26 Energy Forms & Global Relevance (M6Q1)


This module will introduce the basic ideas of an important area of science concerned with the amount of heat absorbed or released during chemical and physical changes—an area called thermochemistry. The concepts introduced in this module are widely used in almost all scientific and technical fields and therefore their mastery is an essential learning goal for all of us. The ability to harness the energy produced by chemical reactions to do useful work is at the heart of our natural world and industrial society, from the twitch of a muscle to the batteries of a cell phone. This section includes sample problems and a glossary.

Learning Objectives for Energy Forms & Global Relevance

| Key Concepts and Summary | Glossary | End of Section Exercises |

Global Relevance of Energy

Chemical changes and their accompanying changes in energy are important parts of our everyday world (Figure 1). The macronutrients in food (proteins, fats, and carbohydrates) undergo metabolic reactions that provide the energy to keep our bodies functioning. We burn a variety of fuels (gasoline, natural gas, coal) to produce energy for transportation, heating, and the generation of electricity. Industrial chemical reactions use enormous amounts of energy to produce raw materials (such as iron and aluminum). We also use energy from fuels to manufacture these materials into useful products, such as cars, skyscrapers, and bridges.

Three pictures are shown and labeled a, b, and c. Picture a is a cheeseburger. Picture b depicts a highway that is full of traffic. Picture c is a view into an industrial metal furnace. The view into the furnace shows a hot fire burning inside.
Figure 1. The energy involved in chemical changes is important to our daily lives: (a) A cheeseburger for lunch provides the energy you need to get through the rest of the day (b) The combustion of gasoline provides the energy that moves your car (and you) between home, work, and school (c) Coke, a processed form of coal, provides the energy needed to convert iron ore into iron, which is essential for making many of the products we use daily. (credit a: modification of work by “Pink Sherbet Photography”/Flickr; credit b: modification of work by Jeffery Turner)

Over 90% of the energy that we use comes originally from the sun. Every day, the sun provides the earth with almost 10,000 times the amount of energy necessary to meet all of the world’s energy needs for that day. Our challenge is to find ways that are convenient and nonpolluting to convert and store incoming solar energy so that the energy can be used in reactions or chemical processes. Many plants and bacteria capture solar energy through photosynthesis. We utilize the energy stored in plants when we burn wood or plant products as fuels. We also use this energy to fuel our bodies by eating food that comes directly from plants or from animals that got their energy by eating plants. Burning coal and petroleum also releases stored energy originally from the sun: these fuels are fossilized plant and animal matter.

Scientists from across all disciplines utilize fundamental concepts involving energy in their research. Food scientists use them to determine the energy content of foods. Biologists study the energetics of living organisms, such as the metabolic combustion of sugar into carbon dioxide and water. The oil, gas, and transportation industries, renewable energy providers, and many others seek to find better methods to produce energy for our commercial and personal needs. Engineers strive to improve energy efficiency, find better ways to heat and cool our homes, refrigerate our food and drinks, and meet the energy and cooling needs of computers and electronics, among other applications. Understanding thermochemical principles is essential for chemists, physicists, biologists, geologists, every type of engineer, and just about anyone who studies or does any kind of science.

Forms of Energy

Like matter, energy comes in different types. One scheme classifies energy into two types: potential energy, the energy an object has because of its relative position, composition, or condition, and kinetic energy, the energy that an object possesses because of its motion. You may have talked about kinetic energy in a physics class and used the equation KE  = ½mv2, where m is the mass and v is the velocity. Water at the top of a waterfall or dam has potential energy because of its position; when it flows downward through generators, the potential energy is converted into kinetic energy that can be used to do work and produce electricity in a hydroelectric plant (Figure 2). A battery has potential energy because the chemicals within it can produce electricity that can do work.

Two pictures are shown and labeled a and b. Picture a shows a large waterfall with water falling from a high elevation at the top of the falls to a lower elevation. The second picture is a view looking down into the Hoover Dam. Water is shown behind the high wall of the dam on one side and at the base of the dam on the other.
Figure 2. (a) Water that is higher in elevation, for example, at the top of Victoria Falls, has a higher potential energy than water at a lower elevation. As the water falls, some of its potential energy is converted into kinetic energy. (b) If the water flows through generators at the bottom of a dam, such as the Hoover Dam shown here, its kinetic energy is converted into electrical energy. (credit a: modification of work by Steve Jurvetson; credit b: modification of work by “curimedia”/Wikimedia commons)

Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form after the change is completed. This observation is expressed in the law of conservation of energy: during a chemical or physical change, energy can be neither created nor destroyed, although it can change forms. (This is also one version of the first law of thermodynamics, as you will learn later in this module.)

When one substance is converted into another, there is always an associated conversion of one form of energy into another. Heat is usually released or absorbed, but sometimes the conversion involves light, electrical energy, or some other form of energy. For example, chemical energy (a type of potential energy) is stored in the molecules that compose gasoline. When gasoline is burned within the cylinders of a car’s engine, the rapidly expanding gaseous products of this chemical reaction generate kinetic energy when they move the pistons in an engine, which in turn rotate the wheels and move the car forward.

Units of Energy

Historically, energy was measured in units of calories (cal). A calorie is the amount of energy required to raise one gram of water by 1 degree Celsius (1 kelvin). However, this quantity depends on the atmospheric pressure and the starting temperature of the water. The ease of measurement of energy changes in calories has meant that the calorie is still frequently used. The Calorie (with a capital C), or large calorie, commonly used in quantifying food energy content, is a kilocalorie (1 Cal = 1 kcal = 1000 cal). The SI unit of heat, work, and energy is the joule. A joule (J) is defined as the amount of energy used when a force of 1 newton moves an object 1 meter. It is named in honor of the English physicist James Prescott Joule. One joule is equivalent to 1 kg m2 s-2. A kilojoule (kJ) is 1000 joules. Another useful conversion factor:  1 calorie = 4.184 joules.

In humans, metabolism is typically measured in Calories per day. A nutritional calorie (i.e., Calorie) is the energy unit used to quantify the amount of energy derived from the metabolism of foods; one Calorie is equal the amount of energy needed to heat 1 kg of water by 1 °C.

Chemistry in Real Life: Measuring Nutritional Calories

In your day-to-day life, you may be more familiar with energy being given in Calories, or nutritional calories, which are used to quantify the amount of energy in foods. One calorie (cal) = exactly 4.184 joules, and one Calorie (note the capitalization) = 1000 cal, or 1 kcal. (This is approximately the amount of energy needed to heat 1 kg of water by 1 °C.)

The macronutrients in food are proteins, carbohydrates, and fats or oils. Proteins provide about 4 Calories per gram, carbohydrates also provide about 4 Calories per gram, and fats and oils provide about 9 Calories/g. Nutritional labels on food packages show the caloric content of one serving of the food, as well as the breakdown into Calories from each of the three macronutrients.

Exothermic and Endothermic Processes

Matter undergoing chemical reactions and physical changes can release or absorb heat. A change that releases heat is called an exothermic process. For example, the combustion reaction that occurs when using an oxyacetylene torch is an exothermic process—this process also releases energy in the form of light as evidenced by the torch’s flame (Figure 3). A reaction or change that absorbs heat is an endothermic process. A cold pack used to treat muscle strains provides an example of an endothermic process. When the substances in the cold pack (water and a salt such as ammonium nitrate) are brought together, the resulting process absorbs heat from your skin, lowering its temperature and leading to the sensation of cold.

Two pictures are shown and labeled a and b. Picture a shows a metal railroad tie being cut with the flame of an acetylene torch. Picture b shows a chemical cold pack containing ammonium nitrate.
Figure 3. (a) An oxyacetylene torch produces heat by the combustion of acetylene in oxygen. The energy released by this exothermic reaction heats and then melts the metal being cut. The sparks are tiny bits of the molten metal flying away. (b) A cold pack uses an endothermic process to create the sensation of cold. (credit a: modification of work by “Skatebiker”/Wikimedia commons)

Energy is an Intrinsic Component of Physical and Chemical Changes

Later in this module we will examine physical and chemical changes more quantitatively. Here we introduce the concepts from a qualitative perspective.

Recall from the section on Classification of Matter at the beginning of the semester, when a substance undergoes a physical change, such as melting or freezing, the physical state of that substance changes. The process can be endothermic or exothermic. For example, the melting of water is endothermic and requires the addition of energy. Energy can be thought of as a reactant in the process.

energy  +  H2O(s)   ⟶   H2O(l)      endothermic process

In contrast, the freezing of water is exothermic and releases energy. Energy can be thought of as a product in the process.

H2O(l)   ⟶   H2O(s)  +  energy      exothermic process

During a chemical change, bonds are broken and formed. For example, when we ignite a balloon containing hydrogen and oxygen gases, water is formed. In simplest terms, one O-O bond and two H-H bonds must be broken in the reactants, and four O-H bonds must be formed in the products. (In reality, it’s more complicated.) Energy is released during this reaction as light, heat, and sound.

O2(g)  +  2 H2(g)   ⟶   2 H2O(l)  +  energy      exothermic reaction

Demonstration: An exothermic reaction releases energy

Set up. In this demonstration, we want to focus on the balloons containing a mix of H2 and O2 gases (starting around 1:20 in the movie). When these balloons are ignited, a chemical reaction takes place. During this chemical reaction, bonds are broken and formed.

Explanation. This demonstration shows that this reaction is an exothermic reaction. We observe energy being released in the forms of light, heat, and sound. While the one O-O bond and two H-H bonds require energy to be broken, the formation of the four O-H bonds in the products release energy. Since more energy is released upon formation of the products than is required for the breaking up of the reactants, this reaction is exothermic overall.

The electrolysis of water, in contrast, is an endothermic process. In simplest terms, energy is required to break four O-H bonds in the reactants. Next, energy is released upon the formation of one O-O bond and two H-H bonds in the products. The energy released upon bond formation is less than the amount of energy required to break the bonds in the reactants. A battery must be hooked up to continuously supply energy to make this reaction occur.

2 H2O(l)  +  energy   ⟶   O2(g)  +  2 H2(g)      endothermic reaction


Demonstration: An endothermic reaction requires energy

Set up. In this demonstration (which we also looked at in an earlier module), electrolysis is performed on a solution of water that has been slightly acidified with H2SO4. The sulfuric acid acts as the electrolyte in this demonstration, carrying the charge between the anode and cathode. A DC voltage is applied to two electrodes, labeled “DC (-)” and “DC (+)”. These are the anode (positive electrode) and cathode (negative electrode), respectively. The following image explains the different parts of the apparatus that you will see in the video.

A screenshot of the demonstration is shown and the parts of the electrolysis setup are labeled. There is an energy source in the lower left. Two inverted burets are clamped on a ring stand side by side. The burets are filled with a solution of water and sulfuric acid. A positive electrode is inserted into the bottom of the right buret, and a negative electrode is inserted into the bottom of the left buret. A bridge near the bottom of the burets connects the two burets and acidified water enters the apparatus at the bridge. The electrodes are connected to the energy source. Gas will collect at the top of the burets during the electrolysis. Tubes connected to the stems of the burets will be used to collect samples.

When the energy source is turned on, you will see small bubbles appear in the two columns of solution. These bubbles are filled with H2 and O2 gas. These bubbles will float upward and gather at the top of the columns. We can then collect samples of the gas and ignite them to show that they are indeed H2 and O2 gas.

Explanation. At the frame that is labeled “20 minutes later”, you can see how much gas has been collected in each column. These columns were filled with solution at the beginning of the experiment, but now one column (the left column, negative electrode) has less solution than the other (and therefore it has more gas). The left column actually contains twice the amount of gas as the right column. This is because H2 is collected in the left column and O2 is collected in the right column. According to the stoichiometry of the reaction, for every one O2 that is produced, two H2 molecules are produced:

2 H2O(l)  +  energy   ⟶   O2(g)  +  2 H2(g)

This reaction would not have occurred if we did not turn on the energy source, though. This reaction required an input of energy, and is therefore an endothermic reaction.

Can you explain why H2 is produced in the left column and O2 is produced in the right column? It may be helpful to look at the overall reaction equation and also review oxidation-reduction reactions from earlier in the semester.

Energy Changes During Chemical Reactions

Breaking bonds is always an endothermic process and requires energy, whether it occurs during a physical or chemical change.  Breaking stronger bonds requires a larger input of energy compared to breaking weaker bonds. Forming bonds is a always an exothermic process and releases energy, whether it occurs during a chemical or physical change. This is perhaps less intuitive than the breaking of bonds requiring energy, but keep in mind that the formation of stronger bonds in the end product will result in a more stable product in comparison to the initial reactant. The formation of stronger bonds releases more energy than the formation of weaker bonds.

For example, melting and evaporation are both endothermic physical changes because hydrogen bonds between water molecules are broken. We shall quantify the energy changes occurring during melting and evaporation later in this module, and we’ll learn about hydrogen bonds in a later module on intermolecular forces. As an introduction and shown in Figure 4, liquid water is lower in energy than water vapor. Upon heating, the hydrogen bonds between water molecules in liquid water are broken and the chemical potential energy of the water increases. Liquid water molecules are close together and interacting through lots of hydrogen bonds. The water vapor molecules are further apart and are no longer interacting through hydrogen bonds. Note that the energy needed to break the hydrogen bonds does not “disappear” or is “used up” but manifests itself as an increase in potential energy of the sample of water vapor compared to the liquid water as the molecules are now further apart.

A plot of chemical potential energy vs reaction progress is shown for the reaction of liquid water being heated to water vapor. The chemical potential energy of liquid water is low and a picture shows the liquid water molecules close together. An inset shows the hydrogen bonding that occurs between the liquid water molecules. The chemical potential energy of water vapor is high and an image shows the water molecules spaced further apart than in the liquid form.
Figure 4.  As liquid water is heated, the energy from the heat breaks the hydrogen bonds and the chemical potential energy of the water increases. (Liquid water image modified from “P99am” / Wikimedia Commons / CC-BY-SA-3.0.)

Chemical reactions are more complex than simple physical changes as they usually involve both bond breaking and bond making. A chemical reaction will be endothermic (or exothermic) depending on the overall energy changes that occur during the chemical reaction. For example, in the case of the formation of water from elemental hydrogen and oxygen (which we looked at in the demonstration above), the reactants have greater chemical potential energy than the products (Figure 5). The energy required to break the reactant’s bonds is less than the energy released upon the formation of the bonds in the products. Overall, the change in energy for this chemical reaction is that more energy is released upon formation of the H-O bonds in water than is required to break apart the H-H and O-O bonds in the reactants. The net release of energy is observed as light, heat, and sound.

A plot of chemical energy vs. reaction progress is shown for the reaction H 2 plus O 2 yields H subscript 2 O. The chemical energy for H 2 plus O 2 is at about halfway on the y axis. As the reaction progresses, the chemical energy increases by about 25%, then there is a large decrease in chemical energy and the chemical energy of the products is at the about 10% of the initial chemical energy.
Figure 5. The energy changes during the formation of water from hydrogen and oxygen.

What is the source of chemical potential energy? If in a chemical change the reactants become stable products with a lower chemical potential energy, the electrostatic attractive forces between and within the atoms and molecules of the products are stronger compared to those in the initial reactants. For example, when an H2 molecule forms from two separate H atoms, the electrons in the product molecule will be subject to the attractive forces of the two protons resulting in the formation of a more stable diatomic molecule with a lower chemical potential energy. Because energy is conserved, any energy difference between the potential energies of the reactants and products must be accounted for. It is equal to the energy evolved during the reaction.

Figure 6. Two hydrogen Atoms combining to form more stable diatomic molecule.

Substances with a high chemical potential are not necessarily easy to spot as the energy is not visually apparent unless a chemical reaction takes place and the evolution of energy is obvious. However, knowledge of what a substance is used for can be very helpful. Organic fuels and foods are substances that release large amounts of energy as they are burned or metabolized to form very stable water and carbon dioxide. Substances with a low chemical potential energy are stable and one would expect to find large quantities of them that have been present, unchanged, for a long time. Inorganic rocks have low chemical potential energy. We do not eat rocks!

Demonstration: Exploding gummi bear

Set up. In the following demonstration, we will observe the exothermic reaction of an exploding gummi bear. First, a test tube with KClO3(s) is heated with a bunsen burner to melt the potassium chlorate. Molten potassium chlorate is a strong oxidizing agent that reacts violently with sugar. So once the molten potassium chlorate has been formed, adding a gummi bear to the test tube will cause a highly exothermic reaction.

Explanation. Gummi bears are made primarily of sugar, C6H12O6. When the sugar comes into contact with the strong oxidizing agent KClO3, a violent exothermic reaction occurs, producing heat and light. The reaction for this part is:

C12H22O11(s)  +  8 KClO3(l)   →   12 CO2(g)  +  11 H2O(g)  +  8 KCl(s)

Exothermic reactions like this one have a net decrease in chemical energy as the reaction progresses.

Key Concepts and Summary

Kinetic energy (KE) is the energy of motion; potential energy (PE) is energy due to relative position, composition, or condition. When energy is converted from one form into another, the total energy stays constant (the law of conservation of energy or first law of thermodynamics). Chemical and physical processes can absorb energy (endothermic) or release energy (exothermic). The SI unit of energy, heat, and work is the joule (J). Breaking bonds is always endothermic, while forming bonds is always exothermic. Whether the overall reaction is endothermic or exothermic will depend on the relative amount of energy that is required and released in the reaction process.


calorie (cal)
unit of heat or other energy; the amount of energy required to raise 1 gram of water by 1 degree Celsius; 1 cal is defined as 4.184 J
endothermic process
chemical reaction or physical change that absorbs heat
the capacity to supply heat or do work, which we discuss in detail in the next few sections
exothermic process
chemical reaction or physical change that releases heat
joule (J)
SI unit of energy; 1 joule is the kinetic energy of an object with a mass of 2 kilograms moving with a velocity of 1 meter per second, 1 J = 1 kg m2/s and 4.184 J = 1 cal
kinetic energy
energy of a moving body, in joules, equal to [latex]\frac{1}{2}\text{m}v^2[/latex] (where m = mass and v = velocity)
potential energy
energy of a particle or system of particles derived from relative position, composition, or condition
thermal energy
kinetic energy associated with the random motion of atoms and molecules
study of measuring the amount of heat absorbed or released during a chemical reaction or a physical change

Chemistry End of Section Exercises

  1. Which of  the following substances would you expect to have a relatively high chemical potential energy: Wood, Gasoline, Chalk, Water, Hydrogen Gas?

Answers to Chemistry End of Section Exercises

  1. Wood, Gasoline, Hydrogen Gas


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