Question #62891

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 #E_(act)# and is the energy difference between the reactants and the first transition state in step 1, and the energy difference between the intermediate and the second transition state in step 2.

The step with the largest activation energy will be the rate determining step. In the figure, the second step has a larger #E_(act)# and is therefore the rate determining step.

So, to draw a two step exothermic reaction, with the second step rate determining, keep two things in mind:

1) #E_(act)# of the second step must be larger than #E_(act)# of the first step.
2) Energy of products must be lower than energy of reactants.