Transition state theory (TST) explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium (quasi-equilibrium) between reactants and activated transition state complexes. TST is used primarily to understand qualitatively how chemical reactions take place. TST has been less successful in its original goal of calculating absolute reaction rate constants because the calculation of absolute reaction rates requires precise knowledge of potential energy surfaces. However, it has been successful in calculating the standard enthalpy of activation, the standard entropy of activation, and the standard Gibbs energy of activation Figure 1 for a particular reaction if its rate constant has been experimentally determined. TST is also referred to as "activated-complex theory," "absolute-rate theory," and "theory of absolute reaction rates."
Before the development of TST, the Arrhenius rate law was widely used to determine energies for the reaction barrier. The Arrhenius equation derives from empirical observations and ignores any mechanistic considerations, such as whether one or more reactive intermediates are involved in the conversion of a reactant to a product. Therefore, further development was necessary to understand the two parameters associated with this law, the pre-exponential factor (A) and the activation energy (Ea).
The basic ideas behind the transition state theory are as follows:
- Rates of the reactions are studied by studying activated complexes which lie at the saddle point of a potential energy surface. The details of how the complexes are formed are not important.
- The activated complexes are in a special equilibrium (quasi-equilibrium) with the reactant molecules.
- The activated complexes can convert into products which allow kinetic theory to calculate the rate of this conversion.
When complete equilibrium is achieved between all the species in the system including activated complexes, [AB]‡ , the concentration of [AB]‡ can be calculated in terms of the concentration of A and B. TST assumes that even when the reactants and products are not in equilibrium with each other, the activated complexes are in quasi-equilibrium with the reactants. At any instant of time, there will be a few activated complexes, with some being reactant molecules in the immediate past, which are designated [ABl]‡ (since they are moving from left to right). The remainder are product molecules in the immediate past [ABr]‡. If the system is in complete equilibrium, the concentrations of [ABl] ‡ and [ABr]‡ are equal, so that each concentration is equal to one-half of the total concentration of activated complexes. If the product molecules are suddenly removed from the reaction system, the flow of those activated complexes that began as products ([ABr]‡ ) will stop; however, there will still be a flow from left to right. Therefore, the assumption is that the rate of flow from left to right is unaffected by the removal of the products; in other words, the flux in the two directions are assumed to be independent of each other.