Collision Theory
Collision theory of Rate of Reaction
Collision theory of rates of reactions was first proposed independently by Max Trautzb and William Lewis to explain the rates of bimolecular reactions occurring in gaseous state.According to this theory, the reactant molecules are assumed to be hard spheres and reaction is postulated to occur when molecules collide with each other. The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z). Another factor which affects the rate of chemical reactions is activation energy (as we have already studied).
For a bimolecular elementary reaction
A + B → Products
rate of reaction can be expressed as-
Rate = ZABe−Ea/RT ---Equation-1
where ZAB represents the collision frequency of reactants, A and B and e−Ea/RT represents the fraction of molecules with energies equal to or greater than Ea. Comparing equation-1 with Arrhenius equation, we can say that A is related to collision frequency. Equation-1 predicts the value of rate constants fairly accurately for the reactions that involve atomic species or simple molecules but for complex molecules significant deviations are observed. The reason could be that all collisions do not lead to the formation of products. The collisions in which molecules collide with sufficient kinetic energy called threshold energy and proper orientation, so as to facilitate breaking of bonds between reacting species and formation of new bonds to form products are called as effective collisions.
To account for effective collisions, another factor P, called the probability or steric factor is introduced. It takes into account the fact that in a collision, molecules must be properly oriented i.e.
Rate = PZABe−Ea/RT ---Equation-2
Thus, in collision theory activation energy and proper orientation of the molecules together determine the criteria for an effective collision and hence the rate of a chemical reaction. Collision theory also has certain drawbacks as it considers atoms or molecules to be hard spheres and ignores their structural aspect.
Summary of Collision Theory
1. For a reaction to occur, there must be collisions between the reacting species.2. Only a certain fraction of the total number of collisions are effective in order to products formation.
3. For effective collisions, the molecules should possess sufficient energy as well as orientation. Thus, the rate of reaction is proportional to the number of collisions per unit volume per second (collision frequency, Z) between the reacting species and the fraction of effective collisions (properly oriented and possessing sufficient energy, f).
Rate = z.f
Limitations of Collision Theory
Collision theory of reaction rate is extremely successful in rationalizing the kinetics of many reactions, however, it has some limitations which are given below-1. This theory finds application only to reactions occurring in the gas phase and solution having simple reactant molecules.
2. The rate constants obtained by employing collision theory are found to be comparable to what has been obtained from the Arrhenius equation only for the simple reactions but not for complex reactions.
3. This theory tells nothing about the exact mechanism behind the chemical reaction i.e. making and breaking of chemical bonds.
4. Collision theory considers only the kinetic energy of reacting molecules and just ignored rotational and vibrational energy which also plays an important role in reaction rate.
5. This theory did not consider the steric factor at all i.e. the proper orientation of the colliding molecules needed to result in the chemical change.
Transition State Theory
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