{"id":12360,"date":"2026-09-30T19:39:49","date_gmt":"2026-09-30T18:39:49","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/rate-equations-orders-and-the-rate-constant\/"},"modified":"2026-10-03T11:51:20","modified_gmt":"2026-10-03T10:51:20","slug":"rate-equations-orders-and-the-rate-constant","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/rate-equations-orders-and-the-rate-constant\/","title":{"rendered":"Rate Equations, Orders and the Rate Constant"},"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: 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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: 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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      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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: 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.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      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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<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>3.1.9 Rate Equations<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/\">3.1.9 Rate Equations Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/rate-equations-orders-and-the-rate-constant\/\">Rate Equations, Orders and the Rate Constant<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/techniques-for-measuring-rates\/\">Techniques for Measuring Rates<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/concentration-time-graphs-and-half-life\/\">Concentration\u2013Time Graphs and Half-Life<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/rate-concentration-graphs-and-the-initial-rates-method\/\">Rate\u2013Concentration Graphs and the Initial-Rates Method<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/the-iodine-propanone-reaction\/\">The Iodine\u2013Propanone Reaction<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/rate-determining-step-and-reaction-mechanisms\/\">Rate-Determining Step and Reaction Mechanisms<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/activation-energy-and-the-arrhenius-equation\/\">Activation Energy and the Arrhenius Equation<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Sections<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-5-kinetics\/\">3.1.5 Kinetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-6-chemical-equilibria-le-chateliers-principle-and-kc\/\">3.1.6 Chemical Equilibria and Kc<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-4-energetics\/\">3.1.4 Energetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/required-practicals\/\">Required Practicals<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    sidebar.querySelectorAll('.active, .active-main, 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Constant<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Rate Equations, Orders and the Rate Constant<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the rate equation rate = k[A]\u1d50[B]\u207f: what order with respect to a substance and overall order mean, why orders come only from experiment, what the rate constant is, and how to work out its units for any overall order.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 2 AQA<\/div>\n<div class=\"ols-badge\">3.1.9 Rate Equations<\/div>\n<div class=\"ols-badge\">7405\/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      <section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>AS Recap: Rates and Collision Theory<\/h2>\n<p>Three quick questions on what you already know: rate as a change in concentration per unit time, why collisions need energy above the activation energy, and how a rate is read from a gradient.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1075\"><\/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 the Rate Equation Says<\/h2>\n<\/div>\n<p>At AS you learned that a higher concentration usually gives a faster reaction. The <strong>rate equation<\/strong> makes that relationship exact.<\/p><p>For a reaction between A and B it is written rate = k[A]\u1d50[B]\u207f, and each symbol has a fixed meaning.<\/p><div class=\"ols-table-wrap\"><table class=\"ols-table\"><thead><tr><th>Symbol<\/th><th>Meaning<\/th><\/tr><\/thead><tbody><tr><td><strong>rate<\/strong><\/td><td>The change in concentration of a reactant or product per unit time, usually in mol dm\u207b\u00b3 s\u207b\u00b9<\/td><\/tr><tr><td><strong>[A]<\/strong><\/td><td>The square brackets mean the concentration of A in mol dm\u207b\u00b3<\/td><\/tr><tr><td><strong>m<\/strong> and <strong>n<\/strong><\/td><td>The powers are the orders of reaction with respect to A and B<\/td><\/tr><tr><td><strong>k<\/strong><\/td><td>The rate constant, a number that links the concentrations to the rate at a particular temperature<\/td><\/tr><\/tbody><\/table><\/div>\n<p>The equation is a statement about how the rate responds when a concentration changes. If m is 1, doubling [A] doubles the rate; if m is 2, doubling [A] quadruples it; if m is 0, changing [A] does nothing at all.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> The rate equation, rate = k[A]\u1d50[B]\u207f, gives the rate of reaction in terms of the concentrations of the species that affect it. k is the rate constant and m and n are the orders with respect to A and B; each is 0, 1 or 2 at A Level.<\/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>Order With Respect to a Substance<\/h2>\n<\/div>\n<p>The <strong>order with respect to a substance<\/strong> is the power to which its concentration is raised in the rate equation. Three values are met at A Level.<\/p><div class=\"ols-table-wrap\"><table class=\"ols-table\"><thead><tr><th>Order<\/th><th>Proportionality<\/th><th>Effect of changing [A]<\/th><\/tr><\/thead><tbody><tr><td><strong>Zero order<\/strong><\/td><td>Rate \u221d [A]\u2070, so the rate is independent of [A]<\/td><td>Doubling the concentration leaves the rate unchanged, and the substance does not appear in the rate equation at all, because [A]\u2070 = 1.<\/td><\/tr><tr><td><strong>First order<\/strong><\/td><td>Rate \u221d [A]<\/td><td>Doubling [A] doubles the rate, trebling it trebles the rate.<\/td><\/tr><tr><td><strong>Second order<\/strong><\/td><td>Rate \u221d [A]\u00b2<\/td><td>Doubling [A] multiplies the rate by 2\u00b2, which is 4, and trebling it multiplies the rate by 9.<\/td><\/tr><\/tbody><\/table><\/div>\n<p>The quickest way to find an order from data is the <strong>doubling test<\/strong>: double one concentration while keeping every other concentration the same, and see what happens to the rate. Rate \u00d7 1 means zero order, rate \u00d7 2 means first order, rate \u00d7 4 means second order.<\/p>\n<p>Orders are <strong>found by experiment<\/strong>, and they have nothing to do with the balancing numbers in the equation.<\/p><p>The reaction 2NO + O\u2082 \u2192 2NO\u2082 happens to have the rate equation rate = k[NO]\u00b2[O\u2082].<\/p><p>But the reaction between hydrogen peroxide and iodide ions, H\u2082O\u2082 + 2I\u207b + 2H\u207a \u2192 I\u2082 + 2H\u2082O, is first order in H\u2082O\u2082, first order in I\u207b and zero order in H\u207a even though the equation shows two of each.<\/p><p>The orders reflect the mechanism (page 6), which the balanced equation cannot show.<\/p>\n<p>A substance can be made to behave as zero order by using it in a <strong>large excess<\/strong>. Its concentration then barely changes during the reaction, so it has no measurable effect on the rate and is described as <strong>pseudo-zero order<\/strong>.<\/p><p>This is how experiments isolate the order with respect to one reactant at a time.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Order 0: rate does not change. Order 1: rate \u221d [A]. Order 2: rate \u221d [A]\u00b2. The orders come from experiment, never from the balanced equation.<\/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\/t11x-doublingtest.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\/t11x-doublingtest.jpg\" alt=\"Decision poster linking rate \u00d7 1, \u00d7 2 and \u00d7 4 to orders 0, 1 and 2, with pseudo-zero order from a large excess.\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Double one concentration and watch the rate: unchanged means zero order, doubled means first order and quadrupled means second order, whatever the balanced equation shows.<\/p><\/div>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: What an Order Means<\/h2>\n<p>Decide how the rate changes when a concentration is changed for orders and reactions not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1076\" class=\"h5p-iframe\" data-content-id=\"1076\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Kinetics Summary: What an Order Means\"><\/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>Overall Order and Writing Rate Equations<\/h2>\n<\/div>\n<p>The <strong>overall order<\/strong> is the sum of the individual orders, m + n.<\/p><p>A reaction that is first order in A and second order in B has the rate equation rate = k[A][B]\u00b2 and is third order overall.<\/p><p>A reaction that is first order in A and zero order in B has rate = k[A][B]\u2070, which is written simply as rate = k[A], and is first order overall. Powers of 1 are not written, and a zero-order species is left out.<\/p>\n<p>Two features of rate equations surprise students. First, a reactant in the balanced equation can be <strong>absent<\/strong> from the rate equation, as H\u207a is absent from the hydrogen peroxide and iodide example above.<\/p>\n<p>Second, a species that is not in the balanced equation at all can <strong>appear<\/strong> in the rate equation, most often a catalyst.<\/p>\n<p>The acid-catalysed reaction between iodine and propanone (page 5) has the rate equation rate = k[CH\u2083COCH\u2083][H\u207a], with the catalyst H\u207a in it and the reactant iodine left out.<\/p>\n<div class=\"ols-key-box\"><p><strong>Key idea:<\/strong> Both follow from the rate equation describing the slowest step of the mechanism.<\/p><\/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\/kinetics-t11-01-rate-equation.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\/kinetics-t11-01-rate-equation.jpg\" alt=\"The rate equation rate = k[A]\u1d50[B]\u207f with each term labelled, graphs of rate against concentration for orders 0, 1 and 2, and the units of k for overall orders 1 to 3\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The rate equation with every symbol labelled, the three orders as rate\u2013concentration sketches, and the units of k for overall orders 1, 2 and 3.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> To write a rate equation from given orders, write k, then each species with a non-zero order raised to its order, and leave out any zero-order species: &#8220;first order in A, second order in B&#8221; becomes rate = k[A][B]\u00b2.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>The Rate Constant and Its Units<\/h2>\n<\/div>\n<p>The <strong>rate constant<\/strong>, k, is the proportionality constant in the rate equation. At a fixed temperature it has a fixed value for a given reaction, however the concentrations change and however far the reaction has gone.<\/p><p>It is not a universal constant: <strong>k increases when the temperature rises<\/strong>, which is the real reason reactions speed up on heating.<\/p><p>It also changes if a catalyst is added because the mechanism changes (page 7 puts numbers on the temperature effect).<\/p>\n<p>The <strong>units of k<\/strong> depend on the overall order, so they must be worked out for each rate equation.<\/p><p>The method is always the same: rearrange to make k the subject, substitute the units of rate and of each concentration, and cancel.<\/p><p>For rate = k[A][B]\u00b2, k = rate \u00f7 ([A][B]\u00b2) = mol dm\u207b\u00b3 s\u207b\u00b9 \u00f7 (mol dm\u207b\u00b3 \u00d7 mol\u00b2 dm\u207b\u2076) = mol dm\u207b\u00b3 s\u207b\u00b9 \u00f7 mol\u00b3 dm\u207b\u2079 = mol\u207b\u00b2 dm\u2076 s\u207b\u00b9.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Overall order<\/th><th>Example rate equation<\/th><th>Units of k<\/th><th>Pattern<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>rate = k[A]<\/td><td>s\u207b\u00b9<\/td><td>mol dm\u207b\u00b3 s\u207b\u00b9 \u00f7 mol dm\u207b\u00b3<\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>rate = k[A][B] or k[A]\u00b2<\/td><td>mol\u207b\u00b9 dm\u00b3 s\u207b\u00b9<\/td><td>mol dm\u207b\u00b3 s\u207b\u00b9 \u00f7 (mol dm\u207b\u00b3)\u00b2<\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>rate = k[A][B]\u00b2 or k[A]\u00b2[B]<\/td><td>mol\u207b\u00b2 dm\u2076 s\u207b\u00b9<\/td><td>mol dm\u207b\u00b3 s\u207b\u00b9 \u00f7 (mol dm\u207b\u00b3)\u00b3<\/td><\/tr>\n<tr><td><strong>0<\/strong><\/td><td>rate = k<\/td><td>mol dm\u207b\u00b3 s\u207b\u00b9<\/td><td>the same units as rate<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Always give k a unit. Work it out from the rate equation in the question, because the examiner sets the order; write the units in the order mol, dm, s, with negative powers, for example mol\u207b\u00b9 dm\u00b3 s\u207b\u00b9.<\/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: Units of k<\/h2>\n<p>Work out the units of the rate constant for rate equations that are not the ones in the table above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1077\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;The order with respect to A is the power of [A] in the rate equation.&#8221; &#8220;Doubling [A] doubles the rate, so the reaction is first order with respect to A.&#8221; &#8220;k is constant at a fixed temperature and increases as the temperature rises.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;The order is 2 because there are two moles of it in the equation&#8221; (orders come only from experiment). &#8220;k is constant&#8221; without adding &#8220;at a fixed temperature&#8221;. &#8220;The rate constant has no units&#8221; (only a first-order overall reaction has the simple unit s\u207b\u00b9, and even that is a unit).<\/p>\n<h3>Watch for<\/h3><p>Questions that give a rate equation and ask for the effect of changing two concentrations at once: apply each change separately and multiply the effects.<\/p><p>Questions that ask for the overall order want a single number, the sum of the powers. If a catalyst appears in the rate equation, that is intended: it is in the rate-determining step.<\/p>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Overall Order and Rate Equations<\/h2>\n<p>Write rate equations from given orders and state the overall order for reactions not used on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1078\"><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the rate equation ideas that come up most often.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Can the order with respect to a substance be a fraction or negative?<\/h3>\n<p>At A Level the orders you meet are 0, 1 and 2, and every question is set so the data give one of those three. Check each comparison against the doubling rules: no change, doubles, quadruples.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the rate constant called constant if it changes with temperature?<\/h3>\n<p>It is constant for a given reaction at a given temperature, whatever the concentrations. Change the temperature (or add a catalyst) and you get a new value of k. Concentration changes never alter k.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why can I not read the orders off the balanced equation?<\/h3>\n<p>Because the balanced equation only shows the overall stoichiometry; the rate depends on the slowest step of the mechanism, which the equation does not show. Orders come only from experiment.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I work out the units of k without memorising a table?<\/h3>\n<p>Rearrange to k = rate \u00f7 (concentration terms) and cancel the units. For rate = k[A][B]\u00b2 that is mol dm\u207b\u00b3 s\u207b\u00b9 \u00f7 (mol dm\u207b\u00b3)\u00b3, which simplifies to mol\u207b\u00b2 dm\u2076 s\u207b\u00b9. Each extra order divides by another mol dm\u207b\u00b3.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>If a substance is zero order, is it still needed for the reaction?<\/h3>\n<p>Yes. It still reacts, and it still appears in the balanced equation, but changing its concentration does not change the rate because it is not involved in the rate-determining step. Its concentration term is [A]\u2070 = 1, so it is left out of the rate equation.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related 3.1.9 Rate Equations Pages<\/h2>\n<p>Use these pages to connect the ideas across 3.1.9 Rate Equations and the rest of the course.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/techniques-for-measuring-rates\/\">Techniques for Measuring Rates<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/concentration-time-graphs-and-half-life\/\">Concentration\u2013Time Graphs and Half-Life<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/rate-concentration-graphs-and-the-initial-rates-method\/\">Rate\u2013Concentration Graphs and the Initial-Rates Method<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/the-iodine-propanone-reaction\/\">The Iodine\u2013Propanone Reaction<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/rate-determining-step-and-reaction-mechanisms\/\">Rate-Determining Step and Reaction Mechanisms<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/activation-energy-and-the-arrhenius-equation\/\">Activation Energy and the Arrhenius Equation<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-9-rate-equations\/\">3.1.9 Rate Equations Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/aqa\/3-1-5-kinetics\/\">3.1.5 Kinetics<\/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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