{"id":12385,"date":"2026-09-30T19:40:44","date_gmt":"2026-09-30T18:40:44","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/rate-determining-step-and-reaction-mechanisms\/"},"modified":"2026-10-03T11:51:49","modified_gmt":"2026-10-03T10:51:49","slug":"rate-determining-step-and-reaction-mechanisms","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/cie\/topic-26-reaction-kinetics\/rate-determining-step-and-reaction-mechanisms\/","title":{"rendered":"Rate-Determining Step and Reaction Mechanisms"},"content":{"rendered":"<script src=\"https:\/\/cdnjs.cloudflare.com\/ajax\/libs\/three.js\/r128\/three.min.js\"><\/script>\n\n<section class=\"ols-revision-page ols-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: 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transparent; }\n    .ols-figure-placeholder { background: #ffffff; border: 1px solid rgba(28, 36, 75, 0.12); border-radius: 22px; padding: 14px; margin: 18px 0 6px; }\n    .ols-figure-placeholder .ols-figure-caption p { margin: 10px 0 0; font-size: 14px; color: #667085; text-align: center; }\n\n    \/* inline checks (H5P re-flow, Sep 2026) *\/\n    .ols-h5p-card.ols-h5p-inline { padding: 26px 28px; border-left: 6px solid #7c3aed; }\n    .ols-h5p-card.ols-h5p-inline h2 { font-size: clamp(20px, 2.2vw, 27px); letter-spacing: -0.02em; }\n    .ols-h5p-card.ols-h5p-inline > p { margin: 8px 0 0; }\n    .ols-h5p-card.ols-h5p-inline .ols-h5p-frame { margin-top: 16px; padding: 14px; border-radius: 20px; }\n    .ols-h5p-kicker { display: inline-block; margin-bottom: 10px; padding: 5px 12px; border-radius: 999px; background: #ede9fe; color: #5b21b6; font-size: 12px; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; }\n    .ols-h5p-card.ols-h5p-recap { border-left-color: #c9973a; background: linear-gradient(135deg, #ffffff 0%, #fff8e8 100%); }\n    .ols-h5p-recap .ols-h5p-kicker { background: #fdf0d2; color: #8a5a00; }\n    @media (max-width: 760px) { .ols-h5p-card.ols-h5p-inline { padding: 20px 16px; } }\n<\/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>Rate-Determining Step and Reaction Mechanisms<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Rate-Determining Step and Reaction Mechanisms<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the rate-determining step: why the slow step fixes the rate equation, deducing a rate equation from a mechanism and a mechanism from a rate equation, intermediates, and how the rate equations for the hydrolysis of halogenoalkanes give evidence for the S\u20991 and S\u20992 mechanisms.<\/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>Mechanisms and the Rate-Determining Step<\/h2>\n<\/div>\n<p>A balanced equation shows what goes in and what comes out; it says nothing about how.<\/p><p>Most reactions happen in a series of simpler steps called the <strong>mechanism<\/strong>, each usually involving a collision between just one or two species.<\/p><p>The species formed in one step and used up in a later one are <strong>intermediates<\/strong>: they never appear in the overall equation, and they are often too reactive to isolate.<\/p>\n<p>The steps do not all happen at the same speed. One is much slower than the rest, and because everything after it has to wait for it, that step sets the rate of the whole reaction.<\/p><p>It is the <strong>rate-determining step<\/strong>. In energy terms the rate-determining step is the one with the <strong>largest activation energy<\/strong>, the highest hump on the reaction profile.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> The rate-determining step is the slowest step in a reaction mechanism, and it controls the overall rate of the reaction. An intermediate is a species formed in one step of a mechanism and consumed in a later step.<\/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>From Mechanism to Rate Equation<\/h2>\n<\/div>\n<p>The rate of the overall reaction equals the rate of the rate-determining step, and the rate of any single step depends on the concentrations of the species that collide in it.<\/p><p>So the <strong>rate equation is written from the rate-determining step<\/strong>: each species that takes part in the slow step appears in the rate equation, and its order is the number of that species involved.<\/p><p>Species that first appear in a step <strong>after<\/strong> the slow step are <strong>zero order<\/strong>; they cannot speed up a step they are not part of.<\/p><p>If an intermediate takes part in the slow step, it is replaced by the reactants that form it, because intermediates never appear in a rate equation.<\/p>\n<p>Worked example: nitrogen dioxide and carbon monoxide react at low temperatures, NO\u2082(g) + CO(g) \u2192 NO(g) + CO\u2082(g), by the mechanism<\/p>\n<p style=\"text-align:center\">step 1 (slow): NO\u2082 + NO\u2082 \u2192 NO + NO\u2083<br>step 2 (fast): NO\u2083 + CO \u2192 NO\u2082 + CO\u2082<\/p>\n<p>The slow step involves two molecules of NO\u2082 and nothing else, so <strong>rate = k[NO\u2082]\u00b2<\/strong>.<\/p><p>Carbon monoxide reacts only in the fast second step, so it is zero order: doubling [CO] has no effect on the rate, even though CO is a reactant.<\/p><p>Adding the two steps cancels the intermediate NO\u2083 and one of the NO\u2082 molecules, and gives back the overall equation, which is the check that the mechanism is properly written.<\/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-t11-06-rate-determining-step.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-t11-06-rate-determining-step.jpg\" alt=\"Reaction profile of a two-step mechanism with the slow rate-determining step, and the worked NO\u2082 + CO example giving rate = k[NO\u2082]\u00b2\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>A two-step reaction profile with the intermediate in the dip and the higher barrier of the slow step, beside the worked NO\u2082 + CO example.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Method:<\/strong> 1 Find the slow step. 2 Write rate = k \u00d7 the concentration of each species in it, raised to the number of that species. 3 Replace any intermediate by the reactants that form it. 4 Everything that appears only later is zero order.<\/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: Rate Equation From a Mechanism<\/h2>\n<p>Given a mechanism with its slow step, write the rate equation and pick out the zero-order reactants, for reactions not worked on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1090\"><\/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>From Rate Equation to Mechanism<\/h2>\n<\/div>\n<p>The reverse problem is more open: given the rate equation and the overall equation, propose a mechanism.<\/p><p>Two rules constrain it. The <strong>slow step must contain exactly the species in the rate equation<\/strong>, in the numbers given by the orders. And <strong>the steps must add up to the overall equation<\/strong>, with every intermediate cancelling.<\/p>\n<p>Worked example: 2NO(g) + 2H\u2082(g) \u2192 N\u2082(g) + 2H\u2082O(g) has the rate equation rate = k[NO]\u00b2[H\u2082]. The slow step therefore involves two NO and one H\u2082. A three-body collision is unlikely, so a common answer is<\/p>\n<p style=\"text-align:center\">step 1 (fast): NO + NO \u21cc N\u2082O\u2082<br>step 2 (slow): N\u2082O\u2082 + H\u2082 \u2192 N\u2082O + H\u2082O<br>step 3 (fast): N\u2082O + H\u2082 \u2192 N\u2082 + H\u2082O<\/p>\n<p>The slow step contains the intermediate N\u2082O\u2082, which is replaced by the 2NO that form it, giving [NO]\u00b2[H\u2082] as required. The three steps add to the overall equation, with N\u2082O\u2082 and N\u2082O cancelling.<\/p><p>The second molecule of hydrogen reacts after the slow step, which is why the order in H\u2082 is 1 and not 2.<\/p><p>Other mechanisms can fit the same rate equation, so a question asks for <strong>a<\/strong> mechanism consistent with the data, not <strong>the<\/strong> mechanism.<\/p>\n<p>A question may give a full mechanism and ask you to <strong>identify the intermediate and the catalyst<\/strong>. Both appear in the steps but not in the overall equation.<\/p><p>The difference is the order in which they appear. An intermediate is <strong>formed<\/strong> in an early step and <strong>used up<\/strong> in a later one.<\/p><p>A catalyst is <strong>used<\/strong> in an early step and <strong>re-formed<\/strong> in a later one, so it is present at the start and at the end.<\/p><p>In the mechanism above, N\u2082O\u2082 and N\u2082O are intermediates and there is no catalyst. In the NO\u2082 + CO mechanism on this page, NO\u2083 is an intermediate; NO\u2082 appears on both sides but is a reactant, not a catalyst, because one NO\u2082 is consumed overall.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Check both things before you write: the species in the slow step (with the earlier fast step feeding it) match the rate equation, and the steps sum to the overall 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-mechbuild.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-mechbuild.jpg\" alt=\"Poster building a three-step mechanism for 2NO + 2H\u2082 from rate = k[NO]\u00b2[H\u2082], with intermediate and catalyst compared.\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>A proposed mechanism must put exactly the rate-equation species into the slow step and its steps must add up to the overall equation, as in the three-step route for nitrogen monoxide and hydrogen.<\/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: Mechanism From a Rate Equation<\/h2>\n<p>Propose or choose steps consistent with a given rate equation and overall equation, and pick out the intermediates, for reactions not used above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1091\" class=\"h5p-iframe\" data-content-id=\"1091\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Kinetics Summary: Mechanism from a Rate Equation\"><\/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>Evidence for S\u20991 and S\u20992 in the Hydrolysis of Halogenoalkanes<\/h2>\n<\/div>\n<p>The hydrolysis of halogenoalkanes by hydroxide ions, R\u2013Br + OH\u207b \u2192 R\u2013OH + Br\u207b, is the standard case where rate equations distinguish two mechanisms. The overall equation is the same for every halogenoalkanes; the rate equation is not.<\/p>\n<p><strong>Primary: bromoethane.<\/strong> Experiment gives rate = k[CH\u2083CH\u2082Br][OH\u207b], first order in each and <strong>second order overall<\/strong>.<\/p><p>Both species are in the rate-determining step, so the mechanism is a single step. The hydroxide ion attacks the \u03b4+ carbon at the same time as the C\u2013Br bond breaks, passing through a transition state in which the carbon is partly bonded to both.<\/p><p>This is <strong>S\u20992<\/strong>: substitution, nucleophilic, with two species in the slow step (bimolecular).<\/p>\n<p><strong>Tertiary: 2-bromo-2-methylpropane.<\/strong> Experiment gives rate = k[(CH\u2083)\u2083CBr], <strong>first order overall<\/strong>, with the hydroxide concentration having no effect.<\/p><p>Only the halogenoalkanes is in the rate-determining step, so the mechanism has two steps: a slow breaking of the C\u2013Br bond to give a <strong>carbocation<\/strong>, (CH\u2083)\u2083C\u207a, and Br\u207b, then a fast attack by OH\u207b on the carbocation.<\/p><p>This is <strong>S\u20991<\/strong>: one species in the slow step (unimolecular).<\/p><p>The route is available to tertiary compounds because the tertiary carbocation is <strong>stabilised by the three alkyl groups<\/strong>, which push electron density towards the positive carbon (a positive inductive effect). A primary carbocation would be far too unstable to form.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th><\/th><th>Primary (S\u20992)<\/th><th>Tertiary (S\u20991)<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Rate equation<\/strong><\/td><td>rate = k[R\u2013Br][OH\u207b]<\/td><td>rate = k[R\u2013Br]<\/td><\/tr>\n<tr><td><strong>Overall order<\/strong><\/td><td>2<\/td><td>1<\/td><\/tr>\n<tr><td><strong>Number of steps<\/strong><\/td><td>one, via a transition state<\/td><td>two, via a carbocation intermediate<\/td><\/tr>\n<tr><td><strong>Species in the slow step<\/strong><\/td><td>halogenoalkane and OH\u207b<\/td><td>halogenoalkane only<\/td><\/tr>\n<tr><td><strong>Effect of doubling [OH\u207b]<\/strong><\/td><td>rate doubles<\/td><td>no change<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Secondary halogenoalkanes can react by either route, depending on the solvent and the nucleophile.<\/p>\n<div class=\"ols-figure-card ols-zoom-pop\">\n<div class=\"ols-figure-image\">\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-t11-sn2-bromoethane.svg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-t11-sn2-bromoethane.svg\" alt=\"One-step S\u20992 mechanism of bromoethane with hydroxide ions through a transition state, with the rate equation rate = k[CH\u2083CH\u2082Br][OH\u207b], second order overall\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-t11-sn2-bromoethane.svg\" width=\"1604\" height=\"874\">\n<\/a>\n<a class=\"ols-image-fullscreen-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-t11-sn1-2-bromo-2-methylpropane.svg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-t11-sn1-2-bromo-2-methylpropane.svg\" alt=\"Two-step S\u20991 mechanism of 2-bromo-2-methylpropane: slow loss of bromide to form a tertiary carbocation, then fast attack by hydroxide ions, with the rate equation rate = k[(CH\u2083)\u2083CBr], first order\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/skel-t11-sn1-2-bromo-2-methylpropane.svg\" width=\"1358\" height=\"1264\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The one-step S\u20992 hydrolysis of bromoethane and the two-step S\u20991 hydrolysis of 2-bromo-2-methylpropane, with the rate equation that is evidence for each.<\/p><\/div>\n<\/div>\n<!-- 3D card: sn2-bromoethane (30 Sep 2026, reused) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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font-size:13px;}\n  .ols-sn2-001 .sn2-lab{font-size:10.5px; padding:4px 8px;}\n  .ols-sn2-001 .sn2-caption{font-size:10.5px; padding:5px 9px;}\n  .ols-sn2-001 .sn2-badge{font-size:10.5px; padding:4px 9px;}\n  .ols-sn2-001 .sn2-badge.b-shape{font-size:12px;}\n  .ols-sn2-001 .sn2-badges{max-width:62%;}\n  .ols-sn2-001 .sn2-info{padding:18px;}\n  .ols-sn2-001 .sn2-stack{top:auto; bottom:auto; top:48px; max-width:72%;}\n  .ols-sn2-001 .r-line{font-size:10.5px;}\n  .ols-sn2-001 .r-line{white-space:normal;}\n  .ols-sn2-001 .sn2-lab.l-dp,.ols-sn2-001 .sn2-lab.l-dm{font-size:12px;}\n  .ols-sn2-001 .r-bars{font-size:9.5px;}\n  .ols-sn2-001 .sn2-lab.l-ion,.ols-sn2-001 .sn2-lab.l-prod{font-size:10.5px;}\n  .ols-sn2-001 .sn2-lab.l-rul{font-size:9.5px; padding:1px 5px;}\n  .ols-sn2-001 .sn2-badge.b-eq{font-size:11px;}\n  .ols-sn2-001 .sn2-read{padding:5px 9px;}\n  .ols-sn2-001 .sn2-panel{left:auto; right:14px; transform:none; font-size:11px; padding:6px 10px;}\n}\n@media (prefers-reduced-motion:reduce){\n  .ols-sn2-001 .sn2-seg button,.ols-sn2-001 .sn2-pill{transition:none;}\n}\n\n.ols-cc-sn2-001{\n  font-family:'Poppins',system-ui,-apple-system,'Segoe UI',Roboto,Helvetica,Arial,sans-serif;\n  margin:10px auto 0; text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-sn2-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-sn2-001 a:hover{text-decoration:underline;}\n<\/style>\n\n<div class=\"sn2-head\">\n  <h2 class=\"sn2-title\">Nucleophilic Substitution of Bromoethane by Hydroxide<\/h2>\n  <p class=\"sn2-sub\">Watch a hydroxide ion attack the \u03b4+ carbon of bromoethane from behind, pass through a single transition state and push out the bromide ion, turning the carbon inside out.<\/p>\n<\/div>\n\n<div class=\"sn2-stage\" id=\"sn2Stage\">\n  <canvas class=\"sn2-canvas\" id=\"sn2Canvas\"><\/canvas>\n  <div class=\"sn2-overlay\" id=\"sn2Overlay\"><\/div>\n  <div class=\"sn2-caption\" id=\"sn2Caption\"><\/div>\n  <div class=\"sn2-hint\" id=\"sn2Hint\">Drag to rotate<\/div>\n<\/div>\n\n<div class=\"sn2-eqbar\" id=\"sn2Badges\"><\/div>\n\n<div class=\"sn2-controls\">\n  <div class=\"sn2-row\">\n    <span class=\"sn2-rowlab\">Step<\/span>\n    <div class=\"sn2-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\">Reactants<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\">Attack<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\">Transition state<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\">Products<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"sn2-row\">\n    <span class=\"sn2-rowlab\">View<\/span>\n    <button type=\"button\" class=\"sn2-pill\" id=\"sn2ResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"sn2-info\">\n  <h3 id=\"sn2InfoTitle\"><\/h3>\n  <p id=\"sn2InfoText\"><\/p>\n  <div class=\"sn2-facts\" id=\"sn2Facts\"><\/div>\n  <div class=\"sn2-key\">\n    <span><i style=\"background:#484e5c\"><\/i>Carbon<\/span>\n    <span><i style=\"background:#eef0f5\"><\/i>Hydrogen<\/span>\n    <span><i style=\"background:#8c1e1e\"><\/i>Bromine<\/span>\n    <span><i style=\"background:#cd342c\"><\/i>Oxygen<\/span>\n    <span><i style=\"background:#8c92a0\"><\/i>Bond<\/span>\n    <span><i style=\"background:#ffffff; border:1.5px dashed #5a6070\"><\/i>Partial bond<\/span>\n    <span><i style=\"background:rgba(28,36,75,0.45)\"><\/i>Curly arrow<\/span>\n    <span><i style=\"background:#3b78dc; border-radius:50%\"><\/i>Lone pair<\/span>\n    <span><i style=\"background:#c9961c\"><\/i>Reaction profile<\/span>\n  <\/div>\n<\/div>\n<\/section>\n\n<p class=\"ols-cc-sn2-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/sn2-bromoethane.js?v=20261003e\"><\/script>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> &#8220;Second order overall, first order in both the halogenoalkanes and OH\u207b, so both are in the rate-determining step: S\u20992.&#8221; &#8220;First order, independent of [OH\u207b], so only the halogenoalkanes is in the rate-determining step, which is the formation of the carbocation: S\u20991.&#8221;<\/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: S\u20991 or S\u20992 From the Rate Equation<\/h2>\n<p>Use rate data for halogenoalkanes not discussed above to decide the mechanism and justify it.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1092\" class=\"h5p-iframe\" data-content-id=\"1092\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Kinetics Explain: Evidence for Two Hydrolysis Mechanisms\"><\/iframe><\/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 rate-determining step is the slowest step and controls the overall rate.&#8221; &#8220;The species in the rate equation are those in the rate-determining step (or in steps before it).&#8221; &#8220;The steps add up to the overall equation.&#8221;<\/p>\n<h3>Do not say<\/h3><p>&#8220;The rate equation is written from the balanced equation.&#8221; &#8220;A zero-order reactant is not involved in the reaction&#8221; (it reacts after the slow step). &#8220;First order means S\u20991&#8221; without linking the order to the number of species in the slow step.<\/p>\n<h3>Watch for<\/h3><p>Mechanisms where the slow step contains an intermediate: substitute the reactants that form it before writing the rate equation. Questions that give a mechanism and ask for the order with respect to a species that appears in the slow step twice: the order is 2.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the rate-determining step and mechanisms.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Is the rate-determining step always the first step?<\/h3>\n<p>No. It is the slowest step wherever it comes. If a fast equilibrium comes before the slow step, the species in that fast step also affect the rate, because they set the concentration of the intermediate that enters the slow step.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I know which species is the intermediate?<\/h3>\n<p>It is formed in one step and used up in a later one, so it appears on the product side of an early step and the reactant side of a later step, and it cancels when the steps are added. Intermediates never appear in the overall equation.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does a tertiary halogenoalkane react by S\u20991?<\/h3>\n<p>The three alkyl groups block the approach of the nucleophile to the carbon and, more importantly, stabilise the carbocation that forms when the halogen leaves. The molecule can therefore ionise on its own in the slow step, giving a rate equation that depends only on the halogenoalkane.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What does the &#8220;2&#8221; in S\u20992 mean?<\/h3>\n<p>It means two species are involved in the rate-determining step: the halogenoalkane and the nucleophile collide in a single step, so the rate equation is rate = k[RX][OH\u207b]. In S\u20991 only the halogenoalkane is in the slow step, so the rate equation has one concentration term.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Can a rate equation prove a mechanism?<\/h3>\n<p>No. It can rule out any mechanism that does not fit and support one that does, but several mechanisms can give the same rate equation. Say a mechanism is &#8220;consistent with&#8221; the rate equation, not &#8220;proved by&#8221; it.<\/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\/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\/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\/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<\/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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