Chapter 16, Problem 46A

If $A\to B$ is exothermic, how does the activation energyfor the forward reaction compare with the activationenergy for the reverse reaction $(A\leftarrow B)$ ?

Interpretation Introduction

Interpretation:

Whether $\text{A}\to \text{B}$ is exothermic or not is to be determined. The comparison between the activation energy for the forward reaction with the activation energy for the reverse reaction $\text{B}\to \text{A}$ needs to be done.

Concept introduction:

An activated complex is formed as a result of collision between two particles with a sufficient amount of energy required for the collision. When the colliding particles break their bonds to form new bonds with the atoms of the particles it has undergo collision, a temporary transition state is generated which is an activated complex.

An endothermic reaction is marked by the absorbed of energy and an endothermic reaction is marked by the release of energy.

Answer to Problem 46A

In the reaction, $\text{A}\to \text{B}$ the collision taking place had enough energy to get over the activation energy barrier. While, in the reaction, $\text{B}\to \text{A}$ energy must be added to the system to overcome the activation energy barrier.

Explanation of Solution

The reaction, $\text{A}\to \text{B}$ is an exothermic reaction i.e. energy is released in the reaction. Since it is exothermic it can be concluded that the collision taking place had enough energy to get over the activation energy barrier. The resulting product B is lower in energy than the reactant A.

In case of the reverse reaction, $\text{B}\to \text{A}$ energy is absorbed, i.e. it is endothermic in nature. Since the reaction is endothermic, it can be concluded that to meet the energy required for collision, energy must be added to the system to overcome the activation energy barrier.

By this, it can be said that the activation energy of exothermic reaction is lower than that of endothermic reaction.

Conclusion

The reaction $\text{A}\to \text{B}$ is an exothermic reaction and the reverse reaction, $\text{B}\to \text{A}$ energy is endothermic reaction.