{"id":4983,"date":"2026-06-04T10:49:13","date_gmt":"2026-06-04T09:49:13","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/cp-8-enthalpy-change-via-hess-law\/"},"modified":"2026-06-10T11:06:15","modified_gmt":"2026-06-10T10:06:15","slug":"cp-8-enthalpy-change-via-hess-law","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-8-enthalpy-change-via-hess-law\/","title":{"rendered":"CP 8 \u2013 Enthalpy Change via Hess\u2019s Law"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-cp8-hess-law-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-green: #f0f7f1;\n      --soft-purple: #f7f0ff;\n      --soft-orange: #fff7ed;\n      --soft-red: #fff7f7;\n      --green: #168a2d;\n      --teal: #0f8a94;\n      --orange: #ea7600;\n      --purple: 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Practicals<\/h4>\r\n    <ul class=\"ols-topic-list\">\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-1-molar-volume-of-a-gas\/\">CP1: Molar Volume of a Gas<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-2-prepare-a-standard-solution\/\">CP2: Prepare a Standard Solution<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-3-concentration-of-hcl-by-titration\/\">CP3: Concentration of HCl by Titration<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-4-hydrolysis-of-halogenoalkanes\/\">CP4: Hydrolysis of Halogenoalkanes<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-5-oxidation-of-ethanol\/\">CP5: Oxidation of Ethanol<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-6-chlorination-of-2-methylpropan-2-ol\/\">CP6: Chlorination of 2-methylpropan-2-ol<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-7-analysis-of-inorganic-and-organic-unknowns\/\">CP7: Analysis of Inorganic and Organic Unknowns<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-8-enthalpy-change-via-hess-law\/\">CP8: Enthalpy Change via Hess\u2019s Law<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-9-electrochemical-cells\/\">CP9: Electrochemical Cells<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-10-titration-of-edta\/\">CP10: Titration of EDTA<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-11-rates-of-reaction-clock\/\">CP11: Rates of Reaction (Clock)<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-12-continuous-monitoring-rates\/\">CP12: Continuous Monitoring Rates<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-13-preparation-of-aspirin\/\">CP13: Preparation of Aspirin<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-14-thin-layer-chromatography\/\">CP14: Thin Layer Chromatography<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-15-preparing-organic-compound\/\">CP15: Preparing Organic Compound<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-16-testing-organic-unknowns\/\">CP16: Testing Organic Unknowns<\/a><\/li>\r\n    <\/ul>\r\n  <\/div>\r\n\r\n  <div class=\"ols-topic-group\">\r\n    <h4>Useful Links<\/h4>\r\n    <ul class=\"ols-topic-list\">\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/\">Edexcel A Level Chemistry<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/\">Core Practicals<\/a><\/li>\r\n      <li><a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/topic-5-formulae-equations-and-amounts-of-substance\/\">Formulae, Equations and Amounts of Substance<\/a><\/li>\r\n    <\/ul>\r\n  <\/div>\r\n<\/aside>\r\n\r\n<script>\r\n(function () {\r\n  var sidebar = document.querySelector('.ols-sidebar');\r\n  if (!sidebar) return;\r\n\r\n  var currentPath = window.location.pathname.replace(\/\\\/$\/, '');\r\n\r\n  sidebar.querySelectorAll('.ols-topic-list li').forEach(function (li) {\r\n    li.classList.remove('active');\r\n\r\n    var link = li.querySelector('a');\r\n    if (!link) return;\r\n\r\n    var linkPath = new URL(link.href).pathname.replace(\/\\\/$\/, '');\r\n\r\n    if (linkPath === currentPath) {\r\n      li.classList.add('active');\r\n    }\r\n  });\r\n})();\r\n<\/script>\n\n    <main class=\"ols-main\">\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\/\">Edexcel<\/a> \/\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/\">Core Practicals<\/a> \/\n        <span>CP8: Enthalpy Change via Hess\u2019s Law<\/span>\n      <\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>CP8: Enthalpy Change via Hess\u2019s Law<\/h1>\n        <p class=\"ols-page-intro\">A guided Edexcel A Level Chemistry revision walkthrough for measuring temperature change, calculating enthalpy change from calorimetry data, and using Hess\u2019s Law to find the decomposition enthalpy of potassium hydrogencarbonate.<\/p>\n\n        <div class=\"ols-badges\">\n          <div class=\"ols-badge\">Exam board: Edexcel<\/div>\n          <div class=\"ols-badge\">Paper 3: General and Practical Principles in Chemistry<\/div>\n        <\/div>\n\n        <div class=\"ols-author\">\n          <img decoding=\"async\" class=\"ols-author-avatar-img\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\" alt=\"Dr. Mohammed Al-Fatah\">\n\n          <div class=\"ols-author-content\">\n            <h2 class=\"ols-author-title\">Written by: Dr. Mohammed Al-Fatah<\/h2>\n            <p class=\"ols-author-description\">Chemistry specialist revision notes for A Level Chemistry.<\/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\"><\/path>\n              <\/svg>\n              View LinkedIn Profile\n            <\/a>\n          <\/div>\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>What Is This Practical Trying to Find?<\/h2>\n        <\/div>\n\n        <p>The aim of this core practical is to calculate the enthalpy change for the thermal decomposition of <strong>potassium hydrogencarbonate<\/strong>.<\/p>\n\n        <div class=\"ols-equation-box center\">\n          2KHCO<sub>3<\/sub>(s) \u2192 K<sub>2<\/sub>CO<sub>3<\/sub>(s) + CO<sub>2<\/sub>(g) + H<sub>2<\/sub>O(l)\n        <\/div>\n\n        <p>This target reaction is difficult to measure directly by simple calorimetry because the solid must be heated. If heat is supplied from outside, the measured temperature change is no longer only caused by the chemical reaction.<\/p>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Key idea:<\/strong> Instead of measuring the decomposition directly, two related reactions are measured experimentally. Hess&#8217;s Law is then used to calculate the enthalpy change for the decomposition reaction indirectly.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">2<\/div>\n          <h2>The Two Reactions Measured in CP8<\/h2>\n        <\/div>\n\n        <p>Both measured reactions use hydrochloric acid in a polystyrene cup. The temperature change is measured after adding the solid carbonate or hydrogencarbonate.<\/p>\n\n        <div class=\"ols-pair-grid\">\n          <div class=\"ols-mini-panel\">\n            <h3>Reaction 1: potassium carbonate<\/h3>\n            <p><strong>K<sub>2<\/sub>CO<sub>3<\/sub>(s) + 2HCl(aq) \u2192 2KCl(aq) + CO<sub>2<\/sub>(g) + H<sub>2<\/sub>O(l)<\/strong><\/p>\n            <p>The temperature rises, so this reaction is <strong>exothermic<\/strong>. Its enthalpy change is negative.<\/p>\n          <\/div>\n\n          <div class=\"ols-mini-panel\">\n            <h3>Reaction 2: potassium hydrogencarbonate<\/h3>\n            <p><strong>KHCO<sub>3<\/sub>(s) + HCl(aq) \u2192 KCl(aq) + CO<sub>2<\/sub>(g) + H<sub>2<\/sub>O(l)<\/strong><\/p>\n            <p>The temperature falls, so this reaction is <strong>endothermic<\/strong>. Its enthalpy change is positive.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-warning-box\">\n          <p><strong>Sign trap:<\/strong> A temperature rise gives a negative \u0394H. A temperature fall gives a positive \u0394H. Students often calculate the size of q correctly but lose marks by giving the wrong sign.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card green\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">3<\/div>\n          <h2>Method: What Each Step Is Really For<\/h2>\n        <\/div>\n\n        <p>The method is designed to measure a temperature change accurately while also finding the actual mass of solid transferred into the acid.<\/p>\n\n        <div class=\"ols-method-grid\">\n          <div class=\"ols-method-step\">\n            <div class=\"ols-step-number\">1<\/div>\n            <p>Place about <strong>3 g of K<sub>2<\/sub>CO<sub>3<\/sub><\/strong> in a test tube and weigh the test tube with its contents. This gives the starting mass before transfer.<\/p>\n          <\/div>\n\n          <div class=\"ols-method-step\">\n            <div class=\"ols-step-number\">2<\/div>\n            <p>Use a burette to place <strong>30 cm<sup>3<\/sup> of 2 mol dm<sup>-3<\/sup> HCl<\/strong> into a polystyrene cup supported in a beaker.<\/p>\n          <\/div>\n\n          <div class=\"ols-method-step\">\n            <div class=\"ols-step-number\">3<\/div>\n            <p>Record the starting temperature of the acid. This is the baseline used to calculate \u0394T.<\/p>\n          <\/div>\n\n          <div class=\"ols-method-step\">\n            <div class=\"ols-step-number\">4<\/div>\n            <p>Add the potassium carbonate gradually, stir continuously, and record the highest temperature reached.<\/p>\n          <\/div>\n\n          <div class=\"ols-method-step\">\n            <div class=\"ols-step-number\">5<\/div>\n            <p>Reweigh the empty test tube. The difference between the two masses gives the actual mass of K<sub>2<\/sub>CO<sub>3<\/sub> added.<\/p>\n          <\/div>\n\n          <div class=\"ols-method-step\">\n            <div class=\"ols-step-number\">6<\/div>\n            <p>Repeat the method using about <strong>3.5 g of KHCO<sub>3<\/sub><\/strong>. This time record the lowest temperature reached because the reaction is endothermic.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Why use a polystyrene cup?<\/strong> Polystyrene is a better insulator than glass, so less heat is lost to or gained from the surroundings. This gives a more reliable temperature change.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">4<\/div>\n          <h2>Measuring Temperature Change Accurately<\/h2>\n        <\/div>\n\n        <p>In slow calorimetry experiments, the maximum or minimum temperature may not be measured exactly because heat transfer to the surroundings occurs while the reaction is still taking place.<\/p>\n\n        <p>The accurate method is to take temperature readings before mixing, continue taking readings after mixing, then extrapolate the cooling or warming line back to the mixing time.<\/p>\n\n        <div class=\"ols-figure-card\">\n          <a class=\"ols-figure-link ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/measuring-temperature-change-accurately.webp\" aria-label=\"Open Measuring Temperature Change Accurately infographic\">\n            <div class=\"ols-figure-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/measuring-temperature-change-accurately.webp\" alt=\"Graph showing how temperature readings are extrapolated back to the mixing time to measure temperature change accurately.\">\n            <\/div>\n          <\/a>\n          <div class=\"ols-figure-caption\">\n            <p>This visual shows why temperature readings are extrapolated back to the mixing point. 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       .ols-temp-graph-card .ols-temp-graph-result {\n              padding-left: 18px;\n              padding-right: 18px;\n            }\n\n            .ols-temp-graph-card .ols-temp-graph-workspace {\n              padding-left: 12px;\n              padding-right: 12px;\n              padding-bottom: 16px;\n            }\n\n            .ols-temp-graph-card .ols-temp-graph-key {\n              grid-template-columns: 1fr;\n            }\n          }\n        <\/style>\n\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">5<\/div>\n          <h2>Interactive Graph: Finding the Correct Temperature Change<\/h2>\n        <\/div>\n\n        <p>This interactive graph shows how calorimetry data can be plotted and extrapolated back to the mixing time. It demonstrates why the highest measured temperature is not always the true temperature at the moment of mixing.<\/p>\n\n        <p><strong>Important:<\/strong> this is a <strong>model dataset<\/strong> used to demonstrate the extrapolation method. It is included to support the practical technique and is not presented as the actual CP8 sample data table used later on this page.<\/p>\n\n        <div class=\"ols-temp-graph-wrap\">\n          <div class=\"ols-temp-graph-top\">\n            <p class=\"ols-temp-graph-note\"><strong>How to use it:<\/strong> follow the numbered buttons on the right. Plot the readings, draw the cooling line, extrapolate back to the mixing time, and then reveal the corrected \u0394T.<\/p>\n          <\/div>\n\n          <div class=\"ols-temp-graph-workspace\">\n            <div class=\"ols-temp-graph-main\">\n              <div class=\"ols-temp-graph-figure\">\n                <svg id=\"olsTempGraphSvg\" class=\"ols-temp-graph-svg\" viewBox=\"0 0 760 560\" aria-label=\"Interactive temperature against time graph showing extrapolation back to the mixing time\">\n                  <defs>\n                    <marker id=\"olsArrowBlue\" markerWidth=\"10\" markerHeight=\"10\" refX=\"5\" refY=\"5\" orient=\"auto-start-reverse\">\n                      <path d=\"M0,0 L10,5 L0,10 z\" fill=\"#2563eb\"><\/path>\n                    <\/marker>\n                    <marker id=\"olsArrowBlack\" markerWidth=\"10\" markerHeight=\"10\" refX=\"5\" refY=\"5\" orient=\"auto-start-reverse\">\n                      <path d=\"M0,0 L10,5 L0,10 z\" fill=\"#111827\"><\/path>\n                    <\/marker>\n                  <\/defs>\n                  <rect x=\"0\" y=\"0\" width=\"760\" height=\"560\" fill=\"#ffffff\"><\/rect>\n                  <text x=\"380\" y=\"36\" text-anchor=\"middle\" font-size=\"25\" font-weight=\"700\" fill=\"#1C244B\">Temperature against time<\/text>\n                  <line x1=\"88\" y1=\"468\" x2=\"675\" y2=\"468\" stroke=\"#111827\" stroke-width=\"3\" marker-end=\"url(#olsArrowBlack)\"><\/line>\n                  <line x1=\"88\" y1=\"468\" x2=\"88\" y2=\"88\" stroke=\"#111827\" stroke-width=\"3\" marker-end=\"url(#olsArrowBlack)\"><\/line>\n                  <text x=\"381\" y=\"525\" text-anchor=\"middle\" font-size=\"18\" font-weight=\"600\" fill=\"#111827\">Time (min)<\/text>\n                  <text x=\"34\" y=\"286\" transform=\"rotate(-90 34 286)\" text-anchor=\"middle\" font-size=\"18\" font-weight=\"600\" fill=\"#111827\">Temperature (\u00b0C)<\/text>\n                  <g id=\"olsTempGraphStatic\"><\/g>\n                  <g id=\"olsTempGraphDynamic\"><\/g>\n                <\/svg>\n              <\/div>\n            <\/div>\n\n            <aside class=\"ols-temp-graph-side\" aria-label=\"Interactive graph controls\">\n              <h3>Follow the steps<\/h3>\n              <p>Press each numbered button from top to bottom to build the graph and reveal the corrected temperature change.<\/p>\n\n              <div class=\"ols-temp-graph-rail\">\n                <div class=\"ols-temp-graph-arrow\">\u2193<\/div>\n\n                <div class=\"ols-temp-graph-step\">\n                  <div class=\"ols-temp-step-number\">1<\/div>\n                  <button type=\"button\" class=\"ols-temp-graph-btn primary\" id=\"olsPlotInitialBtn\">Plot initial readings<\/button>\n                <\/div>\n\n                <div class=\"ols-temp-graph-step\">\n                  <div class=\"ols-temp-step-number\">2<\/div>\n                  <button type=\"button\" class=\"ols-temp-graph-btn\" id=\"olsPlotPostBtn\" disabled=\"\">Add post-mixing readings<\/button>\n                <\/div>\n\n                <div class=\"ols-temp-graph-step\">\n                  <div class=\"ols-temp-step-number\">3<\/div>\n                  <button type=\"button\" class=\"ols-temp-graph-btn\" id=\"olsCoolingLineBtn\" disabled=\"\">Draw cooling line<\/button>\n                <\/div>\n\n                <div class=\"ols-temp-graph-step\">\n                  <div class=\"ols-temp-step-number\">4<\/div>\n                  <button type=\"button\" class=\"ols-temp-graph-btn\" id=\"olsExtrapolateBtn\" disabled=\"\">Extrapolate to mixing time<\/button>\n                <\/div>\n\n                <div class=\"ols-temp-graph-step\">\n                  <div class=\"ols-temp-step-number\">5<\/div>\n                  <button type=\"button\" class=\"ols-temp-graph-btn\" id=\"olsDeltaTBtn\" disabled=\"\">Calculate \u0394T<\/button>\n                <\/div>\n              <\/div>\n\n              <div class=\"ols-temp-graph-reset\">\n                <button type=\"button\" class=\"ols-temp-graph-btn\" id=\"olsResetGraphBtn\">Reset interactive graph<\/button>\n              <\/div>\n            <\/aside>\n          <\/div>\n\n          <div id=\"olsTempGraphStage\" class=\"ols-temp-graph-stage\"><strong>Stage:<\/strong> Start by plotting the initial temperature readings before the reactants are mixed.<\/div>\n\n          <div class=\"ols-temp-graph-key\">\n            <div class=\"ols-temp-graph-key-item\">\n              <div class=\"ols-temp-graph-chip\"><span class=\"ols-temp-graph-dot green\"><\/span>Pre-mixing readings<\/div>\n              <p>These readings establish the starting temperature before the reaction begins.<\/p>\n            <\/div>\n            <div class=\"ols-temp-graph-key-item\">\n              <div class=\"ols-temp-graph-chip\"><span class=\"ols-temp-graph-dot blue\"><\/span>Post-mixing readings<\/div>\n              <p>These are the measured temperatures after the reactants are mixed and heat loss has already started.<\/p>\n            <\/div>\n            <div class=\"ols-temp-graph-key-item\">\n              <div class=\"ols-temp-graph-chip\"><span class=\"ols-temp-graph-dot black\"><\/span>Mixing time<\/div>\n              <p>The vertical guide at <strong>t = 3 min<\/strong> marks when the reactants are mixed.<\/p>\n            <\/div>\n            <div class=\"ols-temp-graph-key-item\">\n              <div class=\"ols-temp-graph-chip\"><span class=\"ols-temp-graph-dot delta\"><\/span>Corrected \u0394T<\/div>\n              <p>The corrected temperature change is found by extrapolating the line back to the mixing time rather than just taking the highest measured point.<\/p>\n            <\/div>\n          <\/div>\n\n          <div class=\"ols-temp-graph-result\">\n            <p><strong>What this shows:<\/strong> the extrapolated temperature at the mixing time is higher than the measured peak, so the corrected \u0394T is larger and gives a more accurate energy change.<\/p>\n          <\/div>\n        <\/div>\n\n        <script>\n          (function(){\n            var svg = document.getElementById('olsTempGraphSvg');\n            if (!svg) { return; }\n\n            var staticLayer = document.getElementById('olsTempGraphStatic');\n            var dynamicLayer = document.getElementById('olsTempGraphDynamic');\n            var stageText = document.getElementById('olsTempGraphStage');\n\n            var btnInitial = document.getElementById('olsPlotInitialBtn');\n            var btnPost = document.getElementById('olsPlotPostBtn');\n            var btnCooling = document.getElementById('olsCoolingLineBtn');\n            var btnExtrapolate = document.getElementById('olsExtrapolateBtn');\n            var btnDelta = document.getElementById('olsDeltaTBtn');\n            var btnReset = document.getElementById('olsResetGraphBtn');\n            var stepButtons = [btnInitial, btnPost, 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    staticLayer.appendChild(createEl('line', {x1:x0-8, y1:y, x2:x0, y2:y, stroke:'#111827', 'stroke-width':'2'}));\n                staticLayer.appendChild(createEl('text', {x:x0-14, y:y+6, 'text-anchor':'end', 'font-size':'16', 'font-weight':'600', fill:'#111827'})).textContent = temp.toString();\n              });\n\n              var mixX = xScale(mixingTime);\n              staticLayer.appendChild(createEl('line', {x1:mixX, y1:y0, x2:mixX, y2:yScale(38), stroke:'#111827', 'stroke-width':'2', 'stroke-dasharray':'8 8'}));\n              staticLayer.appendChild(createEl('text', {x:mixX, y:y0+48, 'text-anchor':'middle', 'font-size':'14', 'font-weight':'700', fill:'#1d4ed8'})).textContent = 'Mixing point';\n              staticLayer.appendChild(createEl('text', {x:mixX, y:y0+66, 'text-anchor':'middle', 'font-size':'14', 'font-weight':'700', fill:'#1d4ed8'})).textContent = '(t = 3 min)';\n            }\n\n            function clearDynamic() {\n              dynamicLayer.innerHTML = '';\n            }\n\n            function drawCircle(x, y, fill, r) {\n              dynamicLayer.appendChild(createEl('circle', {cx:x, cy:y, r:r || 6, fill:fill, stroke:'#ffffff', 'stroke-width':'2'}));\n            }\n\n            function drawLine(x1, y1, x2, y2, stroke, width, dash) {\n              var attrs = {x1:x1, y1:y1, x2:x2, y2:y2, stroke:stroke, 'stroke-width':width || 3, 'stroke-linecap':'round'};\n              if (dash) { attrs['stroke-dasharray'] = dash; }\n              dynamicLayer.appendChild(createEl('line', attrs));\n            }\n\n            function drawText(x, y, text, fill, size, weight, anchor) {\n              var node = createEl('text', {x:x, y:y, fill:fill || '#111827', 'font-size':size || 16, 'font-weight':weight || 600, 'text-anchor':anchor || 'start'});\n              node.textContent = text;\n              dynamicLayer.appendChild(node);\n            }\n\n            function drawPreData() {\n              preData.forEach(function(point){\n              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'2.5');\n              dynamicLayer.lastChild.setAttribute('fill', '#ffffff');\n            }\n\n            function drawDelta() {\n              var x = xScale(2.15);\n              drawLine(x, yScale(correctedTemp), x, yScale(baselineTemp) - 11, '#2563eb', 3);\n              dynamicLayer.lastChild.setAttribute('marker-start', 'url(#olsArrowBlue)');\n              dynamicLayer.lastChild.setAttribute('marker-end', 'url(#olsArrowBlue)');\n              drawText(x-18, (yScale(correctedTemp)+yScale(baselineTemp))\/2 + 2, '\u0394T', '#2563eb', 32, 700, 'end');\n              drawText(122, 120, 'Corrected temperature = 38.0 \u00b0C', '#111827', 14, 700, 'start');\n              drawText(306, 438, 'Initial temperature = 19.0 \u00b0C', '#111827', 14, 700, 'start');\n            }\n\n            function updateButtons(step) {\n              btnInitial.disabled = step > 0;\n              btnPost.disabled = step < 1;\n              btnCooling.disabled = step < 2;\n              btnExtrapolate.disabled = step < 3;\n              btnDelta.disabled = step < 4;\n\n              stepButtons.forEach(function(button, index){\n                button.classList.remove('primary');\n                button.classList.remove('completed');\n                if (index < step) {\n                  button.classList.add('completed');\n                }\n              });\n\n              if (step < stepButtons.length) {\n                stepButtons[step].classList.add('primary');\n              }\n            }\n\n            function render(step) {\n              clearDynamic();\n              if (step >= 1) {\n                drawPreData();\n                stageText.innerHTML = '<strong>Stage:<\/strong> Initial readings plotted. These give the baseline temperature before mixing.';\n              }\n              if (step >= 2) {\n                drawPostData();\n                stageText.innerHTML = '<strong>Stage:<\/strong> Post-mixing readings added. Notice that the temperature has already started to fall after the reaction.';\n              }\n              if (step >= 3) {\n                drawCoolingMeasuredOnly();\n                stageText.innerHTML = '<strong>Stage:<\/strong> A cooling line has been drawn through the post-mixing readings.';\n              }\n              if (step >= 4) {\n                drawCoolingExtrapolated();\n                stageText.innerHTML = '<strong>Stage:<\/strong> The cooling line has been extrapolated back to the mixing time at <strong>t = 3 min<\/strong>.';\n              }\n              if (step >= 5) {\n                drawDelta();\n                stageText.innerHTML = '<strong>Stage:<\/strong> Corrected temperature change revealed. <strong>\u0394T = 38.0 - 19.0 = 19.0 \u00b0C<\/strong>.';\n              }\n              updateButtons(step);\n            }\n\n            btnInitial.addEventListener('click', function(){ render(1); });\n            btnPost.addEventListener('click', function(){ render(2); });\n            btnCooling.addEventListener('click', function(){ render(3); });\n            btnExtrapolate.addEventListener('click', function(){ render(4); });\n            btnDelta.addEventListener('click', function(){ render(5); });\n            btnReset.addEventListener('click', function(){\n              clearDynamic();\n              stageText.innerHTML = '<strong>Stage:<\/strong> Start by plotting the initial temperature readings before the reactants are mixed.';\n              updateButtons(0);\n            });\n\n            drawStatic();\n            updateButtons(0);\n          })();\n        <\/script>\n      <\/article>\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">6<\/div>\n          <h2>Using the Sample Data<\/h2>\n        <\/div>\n\n        <p>The sample data below shows how the mass of each solid and the temperature change are obtained from the practical measurements.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Measurement<\/th>\n                <th>K<sub>2<\/sub>CO<sub>3<\/sub> reaction<\/th>\n                <th>KHCO<sub>3<\/sub> reaction<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td>Mass of test tube with solid \/ g<\/td>\n                <td>25.12<\/td>\n                <td>25.67<\/td>\n              <\/tr>\n              <tr>\n                <td>Mass of test tube after emptying \/ g<\/td>\n                <td>23.04<\/td>\n                <td>22.34<\/td>\n              <\/tr>\n              <tr>\n                <td>Mass of solid used \/ g<\/td>\n                <td>2.08<\/td>\n                <td>3.33<\/td>\n              <\/tr>\n              <tr>\n                <td>Start temperature \/ \u00b0C<\/td>\n                <td>23.2<\/td>\n                <td>23.1<\/td>\n              <\/tr>\n              <tr>\n                <td>Final extreme temperature \/ \u00b0C<\/td>\n                <td>28.4 highest<\/td>\n                <td>19.2 lowest<\/td>\n              <\/tr>\n              <tr>\n                <td>Temperature change \/ \u00b0C<\/td>\n                <td>+5.2<\/td>\n                <td>-3.9<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\n        <div class=\"ols-exam-box\">\n          <p><strong>Exam focus:<\/strong> The sign of the temperature change tells you whether the process is exothermic or endothermic, but q = mc\u0394T is usually calculated using the magnitude of \u0394T.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">7<\/div>\n          <h2>Core Equation: q = mc\u0394T<\/h2>\n        <\/div>\n\n        <p>For reactions in aqueous solution, the energy change for the quantities used is calculated using:<\/p>\n\n        <div class=\"ols-equation-box center\">\n          q = m \u00d7 c<sub>p<\/sub> \u00d7 \u0394T\n        <\/div>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Symbol<\/th>\n                <th>Meaning<\/th>\n                <th>In this practical<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td>q<\/td>\n                <td>energy change in joules<\/td>\n                <td>Calculated from the temperature change<\/td>\n              <\/tr>\n              <tr>\n                <td>m<\/td>\n                <td>mass of solution in grams<\/td>\n                <td>30 cm<sup>3<\/sup> solution is treated as 30 g<\/td>\n              <\/tr>\n              <tr>\n                <td>c<sub>p<\/sub><\/td>\n                <td>specific heat capacity<\/td>\n                <td>4.2 J g<sup>-1<\/sup> \u00b0C<sup>-1<\/sup><\/td>\n              <\/tr>\n              <tr>\n                <td>\u0394T<\/td>\n                <td>temperature change<\/td>\n                <td>Highest or lowest temperature minus starting temperature<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <a class=\"ols-figure-link ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/measuring-enthalpy-change-by-calorimetry.webp\" aria-label=\"Open Measuring Enthalpy Change by Calorimetry infographic\">\n            <div class=\"ols-figure-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/measuring-enthalpy-change-by-calorimetry.webp\" alt=\"Infographic explaining q equals m c p delta T, calorimetric method, enthalpy calculation steps and common calorimetry errors.\">\n            <\/div>\n          <\/a>\n          <div class=\"ols-figure-caption\">\n            <p>This visual summarises the core calorimetry equation, the insulated cup method, the steps for calculating \u0394H and the main experimental limitations.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">8<\/div>\n          <h2>Worked Calculation: Energy Change for Each Reaction<\/h2>\n        <\/div>\n\n        <p>Use the mass of solution, specific heat capacity and temperature change to calculate the energy transferred in each experiment.<\/p>\n\n        <div class=\"ols-pair-grid\">\n          <div class=\"ols-mini-panel\">\n            <h3>Reaction 1: K<sub>2<\/sub>CO<sub>3<\/sub><\/h3>\n            <div class=\"ols-equation-box\">\n              q = 30 \u00d7 4.2 \u00d7 5.2<br>\n              q = 655.2 J<br>\n              q = 0.6552 kJ\n            <\/div>\n            <p>The temperature rises, so the reaction releases heat. The enthalpy change will be negative.<\/p>\n          <\/div>\n\n          <div class=\"ols-mini-panel\">\n            <h3>Reaction 2: KHCO<sub>3<\/sub><\/h3>\n            <div class=\"ols-equation-box\">\n              q = 30 \u00d7 4.2 \u00d7 3.9<br>\n              q = 491.4 J<br>\n              q = 0.4914 kJ\n            <\/div>\n            <p>The temperature falls, so the reaction absorbs heat. The enthalpy change will be positive.<\/p>\n          <\/div>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card green\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">9<\/div>\n          <h2>Worked Calculation: Molar Enthalpy Change<\/h2>\n        <\/div>\n\n        <p>To calculate \u0394H in kJ mol<sup>-1<\/sup>, divide the energy change by the moles of the solid reactant used.<\/p>\n\n        <div class=\"ols-pair-grid\">\n          <div class=\"ols-mini-panel\">\n            <h3>Reaction 1: K<sub>2<\/sub>CO<sub>3<\/sub><\/h3>\n            <div class=\"ols-equation-box\">\n              M<sub>r<\/sub>(K<sub>2<\/sub>CO<sub>3<\/sub>) = 138.2<br>\n              moles = 2.08 \u00f7 138.2<br>\n              moles = 0.0151 mol<br><br>\n              \u0394H<sub>1<\/sub> = -0.6552 \u00f7 0.0151<br>\n              \u0394H<sub>1<\/sub> = -43.5 kJ mol<sup>-1<\/sup>\n            <\/div>\n          <\/div>\n\n          <div class=\"ols-mini-panel\">\n            <h3>Reaction 2: KHCO<sub>3<\/sub><\/h3>\n            <div class=\"ols-equation-box\">\n              M<sub>r<\/sub>(KHCO<sub>3<\/sub>) = 100.1<br>\n              moles = 3.33 \u00f7 100.1<br>\n              moles = 0.0333 mol<br><br>\n              \u0394H<sub>2<\/sub> = +0.4914 \u00f7 0.0333<br>\n              \u0394H<sub>2<\/sub> = +14.8 kJ mol<sup>-1<\/sup>\n            <\/div>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Important:<\/strong> \u0394H<sub>1<\/sub> is for one mole of K<sub>2<\/sub>CO<sub>3<\/sub> reacting. \u0394H<sub>2<\/sub> is for one mole of KHCO<sub>3<\/sub> reacting. Hess&#8217;s Law must account for this mole ratio.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card soft\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">10<\/div>\n          <h2>Checking That Hydrochloric Acid Is in Excess<\/h2>\n        <\/div>\n\n        <p>The acid must be in excess so that the carbonate or hydrogencarbonate is the limiting reagent. This means the calculated enthalpy change is based on the moles of solid used.<\/p>\n\n        <div class=\"ols-equation-box\">\n          moles HCl = concentration \u00d7 volume<br>\n          moles HCl = 2.00 \u00d7 30 \u00f7 1000 = 0.0600 mol\n        <\/div>\n\n        <div class=\"ols-pair-grid\">\n          <div class=\"ols-mini-panel\">\n            <h3>For K<sub>2<\/sub>CO<sub>3<\/sub><\/h3>\n            <div class=\"ols-equation-box\">\n              moles K<sub>2<\/sub>CO<sub>3<\/sub> = 2.08 \u00f7 138.2 = 0.0151 mol<br>\n              HCl needed = 2 \u00d7 0.0151 = 0.0302 mol<br>\n              HCl available = 0.0600 mol\n            <\/div>\n          <\/div>\n\n          <div class=\"ols-mini-panel\">\n            <h3>For KHCO<sub>3<\/sub><\/h3>\n            <div class=\"ols-equation-box\">\n              moles KHCO<sub>3<\/sub> = 3.33 \u00f7 100.1 = 0.0333 mol<br>\n              HCl needed = 1 \u00d7 0.0333 = 0.0333 mol<br>\n              HCl available = 0.0600 mol\n            <\/div>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Conclusion:<\/strong> 0.0600 mol of HCl is greater than the amount needed in both reactions, so hydrochloric acid is in excess.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card purple\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">11<\/div>\n          <h2>Using Hess&#8217;s Law to Find the Target \u0394H<\/h2>\n        <\/div>\n\n        <p>The target reaction contains <strong>2 moles of KHCO<sub>3<\/sub><\/strong>, so reaction 2 must be doubled before combining the measured enthalpy changes.<\/p>\n\n        <div class=\"ols-hess-cycle\">\n          <div class=\"ols-hess-cycle-grid\">\n            <div class=\"ols-hess-row\">\n              <div class=\"ols-hess-species\">\n                2KHCO<sub>3<\/sub>(s)\n              <\/div>\n              <div class=\"ols-hess-arrow\">\u2192<\/div>\n              <div class=\"ols-hess-species\">\n                K<sub>2<\/sub>CO<sub>3<\/sub>(s) + CO<sub>2<\/sub>(g) + H<sub>2<\/sub>O(l)\n              <\/div>\n            <\/div>\n            <div class=\"ols-hess-label\">Target reaction: \u0394H<sub>3<\/sub><\/div>\n\n            <div class=\"ols-hess-stack\">\n              <div class=\"ols-hess-step\">\n                <strong>Double reaction 2:<\/strong><br>\n                2KHCO<sub>3<\/sub>(s) + 2HCl(aq) \u2192 2KCl(aq) + 2CO<sub>2<\/sub>(g) + 2H<sub>2<\/sub>O(l)<br>\n                Enthalpy change = 2\u0394H<sub>2<\/sub>\n              <\/div>\n\n              <div class=\"ols-hess-step\">\n                <strong>Reverse reaction 1:<\/strong><br>\n                2KCl(aq) + CO<sub>2<\/sub>(g) + H<sub>2<\/sub>O(l) \u2192 K<sub>2<\/sub>CO<sub>3<\/sub>(s) + 2HCl(aq)<br>\n                Enthalpy change = -\u0394H<sub>1<\/sub>\n              <\/div>\n\n              <div class=\"ols-hess-step\">\n                <strong>Add the two routes:<\/strong><br>\n                2KHCO<sub>3<\/sub>(s) \u2192 K<sub>2<\/sub>CO<sub>3<\/sub>(s) + CO<sub>2<\/sub>(g) + H<sub>2<\/sub>O(l)\n              <\/div>\n            <\/div>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-equation-box center\">\n          \u0394H<sub>3<\/sub> = 2\u0394H<sub>2<\/sub> &#8211; \u0394H<sub>1<\/sub>\n        <\/div>\n\n        <div class=\"ols-equation-box\">\n          \u0394H<sub>3<\/sub> = 2(+14.8) &#8211; (-43.5)<br>\n          \u0394H<sub>3<\/sub> = 29.6 + 43.5<br>\n          \u0394H<sub>3<\/sub> = +73.1 kJ mol<sup>-1<\/sup>\n        <\/div>\n\n        <div class=\"ols-key-box\">\n          <p><strong>Final answer:<\/strong> The decomposition of potassium hydrogencarbonate is endothermic, so the enthalpy change is positive.<\/p>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card orange\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">12<\/div>\n          <h2>Errors, Limitations and Improvements<\/h2>\n        <\/div>\n\n        <p>Calorimetry is useful, but it is not perfectly accurate. Exam questions often ask students to explain why the measured value differs from the accepted value.<\/p>\n\n        <div class=\"ols-table-wrap\">\n          <table class=\"ols-table\">\n            <thead>\n              <tr>\n                <th>Issue<\/th>\n                <th>Effect on result<\/th>\n                <th>Improvement<\/th>\n              <\/tr>\n            <\/thead>\n            <tbody>\n              <tr>\n                <td>Heat transfer to or from the surroundings<\/td>\n                <td>The measured temperature change is smaller than the true temperature change.<\/td>\n                <td>Use a lid, insulate the cup better, and extrapolate the temperature curve.<\/td>\n              <\/tr>\n              <tr>\n                <td>Specific heat capacity of solution assumed to equal water<\/td>\n                <td>The calculated q value is only an approximation.<\/td>\n                <td>Use a more accurate heat capacity value for the solution if available.<\/td>\n              <\/tr>\n              <tr>\n                <td>Heat absorbed by the cup and thermometer ignored<\/td>\n                <td>Some energy transfer is not included in q = mc\u0394T.<\/td>\n                <td>Calibrate the calorimeter or use a correction factor.<\/td>\n              <\/tr>\n              <tr>\n                <td>Reaction may be incomplete or slow<\/td>\n                <td>The full temperature change may not be recorded directly.<\/td>\n                <td>Stir continuously and take regular temperature readings over time.<\/td>\n              <\/tr>\n              <tr>\n                <td>Potassium carbonate may be hydrated<\/td>\n                <td>The calculated moles of K<sub>2<\/sub>CO<sub>3<\/sub> are inaccurate, and the temperature rise may be smaller than expected.<\/td>\n                <td>Use dry solid and store the carbonate properly before the practical.<\/td>\n              <\/tr>\n            <\/tbody>\n          <\/table>\n        <\/div>\n      <\/article>\n\n      <article class=\"ols-note-card\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">13<\/div>\n          <h2>Common Exam Points<\/h2>\n        <\/div>\n\n        <ul>\n          <li>State the two measured reactions correctly, including state symbols where required.<\/li>\n          <li>Explain why the decomposition reaction cannot be measured directly using simple calorimetry.<\/li>\n          <li>Use q = mc\u0394T with the correct mass, specific heat capacity and temperature change.<\/li>\n          <li>Convert joules to kilojoules before calculating \u0394H in kJ mol<sup>-1<\/sup>.<\/li>\n          <li>Use the correct sign for exothermic and endothermic reactions.<\/li>\n          <li>Show that hydrochloric acid is in excess by comparing moles available with moles required.<\/li>\n          <li>Double reaction 2 before using Hess&#8217;s Law because the target reaction contains 2 moles of KHCO<sub>3<\/sub>.<\/li>\n          <li>Explain heat loss, calorimeter heat absorption and incomplete reaction as limitations.<\/li>\n        <\/ul>\n      <\/article>\n\n      <article class=\"ols-note-card red\">\n        <div class=\"ols-note-title\">\n          <div class=\"ols-note-icon\">14<\/div>\n          <h2>Extra Hess&#8217;s Law and Calorimetry Examples<\/h2>\n        <\/div>\n\n        <p>After the main CP8 calculation, students should recognise the same calculation pattern in other enthalpy questions. These examples extend the core method using visual worked examples rather than long text blocks.<\/p>\n\n        <div class=\"ols-figure-card\">\n          <a class=\"ols-figure-link ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/calculating-enthalpy-change-from-calorimetry-data.webp\" aria-label=\"Open calculating enthalpy change from calorimetry data infographic\">\n            <div class=\"ols-figure-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/calculating-enthalpy-change-from-calorimetry-data.webp\" alt=\"Worked infographic showing how to calculate enthalpy change from copper sulfate and excess zinc calorimetry data.\">\n            <\/div>\n          <\/a>\n          <div class=\"ols-figure-caption\">\n            <p>Use this example to reinforce the standard three-step method: calculate q, calculate moles of the reactant not in excess, then divide q by moles and apply the correct sign.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <a class=\"ols-figure-link ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Calculating-enthalpy-change-of-neutralisation.webp\" aria-label=\"Open calculating enthalpy change of neutralisation infographic\">\n            <div class=\"ols-figure-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Calculating-enthalpy-change-of-neutralisation.webp\" alt=\"Worked infographic showing enthalpy change of neutralisation using hydrochloric acid and sodium hydroxide data.\">\n            <\/div>\n          <\/a>\n          <div class=\"ols-figure-caption\">\n            <p>Neutralisation examples use the total mass of both solutions. The final value is given per mole of acid or alkali reacted, with a negative sign because the temperature increases.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <a class=\"ols-figure-link ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Using-Hesss-law-to-find-enthalphy-changes.webp\" aria-label=\"Open using Hess&#039;s Law to find enthalpy changes infographic\">\n            <div class=\"ols-figure-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/Using-Hesss-law-to-find-enthalphy-changes.webp\" alt=\"Infographic showing a Hess cycle for hydrated copper sulfate and the practical method for measuring enthalpy of solution.\">\n            <\/div>\n          <\/a>\n          <div class=\"ols-figure-caption\">\n            <p>This example shows how Hess&#8217;s Law can be used when the target enthalpy change cannot be measured directly, such as forming a hydrated salt from an anhydrous salt.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-figure-card\">\n          <a class=\"ols-figure-link ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/calculatig-enthalpy-change-of-combustion.webp\" aria-label=\"Open calculating enthalpy change of combustion infographic\">\n            <div class=\"ols-figure-image\">\n              <img decoding=\"async\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/06\/calculatig-enthalpy-change-of-combustion.webp\" alt=\"Worked infographic showing how to calculate enthalpy change of combustion for propan-1-ol using flame calorimetry data.\">\n            <\/div>\n          <\/a>\n          <div class=\"ols-figure-caption\">\n            <p>Combustion calorimetry still uses q = mc\u0394T, but the mass in the equation is the water heated by the flame, not the alcohol burned.<\/p>\n          <\/div>\n        <\/div>\n\n        <div class=\"ols-exam-box\">\n          <p><strong>Exam strategy:<\/strong> Identify the target enthalpy change first. Then decide whether the measured equations need to be reversed, multiplied or combined to produce the target equation.<\/p>\n        <\/div>\n      <\/article>\n\n      <section class=\"ols-faq-card\">\n        <h2>CP8 FAQs<\/h2>\n        <p>These questions focus on the main exam points for calorimetry, enthalpy calculations and Hess&#8217;s Law.<\/p>\n\n        <div class=\"ols-faq-list\">\n          <div class=\"ols-faq-item\">\n            <h3>Why can the decomposition of potassium hydrogencarbonate not be measured directly?<\/h3>\n            <p>The solid must be heated to decompose. This means any temperature change would include energy supplied by the heating source, so the measured value would not only be due to the decomposition reaction.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>Why is a polystyrene cup used?<\/h3>\n            <p>A polystyrene cup reduces heat transfer between the reaction mixture and the surroundings. This makes the measured temperature change more accurate than using a glass beaker alone.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>Why is hydrochloric acid in excess?<\/h3>\n            <p>Hydrochloric acid is in excess so that the carbonate or hydrogencarbonate is the limiting reagent. This allows the enthalpy change to be calculated using the moles of solid added.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>Why is reaction 2 doubled in the Hess calculation?<\/h3>\n            <p>The target equation contains 2 moles of KHCO<sub>3<\/sub>, but reaction 2 is written for 1 mole of KHCO<sub>3<\/sub>. Therefore, \u0394H<sub>2<\/sub> must be multiplied by 2.<\/p>\n          <\/div>\n\n          <div class=\"ols-faq-item\">\n            <h3>What is the most common calculation mistake in this practical?<\/h3>\n            <p>The most common mistake is sign error. A temperature rise means the reaction is exothermic and \u0394H is negative. 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This allows the enthalpy change to be calculated using the moles of solid added.\"\n            }\n          },\n          {\n            \"@type\": \"Question\",\n            \"name\": \"Why is reaction 2 doubled in the Hess calculation?\",\n            \"acceptedAnswer\": {\n              \"@type\": \"Answer\",\n              \"text\": \"The target equation contains 2 moles of potassium hydrogencarbonate, but reaction 2 is written for 1 mole of potassium hydrogencarbonate. Therefore, the enthalpy change for reaction 2 must be multiplied by 2.\"\n            }\n          },\n          {\n            \"@type\": \"Question\",\n            \"name\": \"What is the most common calculation mistake in this practical?\",\n            \"acceptedAnswer\": {\n              \"@type\": \"Answer\",\n              \"text\": \"The most common mistake is sign error. A temperature rise means the reaction is exothermic and enthalpy change is negative. A temperature fall means the reaction is endothermic and enthalpy change is positive.\"\n            }\n          }\n        ]\n      },\n      {\n        \"@type\": \"Course\",\n        \"@id\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-8-enthalpy-change-via-hess-law\/#course\",\n        \"name\": \"CP8: Enthalpy Change via Hess\u2019s Law\",\n        \"description\": \"A revision lesson for Edexcel A Level Chemistry Core Practical 8, covering calorimetry, q = mc\u0394T, enthalpy signs, sample data and Hess's Law calculations.\",\n        \"provider\": {\n          \"@type\": \"Organization\",\n          \"name\": \"Online Learning System\",\n          \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/\"\n        },\n        \"educationalLevel\": \"A Level\",\n        \"teaches\": [\n          \"Calorimetry\",\n          \"Enthalpy change\",\n          \"Hess's Law\",\n          \"Core practical skills\",\n          \"Edexcel A Level Chemistry\"\n        ],\n        \"url\": \"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/edexcel\/core-practicals\/cp-8-enthalpy-change-via-hess-law\/\"\n      }\n    ]\n  }\n  <\/script>\n<\/section>\n","protected":false},"excerpt":{"rendered":"<p>Revision Notes \/ A Level Chemistry \/ Edexcel \/ Core Practicals \/ CP8: Enthalpy Change via Hess\u2019s Law CP8: Enthalpy Change via Hess\u2019s Law A guided Edexcel A Level Chemistry revision walkthrough for measuring temperature change, calculating enthalpy change from calorimetry data, and using Hess\u2019s Law to find the decomposition enthalpy of potassium hydrogencarbonate. Exam [&hellip;]<\/p>\n","protected":false},"author":12,"featured_media":0,"parent":4999,"menu_order":7,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-4983","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages\/4983","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/users\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/comments?post=4983"}],"version-history":[{"count":0,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages\/4983\/revisions"}],"up":[{"embeddable":true,"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/pages\/4999"}],"wp:attachment":[{"href":"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-json\/wp\/v2\/media?parent=4983"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}