# 4.6: Exothermic and Endothermic Reactions


Learning Objectives

• Describe three ways to speed up a reaction.
•  Differentiate between reversible exothermic, irreversible exothermic, and endothermic reactions and draw appropriate graphs to represent each which include axis labels, activation energy, and the effect a catalyst would have on the graph.

## Energy

Energy can be defined as the capacity to supply heat or do work. One type of work (w) is the process of causing matter to move against an opposing force. For example, we do work when we inflate a bicycle tire—we move matter (the air in the pump) against the opposing force of the air surrounding the tire.

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. Water at the top of a waterfall or dam has potential energy because of its position; when it flows downward through generators, it has kinetic energy that can be used to do work and produce electricity in a hydroelectric plant (Figure $$\PageIndex{2}$$). A battery has potential energy because the chemicals within it can produce electricity that can do work.

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 be changed in form. (This is also one version of the first law of thermodynamics, as you will learn later.)

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 combusted within the cylinders of a car’s engine, the rapidly expanding gaseous products of this chemical reaction generate mechanical energy (a type of kinetic energy) when they move the cylinders’ pistons.

According to the law of conservation of matter (seen in an earlier chapter), there is no detectable change in the total amount of matter during a chemical change. When chemical reactions occur, the energy changes are relatively modest and the mass changes are too small to measure, so the laws of conservation of matter and energy hold well. However, in nuclear reactions, the energy changes are much larger (by factors of a million or so), the mass changes are measurable, and matter-energy conversions are significant. This will be examined in more detail in a later chapter on nuclear chemistry. To encompass both chemical and nuclear changes, we combine these laws into one statement: The total quantity of matter and energy in the universe is fixed.

## Thermal Energy, Temperature, and Heat

Thermal energy is kinetic energy associated with the random motion of atoms and molecules. Temperature is a quantitative measure of “hot” or “cold.” When the atoms and molecules in an object are moving or vibrating quickly, they have a higher average kinetic energy (KE), and we say that the object is “hot.” When the atoms and molecules are moving slowly, they have lower KE, and we say that the object is “cold” (Figure $$\PageIndex{3}$$). Assuming that no chemical reaction or phase change (such as melting or vaporizing) occurs, increasing the amount of thermal energy in a sample of matter will cause its temperature to increase. And, assuming that no chemical reaction or phase change (such as condensation or freezing) occurs, decreasing the amount of thermal energy in a sample of matter will cause its temperature to decrease.

'

Figure $$\PageIndex{3}$$: (a) The molecules in a sample of hot water move more rapidly than (b) those in a sample of cold water.

Most substances expand as their temperature increases and contract as their temperature decreases. This property can be used to measure temperature changes, as shown in Figure $$\PageIndex{4}$$. The operation of many thermometers depends on the expansion and contraction of substances in response to temperature changes.

Heat (q) is the transfer of thermal energy between two bodies at different temperatures. Heat flow (a redundant term, but one commonly used) increases the thermal energy of one body and decreases the thermal energy of the other. Suppose we initially have a high temperature (and high thermal energy) substance (H) and a low temperature (and low thermal energy) substance (L). The atoms and molecules in H have a higher average KE than those in L. If we place substance H in contact with substance L, the thermal energy will flow spontaneously from substance H to substance L. The temperature of substance H will decrease, as will the average KE of its molecules; the temperature of substance L will increase, along with the average KE of its molecules. Heat flow will continue until the two substances are at the same temperature (Figure $$\PageIndex{5}$$).

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 $$\PageIndex{6a}$$). 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 like ammonium nitrate) are brought together, the resulting process absorbs heat, leading to the sensation of cold.

Figure $$\PageIndex{6}$$: (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).

Each chemical bond is stored energy.  For a reaction to occur, the bonds of the reactants must be broken.  This process always requires energy.  At the same time, the bonds between the products must form, this process always releases energy.   If the energy required to break the bonds of the reactants is less than that given off by the formation of bonds in the products, the excess energy will be released from the reaction.  If the energy required to break the bonds of the reactants is greater than that given off by the formation of bonds in the products, the reaction will need an external supply of energy and the reaction will be endothermic.

Activation energy: Also called energy of activation.  It is the amount of energy required to get the reaction started.   Some reactions require very little energy, in others a great deal of energy is required.

Catalyst: Is a substance that when added to the reaction reduces the amount of energy required to start the reaction, but itself does not become part of the product and thus can be re-used in the reaction. If a catalyst is involved in a reaction, it is written above the arrow, as it does not become a part of the product.

HCl

Sucrose    +       H2O       →        Glucose     +    fructose

Reversible reaction: The energy hill may be approached from either side.

Irreversible reaction: The change in energy is so great, that the energy hill cannot be approached from the opposite side.

## Glossary

endothermic process
chemical reaction or physical change that absorbs heat
exothermic process
chemical reaction or physical change that releases heat
heat (q)
transfer of thermal energy between two bodies
kinetic energy
energy of a moving body, in joules, equal to $$\dfrac{1}{2}mv^2$$ (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
thermochemistry
study of measuring the amount of heat absorbed or released during a chemical reaction or a physical change

## Contributors

4.6: Exothermic and Endothermic Reactions is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.