Download a Printable Version Here (Adobe Acrobat Format)

Reaction Kinetics & Chemical Reaction Models

1. Literature Data & Order-of-Magnitude Estimates

Very often, the best value to use for the rate coefficient of the reaction is the literature value, i.e. experimentally determined coefficient, if available.  Consideration must be given to the temperature and pressure conditions since as the discussion above has illustrated, they have an effect on the rate coefficient.  This is true, for example, in the case of unimolecular reactions, and chemically activated reactions.

There several sources of chemical kinetic data, some of which are as follows:

(a) High Temperature Reactions- (for example, Methane Combustion) in chronological order:

Baulch et al. (1992), Wang (1992), Miller & Bowman (1989), Warnatz (1984), Westbrook & Dryer (1984).

(b) Chlorinated Hydrocarbons- High Temperature Reactions:

Qun & Senkan (1994), Senkan (1993).

(c ) For lower temperatures and reactions occurring in the ambient air:

DeMore et al. (1990).

An extensive data base for chemical reaction kinetics can be found in the National Institute of Standards Chemical Kinetics Database (1994).  The reference can be found at the end of the manuscript. 

Sometimes, a rough-order-of magnitude value for the rate constant is needed for two reasons: either no other value is available or the aim is to scan the mechanism for reactions that have small impact on the consumption of the reactant, product formation, or formation of any other species of interest.  One way to make such an estimate is by the method of analogous reactions as depicted in Table 2 taken form Senkan (1992).  On inspection of Table 2, there are several issues that are of importance in estimating rate coefficients.  Unimolecular fission reactions are endothermic, and the heat of reaction corresponds to the minimum activation energy that could be expected for the reaction.

Simple kinetic theory of bimolecular reactions gives the following expression for the rate coefficient of the reaction between molecules A and B ( Laidler, 1987),

(25)

Z_AB is the molar collision frequency, s_AB is the mean collision diameter or rigid sphere collision cross-section, m_AB is the reduced mass, N_A is Avogadro’s number, and k is Boltzmann’s constant.  The estimate of the molar collision frequency at 300 K turns out to be 1.0 x 10¹³ cm³/mol-sec, and it represents the upper limit for the bimolecular rate coefficient without accounting for the activation energy or steric factor.  As stated above, a lower limit for the activation energy for endothermic reactions is the heat of reaction.

Another method that can be used to estimate the activation energy of metathesis reactions such as,

is the Evans-Polanyi relationship for similar reactions or,

(26)

where - delta H_rn is the heat of reaction, which is defined as positive for an exothermic reaction, and a and b are the Evans-Polanyi empirical constants for the family of reactions.  Polanyi relationships often fail when there is charge separation involved in the transition state, such is the case when atoms or groups involved in the reaction differ in electronegativities.

Transition State Theory of Unimolecular/Bimolecular Reactions.