How can I represent the activation energy in a potential energy diagram?

I don't want the answer to become too long, so I won't go into too much detail about activation energy and threshold energy. If you want to read more on that, check out this answer:

http://socratic.org/questions/what-is-activation-energy-what-is-threshold-energy-i-want-to-know-the-difference

In order to be able to correctly represent the activation energy on a potential energy diagram, you need to know two things

• the energy level of the reactants (if you're interested in th forward reaction) or the energy level of the products (for the reverse reaction);
• the threshold energy level.

For a forward reaction, the activation energy is equal to the difference between the threshold energy and the energy level of the reactants.

Once you identify the threshold energy and the energy level of the reactants, use a double arrowhead line to connect these two points on the potential energy diagram.

Here's how that's supposed to look on the potential energy diagram for an exothermic reaction and for an endothermic reaction

For a reverse reaction, you have the exact same opposite situation. This time, the activation energy must by drawn to include the threshold energy, which is the same for both the forward, and the reverse reactions, and the energy level of the products.

The reverse reaction for an exothermic process will be an endothermic process, and vice versa.

So, as a conclusion, try to identify the threshold energy first, since that's the same for both the forward, and the reverse reactions.

Next focus on finding the energy level of the reactants (or products) and simply draw a double arrowhead line that connects the two energy levels, i.e. the threshold energy and the energy of the reactants.