{"id":12478,"date":"2026-10-03T08:32:24","date_gmt":"2026-10-03T07:32:24","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/"},"modified":"2026-10-04T23:04:45","modified_gmt":"2026-10-04T22:04:45","slug":"enthalpy-of-solution-and-hydration","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/","title":{"rendered":"Enthalpy of Solution and Hydration"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-kinetics-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      --gold: #c9973a;\n      --grey-text: #667085;\n      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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility and Temperature<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/\">Lattice Energy Trends and Covalent Character<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\">Enthalpy of Solution and Hydration<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\">Predicting Solubility<\/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\/edexcel-international\/topic-6-energetics\/\">Topic 6 Energetics (AS)<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-11-kinetics\/\">Topic 11 Kinetics<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/core-practicals\/\">Core Practicals<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = normalisePath(window.location.pathname);\r\n    var links = sidebar.querySelectorAll('a[href]');\r\n    var matched = null;\r\n    var matchedLength = 0;\r\n\r\n    sidebar.querySelectorAll('.active, .active-main, .parent-active').forEach(function(item) {\r\n      item.classList.remove('active', 'active-main', 'parent-active');\r\n    });\r\n\r\n    sidebar.querySelectorAll('a[data-ols-disabled-parent=\"1\"], 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true;\r\n  }\r\n\r\n  if (!highlightActive()) {\r\n    document.addEventListener('DOMContentLoaded', highlightActive);\r\n  }\r\n})();\r\n<\/script>\r\n\n\n    <main class=\"ols-main\">\n\n      <!-- BREADCRUMBS - updated with full path and correct links -->\n      <nav 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\/edexcel-international\/\">Edexcel International<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics<\/a> \/\n<span>Enthalpy of Solution and Hydration<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Enthalpy of Solution and Hydration<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to the enthalpy changes of solution and hydration: their definitions, the energy cycle and energy level diagram that link them to lattice energy, the calculation for sodium chloride, and how ionic charge and radius decide the size of the hydration enthalpy.<\/p>\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Exam board: Edexcel International<\/div>\n<div class=\"ols-badge\">Unit 4: WCH14\/01<\/div>\n<div class=\"ols-badge\">Topic 12: Entropy and Energetics<\/div>\n<\/div>\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 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>Two Definitions<\/h2>\n<\/div>\n<p>Dissolving an ionic solid in water is a two-part process.<\/p>\n<ol>\n<li>The lattice is pulled apart into separate ions, which costs energy.<\/li>\n<li>Each ion is then surrounded by water molecules, which releases energy.<\/li>\n<\/ol>\n<p>The overall change and the second part each have a named enthalpy change.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Enthalpy change of solution, \u0394solH: the enthalpy change when one mole of a solute dissolves in enough water that further dilution causes no further enthalpy change. Example: NaCl(s) + aq \u2192 Na\u207a(aq) + Cl\u207b(aq).<\/p>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> Enthalpy change of hydration, \u0394hydH: the enthalpy change when one mole of gaseous ions forms one mole of aqueous ions. Example: Na\u207a(g) + aq \u2192 Na\u207a(aq). It is always exothermic.<\/p>\n<\/div>\n<p>Notice the states.<\/p>\n<ul>\n<li>Hydration starts from <strong>gaseous<\/strong> ions, exactly where the lattice energy ends, which is what lets the three quantities be joined in one cycle.<\/li>\n<li>Solution starts from the <strong>solid<\/strong> and may be exothermic or endothermic, depending on which of the two parts is larger.<\/li>\n<li>Each ion has its own hydration enthalpy, so a compound has one \u0394solH but a separate \u0394hydH for each kind of ion.<\/li>\n<li>Each \u0394hydH is multiplied by the number of that ion in the formula.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> Solution: &#8220;one mole of solute dissolves in enough water for no further enthalpy change on dilution&#8221; (the &#8220;infinitely dilute&#8221; condition). Hydration: &#8220;one mole of gaseous ions is converted to one mole of aqueous ions&#8221;.<\/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>The Energy Cycle<\/h2>\n<\/div>\n<p>The cycle has three corners: the solid, the gaseous ions and the aqueous ions.<\/p>\n<ul>\n<li>Solid to gaseous ions is the <strong>reverse<\/strong> of forming the lattice, so its enthalpy change is \u2212LE.<\/li>\n<li>Gaseous ions to aqueous ions is the sum of the hydration enthalpies.<\/li>\n<li>By Hess\u2019s law the direct route, \u0394solH, equals the sum of the two.<\/li>\n<\/ul>\n<p style=\"text-align:center\"><strong>\u0394solH = \u2212LE + \u03a3\u0394hydH<\/strong><\/p>\n<p>For sodium chloride: LE = \u2212787 kJ mol\u207b\u00b9, \u0394hydH(Na\u207a) = \u2212406 and \u0394hydH(Cl\u207b) = \u2212378.<\/p>\n<p><strong>Step 1:<\/strong> break the lattice. NaCl(s) \u2192 Na\u207a(g) + Cl\u207b(g) needs +787.<\/p>\n<p><strong>Step 2:<\/strong> hydrate the ions. Na\u207a(g) + Cl\u207b(g) \u2192 Na\u207a(aq) + Cl\u207b(aq) releases \u2212406 + (\u2212378) = \u2212784.<\/p>\n<p><strong>Step 3:<\/strong> add the two.<\/p>\n<p style=\"text-align:center\">\u0394solH = +787 \u2212 784 = <strong>+3 kJ mol\u207b\u00b9<\/strong><\/p>\n<p>Dissolving sodium chloride is very slightly endothermic, which is why the water cools a little.<\/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\/entropy-t12-09-solution.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-09-solution.jpg\" alt=\"Energy cycle and energy level diagram for dissolving sodium chloride: lattice energy up to the gaseous ions, hydration enthalpies down to the aqueous ions, and the small enthalpy change of solution\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-09-solution.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>The energy cycle and the energy level diagram for dissolving sodium chloride: up by \u2212LE to the gaseous ions, down by the two hydration enthalpies to the aqueous ions, and the small enthalpy of solution as the direct route.<\/p><\/div>\n<\/div>\n<p>The same data draw as an <strong>energy level diagram<\/strong>.<\/p>\n<ul>\n<li>The gaseous ions sit at the top, the solid at the bottom, and the aqueous ions just 3 kJ mol\u207b\u00b9 above the solid.<\/li>\n<li>The arrow up from the solid is the lattice energy reversed.<\/li>\n<li>The arrow down is the total hydration enthalpy.<\/li>\n<li>\u0394solH is the small gap between the two lower levels.<\/li>\n<\/ul>\n<p>Questions use either form, and the arithmetic is identical.<\/p>\n<p>The sign of the first step trips people up.<\/p>\n<ul>\n<li>The lattice energy is the enthalpy change for <strong>forming<\/strong> the lattice, \u2212787 kJ mol\u207b\u00b9.<\/li>\n<li>The cycle uses the reverse, breaking it, so the value goes in as +787.<\/li>\n<li>Write \u2212LE and substitute, rather than trying to remember whether the answer &#8220;should&#8221; be positive.<\/li>\n<\/ul>\n<div class=\"ols-key-box\">\n<p><strong>Worked example:<\/strong> \u0394solH = \u2212LE + \u03a3\u0394hydH = \u2212(\u2212787) + (\u2212406 \u2212 378) = +787 \u2212 784 = +3 kJ mol\u207b\u00b9. For MgCl\u2082 the chloride term is doubled: \u0394solH = \u2212LE + \u0394hydH(Mg\u00b2\u207a) + 2\u0394hydH(Cl\u207b).<\/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: The Cycle and the Calculation<\/h2>\n<p>Build the solution cycle for a salt not used on this page, decide the direction of each arrow, and calculate the missing enthalpy change from the other two.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1117\"><\/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>Why Hydration Is Exothermic and What Decides Its Size<\/h2>\n<\/div>\n<p>Water is polar, so its molecules turn to face an ion.<\/p>\n<ul>\n<li>Around a <strong>cation<\/strong> the water molecules turn their \u03b4\u2212 oxygen atoms inwards.<\/li>\n<li>Around an <strong>anion<\/strong> they turn their \u03b4+ hydrogen atoms inwards.<\/li>\n<li>The <strong>ion\u2013dipole attractions<\/strong> that form release energy, so the hydration enthalpy is always exothermic.<\/li>\n<\/ul>\n<p>How exothermic depends on how strongly the ion attracts the water dipoles, which is the same charge density argument that decides the lattice energy.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Ion<\/th><th>Ionic radius \/ pm<\/th><th>\u0394hydH \/ kJ mol\u207b\u00b9<\/th><th>What the comparison shows<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Mg\u00b2\u207a<\/strong><\/td><td>72<\/td><td>\u22121926<\/td><td>small and 2+: very strong attraction to water<\/td><\/tr>\n<tr><td><strong>Ba\u00b2\u207a<\/strong><\/td><td>135<\/td><td>\u22121305<\/td><td>same charge, larger radius: weaker attraction<\/td><\/tr>\n<tr><td><strong>Na\u207a<\/strong><\/td><td>102<\/td><td>\u2212406<\/td><td>similar size to Mg\u00b2\u207a but only 1+: far weaker<\/td><\/tr>\n<tr><td><strong>F\u207b<\/strong><\/td><td>133<\/td><td>\u2212506<\/td><td>small anion: strong attraction to the \u03b4+ hydrogens<\/td><\/tr>\n<tr><td><strong>Cl\u207b<\/strong><\/td><td>181<\/td><td>\u2212364<\/td><td>same charge, larger radius: weaker attraction<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Two rules follow.<\/p>\n<ul>\n<li>A <strong>smaller ion<\/strong> has a more exothermic hydration enthalpy, because the water dipoles get closer to the centre of charge: F\u207b against Cl\u207b, Mg\u00b2\u207a against Ba\u00b2\u207a.<\/li>\n<li>A <strong>higher charge<\/strong> gives a much more exothermic hydration enthalpy, because the field around the ion is stronger and more water molecules are held more tightly: Mg\u00b2\u207a against Na\u207a.<\/li>\n<\/ul>\n<p>Charge matters more than radius, exactly as it does for the lattice energy.<\/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\/entropy-t12-10-hydration.jpg\" target=\"_blank\" rel=\"noopener\" aria-label=\"Open image fullscreen\">\n<img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-10-hydration.jpg\" alt=\"Why hydration enthalpy depends on charge and radius: water molecules orienting their \u03b4\u2212 oxygens around a cation and \u03b4+ hydrogens around an anion, with Mg\u00b2\u207a against Ba\u00b2\u207a and F\u207b against Cl\u207b compared\" data-fullscreen-src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/entropy-t12-10-hydration.jpg\">\n<\/a>\n<\/div>\n<div class=\"ols-figure-caption\"><p>Water molecules oriented around a cation (oxygen inwards) and an anion (hydrogen inwards), with Mg\u00b2\u207a against Ba\u00b2\u207a and F\u207b against Cl\u207b compared.<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> Both the lattice energy and the hydration enthalpy become more exothermic with higher ionic charge and smaller ionic radius. The enthalpy of solution is the difference between them, so it depends on which of the two responds more.<\/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: Charge and Radius Effects<\/h2>\n<p>Compare the hydration enthalpies of pairs of ions not used on this page and explain the difference in terms of charge, radius and ion\u2013dipole attraction.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"1118\"><\/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>Reading \u0394solH<\/h2>\n<\/div>\n<p>An enthalpy of solution is a <strong>small difference between two large numbers<\/strong>.<\/p>\n<ul>\n<li>Sodium chloride needs 787 kJ mol\u207b\u00b9 to break the lattice and gets 784 back from hydration.<\/li>\n<li>The result, +3, is smaller than the uncertainty in either figure.<\/li>\n<li>So questions expect you to use the data given exactly and not to round early.<\/li>\n<\/ul>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>\u0394solH<\/th><th>What it usually means<\/th><th>Example<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>Exothermic<\/strong><\/td><td>almost always a soluble salt<\/td><td>lithium chloride, anhydrous magnesium chloride<\/td><\/tr>\n<tr><td><strong>Slightly endothermic<\/strong><\/td><td>often freely soluble: the entropy gain outweighs the small enthalpy cost<\/td><td>sodium chloride<\/td><\/tr>\n<tr><td><strong>Very endothermic<\/strong><\/td><td>usually insoluble: dissolving is not feasible<\/td><td>silver chloride<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>A slightly endothermic \u0394solH does not stop a salt dissolving. The ions spreading through the water bring a large gain in entropy that outweighs the small enthalpy cost.<\/p>\n<p>A <strong>very endothermic<\/strong> \u0394solH means the lattice energy is so much larger than the hydration enthalpies can repay that dissolving is not feasible.<\/p>\n<p>Silver chloride is a familiar case, with a lattice energy strengthened by covalent character that hydration cannot match.<\/p>\n<p>Whether a salt actually dissolves depends on the entropy change as well as the enthalpy change; the next page puts the two together and uses them to explain the solubility trends of the Group 2 hydroxides and sulfates.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Explain a difference in \u0394solH by naming the two terms that make it up, saying which one changed more and why (charge, radius) and then giving the sign of the result.<\/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: Interpreting Solution Enthalpies<\/h2>\n<p>Use given values of the lattice energy and hydration enthalpies for salts not on this page to decide the sign of \u0394solH and to comment on what it suggests about solubility.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-1119\" class=\"h5p-iframe\" data-content-id=\"1119\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Entropy and Energetics Summary: Interpreting Solution Enthalpies\"><\/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>\n<ul>\n<li>&#8220;Enthalpy of hydration: one mole of gaseous ions forms one mole of aqueous ions.&#8221;<\/li>\n<li>&#8220;The hydration enthalpy is exothermic because ion\u2013dipole attractions form between the ion and water molecules.&#8221;<\/li>\n<li>&#8220;\u0394solH = \u2212(lattice energy) + the sum of the hydration enthalpies.&#8221;<\/li>\n<\/ul>\n<h3>Do not say<\/h3>\n<ul>\n<li>&#8220;Hydration is when the solid dissolves in water&#8221; (that is solution; hydration starts from gaseous ions).<\/li>\n<li>&#8220;The lattice energy is added&#8221; when the cycle needs its reverse.<\/li>\n<li>&#8220;Mg\u00b2\u207a has a more exothermic hydration enthalpy because it is more reactive&#8221; (it is smaller and more highly charged).<\/li>\n<\/ul>\n<h3>Watch for<\/h3>\n<ul>\n<li>A formula with two anions (MgCl\u2082, CaBr\u2082): double the anion hydration enthalpy.<\/li>\n<li>A value quoted as an enthalpy of lattice dissociation, which goes into the cycle positive as it is.<\/li>\n<li>A question that gives \u0394solH and asks for a hydration enthalpy: rearrange the same equation.<\/li>\n<\/ul>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the enthalpy of solution and hydration ideas.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>Why is hydration always exothermic?<\/h3>\n<p>Because new attractions are being made without any being broken. Water is polar, so its \u03b4\u2212 oxygen atoms cluster round a cation and its \u03b4+ hydrogen atoms round an anion, and forming those ion\u2013dipole attractions releases energy. There is no lattice to break in the hydration step; that cost belongs to the lattice term.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>How are enthalpy of solution, lattice energy and hydration enthalpy linked?<\/h3>\n<p>Dissolving can be imagined as two steps: pull the lattice apart into gaseous ions, then hydrate each ion. So \u0394solH = \u2212(lattice energy) + sum of the hydration enthalpies. Remember to count each hydration term once per mole of that ion in the formula.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is the enthalpy of solution often close to zero?<\/h3>\n<p>Because it is the small difference between two very large numbers. Breaking the lattice might cost about +750 kJ mol\u207b\u00b9 and hydrating the ions might release about \u2212740 kJ mol\u207b\u00b9, leaving a solution enthalpy of only about +10 kJ mol\u207b\u00b9. That is why many salts dissolve with barely any temperature change.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why does Mg\u00b2\u207a have a much more exothermic hydration enthalpy than Na\u207a?<\/h3>\n<p>Because it has twice the charge in a smaller ion, so its charge density is far higher and it attracts the \u03b4\u2212 oxygen of water much more strongly. Hydration enthalpy becomes more exothermic as the charge increases and as the radius decreases, the same two factors that control the size of the lattice term.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Does a positive enthalpy of solution mean the salt will not dissolve?<\/h3>\n<p>No. Many salts with a small positive \u0394solH, such as potassium nitrate, dissolve readily because the entropy of the ions and water increases when the lattice breaks up. A positive \u0394solH only makes dissolving less likely and, usually, makes the solubility rise with temperature.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related Topic 12 Entropy and Energetics Pages<\/h2>\n<p>Use these pages to connect the ideas across Topic 12 Entropy and Energetics 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\/edexcel-international\/topic-12-entropy-and-energetics\/entropy-and-the-direction-of-change\/\">Entropy and the Direction of Change<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/calculating-entropy-changes\/\">Calculating Entropy Changes<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/feasibility-gibbs-energy-and-temperature\/\">Feasibility and Temperature<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/thermodynamic-and-kinetic-stability\/\">Thermodynamic and Kinetic Stability<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-and-born-haber-cycles\/\">Lattice Energy and Born\u2013Haber Cycles<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/lattice-energy-trends-and-covalent-character\/\">Lattice Energy Trends and Covalent Character<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/predicting-solubility\/\">Predicting Solubility<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/\">Topic 12 Entropy and Energetics Overview<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-8-redox-chemistry-and-groups-1-2-and-7\/8b-groups-1-and-2\/\">Topic 8B Groups 1 and 2<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\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\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\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/#webpage\",\n      \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel-international\/topic-12-entropy-and-energetics\/enthalpy-of-solution-and-hydration\/\",\n      \"name\": \"Enthalpy of Solution and Hydration | Topic 12 Entropy and Energetics | Online Learning System\",\n      \"description\": \"Edexcel International A Level Chemistry revision notes on enthalpy of solution and hydration: definitions, energy cycles with lattice energy, worked calculations and the effect of ionic 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