Question: The bromination of methane proceeds through the following steps:

(a) Draw a complete reaction-energy diagram for this reaction.

(b) Label the rate-limiting step.

(c) Draw the structure of each transition state

(d) Compute the overall value of$∆H^0$for the bromination.

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. The transition state isthemost unstable. The transition state or activated complex gets converted into product molecules. During the formation of this complex, old bonds start breaking,and new bonds start making.

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

A reaction energy diagram is used to understand the concepts of activation energy and transition state graphically. 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.

Reaction-energy diagram

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

Rate-limiting step is shown in the reaction energy diagram

(c) Two transition states can be seen in the reaction energy diagrams. At least heat must be placed for the reaction to take place to overcome the transition state.

Transition states are shown in the reaction-energy diagram

(d) Based on the reaction-energy diagram, the free radical bromination of methane is exothermic. As can be seen, the reaction releases of heat when one mole of methane undergoes free-radical bromination.

Overall value of$∆H^0=+73KJ+(-112KJ)=-39KJ$