Lesson Video: Exothermic and Endothermic Reactions Chemistry

In this video, we will explore the meanings of ‘exothermic’ and ‘endothermic’ as they apply to reactions, and examine the energy transfers involved.

10:21

Video Transcript

In this video, we will explore the meanings of exothermic and endothermic as they apply to reactions and examine the energy transfers involved.

Let’s consider the simplest possible reaction, the formation of a single bond between two hydrogen atoms. Typically, a hydrogen atom has a single proton and no neutrons as 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.

However, energy cannot be created or destroyed. So when the two atoms bond together, all the energy must still be there. All the chemical potential they had when they were separated 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. So overall, chemical potential energy in our reactants becomes kinetic energy, which is lost to the surroundings, likely as heat. Taken in isolation, the hydrogen molecule is more stable than the separated atoms. So broadly speaking, we can say that bond formation releases energy.

So, what about the reverse? The reverse of bond formation is bond dissociation, otherwise known as bond breaking. Let’s take our stable hydrogen molecule from the last section. We can send a particle at it with a lot of chemical energy, exciting the molecule. If the conditions are right, we can break apart the hydrogen molecule, turning kinetic energy into chemical potential energy. This means that if we’re just speaking about the molecule and the atoms, bond dissociation requires external energy, so we often describe it as absorbing energy. This energy will often be delivered from the surroundings. The words we’re going to use from here on are exothermic and endothermic. But what exactly do they mean?

The two words both contain -thermic. This is because, most of the time, most of the energy transferred in reactions is thermal energy, what we commonly call heat. A hot object has more thermal energy than a cold object. This energy, naturally, will flow from the hot object to the cold object, so it’s quite natural to think about the energy entering or leaving a system as thermal energy. The exo- in exothermic means outside or simply out. The endo- in endothermic means within or simply in. So it would make perfect sense if exothermic meant thermal energy out and endothermic meant thermal energy in.

But chemists recognize that there are many forms of energy like light and sound that are involved in reactions. So, in fact, exothermic means energy of any kind out and endothermic means energy of any kind in. You’ll just have to remember that the thermic bit doesn’t really mean what you expect it to mean. Going back to what we were talking about in terms of bonds, during bond formation, chemical potential energy is converted to other forms of energy that are released, so bond formation is exothermic. Meanwhile, bond dissociation, the opposite, is endothermic. So we’ve looked at exothermic and endothermic as they apply to single bonds. But what about more complicated reactions?

Typically, when chemical reactions happen, bonds in the reactants break and subsequently bonds in the products are formed. We call any substances that exist along the way intermediates although the name doesn’t matter that much. We can label the total energy needed to break the old bonds energy in and the total energy we get out when the new bonds form energy out. If the energy out is more than the energy in, then, overall, energy has been released and the reaction is exothermic. However, if the energy in is greater than the energy out, then, overall, energy has bean absorbed and the reaction is endothermic.

Combustion reactions, like the combustion of methane, are good examples of exothermic reactions. In the products, the bonds are stronger than in the reactants, so overall we see a release of energy into the surroundings. We also see releases of energy during state changes; for instance, when a liquid turns into a solid, when water freezes, a small amount of energy is released to the surroundings. A good example of an endothermic reaction is the decomposition of calcium carbonate, a vital process in the production of cement. This requires a significant amount of energy. On the other hand, ice melting is much more mild and doesn’t require anywhere near as much energy as the decomposition of calcium carbonate, but it is an endothermic process.

When talking about reactions, we often only care about the reactants and products and nothing else. We call the reactants and products the system. Anything else, we simply call the surroundings. Quite often, when we’re talking about energy, we’re only really talking about the energy of the system. Energy in increases the energy of the system, and energy out decreases it. So, for energy in, the change in energy is positive. And for energy out, the change in energy is negative. So overall, if energy is released, the energy of the system goes down, the change in energy is negative, and the reaction is exothermic. On the other hand, overall, if energy is absorbed, then the energy of the system goes up, the change in energy is positive, and the reaction is endothermic.

We can also talk about the energy of the system in terms of its enthalpy. It’s beyond the scope of this video but exactly the same principle applies when we’re talking about exothermic and endothermic reactions. So for an exothermic reaction, the change in enthalpy will be negative; the enthalpy of the system will have decreased because energy has been released to the surroundings, while in an endothermic reaction, the change in enthalpy is positive; the enthalpy of the system will have increased because energy will have been absorbed from the surroundings.

So, we’ve already discussed that energy doesn’t need to be thermal energy 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’s pretty certain that the reaction is exothermic and vice versa. But if the surroundings get colder, then the reaction is endothermic. Now that we know so much about endothermic and exothermic processes, let’s have some practice.

Consider the chemical reaction equation shown. H plus H react to form H single bond H. Is this reaction endothermic or exothermic?

The reaction equation shows that we’re forming a bond between two hydrogen atoms, so this is an example of bond formation. Generally, energy is released to the surroundings when bonds are formed. In an endothermic reaction, energy is absorbed. And in exothermic reaction, energy is released. Therefore, in this reaction, since we are only forming a single bond and we are breaking no bonds at all, this reaction is exothermic.

So, that one was relatively straightforward. Let’s do a tougher one.

A chemical reaction has a reaction enthalpy of positive 430 kilojoules per mole. Is it endothermic or exothermic?

So what we have here is an unknown chemical reaction where reactants are transforming to products and the change in enthalpy is positive. What this means is that because of the reaction, more energy must have entered the system than has left it. An endothermic reaction is a reaction where the energy in exceeds the energy out. This is usually accompanied by a decrease in the temperature of the surroundings. Meanwhile, an exothermic reaction is one where the energy released is greater than the energy absorbed. This is usually accompanied by an increase in the temperature of the surroundings.

Since the enthalpy change is positive, the enthalpy after the reaction is greater than the enthalpy before the reaction. The reaction is, by definition, endothermic even though we don’t know what the reactants and products are.

Let’s finish up with the key points. In exothermic reactions, more energy is released than it’s absorbed. In exothermic reactions, the total energy of the system goes down. So the change in energy is negative, the change in enthalpy is negative, and the surroundings generally heat up. The formation of bonds is an example of an exothermic process. On the other hand, in endothermic reactions, more energy is absorbed than it’s released. In endothermic processes, the total energy of the system goes up. So the change in energy of the system is positive, the change in enthalpy is positive, and the surroundings generally cool down. And finally, bond dissociation or bond breaking is an example of an endothermic process.

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