Consider the following reaction-energy diagram.

(a) Label the reactants and the products. Label the activation energy for the first step and the second step.

(b) Is the overall reaction endothermic or exothermic? What is the sign of ?

(c) Which points in the curve correspond to intermediates? Which correspond to transition states?

(d) Label the transition state of the rate-limiting step. Does its structure resemble the reactants, the products, or an intermediate?

The activation energy may be defined as the extra energy which the molecules of the reactants have to absorb so that their energy becomes equal to the threshold energy.

The highest energy state in a molecular collision that leads to reaction is the transition state. Transition state is most unstable. The transition state or activated complex gets converted into product molecules. During the formation of this complex, old bonds start breaking and the new bonds start making.

It may be defined as a species that exists for a finite amount of time, although it might be short but it has some stability.

To understand the concepts of activation energy and transition state graphically, reaction energy diagram is used. The vertical axis of the diagram represents the total potential energy of all the species present in the reaction. The horizontal axis represents the reaction coordinate that gives the progress of the reaction, proceeding from reactants on the left to products on the right. The highest point on the graph is the transition state and the activation energy is the difference in energy between the reactants and the transition state.

It may be defined as the slowest step of a chemical reaction that determines the rate at which the overall reaction takes place.

(a)

Reaction energy diagram with reactants, products and activation energies labelled.

(b)

The reaction is exothermic and the value of △H⁰ is smaller than zero (△H⁰< 0) .

(c)

Reaction energy diagram with intermediate and both transition states labelled.

(d) The highest peak in the diagram indicates the transition state. It is the rate-limiting step of the reaction since the reaction involved here takes place at the slowest rate.

Reaction energy diagram with rate-limiting step labelled

In an exothermic reaction, the transition state is closer to the reactants in both energy and structure. Hence, the transition state of the rate-limiting step resembles reactants.