Shock theory
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 concentrations of the reacting substances in square brackets
- the impact factor k .
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 activation energy E A
- the gas constant R
- the temperature T in Kelvin
- the impact density, i.e. the number of impacts between A and B per unit of time and volume:
- the impact cross section σ from the diameter of the two particles
- the Boltzmann constant
- the reduced mass µ of the particles A and B.
- the Avogadro constant N A , which indicates the number of particles in a mole .
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
- ↑ 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 .