{"id":66535,"date":"2026-10-07T08:29:45","date_gmt":"2026-10-07T05:29:45","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=66535"},"modified":"2026-10-07T08:29:45","modified_gmt":"2026-10-07T05:29:45","slug":"reaction-enthalpy-from-bond-energies-calculator","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/chemistry\/reaction-enthalpy-from-bond-energies-calculator\/","title":{"rendered":"Reaction Enthalpy from Bond Energies Calculator"},"content":{"rendered":"<section class=\"not-prose calculator-box\" aria-labelledby=\"calculator-title\"><div class=\"flex items-baseline sm:items-center justify-between gap-2 sm:gap-3\"><div class=\"flex flex-col sm:flex-row sm:items-center gap-2\"><span class=\"icon icon-calculator text-primary-700! text-base! dark:text-primary-300!\" aria-hidden=\"true\"><\/span><h2 id=\"calculator-title\" class=\"text-lg font-display font-bold\">Estimate \u0394H from Bonds Broken and Formed<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"66535\" data-favorite-title=\"Bond Energy Reaction Enthalpy\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/chemistry\/reaction-enthalpy-from-bond-energies-calculator\/\" data-favorite-excerpt=\"Estimate the enthalpy change (\u0394H) of a chemical reaction from average bond energies. Enter the bonds broken and bonds formed as &quot;energy \u00d7 count&quot;, and see whether the reaction is exothermic or endothermic.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-molecule\">\n    <span class=\"icon icon-shape-star-empty\" aria-hidden=\"true\"><\/span>\n  <\/button>\n  <div class=\"calcdash-favorite__tip\" aria-hidden=\"true\">\n   <span class=\"favorite-tooltip\" id=\"favorite-tooltip\"><\/span>\n  <\/div>\n  <span class=\"sr-only\" role=\"status\" id=\"favorite-status\"><\/span>\n<\/div><\/div><form action=\"https:\/\/wpcalc.com\/en\/chemistry\/reaction-enthalpy-from-bond-energies-calculator\/\" method=\"POST\" class=\"calculator\" id=\"calculator-66535\" data-post=\"66535\"><fieldset class=\"fieldset-input is-1\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"field is-single\" id=\"input-1\"><label for=\"field-1\">Energy Unit<\/label><div class=\"grid grid-cols-1\"><select id=\"field-1\" name=\"energy_unit\"><option value=\"kJ\" selected>kJ\/mol<\/option><option value=\"kcal\">kcal\/mol<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><div class=\"field is-single\" id=\"input-2\"><label for=\"field-2\">Bonds Broken \u2014 Energy \u00d7 Count<\/label><textarea id=\"field-2\" name=\"bonds_broken_energy_count\">436 \u00d7 1; 243 \u00d7 1<\/textarea><\/div><div class=\"field is-single\" id=\"input-3\"><label for=\"field-3\">Bonds Formed \u2014 Energy \u00d7 Count<\/label><textarea id=\"field-3\" name=\"bonds_formed_energy_count\">431 \u00d7 2<\/textarea><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"66535\">Calculate<\/button><button type=\"reset\">Reset<\/button><\/div><div class=\"field is-checkbox hidden!\" id=\"field-auto-calc\"><input type=\"checkbox\" id=\"auto-calc\"><label for=\"auto-calc\">Calculate automatically<\/label><small>If enabled, the result will update automatically when you change any value.<\/small><\/div><div class=\"fieldset-result is-hidden\" aria-labelledby=\"result-title\" aria-live=\"polite\" role=\"region\"> <h3 class=\"result-title bg-gradient-to-r from-primary-50 to-gray-50 dark:from-primary-900 dark:to-gray-800\" id=\"result-title\"><span class=\"icon icon-s-pulse\" aria-hidden=\"true\"><\/span> Reaction Enthalpy Estimate<\/h3><div class=\"result-box\"><div class=\"field-result\" id=\"output-1\"><span class=\"field-title\"><span>Energy Absorbed (Bonds Broken)<\/span><span class=\"result-term\">\u03a3D<\/span><\/span><span class=\"field-value\" id=\"result-1\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result\" id=\"output-2\"><span class=\"field-title\"><span>Energy Released (Bonds Formed)<\/span><span class=\"result-term\">\u03a3D<\/span><\/span><span class=\"field-value\" id=\"result-2\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result\" id=\"output-3\"><span class=\"field-title\"><span>Reaction Enthalpy<\/span><span class=\"result-term\">\u0394H<\/span><\/span><span class=\"field-value\" id=\"result-3\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result is-column\" id=\"output-4\"><span class=\"field-title\"><span>Verdict<\/span><\/span><span class=\"field-value\" id=\"result-4\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result is-column\" id=\"output-5\"><span class=\"field-title\"><span>Calculation Details<\/span><\/span><span class=\"field-value\" id=\"result-5\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result is-column\" id=\"output-6\"><span class=\"field-title\"><span>Common Bond Energies<\/span><\/span><span class=\"field-value\" id=\"result-6\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><\/div><div class=\"result-actions\"><button type=\"button\" class=\"copy-link\"><svg viewBox=\"0 0 20 20\" fill=\"currentColor\" aria-hidden=\"true\"><path d=\"M12.232 4.232a2.5 2.5 0 0 1 3.536 3.536l-1.225 1.224a.75.75 0 0 0 1.061 1.06l1.224-1.224a4 4 0 0 0-5.656-5.656l-3 3a4 4 0 0 0 .225 5.865.75.75 0 0 0 .977-1.138 2.5 2.5 0 0 1-.142-3.667l3-3Z\"\/><path d=\"M11.603 7.963a.75.75 0 0 0-.977 1.138 2.5 2.5 0 0 1 .142 3.667l-3 3a2.5 2.5 0 0 1-3.536-3.536l1.225-1.224a.75.75 0 0 0-1.061-1.06l-1.224 1.224a4 4 0 1 0 5.656 5.656l3-3a4 4 0 0 0-.225-5.865Z\"\/><\/svg><span class=\"copy-link__text\">Copy link to this calculation<\/span><\/button><span class=\"sr-only\" role=\"status\" id=\"copy-link-status\"><\/span><\/div><\/div><a href=\"#respond\" class=\"hidden transition-opacity duration-300 opacity-0 w-50 text-sm justify-center items-center gap-2 px-4 py-2 rounded bg-gray-200 text-gray-700 hover:bg-gray-300\" id=\"leave-comment\"><span class=\"icon icon-comments\" aria-hidden=\"true\"><\/span>Leave a Comment<\/a><\/form><\/section><section id=\"calc-reactions\" class=\"not-prose hidden my-12 bg-gradient-to-r from-primary-50 to-gray-50 border border-indigo-100 rounded-xl px-6 py-4 shadow-sm\" data-post=\"66535\" aria-live=\"polite\" aria-labelledby=\"calc-reactions-title\"><p id=\"calc-reactions-title\" class=\"text-sm text-gray-500 text-center mb-2\">How did this result make you feel?<\/p><div role=\"group\" aria-labelledby=\"calc-reactions-title\" class=\"grid grid-cols-3 sm:flex sm:flex-row sm:justify-around gap-4 sm:items-center sm:flex-wrap\"><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"like\"><span class=\"reaction-count\" data-reaction-count=\"like\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude00 <\/span><span class=\"reaction-description font-medium\">Like <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"helpful\"><span class=\"reaction-count\" data-reaction-count=\"helpful\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\udca1 <\/span><span class=\"reaction-description font-medium\">Helpful <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"confused\"><span class=\"reaction-count\" data-reaction-count=\"confused\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude15 <\/span><span class=\"reaction-description font-medium\">Confused <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"disappointed\"><span class=\"reaction-count\" data-reaction-count=\"disappointed\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude1e <\/span><span class=\"reaction-description font-medium\">Disappointed <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"inaccurate\"><span class=\"reaction-count\" data-reaction-count=\"inaccurate\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\u274c <\/span><span class=\"reaction-description font-medium\">Inaccurate <\/span><\/button><\/div><div id=\"reaction-message\" class=\"hidden mt-8 rounded-md border border-primary-100 bg-white\/50 backdrop-blur-sm px-4 py-2 text-sm text-center shadow-sm\"><\/div><\/section><div class=\"calcdash-ad not-prose calcdash-ad--calc_result calcdash-ad--in-article\"><ins class=\"adsbygoogle\" style=\"display:block\" data-ad-client=\"ca-pub-1721569815777345\" data-ad-slot=\"6317458308\" data-ad-layout=\"in-article\" data-ad-format=\"fluid\"><\/ins><script>(adsbygoogle = window.adsbygoogle || []).push({});<\/script><\/div>\n<style id=\"wpcoderpro-inline-1\">.formula-box {\r\n\tborder-bottom: none !important;\r\n}<\/style><section class=\"formula-box\" aria-labelledby=\"formula-title\">\r\n\t<div id=\"formula-title\" class=\"not-prose font-display mb-4\"><span class=\"icon icon-formula\" aria-hidden=\"true\"><\/span><h2>Bond Energy Reaction Enthalpy Formula<\/h2><\/div>\r\n\t<figure class=\"not-prose formula\">\r\n\t\t<figcaption class=\"formula-title\">Formula<\/figcaption>\r\n\t\t<div class=\"text-base text-gray-800\" id=\"formula\">\n\\[ \\Delta H^\\circ_{\\text{rxn}} \\approx \\sum D(\\text{bonds broken}) - \\sum D(\\text{bonds formed}) \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(\\Delta H^\\circ_{\\text{rxn}}\\) \u2014 estimated standard reaction enthalpy (kJ\/mol or kcal\/mol per mole of reaction as written)<\/li>\n<li>\\(D(\\text{bonds broken})\\) \u2014 sum of the average bond energies of all bonds that must be broken in the reactants<\/li>\n<li>\\(D(\\text{bonds formed})\\) \u2014 sum of the average bond energies of all new bonds formed in the products<\/li>\n<\/ul>\n<p>Breaking a bond always costs energy, and forming a bond always releases it. The enthalpy change of the reaction is the difference: whatever is left over is either released as heat (negative \u0394H) or must be absorbed from the surroundings (positive \u0394H).<\/p>\n<p>\r\n<\/section>\n<style id=\"wpcoderpro-inline-459\">.calculation-box {\r\n\tborder-bottom: none !important;\r\n}<\/style><section id=\"calculation\" class=\"calculation-box\" aria-labelledby=\"calculation-title\">\r\n\t<div id=\"calculation-title\" class=\"not-prose font-display\"><span class=\"icon icon-unordered-list\" aria-hidden=\"true\"><\/span><h2>Bond Energy Reaction Enthalpy \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> hydrogen gas reacts with chlorine gas to form hydrogen chloride: H\u2082(g) + Cl\u2082(g) \u2192 2 HCl(g). One H\u2013H bond (436 kJ\/mol) and one Cl\u2013Cl bond (243 kJ\/mol) are broken, and two H\u2013Cl bonds (431 kJ\/mol each) are formed.<\/p>\n<p>\\[ \\sum D(\\text{broken}) = 436 \\times 1 + 243 \\times 1 = 679 \\text{ kJ\/mol} \\]<\/p>\n<p>\\[ \\sum D(\\text{formed}) = 431 \\times 2 = 862 \\text{ kJ\/mol} \\]<\/p>\n<p>\\[ \\Delta H \\approx 679 - 862 = -183 \\text{ kJ\/mol} \\]<\/p>\n<p><strong>Result:<\/strong> \u0394H \u2248 \u2212183 kJ\/mol (about \u221243.7 kcal\/mol). The reaction is exothermic: forming the two H\u2013Cl bonds releases more energy than was needed to break the H\u2013H and Cl\u2013Cl bonds, so about 183 kJ of heat is released per mole of reaction.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to Use the Calculator<\/h2>\n<ol>\n<li>Choose the <strong>Energy Unit<\/strong> \u2014 kJ\/mol or kcal\/mol. This applies to every value you enter and to all results.<\/li>\n<li>In <strong>Bonds Broken \u2014 Energy \u00d7 Count<\/strong>, list each type of bond that is broken in the reactants. Write the average bond energy, then how many such bonds are broken, e.g. <code>436 \u00d7 1; 243 \u00d7 1<\/code>. Separate entries with a semicolon or a new line. If you enter just a number, it counts as one bond of that energy.<\/li>\n<li>In <strong>Bonds Formed \u2014 Energy \u00d7 Count<\/strong>, do the same for the new bonds in the products, e.g. <code>431 \u00d7 2<\/code> for two H\u2013Cl bonds.<\/li>\n<li>Read the results: <strong>Energy Absorbed (Bonds Broken)<\/strong>, <strong>Energy Released (Bonds Formed)<\/strong>, the <strong>Reaction Enthalpy<\/strong> \u0394H in your unit (with the equivalent in the other unit), and a verdict telling you whether the reaction is exothermic or endothermic.<\/li>\n<li>Check the <strong>Calculation Details<\/strong> for the entry-by-entry arithmetic, and the <strong>Common Bond Energies<\/strong> table if you need typical values to plug in.<\/li>\n<\/ol>\n<h2>What Is Reaction Enthalpy from Bond Energies?<\/h2>\n<p>The enthalpy change of a reaction, \u0394H, tells you how much heat is exchanged with the surroundings when the reaction proceeds at constant pressure. A negative \u0394H means the reaction is exothermic \u2014 it releases heat \u2014 while a positive \u0394H means it is endothermic and must absorb heat to keep going. \u0394H is one of the key quantities in thermochemistry, alongside entropy and Gibbs free energy.<\/p>\n<p>Instead of measuring \u0394H in a calorimeter, you can estimate it from bond energies. The idea rests on a simple bookkeeping picture: imagine the reaction happening in two steps. First, all the bonds in the reactant molecules are broken, atomising them completely; this step absorbs energy equal to the sum of the bond energies of the broken bonds. Second, the atoms recombine into the product molecules, forming new bonds; this step releases energy equal to the sum of the bond energies of the formed bonds. The net difference is the reaction enthalpy.<\/p>\n<p>Because the same atoms end up on both sides of a balanced equation, every bond that exists before the reaction must either survive unchanged or be replaced by a new one. Bonds that appear in both reactants and products (spectator bonds) cancel out, so you only need to count bonds that actually change. This makes the method quick: draw the structures, identify which bonds break and which form, look up their average energies, and apply the formula.<\/p>\n<p>The sign of the result is its most useful feature. If \u0394H is clearly negative, the products have stronger bonds overall than the reactants, and the excess energy leaves as heat. If \u0394H is positive, the reactants hold together more strongly, so energy must be supplied from outside. Note, however, that \u0394H alone does not tell you whether a reaction will actually occur spontaneously \u2014 that depends on Gibbs free energy, and the equilibrium position depends on the temperature (see the <a href=\"https:\/\/wpcalc.com\/en\/chemistry\/equilibrium-constant-calculator\/\">equilibrium constant calculator<\/a> and the <a href=\"https:\/\/wpcalc.com\/en\/chemistry\/van-t-hoff-equation-calculator\/\">Van 't Hoff equation calculator<\/a> for those aspects).<\/p>\n<h2>Assumptions, Limitations and Practical Tips<\/h2>\n<p><strong>Average bond energies are approximations.<\/strong> The tabulated values are averages over many different molecules. A real C\u2013H bond in methane, ethene and benzene does not have exactly the same strength, so any \u0394H built from average values carries an uncertainty of roughly 10\u201320 kJ\/mol, sometimes more. For the H\u2082 + Cl\u2082 example above the estimate (\u2212183 kJ\/mol) happens to land close to the experimental value (\u2212184.6 kJ\/mol), but do not expect that agreement every time.<\/p>\n<p><strong>Everything is treated as a gas.<\/strong> Bond energies refer to gas-phase molecules. If any reactant or product is a liquid or solid, the enthalpies of vaporisation or sublimation are ignored, which can shift the result noticeably. For reactions in solution, additional interaction energies are also left out.<\/p>\n<p><strong>Count bonds, not molecules.<\/strong> Draw Lewis structures for every species before you start. A common mistake is counting a double bond as two single bonds \u2014 a C=C double bond (614 kJ\/mol) is <em>not<\/em> twice a C\u2013C single bond (347 kJ\/mol). Also make sure the equation is balanced first, so the bond counts on each side are correct.<\/p>\n<p><strong>When accuracy matters, use standard enthalpies of formation.<\/strong> Bond-energy estimates are excellent for quick reasoning, classroom problems and comparing rough magnitudes, but tabulated \u0394H\u00b0f values combined via Hess's law give far more accurate results. If you then want to verify your prediction experimentally, the <a href=\"https:\/\/wpcalc.com\/en\/chemistry\/calorimetry-calculator\/\">calorimetry calculator<\/a> shows how \u0394H is measured from temperature changes.<\/p>\n<p><strong>Unit conversion:<\/strong> bond energy tables are usually given in kJ\/mol; divide by 4.184 to get kcal\/mol. The calculator displays the result in both units automatically, so you can cross-check.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Estimate the enthalpy change (\u0394H) of a chemical reaction from average bond energies. Enter the bonds broken and bonds formed as \"energy \u00d7 count\", and see whether the reaction is exothermic or endothermic.<\/p>\n","protected":false},"author":3168,"featured_media":0,"comment_status":"open","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[54],"tags":[61],"class_list":["post-66535","post","type-post","status-publish","format-standard","hentry","category-chemistry","tag-bonding"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66535","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/users\/3168"}],"replies":[{"embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/comments?post=66535"}],"version-history":[{"count":1,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66535\/revisions"}],"predecessor-version":[{"id":66536,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66535\/revisions\/66536"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=66535"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=66535"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=66535"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}