What is transition state theory? How transition state theory is superior to collision theory? Discuss the limitations of this theory.
Transition State Theory (TST)
This theory was proposed by Erying and Polanyl in 1935. Transition state theory is also called Activated Complex Theory or Theory of Absolute Reaction Rate.
According to this theory, chemical reaction is simply a continuous series of changes in bond length and other rearrangement of atoms in reactant molecules as soon as the reactant molecules with sufficient energy begin to approach each other. Some bonds start getting longer and ultimately break while new bonds start forming. Reactant molecules thus lead to the formation of particular configuration of atoms. This critical configuration corresponds to maximum energy of the reacting system and is called transition state which is always in equilibrium with the reactant molecules.
The transition state is assumed to have properties of a ordinary reactant molecules but it has only transient existence (i.e. unstable). Once it is formed, it decomposes to products and hence, the rate of reaction is the rate of decomposition of transition state. The entire energy changes from reactant to products can be shown with the help of potential energy diagram.
Transition state theory is superior to collision theory because transition state theory can explain the reactions involving molecules having a large number of atoms. The formation entropy and hence the frequency factor is much smaller in such reactions.
Limitations of Transition State Theory
Transition state theory is mostly applicable for simple reactions in which the transition state is single and well-defined.
This theory does not consider all temperature-dependent effects. At very low temperatures, quantum mechanical effects can become important, which are not considered in this theory.
Transition state theory assumes that the transition state is in quasi-equilibrium with the reactants and products. This assumption may not always hold true, especially for highly reactive or unstable transition states.
Transition state theory is less applicable to enzymatic reactions where the enzyme-substrate interactions and the nature of the active site can significantly change the reaction pathway and transition state structure.
This theory assume an isolated gas-phase reaction. In solution or in the presence of a solvent, solvation effects and interactions with the environment can significantly change the rate of reaction.
