Properties of Rate Coefficients

Temperature and pressure represent external variables that can be controlled in experimental studies of reaction rates. It has been found that the rate coefficients associated with the three basic types of elementary reactions in Tables 2-2 behave quite differently with regard to both variables, so that it is convenient to treat each reaction type separately. Bimolecular reactions will be discussed first, and decomposition and recombination reactions subsequently. 

Bimolecular reactions are basically independent of pressure. If an influence of pressure is observed, it is an indication that the reaction is not truly elementary but involves more than one chemical process. This complication will be ignored here. The rate of a bimolecular reaction increases as the temperature is raised. Laboratory experience shows that the increase of the rate coefficient with temperature over not too wide a range can be expressed almost always by an equation of the form 

Thermal decomposition reactions may be considered briefly. The breakage of a chemical bond requires an energy input so that these reactions are endothermic. Again, the rate coefficient displays an exponential temperature dependence. The activation energy is related to the endothermicity of the 

Recombination reactions are the inverse of unimolecular dissociation processes, and the associated rate coefficients correspondingly exhibit also a pressure dependence. Like thermal decomposition processes, recombination reactions require an energy transfer by collision. The pressure dependence results from the change in efficiency with which the excess energy is removed from the incipient product molecule by the third body M. The situation can be made clearer by writing the reaction as a sequence of two steps 

It is apparent that two limits exist. At low pressures when fcsn(M) kr, the overall rate law of the reaction will follow a termolecular rate law with an effective rate coefficient k0= kskq/kr, whereas at high pressures, when M(M) kr, the n(M) cancel and the formation of the product C proceeds in accordance with a bimolecular rate law. The effective rate coefficient then is kaD= kq. In the intermediate pressure region neither rate law applies, and the rate coefficient becomes pressure-dependent. Usually, the rate of the reaction depends somewhat on the nature of the third body M as well. 

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