Question

You have worked on a complex atmospheric mechanism which you now want to recreate in the lab. The mechanism includes multiple elementary steps and intermediates.

1. Describe what experiments you would prepare to determine the overall mechanism knowing that an analytical solution is not feasible. How would those experiments help you determine the overall rate of the reaction and the rate with respect each molecule?
2. For one of the steps in the reaction, you have identified an important intermediate which you want to isolate. Is it possible to isolate such an intermediate? Why?
3. In some cases, intermediates are short lived. Is it possible to stabilize such intermediates?
4. What about the transition state? Would we be able to stabilize it in a similar way as the intermediates?

Answer 1

a) The easiest way to find the mechanism is by ploting the initial rate of the reaction for different initial concentration of a particular species keeping concentration of all other species constant. The same can be done for all reactants involved.

Example:

If a reaction is like this,

A+B->C+D

We have to take different concentrations of A keeping the concentration of B constant to obtain a graph like this.

similarly by ploting for B, the rate as a function of concentrations of A and B can be obtained. With the overall kinetics the mechanism can be determined easily. From the kinetics obtained, with reference kinetics from models like Langmuir-Hinshelwood and Eley-Rideal the mechanism can be obtained.

b) An intermediate differs from a transition state in that the intermediate has a discrete lifetime (be it a few nanoseconds or many days), whereas a transition state lasts for just one bond vibration cycle. An intermediate may be an unstable molecule (a reactive intermediate) or a highly stable molecule.

c) Yes, intermediates can be stabilized easily.

d) No, the transitional state is of higher energy than both the products and the reactants and has partial bonds. So, it can not be isolated.

This graph below will give you the difference between intermediates and transition states.