The figure above represents the reaction profile of a two step, exothermic reaction. The y-axis represents the potential energy of the reaction species, and the x-axis represents the progress of the reaction.
The reaction is exothermic because the energies of the products are lower than those of the reactants. The reactants are represented by the horizontal line at the far left of the graph; the products by the horizontal line at the far right. The intermediate species is represented by the trough in the middle of the graph, and the transition states by the two crests.
The activation energy of each step is given by the
The step with the largest activation energy will be the rate determining step. In the figure, the second step has a larger
So, to draw a two step exothermic reaction, with the second step rate determining, keep two things in mind:
2) Energy of products must be lower than energy of reactants.