Collision theory

The collision theory, or also called collision theory, is a simple mechanical theory for the course of chemical reactions. It is assumed that for a reaction to take place, a collision between the two reaction partners, in which a certain so-called threshold energy must be exceeded along the internuclear axis connection. For a simple bimolecular reaction of the type A B → P can create the following rate law:

The square brackets to indicate that one must consider the concentrations of the reacting substances. The collision theory now deals with the calculation of the factor k (impact factor).

This requires the collision density, ie the number of collisions between A and B per unit time and unit volume.

Here σ is the collision cross section of the diameter of the particles, μ is the reduced mass of particles A and B, and NA is the Avogadro constant that specifies the number of particles in a mole. The collision theory now is the change in concentration of A together with a factor for the probability of a successful response. In the factor EA is the activation energy, R is the gas constant and T is the temperature in Kelvin.

If at this point both equations together, we obtain the following equation:

In simple terms, this is now put in the form of an Arrhenius equation:

The collision theory provides for simple gas phase reactions between spherical ion usable values ​​for topologically complex systems it is unusable. The values ​​are the poorer, the more the shape deviates of the reactants from the ideal spherical shape. Empirically, one can address this need that you extended the equation to a so-called steric factor P:

P is less than 1 in order to calculate for most molecules several orders of magnitude reasonable values ​​for P in theory, must be resorted to more complicated theories such as the theory of the transition state.