{"id":7988,"date":"2013-09-07T00:23:00","date_gmt":"2013-09-07T00:23:00","guid":{"rendered":"http:\/\/www.mometrix.com\/academy\/?page_id=7988"},"modified":"2026-03-26T11:35:26","modified_gmt":"2026-03-26T16:35:26","slug":"enthalpy","status":"publish","type":"page","link":"https:\/\/www.mometrix.com\/academy\/enthalpy\/","title":{"rendered":"Enthalpy"},"content":{"rendered":"\n\t\t\t<div id=\"mmDeferVideoEncompass_Q7FFjZU9OJU\" style=\"position: relative;\">\n\t\t\t<picture>\n\t\t\t\t<source srcset=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2023\/01\/circle-play-duotone.webp\" type=\"image\/webp\">\n\t\t\t\t<source srcset=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2023\/01\/circle-play-duotone.png\" type=\"image\/jpeg\"> \n\t\t\t\t<img fetchpriority=\"high\" decoding=\"async\" loading=\"eager\" id=\"videoThumbnailImage_Q7FFjZU9OJU\" data-source-videoID=\"Q7FFjZU9OJU\" src=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2023\/01\/circle-play-duotone.png\" alt=\"Enthalpy Video\" height=\"464\" width=\"825\" class=\"size-full\" data-matomo-title = \"Enthalpy\">\n\t\t\t<\/picture>\n\t\t\t<\/div>\n\t\t\t<style>img#videoThumbnailImage_Q7FFjZU9OJU:hover {cursor:pointer;} img#videoThumbnailImage_Q7FFjZU9OJU {background-size:contain;background-image:url(\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2023\/01\/1451-thumb-final-2.webp\");}<\/style>\n\t\t\t<script defer>\n\t\t\t  jQuery(\"img#videoThumbnailImage_Q7FFjZU9OJU\").click(function() {\n\t\t\t\tlet videoId = jQuery(this).attr(\"data-source-videoID\");\n\t\t\t\tlet helpTag = '<div id=\"mmDeferVideoYTMessage_Q7FFjZU9OJU\" style=\"display: none;position: absolute;top: -24px;width: 100%;text-align: center;\"><span style=\"font-style: italic;font-size: small;border-top: 1px solid #fc0;\">Having trouble? <a href=\"https:\/\/www.youtube.com\/watch?v='+videoId+'\" target=\"_blank\">Click here to watch on YouTube.<\/a><\/span><\/div>';\n\t\t\t\tlet tag = document.createElement(\"iframe\");\n\t\t\t\ttag.id = \"yt\" + videoId;\n\t\t\t\ttag.src = \"https:\/\/www.youtube-nocookie.com\/embed\/\" + videoId + \"?autoplay=1&controls=1&wmode=opaque&rel=0&egm=0&iv_load_policy=3&hd=0&enablejsapi=1\";\n\t\t\t\ttag.frameborder = 0;\n\t\t\t\ttag.allow = \"autoplay; fullscreen\";\n\t\t\t\ttag.width = this.width;\n\t\t\t\ttag.height = this.height;\n\t\t\t\ttag.setAttribute(\"data-matomo-title\",\"Enthalpy\");\n\t\t\t\tjQuery(\"div#mmDeferVideoEncompass_Q7FFjZU9OJU\").html(tag);\n\t\t\t\tjQuery(\"div#mmDeferVideoEncompass_Q7FFjZU9OJU\").prepend(helpTag);\n\t\t\t\tsetTimeout(function(){jQuery(\"div#mmDeferVideoYTMessage_Q7FFjZU9OJU\").css(\"display\", \"block\");}, 2000);\n\t\t\t  });\n\t\t\t  \n\t\t\t<\/script>\n\t\t\n<p><script>\nfunction LGW_Function() {\n  var x = document.getElementById(\"LGW\");\n  if (x.style.display === \"none\") {\n    x.style.display = \"block\";\n  } else {\n    x.style.display = \"none\";\n  }\n}\n<\/script><\/p>\n<div class=\"moc-toc hide-on-desktop hide-on-tablet\">\n<div><button onclick=\"LGW_Function()\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2024\/12\/toc2.svg\" width=\"16\" height=\"16\" alt=\"show or hide table of contents\"><\/button><\/p>\n<p>On this page<\/p>\n<\/div>\n<nav id=\"LGW\" style=\"display:none;\">\n<ul>\n<li class=\"toc-h2\"><a href=\"#Enthalpy_in_Chemical_Reactions\" class=\"smooth-scroll\">Enthalpy in Chemical Reactions<\/a><\/li>\n<li class=\"toc-h2\"><a href=\"#Internal_Energy,_Heat,_and_Work\" class=\"smooth-scroll\">Internal Energy, Heat, and Work<\/a><\/li>\n<li class=\"toc-h2\"><a href=\"#Deriving_Enthalpy_from_Internal_Energy\" class=\"smooth-scroll\">Deriving Enthalpy from Internal Energy<\/a><\/li>\n<li class=\"toc-h2\"><a href=\"#Enthalpy_Changes_and_Reaction_Types\" class=\"smooth-scroll\">Enthalpy Changes and Reaction Types<\/a><\/li>\n<li class=\"toc-h2\"><a href=\"#Review\" class=\"smooth-scroll\">Review<\/a><\/li>\n<\/ul>\n<\/nav>\n<\/div>\n<div class=\"accordion\"><input id=\"transcript\" type=\"checkbox\" class=\"spoiler_button\" \/><label for=\"transcript\">Transcript<\/label>\n<div class=\"spoiler\" id=\"transcript-spoiler\">\n<p>Hi, and welcome to this video on enthalpy!<\/p>\n<p>There are a couple ways to approach this topic; often, textbooks begin with its derivation from internal energy. We will get to that briefly along with how to measure enthalpy, but first, we want to give enthalpy a more tangible definition before diving into the details.<\/p>\n<h2><span id=\"Enthalpy_in_Chemical_Reactions\" class=\"m-toc-anchor\"><\/span>Enthalpy in Chemical Reactions<\/h2>\n<p>\nTo start, imagine any chemical reaction. What properties would you want to know about that reaction? Well, of course, you\u2019d want to know the chemical components: the reactants, intermediates, and products. You\u2019d also want to know the kinetics of the reaction; how fast is this reaction going to proceed?<\/p>\n<p>And, lastly, you\u2019d want to know about the thermodynamics. The thermodynamics would tell us about the equilibrium of the reaction, such as how much of the reactants will become products. It would also tell us whether the reaction would release or absorb heat (that is, whether it\u2019s exothermic or endothermic). This is a really noticeable and easily measured feature of a reaction; you can often feel the temperature change of the reaction vessel with your hand! That release or absorbance of heat is the change of enthalpy for a reaction (with a few assumptions that we\u2019ll get to in a minute).<\/p>\n<h3><span id=\"Defining_Enthalpy\" class=\"m-toc-anchor\"><\/span>Defining Enthalpy<\/h3>\n<p>\nNow that we have a physical idea of what enthalpy is, let\u2019s turn to the more formal definition.<\/p>\n<p>To begin with, there are lots of properties that can describe a system. So, how do we go about deciding which to measure? Well, our hands are often tied on this point because many properties are simply too difficult to measure. So even if they\u2019re potentially very useful, if we can\u2019t measure them quickly and reliably, they aren\u2019t of much use. For example, the internal energy, denoted as U, is the total potential and kinetic energy of a system.<\/p>\n<p>That\u2019s an incredibly hard number to actually calculate. We\u2019d have to infinitely separate out all the zillions of molecules and their subatomic particles and account for their kinetic energy. It\u2019s basically impossible. So, rather than trying to measure the absolute internal energy of a system, we measure the change of the internal energy denoted by the capital delta, for instance, \u201cthe change in internal energy is equal to heat plus work\u201d.<\/p>\n<div class=\"examplesentence\">\\(\u2206U=q+w\\)<\/div>\n<p>\n&nbsp;<\/p>\n<h2><span id=\"Internal_Energy,_Heat,_and_Work\" class=\"m-toc-anchor\"><\/span>Internal Energy, Heat, and Work<\/h2>\n<p>\nHere, it\u2019s important to remember that the first law of thermodynamics, energy cannot be created or destroyed, is always conserved. So heat and work, which could be mechanical work, electrical work, radiant work, basically any non-thermal energy, are the only two ways in which energy is transferred in or out of the system.<\/p>\n<p>And we should remind ourselves of a second point here. While the energy in the universe is conserved, in most of our problems, we\u2019ll split the universe into the system and its surroundings. Oftentimes in chemistry, we\u2019ll define our reaction vessel as the system and the lab we\u2019re in as the surroundings (since we don\u2019t really need to consider the entire universe).<\/p>\n<p>So, by measuring the released or absorbed heat and work done on or by a system over the course of a reaction, we can track the change in internal energy. Of those two quantities, we\u2019re often more interested in the heat absorbed or released by a chemical reaction. However, heat and work are path functions, meaning they depend on the path the system takes to go from the initial to final state. This means that there are infinite combinations of heat and work that could account for a change in internal energy.<\/p>\n<p>Other properties, like internal energy, pressure, and volume, on the other hand, are state functions, which means they only depend on the initial and final states, not on the pathway, of which there could be many. So, what we\u2019d actually like is a state function that measures the heat released or absorbed by a system (spoiler- it\u2019s enthalpy). But how do we get there?<\/p>\n<h2><span id=\"Deriving_Enthalpy_from_Internal_Energy\" class=\"m-toc-anchor\"><\/span>Deriving Enthalpy from Internal Energy<\/h2>\n<p>\nFirst, we\u2019ll begin with the change in internal energy and start by assuming that the only type of work done by our system is pressure-volume work, which means we\u2019re ignoring all other types of work the system could be doing (like electrical, radiant, etc). So, we rewrite the change in internal energy as equal to heat minus pressure times the change in volume.<\/p>\n<div class=\"examplesentence\">\\(\u2206U=q+w=q-P\u2206V\\)<\/div>\n<p>\n&nbsp;<br \/>\nVery briefly, here\u2019s how to think about PV work. Imagine a piston at the initial volume with a weightless and frictionless lid. A reaction occurs that increases the volume to the final volume, maintaining the internal pressure of the container. This work is considered negative because the system is <em>doing<\/em> work on the surroundings, so energy is transferred out of the system. This work is defined as the external pressure times the change in volume, which makes some intuitive sense because it\u2019s how much the system pushed on the atmosphere.<\/p>\n<p>Alright, let\u2019s return to our definition of delta \\(U\\). Rearrange this equation to solve for heat, \\(q\\).<\/p>\n<div class=\"examplesentence\">\\(q= \u2206U+P\u2206V\\)<\/div>\n<p>\n&nbsp;<br \/>\nAnd now note that \\(q\\) is defined by the internal energy, pressure, and volume, which are all state functions. That means q can be expressed as a state function, which we will actually name enthalpy. This is represented by capital H and gets a delta here as well, because we\u2019re measuring the change in enthalpy!<\/p>\n<div class=\"examplesentence\">\\(\u2206H=q_{P}=\u2206U+P\u2206V\\)<\/div>\n<p>\n&nbsp;<br \/>\nImportantly, notice that throughout this derivation we\u2019ve been assuming constant pressure- if it changed, there would be a capital delta in front of \\(P\\) as well.  This was especially noticeable when we defined work.  The volume of the system changed to maintain the internal pressure.<\/p>\n<p>This assumption that pressure isn\u2019t changing is fundamental to the definition of enthalpy, which is formally written as the amount of heat absorbed or released by a system at constant pressure. This is denoted in the equation with the subscript \\(P\\) on heat.  <\/p>\n<p>This assumption turns out to be fine for most chemical reactions, as we often carry out reactions in open vessels at atmospheric pressure, which doesn\u2019t change over the course of the reaction. <\/p>\n<p>Just like internal energy, we could also define the absolute enthalpy of a system as:<\/p>\n<div class=\"examplesentence\">\\(H=U+PV\\)<\/div>\n<p>\n&nbsp;<br \/>\nBut measuring the total enthalpy of a system would be really hard and isn\u2019t super useful anyway. To illustrate this point with an everyday example, consider a mountain hike. While maybe you\u2019re interested in the absolute altitude of your starting and ending point, the more important metric for you at that moment is the change in altitude. <\/p>\n<p>It\u2019s the same for enthalpy and internal energy- we\u2019re most interested in their change during a chemical reaction rather than the absolute value.  <\/p>\n<h2><span id=\"Enthalpy_Changes_and_Reaction_Types\" class=\"m-toc-anchor\"><\/span>Enthalpy Changes and Reaction Types<\/h2>\n<p>\nSo, for a chemical reaction, instead of a mountain, we\u2019ll define our stationary points as the reactants and products. The \\(x\\)-axis here is the reaction coordinate (which is essentially the progress of the reaction) and the y-axis is energy. Remember, heat is the transfer of thermal energy, so it should make some sense that enthalpy is measured in units of energy.  <\/p>\n<p>If the enthalpy of the system increases from the reactants to products, the change in enthalpy is positive and the reaction is endothermic.  <\/p>\n<p><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Endothermic-Reaction-Correct-e1657724577338.jpg\" alt=\"endothermic reaction\" width=\"652\" height=\"375\" class=\"alignnone size-full wp-image-130063\" srcset=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Endothermic-Reaction-Correct-e1657724577338.jpg 652w, https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Endothermic-Reaction-Correct-e1657724577338-300x173.jpg 300w\" sizes=\"auto, (max-width: 652px) 100vw, 652px\" \/><\/p>\n<div class=\"examplesentence\">\\(\u2206_{r}H=H_{prod}-H_{react}\\)<\/p>\n<p>when \\(H_{prod}\\gt H_{react}\\)<\/p>\n\\(\u2206_{r}H\\gt 0\\)\n<\/div>\n<p>\n&nbsp;<br \/>\nThe bonds of the reactants are more stable than the bonds of the products, so the system requires energy to form those new higher energy bonds in the products.  In this case, the reaction vessel would feel cold to the touch.  <\/p>\n<p><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Endothermic-vs-exothermic-e1657724627921.jpg\" alt=\"endothermic is cold to the touch\" width=\"652\" height=\"526\" class=\"alignnone size-full wp-image-130066\" srcset=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Endothermic-vs-exothermic-e1657724627921.jpg 652w, https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Endothermic-vs-exothermic-e1657724627921-300x242.jpg 300w\" sizes=\"auto, (max-width: 652px) 100vw, 652px\" \/><\/p>\n<p>Conversely, if the enthalpy of the system decreases from the reactants to products, the change in enthalpy is negative, and the reaction is exothermic. <\/p>\n<p><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Exothermic-Reaction-e1657724691843.jpg\" alt=\"exothermic chemical reaction\" width=\"652\" height=\"408\" class=\"alignnone size-full wp-image-130069\" srcset=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Exothermic-Reaction-e1657724691843.jpg 652w, https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Exothermic-Reaction-e1657724691843-300x188.jpg 300w\" sizes=\"auto, (max-width: 652px) 100vw, 652px\" \/><\/p>\n<div class=\"examplesentence\">when \\(H_{prod}\\lt H_{react}\\)<\/p>\n\\(\u2206_{r}H\\lt 0\\)\n<\/div>\n<p>\n&nbsp;<br \/>\nThe bonds of the reactants are less stable than the bonds of the products, so the system releases that excess energy as heat when the products are formed. In this case, the reaction vessel would feel hot to the touch.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Exothermic-releases-heat-e1657724727357.jpg\" alt=\"exothermic chemical reaction releases heat\" width=\"652\" height=\"467\" class=\"alignnone size-full wp-image-130072\" srcset=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Exothermic-releases-heat-e1657724727357.jpg 652w, https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Exothermic-releases-heat-e1657724727357-300x215.jpg 300w\" sizes=\"auto, (max-width: 652px) 100vw, 652px\" \/><\/p>\n<p>And this is a really important point.  Many, many times students get confused about \u201cbreaking bonds to release energy.\u201d That sentence is a very common misconception. Breaking bonds costs energy. Energy is only released when you form bonds that are more stable than the ones you have broken. So the change in enthalpy for a reaction, whether it\u2019s exo- or endothermic, is telling us directly about the relative energy of the reactant and product bonds. <\/p>\n<p>To experimentally measure the change in enthalpy for a reaction, we could run a calorimetry experiment. We could measure the change in temperature during the course of the reaction and then using the specific heat capacity and mass of the substance, could convert that temperature change to the change in enthalpy.  <\/p>\n<p><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Calorimetry--e1657724786396.jpg\" alt=\"calorimetry experiment \" width=\"652\" height=\"366\" class=\"alignnone size-full wp-image-130075\" srcset=\"https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Calorimetry--e1657724786396.jpg 652w, https:\/\/www.mometrix.com\/academy\/wp-content\/uploads\/2022\/07\/Calorimetry--e1657724786396-300x168.jpg 300w\" sizes=\"auto, (max-width: 652px) 100vw, 652px\" \/><\/p>\n<hr>\n<h2><span id=\"Review\" class=\"m-toc-anchor\"><\/span>Review<\/h2>\n<p>\nOkay, we\u2019ve covered a lot of information in this video, so let\u2019s go back over everything with a quick review.<\/p>\n<p>We started by deriving the standard definition of enthalpy from the change in internal energy, which is equal to the sum of heat and work. To do so, we assumed that the only work the system does is mechanical pressure-volume work and that pressure is constant during the course of the reaction, two assumptions that are usually safe to make. We then defined endo- and exothermic reactions in terms of their enthalpy change.<\/p>\n<p>Throughout, we discussed how for these state functions, we\u2019re rarely interested in absolute values and usually calculate the change in enthalpy and internal energy. And we finished with a discussion of how enthalpy is calculated experimentally by touching on the concept of calorimetry.<\/p>\n<p>Alright, that\u2019s it for today\u2019s video!  Thanks for watching, and happy studying!<\/p>\n<\/div>\n<\/div>\n\n<div class=\"home-buttons\">\n<p><a href=\"https:\/\/www.mometrix.com\/academy\/chemistry\/\">Return to Chemistry Videos<\/a><\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>Return to Chemistry Videos<\/p>\n","protected":false},"author":1,"featured_media":130051,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"open","template":"","meta":{"footnotes":""},"class_list":{"0":"post-7988","1":"page","2":"type-page","3":"status-publish","4":"has-post-thumbnail","6":"page_category-chemistry-thermodynamics","7":"page_type-video","8":"subject_matter-science"},"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/pages\/7988","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/comments?post=7988"}],"version-history":[{"count":7,"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/pages\/7988\/revisions"}],"predecessor-version":[{"id":281741,"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/pages\/7988\/revisions\/281741"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/media\/130051"}],"wp:attachment":[{"href":"https:\/\/www.mometrix.com\/academy\/wp-json\/wp\/v2\/media?parent=7988"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}