Classification of reaction rates

Reactions are roughly classified as fast reactions - and the rest. The borderline is indistinct, but the general consensus is that a fast reaction is one which is over in one second or less. Reactions slower than this lie in the conventional range of rates, and any of the techniques described previously can be adapted to give rate measurements. Fast reactions require special techniques. 

A very rough general classification of rates can also be given in terms of the time taken for reaction to appear to be virtually complete, or in terms of half-lives. 

Type of reaction Time span for apparent completion Half-life 

The half-life is the time taken for the concentration to drop to one-half of its value. If the concentration is 6 x 10 2 mol dm 3, then the first half-life is the time taken for the concentration to fall to 3 x 10-2 mol dm-3. The second half-life is the time taken for the concentration to fall from 3 x 10 2 mol dm-3 to 1.5 x 10 2 mol dm-3, and so on. 

The dependence of the half-life on concentration reflects the way in which the rate of reaction depends on concentration. 

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The functional group tree classification of reaction rate parameters is conceptually related to an extension of the thermochemical group-additivity to transition states. In conventional transition state theory, for ideal gases and neglecting tunneling and recrossings, the reaction rate coefficient is given by ... 

Within each of the classifications of reaction-rate methods, there are many different methods of display or mathematical manipulation of the data or equations used to calculate the initial concentration of the species being determined. The calculating technique used can have very significant effects on the accuracy of the analysis. For example, the kinetic role of the species being determined in methods employing first-order or enzymatic or other catalyzed reactions has a strong effect on the choice of measurement of the reaction rate. For the simultaneous, in situ, analysis of several components of a mixture, the choice of method is even more critical with respect to accuracy. Both the relative and absolute values of the rate constants, as well as the initial concentrations of the species to be determined, dictate the choice of method. Furthermore, within the mathematical framework of each of these calculation procedures, there are generally optimum or limited times at which rate data should be taken in order to minimize the effects of random and absolute error in measurement. The choice of procedure and optimization of the measurement... 

A rational nomenclature suggested for the classification of reaction rates. 

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