Unit 5: Topic 6 - Reaction Energy Profile

Question 1

What are elementary reactions?

Answer 1

An elementary reaction is a reaction that occurs in one step. It has one transition state and no intermediates. In the process, bonds are broken and new bonds are formed which allows for products to form. However, many reactions are multi-step because they consist of multiple elementary reactions and could have intermediates.

Question 2

What are intermediates and catalysts?

Answer 2

Here is the example. An intermediate is a molecule that is a product of one elementary reaction and a reactant in another; NO3 is the intermediate in this case. A catalyst is a molecule that is a reactant of one elementary step and the product of another; in this case, it is NO2. Intermediates and catalysts don’t appear in the overall net reaction since they are canceled out.

Question 3

What is the x-axis of a reaction energy profile graph?

Answer 3

Here is the graph. On the x-axis, it is labeled reaction progress/reaction coordinate. This is the progress of a reaction (how the reactant changes to the product over time) and thus has no units.

Question 4

What is the y-axis of a reaction energy profile graph?

Answer 4

The y-axis is potential energy. The potential energy of the reactants is on the left side and that of the products is on the right. The highest point on the graph is the transition state/activated complex. This is like a hybrid between the reactants and the products where some bonds have been formed but the reactant is still transitioning into the product.

Activation energy is the difference of energy between the transition state and the reactant. It is the energy needed for the reaction to occur.

Question 5

What does the Arrhenius equation say about reaction rates?

Answer 5

Here is the Arrhenius equation. It relates the temperature to the rate of the elementary reaction. As temperature increases, the rate of the reaction increases as well because there are more collisions with enough energy to overcome the activation energy. You won’t need to use this equation for calculations on the AP exam, but you need to understand it conceptually.