The collision theory of reaction rates

The height of this barrier is H — this is the activation energy for the reaction in the forward direction EJ. Reactants which, upon collision, do not have sufficient energy to surmount the barrier will remain as reactants. 

Viewed from the product side, the height of the barrier is H — Jfp. This is the activation energy for the reverse reaction E. The relation between the two activation energies is obtained very simply. We write 

In its simplest form the collision theory is applicable only to bimolecular elementary reactions. With additional assumptions it can be applied to first-order reactions, and with some elaboration it is applicable to termolecular elementary reactions. As an example, we choose an elementary reaction of the type 

It is obvious that this reaction cannot occur more often than the number of times molecules A and B collide. The number of collisions between molecules A and B in one cubic metre per second is given by Eq. (30.23)  

Every collision does not, in fact, result in the reaction of A and B, but only those collisions in which the energy of the colliding molecules exceeds E. The fraction of collisions in which the energy exceeds E is proportional to exp (-E /RT) so that the rate of the reaction is (after setting N = cNa) 

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Why does the collision theory of reaction rates conflict with equilibrium... 

The collision theory of reaction rates states that molecules, atoms or ions must collide effectively in order to react. For an effective collision to occur, the reacting species must have (1) at least a minimum amount of energy in order to break old bonds and make new ones, and (2) the proper orientation toward each other. 

The collision theory of reaction rates in its simplest form (the simple collision theory or SCT) is one of two theories discussed in this chapter. Collision theories are based on the notion that only when reactants encounter each other, or collide, do they have the chance to react. The reaction rate is therefore based on the following expressions ... 

Although the collision theory of reaction rates is evidently satisfactory when applied to a number of reactions, it fails conspicuously in many cases such as rapid chain reactions, reactions involving complex molecules etc. The collision theory has been oversimplified and suffers from following weaknesses ... 

Application of the collision theory of reaction rates to surface processes is not straightforward. The meaningful definition of a surface collision is difficult and the necessary assumptions, inherent in any quantitative treatment based on this approach, make the results of dubious validity and restricted usefulness. The movement of surface entities within the temperature range of interest could necessitate activation, but (in different systems) may alternatively be a rapid and facile process, and the expression defining the... 

The collision theory of reaction rates dealt with earlier gives a useful, even if a crude, picture of reaction rates and permits us to calculate the rates of reactions between simple molecules when the activation energies are known. However, this theory leaves much to be desired. It does not furnish a method of calculating activation energies theoretically. It provides no information on the details of reactive collisions. It also does not account for the role that the internal energy might play in the reaction. 

The idea that reaction rates are tied to the rate of collision is aptly called the collision theory of reaction rates. But nature s mysteries are not so easily solved. During a chemical reaction, the electron orbitals (that is, the electron clouds) on individual reactants overlap and coalesce, the way two bubbles come together and merge into one. Once the conditions are right, it takes about a quadrillionth of a second for the electrons in the individual orbitals to readjust themselves into the orbitals around the products. But if all reactions took place as fast as collisions, then food would cook before... 

Originally E was regarded as a constant quantity which represented the difference between the energies of normal and activated molecules (Arrhenius, 1889), a conclusion which also arises from the collision theory of reaction rates. The linear relation between In and 1/T then required by equation (1) has been found to be valid within the limits of the experimental error on innumerable occasions. However, Hinshel-wood (1926) pointed outiihat E would vary with the temperature if the additional energy of the activated molecules was distributed among more than two square terms, and even before the development of... 

The fundamental notion of the collision theory of reaction rates is that for a reaction to occur, molecules, atoms, or ions must first collide. Increased concentrations of reacting species result in greater numbers of collisions per unit time. However, not all collisions result in reaction that is, not all collisions are effective collisions. For a collision to be effective, the reacting species must (1) possess at least a certain minimum energy necessary to rearrange outer electrons in breaking bonds and forming new ones and (2) have the proper orientations toward one another at the time of collision. 

The Eaci is an important parameter in the collision theory of reaction rates, and it approximates the energy of activation in the transition state theory of reaction rates [94]. As indicated earlier, diffusion reactions for keratin fibers generally involve mutual diffusion coefficients because... 

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