In this explainer, we will learn how to describe exothermic and endothermic reactions and examine the energy transfers involved.
The simplest possible chemical reaction is the formation of a single bond between two hydrogen atoms:
Typically, a hydrogen atom has a single proton and no neutrons in its nucleus, with one electron in the electron cloud. When the hydrogen atoms approach one another, the electron of one hydrogen atom is attracted to the nucleus of the other. When they get close enough, a covalent bond is formed: the two electrons are shared between the two nuclei:
Energy cannot be created or destroyed, so the atoms bonded together must have the same total energy as the two separate hydrogen atoms.
Potential energy (PE) is converted to kinetic energy (KE) as the bond forms, but the total amount of energy stays constant:
All the chemical potential the hydrogen atoms have at the start becomes kinetic energy, as they rotate or vibrate as a molecule. When this high-energy excited hydrogen molecule impacts on other particles, some of this energy is lost and the hydrogen molecule becomes more stable:
Overall, chemical potential energy in the reactants becomes kinetic energy, which is lost to the surroundings (usually in the form of thermal energy). Taken in isolation, a hydrogen molecule is more stable than two separated hydrogen atoms. So, broadly speaking, we can say that bond formation “releases” energy.
The reverse of bond formation is bond dissociation, otherwise known as bond breaking. If another particle hits the hydrogen molecule with enough energy, the hydrogen molecule may break apart; during this process, kinetic energy is turned into chemical potential energy.
Bond formation produces a form of energy that can be shared with other particles, so we often describe the process as releasing energy. Meanwhile, bond dissociation requires external energy, so we often describe the process as absorbing energy. The words commonly used to describe these processes are “exothermic” and “endothermic.”
The two words contain -thermic because, generally, the energy transferred in reactions is thermal energy, which we commonly call heat. This energy will flow from hot objects to cold objects, so it is quite natural to think about the energy entering or leaving a system as thermal energy.
Exo- means “out” and endo- means “in.” Based on the words, it would make perfect sense if exothermic meant “thermal energy out” and endothermic meant “thermal energy in.” However, there are many categories of energy, like light and sound, that can be involved in reactions. So, exothermic means “energy of any kind out” and endothermic means “energy of any kind in.”
Energy does not need to be thermal to be exchanged between the system and the surroundings. However, we can generally still use energy changes to predict whether the surroundings will get hotter or colder. If we see the surroundings get hotter because of a reaction, then it is pretty certain that the reaction is exothermic. If the surroundings get colder, then the reaction is probably endothermic.
Bond formation is exothermic, and bond dissociation is endothermic.
Definition: Exothermic Reaction
It is a reaction that releases energy to its surroundings.
Definition: Endothermic Reaction
It is a reaction that absorbs energy from its surroundings.
Example 1: Identifying Problems in Descriptions of Exothermic Reactions
When reporting an experiment, a student writes that “combustion is an exothermic reaction because energy is produced in the form of heat.”
- Why is this description not correct?
- There are no energy changes during an exothermic reaction.
- Combustion is an endothermic reaction.
- Heat is not a form of energy.
- Energy cannot be produced.
- Exothermic reactions absorb heat.
- Which of the following is a more accurate description of the combustion reaction?
- Combustion is an endothermic reaction because energy is produced in the form of heat.
- Combustion is an exothermic reaction because energy is not produced.
- Combustion is an exothermic reaction because chemical energy is converted to heat.
- Combustion is an exothermic reaction because heat is converted to chemical energy.
- Combustion is an endothermic reaction because thermal energy is released.
Answer
Part 1
In a combustion reaction, a substance reacts with oxygen. Combustion reactions release energy, typically in the form of heat.
A reaction is exothermic if more energy is released to the surroundings than is taken in; the most common result of an exothermic reaction is an increase in the temperature of the surroundings.
At first glance, the student’s statement appears correct because combustion really is an exothermic reaction. However, the part stating that this is “because energy is produced in the form of heat” is not correct.
We know that energy is never created (produced) or destroyed, only converted from one form to another. Therefore, a reaction cannot produce energy.
Therefore, the answer is D.
Part 2
We have established that exothermic reactions involve an overall transfer of energy to the surroundings and that combustion is an exothermic reaction. We have also established that energy cannot be produced, only converted from one form to another. When substances react with oxygen and combust, chemical potential energy is converted into other forms of energy, like heat. In fact, heat energy typically makes up the majority of the energy converted from chemical potential energy. This is why we often simplify descriptions of exothermic reactions as “the release of heat.”
Answers A and E say combustion is endothermic, so they cannot be correct.
Answer B suggests that energy is not produced in an exothermic reaction; this is true, but it is also true of all reactions. No reaction can produce energy. All reactions involve the conversion of chemical potential energy to other forms of energy or vice versa. Answer B is not the correct answer.
Answers C and D both correctly describe combustion as exothermic and identify that energy is converted from one form to another; however, only answer C correctly identifies that it is chemical potential energy (or “chemical energy” for short) that is converted to heat.
The answer is C.
Example 2: Identifying Whether an Atomic Reaction is Exothermic or Endothermic
Consider the chemical reaction equation shown:
Is this reaction endothermic or exothermic?
Answer
The reactants in this equation are separated hydrogen atoms. This is not a typical reaction as we tend to find pure hydrogen in the form of hydrogen molecules . These atoms are combining to form , meaning that a single covalent bond holds the atoms together; this is the molecule we are familiar with.
No bonds are being broken, but bonds are being formed. Bond formation is an exothermic process as the formation of bonds converts potential energy to other forms of energy, like heat, that can be lost to the surroundings.
Therefore, this reaction is exothermic.
Typically, in chemical reactions, bonds in the reactants break, and bonds in the products then form. We call any substances that exist along the way intermediates. The total energy needed to break the old bonds is energy that must enter the system (energy in, ) and the total energy we get out when the new bonds form is the energy that exits the system (energy out, ).
If , overall, energy has been released and the reaction is exothermic. However, if , overall, energy has been absorbed and the reaction is endothermic.
When talking about reactions, we often only care about the reactants and products, nothing else. We call the reactants and products “the system.” We simply call anything else “the surroundings.”
Often, when we are talking about energy, we are only really talking about the energy of the system. Energy entering the system increases the energy of the system, and energy leaving the system decreases it. So, for energy in, the change in energy is positive; for energy out, the change in energy is negative.
| If, overall, | the energy of the system , | the change in energy is , | and the reaction is . |
|---|---|---|---|
| energy is released | goes down | negative | exothermic |
| energy is absorbed | goes up | positive | endothermic |
We can also talk about the energy of the system in terms of its enthalpy. Exactly the same principle applies when we are talking about exothermic and endothermic reactions. The enthalpy change for a reaction can be described by the difference between the enthalpy of the reactants and that of the products.
For an exothermic reaction, the enthalpy of the products is lower than the enthalpy of the reactants. As a result, the change in enthalpy is negative. However, in an endothermic reaction, the enthalpy of the products is greater than the enthalpy of the reactants, and so the change in enthalpy is positive.
The difference in enthalpy for exothermic and endothermic reactions is shown in the following diagram.
Example 3: Identifying Endothermic and Exothermic Reactions from Reaction Enthalpies
A chemical reaction has a reaction enthalpy of kJ/mol. Is it endothermic or exothermic?
Answer
A reaction enthalpy is the change in enthalpy of the system for a reaction. A positive change in enthalpy means that energy is entering the system. Everything that is not the system is the surroundings. For energy to enter the system, it must leave the surroundings (since energy cannot be created or destroyed).
An endothermic reaction is a reaction in which energy from the surroundings is absorbed by the system. Therefore, since the chemical reaction has a positive reaction enthalpy, it is endothermic.
Here are some examples of exothermic reactions and physical processes:
- The combustion of methane
- Freezing
- Condensing
If, collectively, the bonds in the products are stronger than the bonds in the reactants, the reaction will be exothermic.
Changes of state also involve changes in bond strength. For example, the intermolecular forces in ice are stronger than the intermolecular forces in water, so when water freezes, there is a release of energy into the surroundings.
However, if the bonds in the products are weaker than the bonds in the reactants, the reaction will be endothermic.
Here are some examples of endothermic reactions and physical processes:
- The decomposition of calcium carbonate
- Melting
- Evaporating
Example 4: Comparison of Heat Transfer in Combustion and Freezing
When 10 mL of gasoline freezes at , the total amount of heat transferred is approximately 1 kJ. However, 300 kJ of heat is transferred when the same volume of gasoline is burned at room temperature. Why does combustion result in a greater transfer of heat than freezing?
- Freezing is exothermic, while combustion is endothermic.
- Bonds broken during combustion are weaker than those broken during freezing.
- Bonds formed during combustion are stronger than those formed during freezing.
- Combustion occurs at a higher temperature than freezing.
- Freezing is endothermic, while combustion is exothermic.
Answer
In this question, we are comparing two different quantities of energy. Both reactions are exothermic and as such we can eliminate answers A and E. In addition, while it is likely that combustion does indeed occur at a higher temperature than freezing, it does not help us to compare the quantities of energy transferred.
Energy changes that occur during chemical reactions or physical processes, such as freezing, occur due to the difference in energy required to break the bonds in the reactants and the energy released when new bonds are formed in the products. If more energy is released when 1 g of gasoline is burned, we know that the bonds formed in the products of combustion are stronger than those formed during freezing. As such, the answer is C.
The decomposition of calcium carbonate is a vital process in the production of cement. This requires a significant amount of energy. Melting ice does not require anywhere near as much energy as the decomposition of calcium carbonate, but it is still an endothermic process.
Example 5: Classifying a Named Reaction as Endothermic or Exothermic Based on the Reaction Enthalpy
The complete combustion of methanol proceeds according to the equation
The reaction enthalpy is kJ/mol. Is the combustion of methanol endothermic or exothermic?
Answer
An exothermic reaction is one that, overall, releases more energy than it takes in. This will usually be in the form of heat.
The reaction enthalpy is negative. This means that the energy of the system is lower after the reaction than before the reaction. This would match the description of an exothermic reaction.
The answer is “exothermic.”
Key Points
- In exothermic reactions, more energy is released than is absorbed. This means that the enthalpy of the system goes down and that the enthalpy of products is lower than the enthalpy of the reactants.
- In exothermic reactions, the total energy of the system goes down, the change in energy is negative, the change in enthalpy is negative, and the surroundings generally heat up:
- Bond formation is exothermic.
- In endothermic reactions, more energy is absorbed than is released. This means that the enthalpy of the system goes up and that the enthalpy of products is higher than the enthalpy of the reactants.
- In endothermic reactions, the total energy of the system goes up, the change in energy of the system is positive, the change in enthalpy is positive, and the surroundings generally cool down:
- Bond breaking, also known as bond dissociation, is endothermic.
