{"id":3438,"date":"2026-05-13T07:36:43","date_gmt":"2026-05-13T06:36:43","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/?page_id=3438"},"modified":"2026-10-18T18:02:58","modified_gmt":"2026-10-18T17:02:58","slug":"hydrogen","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/hydrogen\/","title":{"rendered":"Hydrogen"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-alkene-hydrogen-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      --green: #3f8f46;\n      --red: #d94b2b;\n      --gold: #c9973a;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      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<aside class=\"ols-sidebar\">\r\n  <div class=\"ols-sidebar-header\">\r\n    <h3>Revision Notes<\/h3>\r\n    <p>A Level Chemistry<\/p>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Topic 5 Alkenes<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/alkene-bonding\/\">Alkene Bonding<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/geometric-isomerism\/\">Geometric Isomerism<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/electrophilic-addition-mechanism\/\">Electrophilic Addition Mechanism<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/\">Reactions of Alkenes<\/a>\r\n\r\n        <ul class=\"ols-subtopic-list\">\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/halogens\/\">Halogens<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/hydrogen-halide\/\">Hydrogen Halide<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/bromine-water\/\">Bromine Water<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/hydrogen\/\">Hydrogen<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/steam\/\">Steam<\/a>\r\n          <\/li>\r\n          <li>\r\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/reactions-of-alkenes\/kmno4\/\">KMnO4<\/a>\r\n          <\/li>\r\n        <\/ul>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/polymerisation-reactions\/\">Polymerisation Reactions<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/polymer-waste\/\">Polymer Waste<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Next Topics<\/h4>\r\n\r\n    <ul class=\"ols-topic-list\">\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-6-energetics\/\">Energetics<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-7-intermolecular-forces\/\">Intermolecular Forces<\/a>\r\n      <\/li>\r\n      <li>\r\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8A-redox-chemistry\/\">Redox Chemistry<\/a>\r\n      <\/li>\r\n    <\/ul>\r\n  <\/div>\r\n<\/aside>\r\n\r\n<script>\r\n(function() 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 <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/\">Revision Notes<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/\">A Level Chemistry<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Edexcel International<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-5-alkenes\/\">Topic 5 Alkenes<\/a> \/\n        <span>Reactions of Alkenes: Hydrogen<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Reactions of Alkenes with Hydrogen<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the reaction of alkenes with hydrogen, including the change from alkene to alkane, the use of a nickel catalyst and why this reaction is both an addition reaction and a reduction reaction.<\/p>\n\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n          <div class=\"ols-badge\">Topic 5: Alkenes<\/div>\n          <div class=\"ols-badge\">Reaction 4: Hydrogen<\/div>\n        <\/div>\n      <\/header>\n<!-- 3D card: hydrogenation-catalyst (5 Oct 2026, Topics 1-5 upgrade) -->\n<!-- Copyright (c) 2026 Dr. Mohammed Al-Fatah, onlinelearningsystem.net. 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text-align:center; font-size:11px; font-style:italic; color:#aab0c0;\n}\n.ols-cc-hyc-001 a{color:#aab0c0; text-decoration:none;}\n.ols-cc-hyc-001 a:hover{text-decoration:underline;}\n.ols-hyc-001 .hyc-seg button{display:inline-flex; align-items:center;}\n.ols-hyc-001 .hyc-seg button .hyc-n{display:inline-flex; align-items:center; justify-content:center; width:20px; height:20px; margin-right:9px; border-radius:50%; font-size:11px; font-weight:600; background:rgba(37,99,235,0.10); color:#2563eb; flex:0 0 auto;}\n.ols-hyc-001 .hyc-seg button[aria-pressed=\"true\"] .hyc-n{background:#ffffff; color:#2563eb;}\n<\/style>\n\n<div class=\"hyc-head\">\n  <h2 class=\"hyc-title\">Hydrogenation on a Catalyst Surface<\/h2>\n  <p class=\"hyc-sub\">Choose a step to watch ethene and hydrogen meet on a nickel surface and leave as ethane.<\/p>\n<\/div>\n\n<div class=\"hyc-stage\" id=\"hycStage\">\n  <canvas class=\"hyc-canvas\" id=\"hycCanvas\"><\/canvas>\n  <div class=\"hyc-overlay\" id=\"hycOverlay\"><\/div>\n  <div class=\"hyc-caption\" id=\"hycCaption\"><\/div>\n  <div class=\"hyc-hint\" id=\"hycHint\">Drag to rotate<\/div>\n  <div class=\"hyc-badges\" id=\"hycBadges\"><\/div>\n<\/div>\n\n<div class=\"hyc-controls\">\n  <div class=\"hyc-row\">\n    <span class=\"hyc-rowlab\">Step<\/span>\n    <div class=\"hyc-seg\" role=\"group\" aria-label=\"Step\">\n      <button type=\"button\" data-step=\"1\" aria-pressed=\"true\"><span class=\"hyc-n\">1<\/span>Adsorb<\/button>\n      <button type=\"button\" data-step=\"2\" aria-pressed=\"false\"><span class=\"hyc-n\">2<\/span>Split<\/button>\n      <button type=\"button\" data-step=\"3\" aria-pressed=\"false\"><span class=\"hyc-n\">3<\/span>Add<\/button>\n      <button type=\"button\" data-step=\"4\" aria-pressed=\"false\"><span class=\"hyc-n\">4<\/span>Desorb<\/button>\n    <\/div>\n  <\/div>\n\n  <div class=\"hyc-row\">\n    <span class=\"hyc-rowlab\">View<\/span>\n    <button type=\"button\" class=\"hyc-pill\" id=\"hycResetV\">Reset view<\/button>\n  <\/div>\n<\/div>\n\n<div class=\"hyc-info\">\n  <h3 id=\"hycInfoTitle\"><\/h3>\n  <p id=\"hycInfoText\"><\/p>\n  <div class=\"hyc-facts\" id=\"hycFacts\"><\/div>\n  <div class=\"hyc-key\">\n    <span><i class=\"k-ball\" style=\"background:#464a54\"><\/i>Carbon<\/span>\n    <span><i class=\"k-ball\" style=\"background:#ececf0\"><\/i>Hydrogen<\/span>\n    <span><i class=\"k-ball\" style=\"background:#b2b8c4\"><\/i>Nickel<\/span>\n    <span><i class=\"k-ball\" style=\"background:#2f5f9a\"><\/i>Weak bond to the surface<\/span>\n  <\/div>\n<\/div>\n\n<script src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/JS\/hydrogenation-catalyst.js?v=20261018z\"><\/script>\n<\/section>\n\n<p class=\"ols-cc-hyc-001\">&copy; Dr. Mohammed Al-Fatah &#8211; <a href=\"https:\/\/www.onlinelearningsystem.net\" target=\"_blank\" rel=\"noopener\">onlinelearningsystem.net<\/a><\/p>\n\n\n      \n<section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: Catalysts and Hardening Oils<\/h2>\n<p>Answer three quick GCSE questions before you start.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"748\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">1<\/div>\n          <h2>The Overall Reaction<\/h2>\n        <\/div>\n\n        <p>Alkenes react with <strong>hydrogen, H<sub>2<\/sub><\/strong>, in the presence of a <strong>nickel catalyst<\/strong>. The reaction converts an <strong>alkene<\/strong> into an <strong>alkane<\/strong>.<\/p>\n        <p>The C=C double bond opens and one hydrogen atom adds to each carbon atom from the original double bond. This removes the alkene functional group and produces a saturated hydrocarbon.<\/p>\n        <p>For example, <strong>ethene<\/strong> reacts with hydrogen to form <strong>ethane<\/strong>.<\/p>\n\n        <div class=\"ols-reaction-summary\">\n          <div class=\"ols-summary-item\"><span>Functional group change<\/span><strong>Alkene \u2192 alkane<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Reagent<\/span><strong>Hydrogen, H<sub>2<\/sub><\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Conditions<\/span><strong>Nickel catalyst<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Type of reaction<\/span><strong>Addition and reduction<\/strong><\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Key idea:<\/strong> Hydrogen adds across the C=C double bond, so the alkene becomes an alkane.<\/p>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <div class=\"ols-figure-image\">\n            <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-19-hydrogenation.svg\" alt=\"Ethene plus hydrogen over nickel giving ethane, with the added hydrogen atoms highlighted\" width=\"1416\" height=\"756\">\n          <\/div>\n          <div class=\"ols-figure-caption\">\n            <p>Ethene reacts with H<sub>2<\/sub>; the C=C double bond is converted into a C-C single bond as ethane forms.<\/p>\n          <\/div>\n        <\/div>\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\/10\/t4-cd-19b-propenehydrog.svg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-19b-propenehydrog.svg\" alt=\"Ethene plus hydrogen over nickel giving ethane, with the added hydrogen atoms highlighted\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t4-cd-19b-propenehydrog.svg\">\n<\/a>\n<\/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>Quick Check: How Much Hydrogen?<\/h2>\n<p>Apply the conditions to a molecule that has more than one double bond.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-39\" class=\"h5p-iframe\" data-content-id=\"39\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogenation MCQ: A Diene with an Excess of Hydrogen\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">2<\/div>\n          <h2>Why This Is an Addition Reaction<\/h2>\n        <\/div>\n\n        <p>This reaction is an <strong>addition reaction<\/strong> because atoms are added across the carbon-carbon double bond.<\/p>\n        <p>The alkene starts with a C=C double bond, meaning it is <strong>unsaturated<\/strong>. During the reaction, the double bond opens and two new C-H bonds form.<\/p>\n        <p>The product is an alkane, which is <strong>saturated<\/strong> because it contains only single covalent bonds between carbon atoms.<\/p>\n\n        <div class=\"ols-definition-box\">\n          <div class=\"ols-definition-circle\">H<sub>2<\/sub><\/div>\n          <p><strong>Hydrogenation:<\/strong> The addition of hydrogen across a C=C double bond, converting an alkene into an alkane.<\/p>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Remember:<\/strong> In alkene addition reactions, the C=C double bond is the reactive site. The double bond opens so new atoms can bond to the two carbon atoms.<\/p>\n        <\/div>\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\/10\/t5-img-28-nickelsurface.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-28-nickelsurface.jpg\" alt=\"Four frames on a nickel surface: H\u2082 and an alkene adsorb, the H\u2013H bond weakens, H atoms add across the C=C, the alkane leaves\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-28-nickelsurface.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Hydrogen and the alkene are held on the nickel surface, the H\u2013H bond breaks and one hydrogen atom adds to each carbon of the C=C.<\/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>Quick Check: Formulae, Names and Amounts<\/h2>\n<p>Answer as quickly as you can, using alkenes that are not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-40\" class=\"h5p-iframe\" data-content-id=\"40\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogenation Quick-Fire: Formulae, Names and Amounts\"><\/iframe><\/div><\/div>\n<\/section>\n\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">3<\/div>\n          <h2>Why This Is Also a Reduction Reaction<\/h2>\n        <\/div>\n\n        <p>The reaction with hydrogen can also be described as a <strong>reduction reaction<\/strong>.<\/p>\n        <p>In organic chemistry, reduction often means the addition of hydrogen to a molecule. Since the alkene gains hydrogen atoms, the reaction is described as reduction.<\/p>\n        <p>This is why the same reaction can be correctly described in two ways: <strong>addition<\/strong>, because atoms add across the double bond, and <strong>reduction<\/strong>, because hydrogen is added.<\/p>\n\n        <div class=\"ols-mechanism-steps\">\n          <div class=\"ols-step-card\">\n            <h3>Step 1: Hydrogen is the reagent<\/h3>\n            <p>The alkene reacts with H<sub>2<\/sub> in the presence of a nickel catalyst.<\/p>\n          <\/div>\n\n          <div class=\"ols-step-card\">\n            <h3>Step 2: The double bond opens<\/h3>\n            <p>The C=C bond is replaced by a C-C single bond as new C-H bonds form.<\/p>\n          <\/div>\n\n          <div class=\"ols-step-card\">\n            <h3>Step 3: An alkane forms<\/h3>\n            <p>The product is saturated because it no longer contains the alkene C=C functional group.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> If asked for the reaction type, addition is essential. Reduction is also valid because hydrogen is added.<\/p>\n        <\/div>\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\/10\/t5-img-29-hardeningoils.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-29-hardeningoils.jpg\" alt=\"Kinked unsaturated oil chains react with hydrogen on nickel to give straight saturated chains that pack into a solid fat\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/10\/t5-img-29-hardeningoils.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Adding hydrogen across C=C bonds is both addition and reduction; it turns liquid vegetable oils into solid fats.<\/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>Quick Check: Two Names for One Reaction<\/h2>\n<p>Drag the words into place to describe the hydrogenation of a ring alkene that is not on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1273\" class=\"h5p-iframe\" data-content-id=\"1273\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogenation Drag: Cyclohexene, Addition and Reduction\"><\/iframe><\/div><\/div>\n<\/section>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Quick Check: Pick the Accurate Statement<\/h2>\n<p>In each round, choose the one statement that is accurate.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-41\" class=\"h5p-iframe\" data-content-id=\"41\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Hydrogenation Summary: Skeletons, Names and the Nickel\"><\/iframe><\/div><\/div>\n<\/section>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">4<\/div>\n          <h2>Common Exam Mistakes<\/h2>\n        <\/div>\n\n        <p>Students often lose marks by forgetting the <strong>nickel catalyst<\/strong>, or by describing the reaction as substitution. This is not substitution because no atom is replaced.<\/p>\n        <p>Another common error is to leave the product as an alkene. The product must be an <strong>alkane<\/strong>, because the carbon-carbon double bond has been removed.<\/p>\n        <p>For a simple reaction question, the safest answer should include the reagent, condition, product type and reaction type.<\/p>\n\n        <div class=\"ols-reaction-summary\">\n          <div class=\"ols-summary-item\"><span>Do say<\/span><strong>Hydrogen adds across the C=C double bond.<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Condition<\/span><strong>Use a nickel catalyst.<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Product type<\/span><strong>Alkane, not alkene.<\/strong><\/div>\n          <div class=\"ols-summary-item\"><span>Do not say<\/span><strong>Substitution reaction.<\/strong><\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Exam focus:<\/strong> Link the functional group change to the reaction type: alkene to alkane happens because hydrogen adds across the C=C double bond.<\/p>\n        <\/div>\n      <\/article>\n\n      \n\n      <section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>These questions secure the catalyst, reaction-type and quantity points for hydrogenation.<\/p>\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is a catalyst needed if hydrogen simply adds across the C=C bond?<\/h3>\n<p>The reaction has a high activation energy, so it is very slow on its own. Finely divided nickel gives a surface where the alkene and H<sub>2<\/sub> are adsorbed and react, lowering the activation energy. The mixture is usually heated to about 150 \u00b0C.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Why is hydrogenation also called reduction?<\/h3>\n<p>In organic chemistry, gaining hydrogen counts as reduction. The alkene gains two hydrogen atoms, so the same reaction is both an addition and a reduction.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>How much hydrogen reacts with a molecule that has two C=C bonds?<\/h3>\n<p>One mole of H<sub>2<\/sub> adds across each C=C bond. Buta-1,3-diene, C<sub>4<\/sub>H<sub>6<\/sub>, therefore needs 2 mol of H<sub>2<\/sub> to form butane, C<sub>4<\/sub>H<sub>10<\/sub>.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Where is hydrogenation used in industry?<\/h3>\n<p>It is used to harden vegetable oils to make margarine. Hydrogen adds to some of the C=C bonds in the unsaturated fat chains, raising the melting temperature so the oil becomes a soft solid.<\/p>\n<\/div>\n<div class=\"ols-faq-item\">\n<h3>Do I need to draw an electrophilic addition mechanism for hydrogenation?<\/h3>\n<p>No. H<sub>2<\/sub> is non-polar and the reaction happens on the nickel surface, so no curly-arrow mechanism is expected. State that it is an addition, and a reduction, using a nickel catalyst.<\/p>\n<\/div>\n<\/div>\n<\/section>\n\n      <!-- author card (moved to the end, 10 Oct 2026) -->\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: Dr. Mohammed Al-Fatah\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/p>\n\n      <a\n        class=\"ols-linkedin-pill\"\n        href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\"\n        target=\"_blank\"\n        rel=\"noopener noreferrer\"\n      >\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\n\n      <section class=\"ols-related-card\">\n        <h2>Related Alkene Revision Notes<\/h2>\n        <p>Continue through Topic 5 by linking hydrogenation to halogen addition, hydrogen halide addition and hydration with steam.<\/p>\n\n        <div class=\"ols-related-grid\">\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Bromine Water<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Steam<\/a>\n          <a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\">Hydrogen Halide<\/a>\n        <\/div>\n      <\/section>\n\n      <section class=\"ols-attribution-card ols-attr-std\" aria-label=\"Copyright notice\">\n<style id=\"ols-attribution-card-css\">.ols-attribution-card.ols-attr-std{background:linear-gradient(135deg,#ffffff,#f8fbff);border:1px solid rgba(28,36,75,0.14);border-radius:28px;box-shadow:0 18px 45px rgba(28,36,75,0.10);padding:34px;margin:24px 0;overflow:hidden;font-family:Poppins,Arial,sans-serif;box-sizing:border-box}.ols-attribution-card.ols-attr-std *{box-sizing:border-box}.ols-attribution-card.ols-attr-std p{margin:0;font-size:14px;line-height:1.65;font-weight:300;color:#667085}.ols-attribution-card.ols-attr-std strong{font-weight:700;color:#1C244B}@media (max-width:760px){.ols-attribution-card.ols-attr-std{padding:24px 18px;border-radius:22px}}<\/style>\n<p><strong>Copyright notice:<\/strong> This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.<\/p>\n<\/section>\n    <\/main>\n  \n<script type=\"application\/ld+json\">\n{\n    \"@context\": \"https:\/\/schema.org\",\n    \"@graph\": [\n        {\n            \"@type\": \"WebPage\",\n            \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/#webpage\",\n            \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/\",\n            \"name\": \"Reactions of Alkenes with Hydrogen | A Level Chemistry\",\n            \"description\": \"Learn how alkenes react with hydrogen to form alkanes using a nickel catalyst, including hydrogenation, reduction and addition reactions in A Level Chemistry.\",\n            \"inLanguage\": \"en-GB\",\n            \"isPartOf\": {\n                \"@id\": \"https:\/\/www.onlinelearningsystem.net\/#website\"\n            },\n           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