Are the laws of chemistry established?

to directory mode

A. Formulation of the rate of reaction

As an example, consider the type of reaction

In the course of the reaction, the concentration of the starting materials and that of the product increase. Their change over time is described by the following three differential quotients.

The three differential quotients are different, but not independent of one another according to the reaction equation. For example, must have twice the value of, since with every particle A, two particles B convert to product C. The reaction rate is therefore used to characterize the reactions in their entirety, which is also associated with a clear definition of the rate constant. It applies


Usual notation of a general reaction equation:
In contrast to the notation in thermodynamics, it is common in kinetics to define the stoichiometric coefficients for both starting materials and products as positive numbers. This simplifies the notation of the pseudo-rate constants', which can be defined by choosing an equimolar approach for the reactant concentrations.

Accordingly, the general formulation of the reaction rate is:

  1. It should be noted that when using the molar concentrations, the volume must remain constant. This is usually not the case with gas reactions. While it is generally true for reactions in liquid solutions. However, if the reaction is examined at different temperatures, the volume must be determined at the respective temperature! In these cases it is more practical to use the molality as a measure of concentration!
  2. Instead of the amount of substance, the reaction rate can also be defined on the basis of the molality, the particle concentration, the partial pressures, etc. However, the stoichiometric coefficients must always be used accordingly!

B. To determine the RG function

Depending on the type of chemical reaction examined, very different functions occur. The following cases can be distinguished:

  1. RG product approach: A kinetic investigation always begins with the mixture of the starting materials. For times shortly after the start of the reaction, the reaction rate should therefore only be determined by the starting materials, since the influence of the products is still negligible. The approach applies here
  2. Coupled reactions: Often the products react back to the starting materials (reversible reactions) or react further to decomposition products (secondary reactions). The approach often applies here
  3. Complex RG laws and special cases: Reactions also occur in which the concentration of the products influences the reaction rate immediately at the start of the reaction or other special cases apply. The functions here take on a more complex form than in cases 1 and 2.