Shock theory

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The frequency of reactions increases with concentration due to frequent collisions

The impact theory or collision theory is a mechanical theory for the course of chemical reactions , which was developed in 1916 by Max Trautz and in 1918 by the British chemist William Lewis . It is assumed with her that for a reaction a collision must take place between the two reaction partners, in which a certain threshold energy has to be exceeded along the core-core connection axis . The collision theory provides useful values, especially for simple gas phase reactions between spherical ions .

formulation

For spherical reactants

For a simple bimolecular reaction of type A + B → P the following rate equation can be set up:

With

The impact theory deals with the calculation of the impact factor. To do this, another approach is chosen for changing the concentration of A:

With

  • a factor in the likelihood of a successful response
  • the impact density, i.e. the number of impacts between A and B per unit of time and volume:

Inserting the factor and the shock density yields:

By comparison with the first equation above, the Arrhenius equation follows:

with the frequency factor or pre-exponential factor  A , which is composed of the impact number Z and the steric factor P:

The following applies to spherical reactants

For non-spherical reactants

The impact theory is useless for topologically more complicated systems: the values ​​become worse the further the shape of the reactants deviates from the ideal spherical shape. This can be taken into account empirically by adding a steric factor  P to the equation , which takes into account the spatial orientation of the molecules :

This steric factor is several orders of magnitude smaller than 1 for most molecules:

In order to theoretically calculate meaningful values ​​for P, one must refer to more complicated theories such as B. the theory of the transition state can be used.

literature

Web links

Individual evidence

  1. Max Trautz: The law of the reaction rate and the equilibrium in gases. Confirmation of the additivity of Cv-3 / 2R. New determination of the integration constants and the molecular diameter . In: Journal of Inorganic and General Chemistry . tape 96 , no. 1 , 1916, ISSN  1521-3749 , p. 1-28 , doi : 10.1002 / zaac.19160960102 .