Physical models of chemical reactions

A theory appropriate for describing reactions in dilute gases cannot be easily extended to reactions taking place in condensed phase. The spirit of transition state theory can be applied to ion reactions in electrolytes, if the concentrations are replaced by the practically useful, but theoretically not well-founded notion of activity . Since the activity coefficients of each ionic species is a function of the quantities of all ionic species present, the picture of collisions among the individuals of finite discrete qualities cannot be maintained rigorously. 

The velocity of migration of individual ions is characterised by mobility. The mobility of ions directly influences the diffusion constant, and indirectly the reaction rate, through its effect on diffusion. For the connection between the theory of ion reactions and the Debye-Hiickel theory of electrolytes see, for example, Moelwigh-Hughes (1971). 

The overwhelming majority of chemical reactions taking place in living organisms are enzyme reactions. An enzyme reaction is a particular kind of chemical reaction, since the size of the components can be considerable different. A reaction of an enzyme with a substrate molecule cannot be interpreted as collision between spheres of near-equal diameter. From a formal point of view the enzyme reaction can be qualified as a heterogeneous or, more precisely, surface reaction, since the individual velocity of one component is very small, and can even be zero. Another difficulty is the identification and enumeration of the chemical components. The bulk of enzyme molecules are proteins. The primary structure of the proteins, i.e. the 

The quest for the referent of modern scientific theories is nontrivial and the danger of making category mistakes is serious. For example, it is not at all clear, that collision theory (as used for the description of molecular beam experiments) and phenomenological kinetics (as used to describe enzyme kinetics in biochemistry) have reference categories of the same logical type. 

Very often cells contain a small number of individual molecules of a given species. In this case the quantity of these enzymes can be given by an integer number and so the notion of concentration has no sense, even as an approximation. 

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