{"id":8162,"date":"2026-09-16T17:33:41","date_gmt":"2026-09-16T16:33:41","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/permanent-dipole-dipole-forces\/"},"modified":"2026-09-22T09:36:44","modified_gmt":"2026-09-22T08:36:44","slug":"permanent-dipole-dipole-forces","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/permanent-dipole-dipole-forces\/","title":{"rendered":"Permanent Dipole-Dipole Forces"},"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      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border: 1px solid rgba(28, 36, 75, 0.12); border-radius: 22px; padding: 14px; margin: 18px 0 6px; }\n    .ols-figure-placeholder .ols-figure-caption p { margin: 10px 0 0; font-size: 14px; color: #667085; text-align: center; }\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>OCR A Level Chemistry A<\/p>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>2.2.2 Bonding and Structure<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/\">Section overview<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/ionic-bonding\/\">Ionic Bonding<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/ionic-bonding\/nature-of-ionic-bonding\/\">Nature of Ionic Bonding<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/ionic-bonding\/polarisation-of-ions\/\">Polarisation of Ions<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/ionic-bonding\/physical-properties-of-ionic-compounds\/\">Physical Properties of Ionic Compounds<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/covalent-bonding\/\">Covalent Bonding<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/covalent-bonding\/nature-of-covalent-bonding\/\">Nature of Covalent Bonding<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/covalent-bonding\/dative-covalent-bonding\/\">Dative Covalent Bonding<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/covalent-bonding\/electronegativity\/\">Electronegativity<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/covalent-bonding\/giant-covalent-structures\/\">Giant Covalent Structures<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/\">Shapes of Molecules<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/linear\/\">Linear<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/trigonal-planar\/\">Trigonal Planar<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/tetrahedral\/\">Tetrahedral<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/trigonal-pyramidal\/\">Trigonal Pyramidal<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/bent\/\">Bent<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/trigonal-bipyramidal\/\">Trigonal Bipyramidal<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/octahedral\/\">Octahedral<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/square-planar\/\">Square Planar<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/square-pyramidal\/\">Square Pyramidal<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/distorted-t\/\">Distorted T<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/shapes-of-molecules\/seesaw\/\">SeeSaw<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/polar-molecules\/\">Non-Polar and Polar Molecules<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/bond-enthalpy-and-pauling-values\/\">Bond Enthalpy and Pauling Values<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/metallic-bonding\/\">Metallic Bonding<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/metallic-bonding\/metallic-bonding-and-structure\/\">Metallic Bonding and Structure<\/a><\/li>\n          <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/metallic-bonding\/physical-properties-of-metals\/\">Physical Properties of Metals<\/a><\/li>\n        <\/ul>\n      <\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/simple-molecular-structures\/\">Simple Molecular Structures<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/structure-types\/\">Structure Types<\/a><\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/\">Intermolecular Forces<\/a>\n        <ul class=\"ols-subtopic-list\">\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/london-forces\/\">London Forces<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/permanent-dipole-dipole-forces\/\">Permanent Dipole-Dipole Forces<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/hydrogen-bonding\/\">Hydrogen Bonding<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/anomalous-properties-of-water\/\">Anomalous Properties of Water<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/boiling-temperature-trends\/\">Boiling Point Trends<\/a>\n          <\/li>\n          <li>\n            <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/a>\n          <\/li>\n        <\/ul>\n      <\/li>\n    <\/ul>\n  <\/div>\n  <div class=\"ols-topic-group\">\n    <h4>Other OCR Sections<\/h4>\n    <ul class=\"ols-topic-list\">\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-1-basic-concepts-of-organic-chemistry\/\">4.1.1 Basic Concepts of Organic Chemistry<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-2-alkanes\/\">4.1.2 Alkanes<\/a><\/li>\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/4-1-3-alkenes\/\">4.1.3 Alkenes<\/a><\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n   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class=\"ols-breadcrumbs\" aria-label=\"Breadcrumb\">\n<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\/ocr-a\/\">OCR A<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/\">2.2.2 Bonding and Structure<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/\">Intermolecular Forces<\/a> \/\n<span>Permanent Dipole-Dipole Forces<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Permanent Dipole-Dipole Forces<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to permanent dipole\u2013dipole interactions: which molecules have a permanent dipole, how polar molecules attract each other, why the attraction adds to London forces, and how to decide the forces in an unfamiliar molecule, for OCR A A Level Chemistry.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 and 2<\/div>\n<div class=\"ols-badge\">2.2.2: Bonding and Structure<\/div>\n<div class=\"ols-badge\">H432\/01 and H432\/02<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by:<br><span>Dr. Mohammed Al-Fatah<\/span>\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/p>\n\n      <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>Polar Molecules and Permanent Dipoles<\/h2>\n<\/div>\n<p>A <strong>permanent dipole<\/strong> exists in a molecule that contains polar bonds whose dipoles do not cancel, so one end of the molecule is always \u03b4+ and the other always \u03b4-. Permanent dipole\u2013dipole interactions are the attractions between the \u03b4+ end of one such molecule and the \u03b4- end of a neighbour.<\/p>\n<p>Two conditions must both hold. First, the molecule must contain a bond between atoms of different electronegativity, such as H-Cl, C-Cl, C-F or C=O. Second, the shape of the molecule must leave an overall dipole. CO\u2082 has two very polar C=O bonds, but the molecule is linear and the two bond dipoles cancel, so CO\u2082 has no permanent dipole and only London forces act between its molecules. The same applies to CCl\u2084 and BF\u2083.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Molecule<\/th><th>Polar bonds?<\/th><th>Shape<\/th><th>Overall dipole?<\/th><th>Strongest intermolecular force<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>HCl<\/strong><\/td><td>yes<\/td><td>linear, two atoms<\/td><td>yes<\/td><td>permanent dipole\u2013dipole interactions<\/td><\/tr>\n<tr><td><strong>CH\u2083Cl<\/strong><\/td><td>yes<\/td><td>tetrahedral, one Cl<\/td><td>yes<\/td><td>permanent dipole\u2013dipole interactions<\/td><\/tr>\n<tr><td><strong>CO\u2082<\/strong><\/td><td>yes<\/td><td>linear, symmetric<\/td><td>no<\/td><td>London forces<\/td><\/tr>\n<tr><td><strong>CCl\u2084<\/strong><\/td><td>yes<\/td><td>tetrahedral, symmetric<\/td><td>no<\/td><td>London forces<\/td><\/tr>\n<tr><td><strong>SO\u2082<\/strong><\/td><td>yes<\/td><td>bent<\/td><td>yes<\/td><td>permanent dipole\u2013dipole interactions<\/td><\/tr>\n<tr><td><strong>N\u2082<\/strong><\/td><td>no<\/td><td>linear<\/td><td>no<\/td><td>London forces<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Polar bonds are necessary but not sufficient. Ask whether the bond dipoles cancel in three dimensions before deciding that a molecule is polar.<\/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>Quick Check: Do the Bond Dipoles Cancel?<\/h2>\n<p>Four molecules, four shapes, one overall dipole. Draw them before you choose.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-527\" class=\"h5p-iframe\" data-content-id=\"527\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Permanent Dipoles MCQ: Which of These Four Molecules Is Polar\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>How the Molecules Line Up<\/h2>\n<\/div>\n<p>In a liquid such as hydrogen chloride the molecules tumble constantly, but on average they spend more time with their \u03b4+ hydrogen close to the \u03b4- chlorine of a neighbour than in any other arrangement. This partial alignment produces a net attraction that is stronger than London forces alone for molecules of similar size, so more energy is needed to pull the molecules apart.<\/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\/intermolecular-forces-in-hydrogen-chloride-and-fluorine.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\/intermolecular-forces-in-hydrogen-chloride-and-fluorine.jpg\" alt=\"Polar hydrogen chloride molecules aligned by permanent dipole-dipole attractions, beside non-polar fluorine molecules held only by London forces\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>Polar molecules attract each other by the alignment of their permanent dipoles; this attraction acts in addition to the London forces that every molecule has.<\/p><\/div>\n<\/div>\n<p>The alignment is never perfect, because thermal motion keeps disturbing it, which is why permanent dipole\u2013dipole interactions are still weak compared with covalent bonds and why polar liquids such as HCl still boil far below room temperature.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> Permanent dipole\u2013dipole interactions act <strong>in addition to<\/strong> London forces, never instead of them.<\/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>Quick Check: Lined Up or Tumbling?<\/h2>\n<p>Judge one statement about what polar molecules are actually doing in a liquid.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-786\" class=\"h5p-iframe\" data-content-id=\"786\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Permanent Dipoles True or False: How Polar Molecules Sit in a Liquid\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Comparing Molecules With Similar Numbers of Electrons<\/h2>\n<\/div>\n<p>The fair way to show that a permanent dipole raises the boiling point is to compare two molecules with the same number of electrons, so that their London forces are similar. Hydrogen chloride and fluorine both have 18 electrons. F\u2082 is non-polar and boils at -188 \u00b0C; HCl is polar and boils at -85 \u00b0C. The extra 100 \u00b0C comes from the permanent dipole\u2013dipole interactions.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Pair<\/th><th>Electrons<\/th><th>Polar?<\/th><th>boiling point \/ \u00b0C<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>F\u2082<\/strong><\/td><td>18<\/td><td>no<\/td><td>-188<\/td><\/tr>\n<tr><td><strong>HCl<\/strong><\/td><td>18<\/td><td>yes<\/td><td>-85<\/td><\/tr>\n<tr><td><strong>Butane, C\u2084H\u2081\u2080<\/strong><\/td><td>34<\/td><td>no<\/td><td>-1<\/td><\/tr>\n<tr><td><strong>Propanone, CH\u2083COCH\u2083<\/strong><\/td><td>32<\/td><td>yes (C=O)<\/td><td>56<\/td><\/tr>\n<tr><td><strong>Ethane, C\u2082H\u2086<\/strong><\/td><td>18<\/td><td>no<\/td><td>-89<\/td><\/tr>\n<tr><td><strong>Fluoromethane, CH\u2083F<\/strong><\/td><td>18<\/td><td>yes (C-F)<\/td><td>-78<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Propanone and butane make the same point with organic molecules: almost the same electron count, but the C=O dipole in propanone lifts its boiling point by nearly 60 \u00b0C.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam sentence:<\/strong> Propanone has a higher boiling point than butane because propanone molecules have a permanent dipole, so permanent dipole\u2013dipole interactions act between them in addition to London forces, and more energy is needed to separate the molecules.<\/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>Quick Check: A Fair Comparison of Two Small Molecules<\/h2>\n<p>Build the comparison between two molecules of almost equal electron count with a 124 \u00b0C gap in boiling temperature.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-526\" class=\"h5p-iframe\" data-content-id=\"526\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Permanent Dipoles Drag: Propane Against Ethanenitrile\"><\/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: Explain a Boiling Temperature<\/h2>\n<p>Write a short explanation for two isomers that differ only in where their chlorine atoms sit.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-528\" class=\"h5p-iframe\" data-content-id=\"528\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Permanent Dipoles Explain: The Two Isomers of 1,2-Dichloroethene\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Deciding the Forces in an Unfamiliar Molecule<\/h2>\n<\/div>\n<p>Exam questions often give a molecule you have never met and ask which intermolecular forces act between its molecules. Use three questions in order.<\/p>\n<ol>\n<li><strong>Does the molecule contain an H atom bonded directly to N, O or F?<\/strong> If yes, hydrogen bonding is present (see the <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/hydrogen-bonding\/\">Hydrogen Bonding<\/a> page), and so are the two forces below.<\/li>\n<li><strong>Does the molecule have an overall permanent dipole?<\/strong> Check for polar bonds and then check that the shape does not cancel them. If yes, permanent dipole\u2013dipole interactions are present, plus London forces.<\/li>\n<li><strong>Every molecule<\/strong> has London forces. If the answers to questions 1 and 2 are no, these are the only forces.<\/li>\n<\/ol>\n<p>For example, chloromethane, CH\u2083Cl, has no H bonded to N, O or F, but it has a polar C-Cl bond in a tetrahedral molecule with only one chlorine, so it has an overall dipole: permanent dipole\u2013dipole interactions plus London forces. Methanal, HCHO, has a C=O dipole: the same answer. Tetrachloromethane, CCl\u2084, has four polar bonds that cancel: London forces only.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Name every force that is present and say which is the strongest. A list with one force missing loses the mark even if the strongest is right.<\/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>Quick Check: Name Every Force<\/h2>\n<p>Run the three questions in order over five molecules you have not met on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-529\" class=\"h5p-iframe\" data-content-id=\"529\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Permanent Dipoles Rapid Check: Naming the Forces in Five New Molecules\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Permanent Dipoles and Solubility<\/h2>\n<\/div>\n<p>Polar molecules tend to dissolve in polar solvents and non-polar molecules in non-polar solvents, because a solute dissolves when the new attractions it makes with solvent molecules are similar in strength to the attractions it loses. Propanone mixes with water in all proportions because its C=O oxygen can accept a hydrogen bond from water, while hexane does not mix with water because the only forces hexane can offer are London forces. The <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/a> page develops this idea.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Like dissolves like: match the strongest intermolecular force in the solute to the strongest force in the solvent.<\/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>Quick Check: The Ideas That Catch People Out<\/h2>\n<p>Pick the one accurate statement in each round to finish the page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-787\" class=\"h5p-iframe\" data-content-id=\"787\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Permanent Dipoles Summary: Shape, Tumbling and Fair Comparisons\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Explain why HCl boils higher than F\u2082<\/h3><p>Similar electrons, so similar London forces; HCl is polar, so permanent dipole\u2013dipole interactions act as well; more energy is needed to separate HCl molecules.<\/p>\n<h3>Explain why CO\u2082 is non-polar although C=O bonds are polar<\/h3><p>The molecule is linear and symmetrical, so the two bond dipoles cancel and there is no overall dipole.<\/p>\n<h3>State the intermolecular forces in CH\u2083Br<\/h3><p>Permanent dipole\u2013dipole interactions and London forces; no hydrogen bonding because no H is bonded to N, O or F.<\/p>\n<h3>Do not say<\/h3><p>&#8220;Polar molecules have no London forces&#8221;; &#8220;any molecule with a polar bond is polar&#8221;.<\/p>\n<\/article>\n\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check how OCR A expects permanent dipoles to be identified and explained.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>What is a permanent dipole?<\/h3>\n<p>It is a separation of charge that is always present in a molecule, because the molecule contains polar bonds whose dipoles do not cancel. One end of the molecule is permanently \u03b4+ and the other permanently \u03b4-.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Does a polar bond always make a polar molecule?<\/h3>\n<p>No. CO\u2082 and CCl\u2084 both contain very polar bonds, but the molecules are symmetrical, so the bond dipoles cancel and there is no overall dipole. Always check the shape before deciding.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How do I decide which forces act between molecules of an unfamiliar substance?<\/h3>\n<p>Ask three questions in order: is a hydrogen atom bonded directly to N, O or F (hydrogen bonding); does the molecule have an overall permanent dipole (permanent dipole\u2013dipole interactions); and finally, every molecule has London forces.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Which is stronger, a permanent dipole attraction or London forces?<\/h3>\n<p>For molecules of a similar size, the permanent dipole\u2013dipole attraction is stronger. For a much larger molecule the London forces can be bigger in total, which is why HI boils higher than HCl even though HCl is more polar.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How should I show this in an answer?<\/h3>\n<p>Compare molecules with a similar number of electrons so the London forces are similar, then attribute the difference to the permanent dipole and finish with the energy statement.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Intermolecular Forces Pages<\/h2>\n<p>Use these pages to connect the three intermolecular forces with the physical properties they explain.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/london-forces\/\">London Forces<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/hydrogen-bonding\/\">Hydrogen Bonding<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/anomalous-properties-of-water\/\">Anomalous Properties of Water<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/boiling-temperature-trends\/\">Boiling Point Trends<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-2-2-bonding-and-structure\/intermolecular-forces\/choosing-solvents\/\">Choosing Solvents<\/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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