electrons, changes.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Write the ionic equation with state symbols and be ready to say which species is the oxidising agent. In Zn + 2H\u207a \u2192 Zn\u00b2\u207a + H\u2082 the oxidising agent is H\u207a, because the hydrogen ions gain the electrons.<\/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: Metals, Acids and Half-Equations<\/h2>\n<p>Write the ionic equation and half-equations for a metal and acid combination not used on this page, and name the oxidising agent.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"861\"><\/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>Unfamiliar Redox Reactions<\/h2>\n<\/div>\n<p>Exam questions often give an equation for a reaction that has never been taught and ask for the oxidation numbers, the species oxidised and reduced, and the oxidising and reducing agents. The method is always the same and does not need the chemistry to be recognised.<\/p>\n<ol>\n<li>Assign oxidation numbers to every element on both sides.<\/li>\n<li>Find the element whose value rises: it has been oxidised, and the species containing it is the reducing agent.<\/li>\n<li>Find the element whose value falls: it has been reduced, and the species containing it is the oxidising agent.<\/li>\n<li>Check that the total rise equals the total fall; if it does not, the equation is not balanced.<\/li>\n<\/ol>\n<p><strong>Worked example:<\/strong> 3Cu + 8HNO\u2083 \u2192 3Cu(NO\u2083)\u2082 + 2NO + 4H\u2082O. Copper rises from 0 to +2 (oxidised, so copper is the reducing agent). Nitrogen in HNO\u2083 is +5; in NO it is +3, so two of the eight nitrogen atoms have been reduced. Nitric acid is the oxidising agent. Total rise 3 \u00d7 2 = 6; total fall 2 \u00d7 3 = 6, so the equation balances.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Technique point:<\/strong> Not every atom of the oxidising agent is reduced. Six of the eight nitrate ions here are spectators that end up in copper(II) nitrate; only the two that form NO change.<\/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: An Unfamiliar Equation<\/h2>\n<p>Work through an equation for a reaction you have not studied and identify the oxidising and reducing agents.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-862\" class=\"h5p-iframe\" data-content-id=\"862\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Explain: An Unfamiliar Equation\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>State whether a reaction is redox and explain<\/h3><p>Yes or no, then the element and its change: &#8220;redox, because copper changes from 0 to +2 and nitrogen from +5 to +3&#8221;.<\/p>\n<h3>Identify the reducing agent<\/h3><p>The species containing the element that is oxidised, named as the whole species, for example &#8220;copper&#8221; or &#8220;iodide ions&#8221;.<\/p>\n<h3>Do not say<\/h3><p>&#8220;The reaction is redox because oxygen is involved&#8221;; &#8220;the reducing agent is reduced&#8221;; &#8220;nitrogen is reduced&#8221; without giving the values.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check how reactions are classified with oxidation numbers.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>How do I know if a reaction is redox?<\/h3>\n<p>Assign oxidation numbers to every element on both sides. If any value changes, the reaction is redox; if none changes, it is not.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Is neutralisation a redox reaction?<\/h3>\n<p>No. Hydrogen stays at +1, oxygen at \u22122 and the other elements unchanged, so no electrons are transferred.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What is the oxidising agent when magnesium reacts with acid?<\/h3>\n<p>The hydrogen ions, H\u207a, because they gain the electrons that magnesium loses and are reduced to hydrogen gas.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Does every atom of the oxidising agent get reduced?<\/h3>\n<p>Not always. In copper reacting with nitric acid only some of the nitrate ions are reduced to nitrogen monoxide; the rest end up unchanged in copper(II) nitrate.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I check an unfamiliar redox equation is balanced?<\/h3>\n<p>The total increase in oxidation number must equal the total decrease, and the charges on each side must be equal.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 6 Electrochemistry Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 6 Electrochemistry and the rest of the course.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/oxidation-numbers\/\">Oxidation Numbers<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/electron-transfer-and-half-equations\/\">Electron Transfer and Half-Equations<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/disproportionation\/\">Disproportionation<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-6-electrochemistry\/\">Topic 6 Electrochemistry 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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