{"id":12381,"date":"2026-09-30T19:40:38","date_gmt":"2026-09-30T18:40:38","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/techniques-for-measuring-rates\/"},"modified":"2026-10-03T11:51:43","modified_gmt":"2026-10-03T10:51:43","slug":"techniques-for-measuring-rates","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/techniques-for-measuring-rates\/","title":{"rendered":"Techniques for Measuring Rates"},"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      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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; 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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  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}\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   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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 26 Reaction Kinetics<\/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-26-reaction-kinetics\/\">Topic 26 Reaction Kinetics Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/activation-energy-and-the-arrhenius-equation\/\">Activation Energy and the Arrhenius Equation<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/heterogeneous-and-homogeneous-catalysis\/\">Heterogeneous and Homogeneous Catalysis<\/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-8-reaction-kinetics\/\">Topic 8 Reaction Kinetics (AS)<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-7-equilibria\/\">Topic 7 Equilibria<\/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      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/practical-skills\/\">Practical Skills<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() 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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\/cie\/\">Cambridge International (CIE)<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/\">Topic 26 Reaction Kinetics<\/a> \/\n<span>Techniques for Measuring Rates<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Techniques for Measuring Rates<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to choosing and justifying a technique for rate data: gas volume, mass change, colorimetry, sampling with quenching and titration, and other methods, and the two ways of using them, continuous monitoring and the initial-rates method, including clock reactions.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">A Level<\/div>\n<div class=\"ols-badge\">Topic 26: Reaction Kinetics<\/div>\n<div class=\"ols-badge\">9701 Paper 4<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by:<br><span>Dr. Mohammed Al-Fatah<\/span>\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/p>\n\n      <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Choosing a Technique<\/h2>\n<\/div>\n<p>Every rate experiment follows a property of the mixture that changes as the reaction happens, and the property decides the technique.<\/p><div class=\"ols-table-wrap\"><table class=\"ols-table\"><thead><tr><th>What changes<\/th><th>Technique<\/th><\/tr><\/thead><tbody><tr><td>If <strong>a gas<\/strong> is given off<\/td><td>Its <strong>volume<\/strong> can be collected in a gas syringe, or the <strong>loss in mass<\/strong> of the open flask followed on a balance.<\/td><\/tr><tr><td>If a reactant or product is <strong>coloured<\/strong><\/td><td>A <strong>colorimeter<\/strong> measures how the absorbance changes.<\/td><\/tr><tr><td>If <strong>nothing visible<\/strong> changes<\/td><td>Samples can be taken at intervals, the reaction in each sample stopped, and the concentration of one species found by <strong>titration<\/strong>.<\/td><\/tr><tr><td>If the number or type of <strong>ions<\/strong> changes<\/td><td>The <strong>conductivity<\/strong> of the solution changes.<\/td><\/tr><tr><td>If <strong>an acid<\/strong> is made or used<\/td><td>The <strong>pH<\/strong> does.<\/td><\/tr><\/tbody><\/table><\/div>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Technique<\/th><th>What is measured<\/th><th>Suitable for<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Gas volume<\/strong><\/td><td>Volume of gas in a gas syringe or an inverted burette at known times<\/td><td>Reactions that produce a gas, for example Mg(s) + 2HCl(aq) \u2192 MgCl\u2082(aq) + H\u2082(g)<\/td><\/tr>\n<tr><td><strong>Mass change<\/strong><\/td><td>Mass of the flask and contents on a balance, with cotton wool in the neck so the gas escapes but nothing spits out<\/td><td>Reactions that lose a dense gas, for example CaCO\u2083(s) + 2HCl(aq) \u2192 CaCl\u2082(aq) + H\u2082O(l) + CO\u2082(g)<\/td><\/tr>\n<tr><td><strong>Colorimetry<\/strong><\/td><td>Absorbance of the mixture in a cuvette at a wavelength the coloured species absorbs; a calibration curve converts absorbance to concentration<\/td><td>Reactions where one species is coloured, for example the brown iodine fading in the iodine\u2013propanone reaction<\/td><\/tr>\n<tr><td><strong>Sampling, quenching and titration<\/strong><\/td><td>The concentration of one species in a sample whose reaction has been stopped<\/td><td>Reactions of acids, alkalis, iodine or other species that can be titrated, when no gas or colour change is available<\/td><\/tr>\n<tr><td><strong>Conductivity<\/strong><\/td><td>Electrical conductivity of the solution<\/td><td>Reactions that make or remove ions, for example the hydrolysis of 2-chloro-2-methylpropane, which produces H\u207a and Cl\u207b<\/td><\/tr>\n<tr><td><strong>pH<\/strong><\/td><td>pH with a probe and meter<\/td><td>Reactions that produce or consume H\u207a, for example the hydrolysis of an ester by water<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>A question that asks you to <strong>select and justify<\/strong> a technique wants both halves: name the technique, then link it to the property that changes.<\/p><p>&#8220;Use a colorimeter, because iodine is the only coloured species and its absorbance falls as it is used up&#8221; is a full answer; &#8220;use a colorimeter&#8221; alone is not.<\/p><p>The technique must also not disturb the mixture: a titration takes minutes, so a sample must be quenched first.<\/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\/kinetics-t11-02-measuring-techniques.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-t11-02-measuring-techniques.jpg\" alt=\"Six techniques for following a reaction: gas volume, mass change, colorimetry, sampling and titration, conductivity and pH\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Six ways of following a reaction, each with the apparatus, the quantity measured and an example reaction.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Match the technique to the property that changes: gas evolved, mass lost, colour, ions, acidity, or a species that can be titrated after quenching.<\/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: Which Technique?<\/h2>\n<p>Match a technique to a reaction and justify the choice for reactions not listed on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1079\" class=\"h5p-iframe\" data-content-id=\"1079\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Kinetics Drag: Choosing a Technique\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>Continuous Monitoring<\/h2>\n<\/div>\n<p>In <strong>continuous monitoring<\/strong> one reaction mixture is followed from start to finish, with readings taken at regular intervals.<\/p><p>The results are plotted as a <strong>concentration\u2013time graph<\/strong> (or a volume\u2013time or mass\u2013time graph, which can be converted to concentrations).<\/p><p>The gradient of the curve at any time is the rate at that time, so one experiment gives the rate at many different concentrations. Page 3 shows how the shape and half-lives give the order.<\/p>\n<p>The method only gives a clean order if the concentration of <strong>one<\/strong> reactant is changing significantly. The other reactants are therefore used in a <strong>large excess<\/strong>, so that their concentrations are effectively constant throughout and they behave as pseudo-zero order.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Continuous monitoring follows one reaction mixture over time to give a concentration\u2013time graph; the other reactants are in large excess so that only one concentration changes.&#8221;<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>The Initial-Rates Method<\/h2>\n<\/div>\n<p>In the <strong>initial-rates method<\/strong> several separate experiments are run. In each one the initial concentration of one reagent is changed while every other concentration, the temperature and the volume are kept the same, and the initial rate is measured.<\/p><p>The <strong>initial rate<\/strong> is the gradient of the tangent to the concentration\u2013time curve at t = 0, and it is used because it is the only moment when all the concentrations are known exactly: they are the ones that were mixed.<\/p><p>Comparing the initial rates gives the order with respect to the reagent that was varied (page 4).<\/p>\n<p>Drawing tangents for every run is slow, so a <strong>clock reaction<\/strong> is an acceptable approximation. A fixed, small amount of a product is allowed to form, and the time, t, for a sudden visible change to signal it is recorded.<\/p><p>Because the same amount of product forms each time, the average rate over that interval is proportional to <strong>1 \u00f7 t<\/strong>, and 1\/t is used as the initial rate.<\/p><p>In the <strong>iodine clock<\/strong>, hydrogen peroxide oxidises iodide ions to iodine. A known small amount of thiosulfate ions removes the iodine as fast as it forms, and when the thiosulfate is used up the free iodine turns starch blue-black.<\/p><p>In the <strong>thiosulfate and acid<\/strong> reaction the time for the sulfur precipitate to hide a cross under the flask is used in the same way.<\/p>\n<p>The clock method is an approximation because it measures an average rate over the first part of the reaction, not the true gradient at t = 0.<\/p><p>It is accurate provided that only a small fraction of the reactants has been used up when the clock stops, so that the concentrations are still close to their initial values.<\/p>\n<p>Paper 5 planning questions may ask you to plan or evaluate either method, and the practical skills page describes the timing method in detail: see <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/practical-skills\/rates-of-reaction-timing-an-observation\/\">the practical skills pages (rates of reaction: timing an observation)<\/a>.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Worked example:<\/strong> A clock reaction takes 60 s with [I\u207b] = 0.010 mol dm\u207b\u00b3 and 30 s with [I\u207b] = 0.020 mol dm\u207b\u00b3, other concentrations unchanged. Rate \u221d 1\/t, so the rate has doubled when [I\u207b] doubled: first order with respect to I\u207b.<\/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-clockmethod.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-clockmethod.jpg\" alt=\"Poster of the initial-rates method, the iodine clock and the disappearing cross, with a worked 1\/t example.\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The initial-rates method compares separate runs at t = 0, and a clock reaction stands in for the tangent because the rate is proportional to 1\/t.<\/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: Continuous or Initial Rate?<\/h2>\n<p>Decide which method a description is using, and why, for experiments not described on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1080\" class=\"h5p-iframe\" data-content-id=\"1080\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Kinetics Summary: Continuous Monitoring or Initial Rates\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Sampling, Quenching and Titration<\/h2>\n<\/div>\n<p>Titration cannot be done on a reacting mixture, because the concentration would keep changing during the titration itself. Instead a sample is withdrawn with a pipette at a known time and the reaction in it is stopped at once: this is <strong>quenching<\/strong>.<\/p>\n<p>There are three common ways of quenching. The first is to run the sample into a large volume of <strong>ice-cold water<\/strong>, which dilutes the reactants and cools them so that the rate falls almost to zero.<\/p>\n<p>The second is to add a reagent that <strong>removes a catalyst or a reactant<\/strong>, such as sodium hydrogencarbonate solution to neutralise an acid catalyst. The third is simply to <strong>cool<\/strong> the sample rapidly in an ice bath.<\/p>\n<div class=\"ols-key-box\"><p><strong>Remember:<\/strong> The time of quenching, not the time of titration, is the time recorded.<\/p><\/div>\n<p>The quenched sample is then titrated to find the concentration of one species.<\/p><p>Iodine is titrated with <strong>sodium thiosulfate<\/strong> solution using starch as the indicator near the end point; an acid formed or consumed in the reaction is titrated with a standard <strong>alkali<\/strong>.<\/p><p>Repeating at intervals, for example every 2 minutes, gives concentrations at known times for a concentration\u2013time graph.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Quenching stops the reaction in the sample so that the concentration measured is the concentration at the time the sample was taken. Give the method (ice-cold water, sodium hydrogencarbonate) and the reason.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Say<\/h3><p>&#8220;The other reactant is in large excess, so its concentration is effectively constant.&#8221; &#8220;The initial rate is the gradient of the tangent at t = 0.&#8221;<\/p><p>&#8220;Rate is proportional to 1\/t because the same amount of product forms in each run.&#8221; &#8220;The sample is quenched with ice-cold water to stop the reaction before titrating.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;Time is the rate&#8221; (rate \u221d 1\/t, not t). &#8220;Measure the gas with a colorimeter.&#8221; &#8220;Titrate the reaction mixture&#8221; (titrate a quenched sample).<\/p>\n<h3>Watch for<\/h3><p>Justify questions: name the technique, name the property that changes, and say why the alternative would not work. Clock questions: state that the method assumes the concentrations have hardly changed by the time the clock stops.<\/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: Quenching and Clock Reactions<\/h2>\n<p>Explain how a quench works and how a clock reaction gives a rate for experiments not used above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1081\" class=\"h5p-iframe\" data-content-id=\"1081\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Kinetics Explain: Quenching and Clock Times\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the choice of technique.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>What is quenching and why is it needed?<\/h3>\n<p>Quenching stops the reaction in a sample so that it can be titrated at leisure. It works by removing a reactant or catalyst, for example adding sodium hydrogencarbonate to neutralise an acid catalyst, or by cooling the sample in ice. Without it the composition would keep changing during the titration.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>When can I use a colorimeter?<\/h3>\n<p>When one substance in the mixture is coloured and the others are not, such as iodine or bromine being used up. The colorimeter reads absorbance continuously, so you get a concentration\u2013time curve without taking samples.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What is the difference between continuous monitoring and the initial-rates method?<\/h3>\n<p>Continuous monitoring follows one reaction mixture over time and gives a concentration\u2013time graph. The initial-rates method uses several separate mixtures with different starting concentrations and measures the rate at the start of each, giving a rate\u2013concentration relationship directly.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does a clock reaction give the initial rate?<\/h3>\n<p>The time measured is for a small, fixed amount of product to form, during which the reactant concentrations have hardly changed. Rate is then proportional to 1\/t and is a good approximation to the initial rate.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why must the temperature be kept constant during a rate experiment?<\/h3>\n<p>Because the rate constant depends on temperature. A run that warms up as it proceeds would give a curve that is a mixture of the concentration effect and the temperature effect, so the order deduced from it would be wrong.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 26 Reaction Kinetics Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 26 Reaction Kinetics 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-26-reaction-kinetics\/rate-equations-orders-and-the-rate-constant\/\">Rate Equations, Orders and the Rate Constant<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/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\/cie\/topic-26-reaction-kinetics\/heterogeneous-and-homogeneous-catalysis\/\">Heterogeneous and Homogeneous Catalysis<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/\">Topic 26 Reaction Kinetics Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/practical-skills\/rates-of-reaction-timing-an-observation\/\">the practical skills pages (rates of reaction: timing an observation)<\/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>. 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