Chemical Kinetics - Transition State Theory

In the collision theory, one of the requirements for a reaction to take place is that the molecules must possess a certain amount of kinetic energy. This amount must be at least equivalent to the activation energy.

Activation energy can be simply explained as the energy barrier which a reaction has. It is analagous to a hurdle which a runner has to clear before he/she progresses in the run. This concept of activation energy is highlighted by Transition state theory.

In this theory, it explains that when two reacting particles collide with each other, it will form a structure (sometimes books called it 'Activated Complex'), which is unstable, before forming the products. I have used the reaction between OH^- and CH₃CH₂CH₂Cl to illustrate. Details of this reaction can be found here.

What happens in the transition state is that one of the reactant's bond is breaking, which another is forming. Since strength of the covalent bond is dependent on the effectiveness of orbtial overlapping. The weakening of the C-Cl bond has a greater effect than the formation of the C-O bond, thus making the structure more energetically unstable.
To understand why despite a bond is being formed, the transition state is still energetically unstable, we need to visualise what is occur prior to the formation of the transition state.

The C-Cl bond was originally a strong bond but when the Cl moves away from the C, the overlapping of the orbital becomes less and less effective. To do this, energy is required. While at the start, there is no overlapping of orbtials between O and C. It is at the transition state, there is some small amount of overlapping (and not the full extend of what a C-O bond has), thus only a weak bond formed and less energy is evolved.

As a result if we use the energy profile diagram (also known as reaction profile diagram), which tracks the progress of the reaction, we can see that there is a spike in energy when the transition state is formed. The difference in the energy between reactants and transition state is the activation energy, E_a.
Therefore, the significance in transition state theory is that it allows us to understand why there is a need for reactants to possess that minimum energy before reaction can occur.
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Article written by Kwok YL 2009.}

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