Mass action kinetics macroscopic and microscopic approach

1 Mass action kinetics macroscopic and microscopic approach 

Reaction kinetics (it is important to emphasise that it is kinetics, and not dynamics, since no force law exists, at least not according to any traditional interpretation) governs the velocity of the composition changes. Kinetics assigns to each reaction of the network a velocity function. The most common model can be associated with the mass action law. The velocity of the reaction 

If the participating molecules of a chemical reaction go through states that may be identified as separate chemical components, then the reaction is called complex otherwise it is simple, or elementary in the kinetic sense. Reaction (1.1) is an elementary reaction. By an elementary reaction we mean a chemical reaction with some mechanistic significance between reaction participants (e.g. collisions). 

The molecularity of an elementary reaction is the number of molecules that are to collide in order that the elementary reaction take place. In a bimolecular reaction the transformation is the result of the collision of two molecules. The collision of three or more molecules is highly improbable, a seemingly trimolecular reaction usually is the resultant of mono- and bimolecular elementary steps. In our example the molecularities of the chemical species A, B, C and D are a, b, c and d. 

The order of a reaction with respect to a component is the exponent to which the concentration of the components influencing the rate of the reaction are to be raised in order to get the rate expression. The order of the reaction is the sum of the orders with respect to all the components. The basic assumption of mass action kinetics is that the orders of an elementary reaction with respect to the components are given by the stoichiometric coefficients or molecularities as in (1.2). 

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