{"id":151,"date":"2019-06-17T18:10:31","date_gmt":"2019-06-17T22:10:31","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/3-9-energy-in-chemical-reactions\/"},"modified":"2025-08-26T18:03:56","modified_gmt":"2025-08-26T22:03:56","slug":"3-11-energy-in-chemical-reactions","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/053humanbiology\/chapter\/3-11-energy-in-chemical-reactions\/","title":{"raw":"3.11\u00a0Energy in Chemical Reactions","rendered":"3.11\u00a0Energy in Chemical Reactions"},"content":{"raw":"Created by:\u00a0CK-12\/Adapted by Christine Miller<\/span>\r\n

Slow Burn<\/h1>\r\n[caption id=\"attachment_150\" align=\"alignright\" width=\"397\"]\"Image Figure 3.11.1 Rusting is a type of combustion reaction.<\/em>[\/caption]\r\n\r\nThis old truck gives off a small amount of\u00a0heat\u00a0as it rusts. The rusting of iron is a chemical process. It occurs when iron and oxygen go through a\u00a0[pb_glossary id=\"1330\"]chemical reaction[\/pb_glossary]\u00a0similar to burning, or combustion. Obviously, the chemical reaction that occurs when something burns gives off\u00a0[pb_glossary id=\"1342\"]energy[\/pb_glossary]. You can feel the heat, and you may be able to see the light of flames. The rusting of iron is a much slower process, but it still gives off energy. It's just that it releases energy so slowly that you can't detect a change in\u00a0temperature.\r\n
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The Role of\u00a0Energy in Chemical Reactions<\/h1>\r\n<\/div>\r\nMatter rusting or burning are common examples of chemical changes.\u00a0Chemical changes<\/strong>\u00a0involve\u00a0chemical reactions, in which some substances, called reactants, change at the molecular level to form new substances, which are called products. All chemical reactions involve\u00a0energy, but not all chemical reactions\u00a0release<\/em>\u00a0energy,\u00a0like rusting and burning. In some chemical reactions, energy is\u00a0absorbed<\/em>\u00a0rather than released.\r\n

Exothermic Reactions<\/h2>\r\n[caption id=\"attachment_150\" align=\"alignleft\" width=\"400\"]\"A Figure 3.11.2 Exothermic reactions release energy.<\/em>[\/caption]\r\n\r\nA\u00a0chemical reaction\u00a0that releases energy is called an\u00a0[pb_glossary id=\"1222\"]exothermic reaction<\/strong>[\/pb_glossary]. This type of reaction can be represented\u00a0with this\u00a0general chemical equation:\r\n\r\nReactants \u2192 Products +\u00a0Heat<\/strong><\/span>\r\n\r\nAnother example of an exothermic reaction is chlorine combining with sodium to form table salt. The decomposition of organic matter also releases energy because of exothermic reactions. Sometimes on a chilly morning, you can see steam rising from a compost pile because of these chemical reactions (see photo in Figure 3.11.2).\r\n
\r\n\r\nThis compost pile is steaming because it is much warmer than the chilly air around it. The heat comes from all the exothermic chemical reactions taking place inside the compost as it decomposes.\r\n\r\n<\/div>\r\nA special type of exothermic reaction is an [pb_glossary id=\"1937\"]exergonic reaction<\/strong>[\/pb_glossary]- not only do exergonic reactions release energy, but in addition, they occur spontaneously.\u00a0 Many cell processes rely on exergonic reactions: in a chemical process\u00a0called\u00a0cellular respiration, which is\u00a0similar to combustion, the sugar glucose is \"burned\" to provide\u00a0cells\u00a0with energy.\r\n

Endothermic Reactions<\/h2>\r\nA\u00a0chemical reaction\u00a0that absorbs [pb_glossary id=\"1342\"]energy[\/pb_glossary] is called an\u00a0[pb_glossary id=\"1224\"]endothermic reaction[\/pb_glossary]<\/strong>. This type of reaction can also be represented by a general chemical equation:\r\n\r\nReactants +\u00a0Energy<\/span><\/strong>\u00a0\u2192 Products\r\n\r\n[caption id=\"attachment_150\" align=\"alignright\" width=\"334\"]\"Image Figure 3.11.3 This pack gets cold because of an endothermic reaction.<\/em>[\/caption]\r\n\r\nDid you ever use a chemical cold pack like the one pictured? The pack cools down because of an\u00a0endothermic reaction. When a tube inside the pack is broken, it releases ammonium nitrate, a chemical that reacts with\u00a0water inside the pack. This reaction absorbs heat energy and quickly cools down the contents of the pack.\r\n\r\nMany other chemical processes involve endothermic reactions. Most cooking and baking, for example, involves the use of energy to produce chemical reactions. You can't bake a cake or cook an egg without adding heat energy.\r\n\r\nArguably, the most important endothermic reactions occur during\u00a0photosynthesis. When plants produce sugar by photosynthesis, they take in light energy to power the necessary endothermic reactions. The sugar they produce provides plants and virtually all other living things with glucose for\u00a0cellular respiration.\r\n
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Activation Energy<\/h2>\r\n<\/div>\r\nAll chemical reactions\u00a0require\u00a0energy to get started. Even reactions that release energy need a boost of energy in order to begin. The energy needed to start a chemical reaction is called\u00a0[pb_glossary id=\"1225\"]activation energy[\/pb_glossary]<\/strong>. Activation energy is like the push a child needs to start going down a playground slide. The push gives the child enough energy to start moving, but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in the graph in Figure 3.11.4.\r\n\r\n[caption id=\"attachment_150\" align=\"aligncenter\" width=\"338\"]\"Image Figure 3.11.4 Even though this reaction is exothermic, it requires \"help\" to get started. This \"help\" is the activation energy.<\/em>[\/caption]\r\n\r\nWhy do chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must bump into each other, so they must be moving \u2014 and movement requires energy. When reactant molecules bump together, they may repel each other because of intermolecular forces pushing them apart. Energy is also required to overcome these forces so the molecules can come together and react.\r\n
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3.11 Summary<\/span><\/h1>\r\n<\/header>\r\n
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