Question Video: Identifying Steps in a Reaction Profile Diagram for a Two-Step Chemical Reaction Chemistry

The reaction-profile diagram for a two-step chemical reaction is shown below. In step 1, 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?

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Video Transcript

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.

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