Chapter 16, Problem 80A

Interpretation Introduction

Interpretation:

The reaction energy diagrams for the overall decomposition of nitryl chloride needs to be compared with the mechanism in previous problem.

Concept introduction:

Activation energy is the minimum amount of energy needed by interacting particles to form the activated complex and result in a reaction. It is denoted by ${\text{E}}_{\text{a}}$. A high ${\text{E}}_{\text{a}}$ indicates relatively low collisions have required energy to generate activated complex and the reaction rate is low. A low ${\text{E}}_{\text{a}}$ indicates higher collisions have enough energy to create activated complex and the reaction rate is rapid.

Answer to Problem 80A

The peak of the reaction would occur during the activated complex when step 1 is slower than step 2. If step 2 is slower than step 1 then the peak is located higher than step 1.

Explanation of Solution

Two-step mechanisms which decompose the reactant ${\text{NO}}_{\text{2}}\text{Cl}$ are as follows:

$\begin{array}{l}\text{step}\text{1:}{\text{NO}}_{\text{2}}\text{Cl}\left(\text{g}\right)\to {\text{NO}}_{\text{2}}\left(\text{g}\right)\text{+Cl}\left(\text{g}\right)\\ \text{step}\text{2:}{\text{NO}}_{\text{2}}\text{Cl}\left(\text{g}\right)+\text{Cl}\left(\text{g}\right)\to {\text{NO}}_{\text{2}}\left(\text{g}\right){\text{+Cl}}_2\left(\text{g}\right)\end{array}$

Consider if step 1 is slower than step 2. Since step 1 is slower that means that the activation energy is greater than that of step 2. In the energy diagram, the peak in the reaction would occur during the activated complex of step 1.

Consider if step 2 is slower than step 1. Therefore, the activation energy is greater and thus the peak is located higher than the peak for step 1 for the associated complexes.

In either case, however, the energy of the product is lower than both the reactants and the activated complex.

Conclusion

Activated complex, sometimes called a transition state, is a temporary, unstable arrangement of atoms in which old bonds are breaking and new bonds are forming.