Unit 5: Topic 10 - Multistep Reaction Energy Profile

Question 1

What do the different peaks on a reaction energy profile represent?

Answer 1

• Each of the peaks on a reaction energy profile corresponds to an elementary step of the reaction mechanism. Therefore, the number of peaks inside the energy diagram would indicate the number of elementary steps within the reaction.
• For example, in this diagram, the reaction will have 3 elementary steps.

Question 2

What is the rate-determining step and how is it represented in a reaction energy profile?

Answer 2

- The rate-determining step of a reaction mechanism is also known as its slowest step. The rate of the slowest step will also be the rate of the overall reaction, determined by its rate law.
- When looking at a reaction mechanism, the rate-determining step (the slowest step) in a mechanism will have the highest activation energy.
- Since the rate-determining step will have the highest activation energy, it will correspond to the highest peak on the diagram. For example, in this diagram, the second step will be the slowest step.

Question 3

What are the different parts of a reaction energy profile?

Answer 3

Here is the diagram this flashcard will be referring to:
- The beginning and end of the energy diagram will represent the overall reactants and products, respectively.
- The top of every peak represents transition states. During this stage, old bonds are being broken and new bonds are being formed.
- The plateau on the graph, other than the reactants and products, represents reaction intermediates. They are intermediate products generated with each elementary step and then used up in a later step. Therefore, they won’t be represented in the overall reaction.
- The difference in energy between a reactant or reaction intermediate and a transition state represents the activation energy for that particular step, as shown in the diagram.

Question 4

How can you figure out the energy change for a reaction based on its reaction profile?

Answer 4

- If the products are at a higher energy level than the reactants, then the reaction must have absorbed energy, therefore making the reaction endothermic.
- If the products are at a lower energy level than the reactants, then the reaction must have released energy, therefore making the reaction exothermic.
- To figure out the energy change for a particular elementary step, look at the energy states of the initial state(such as the reactants) and the resulting state(such as a reaction intermediate), and then follow the rules above.
- For example, in this energy diagram, the first elementary step(from A to B) is endothermic, the second elementary step(from B to C) is endothermic, while the third elementary step(from C to D) is exothermic. Nevertheless, the overall reaction(from A to D) will be endothermic.