The energy diagram represents the potential energy against the reaction coordinate, and gives us the energy change suffered by the reactants when transformed into the corresponding products.

diagrama-energia-sn2

During the course of the reaction, the reactants increase their energy until they reach the transition state, forming the activated complex. At that point the carbon-nucleophile bond is half formed and the carbon-leaving group half broken.

diagrama-energia-sn2

The energy diagram for this reaction is as follows.

diagrama-energia-sn2-03