Energy Diagrams - Example 1

The following is an example of a two-step reaction mechanism (i.e. Electrophilic Addition of H-X to Alkenes).

Since there are two steps, there will be two humps in the PES. There will also be two transition states.

Action

Hover your mouse over the reaction arrows and observe the reaction energy diagram. Do you see how each step (i.e. reaction arrow) corresponds to a "hump"?
Hover over each molecule one by one and observe where they lie on the reaction energy diagram.
The structures above each arrow are the transition states for each step. Hover over each so you can see where they lie on the reaction energy diagram.