Lesson Video: Reaction Profiles Chemistry • 10th Grade

Video Transcript

Chemical reactions typically involve the release or absorption of energy. Reaction profiles are diagrams that indicate the energy levels at various steps along the reaction pathway. In this video, we will learn how to read reaction-profile energy diagrams and identify and describe the energy transfers involved.

A reaction profile looks something like this. This is the reaction profile for the following chemical equation where hydrogen gas and oxygen gas combine to form water. The 𝑥-axis of a reaction profile represents the reaction coordinate or the progress of the reaction. Rather than being linked to a specific unit of time, we can generally break down the reaction coordinate into a beginning, middle, and end: when there are reactants, when there’s a transition state in the middle, and when there are products.

The 𝑦-axis represents the energy of the particles involved in the reaction. So the height of the line toward the left gives us the energy of the reactants. The height of the line in the middle gives us the energy of the transition state. And the height of the line on the right gives us the energy of the products. We can quantify these amounts of energy and use specific units. But for our purposes here, we will mainly be looking at relative amounts of energy.

Another value of note is the enthalpy change of the reaction, often shortened as △𝐻. This value gives us the change in enthalpy or the change in total energy from the beginning to the end of the reaction. In other words, it’s the difference between the energy of the products and the energy of the reactants. In the reaction we see here, the energy decreases from the beginning to the end of the reaction. That means that the △𝐻 is negative. It also means that this is an exothermic reaction. When an exothermic reaction occurs, the lost energy will be released into the surroundings, mostly in the form of heat, but often in other forms of energy in smaller amounts as well.

Conversely, if there’s an increase in energy from the reactants to the products, that means that the enthalpy change will be a positive value. It also means that it is an endothermic reaction as energy has been absorbed from the surroundings to give the particles involved more energy. It is also worth noting that we call this value △𝐻 when the reaction runs at constant pressure. If the reaction instead runs at a constant volume, for example, inside a propane tank, we call this value △𝐸 or the change in internal energy of the system.

The next value of note is the activation energy. We find this energy by taking the difference between the energy of the transition state and the energy of the reactants. And it is the energy needed to turn the reactants into the highly unstable transition state before eventually becoming the products. Because the activation energy is a high amount of energy that must be reached before the reaction can proceed, we sometimes call it an energy barrier. We can think of this sort of like a ball rolling up and down a hill. If we push the ball up to the top of the hill giving it energy, it can then roll down into the valley on the other side.

The chemical formulas of the products and the reactants included in a chemical equation give us a good idea of what those molecules look like. The structure of the transition state, however, is not clearly indicated. The transition state, also known as an activated complex, is an extremely unstable high-energy state. It’s a necessary middle step toward becoming the products, but it only exists for a tiny fraction of a second. Since they only exist for such a short period of time, we don’t need to focus too much on what specifically an activation complex looks like. But to give us an idea, it might look something like this: an awkward and unstable arrangement of particles that look something like the products and something like the reactants.

Since oxygen and hydrogen molecules are extremely unlikely to spontaneously combine in this way, it takes a lot of energy to bend, twist, break, or otherwise distort the bonds to create the transition state on the way to creating the products. In fact, stability and energy are inversely related. If a particle has more energy, it’s more likely to move and change, making it less stable. Conversely, low-energy particles are more stable. We can think back to our ball-on-a-hill analogy to understand this relationship even better. A low-energy ball at the bottom of the hill is likely to remain in that general area. But a high-energy ball’s position at the top of the hill is unstable because it could easily roll down to find a new position at the bottom of the hill.

So the transition state is an example of something that is high energy and low stability. One way to provide the energy necessary to overcome the energy barrier is by heating or igniting the reactants with a flame. Note that not all reactions need extra energy to proceed as there’s enough energy in the surroundings to make some reactions occur spontaneously.

Another way to overcome the energy barrier is with a catalyst. A catalyst is sort of like a reaction helper. It arranges the particles in particular ways to make it easier for them to react without directly participating in the reaction. One catalyst for the water-producing reaction we’ve looked at here is palladium. When hydrogen gas reaches the surface of palladium, it weakens the bond between the hydrogen atoms. This allows the reaction to take place more easily as less energy is required to overcome the energy barrier. As a result, the reaction profile looks something like this, with a lower activation energy seen in the form of a lower central peak.

Now that we’ve learned a bit about reaction profiles and how to interpret them, let’s do some practice problems to review.

Labeled in the diagram are the chemical energies of three compounds, a through c. Which of the labeled compounds is highest in energy? Which of the labeled compounds is lowest in energy? Which of the labeled compounds is most stable? Which of the labeled compounds is least stable?

Based on the diagram included in the question, this question is asking us to look at the energy and stability of the three compounds. The axis of the diagram indicates that the energy increases as we move up from low energy at the bottom to high energy at the top. With this understanding in mind, we can see that the labeled compound with the highest energy is b, the one furthest up in the diagram, while the one with the lowest energy is a, the one furthest down in the diagram.

The third and fourth questions ask us to find the most and least stable of the three choices. Stability is not directly indicated by the diagram. But if we know the relationship between energy and stability, we can answer this question. In fact, energy and stability are inversely related. Particles with more energy are more likely to move and change, making them less stable. Particles without a lot of energy don’t move around as much, making them more likely to stay the way they are or making them highly stable. So, the choice with the highest energy, compound b, is also the choice that is the least stable. Conversely, the choice with the lowest energy, compound a, is also the choice that is the most stable.

To summarize our answers, we can say that the compound with the highest energy is compound b. The compound with the lowest energy is compound a. The compound that is the most stable is also compound a. And the compound that is the least stable is compound b.

The reaction-profile diagram for a two-step chemical reaction is shown below. In step one, compound a reacts to form compound b, and in step two, compound b reacts to form compound c. Which step has the highest activation energy? Which step is an exothermic reaction?

The reaction profile for this two-step chemical reaction looks a little different than the more commonly seen reaction profile of a single step reaction. But we can simply think of it as two single reaction profiles joined together end to end. The first part of the question asks which of these two steps has the highest activation energy. To find the activation energy, we take the difference between the energy of the reactants and the energy of the transition state. For the reaction profile, that means finding the vertical difference between the starting point, a for step one and b for step two, and the central peak in the step. If we visualize this on the diagram, we can see that step one has a higher activation energy than step two. So our answer to the first part of the question is that step one has the highest activation energy.

The next part of the question asks us to find which step is an exothermic reaction. As a reminder, an exothermic reaction is a reaction where energy is released into the surroundings. That means that the change in enthalpy is negative and the energy of the products will be lower than the energy of the reactants. So, as we move from a to b in step one and b to c in step two, which step involves a release of energy? The answer is step two. We can see that the products c have a lower energy level than the reactants b. This means that some of the chemical energy from b has been released into the surroundings. The opposite is true for step one. Since the energy increases as we move from the reactants to the products, energy is absorbed from the surroundings, making the reaction endothermic.

So, to answer the second part of the question, we can say that step two is the exothermic reaction. Overall, this question relies on us understanding the definitions of activation energy and exothermic reaction as well as being able to recognize the portions of the reaction-profile diagrams that apply to these concepts. In the end, we can say that the step with the highest activation energy is step one and the step that is an exothermic reaction is step two.

Now that we’ve done some practice problems, let’s review the key points of the video. Reaction profiles give the amount of energy in the system at various steps in the reaction. Those steps are the beginning when there are reactants, the middle when there’s a transition state, and the end when there are products. In a reaction-profile diagram, the 𝑥-axis gives you the progress of the reaction and the 𝑦-axis gives you the energy level at that step of the reaction. The enthalpy change of the reaction or △𝐻 is the difference in total energy between the products and the reactants.

The activation energy or 𝐸 a is the energy required to add to the reactants to reach the high-energy transition state. Energy and stability are inversely related, so a high-energy particle has low stability and a low-energy particle has high stability. And the activation energy can be lowered with the help of a catalyst.