The Order of a Rate Law

The order of a reaction (this is the common parlance more precisely,2 the order of a rate law) is the sum of the exponents of the concentration factors in the rate law. One can also refer to the order with respect to a particular species. Consider the reaction in Eq. (1-11), with the rate law given by Eq. (1-12)  

Overall, the reaction is fourth-order. It is first-order with respect to [BrOj ] and [Br-], and second-order with respect to [H+]. 

The reaction between iodine and a benzylchromium complex,3 ArCH2CrL2+ (L s are incidental ligands shown in the original reference), Eq. (1-13), proceeds at a rate given by Eq. (1-14)  

According to the definition given, this is a second-order reaction. Clearly, however, it is not bimolecular, illustrating that there is distinction between the order of a reaction and its molecularity. The former refers to exponents in the rate equation the latter, to the number of solute species in an elementary reaction. The order of a reaction is determined by kinetic experiments, which will be detailed in the chapters that follow. The term molecularity refers to a chemical reaction step, and it does not follow simply and unambiguously from the reaction order. In fact, the methods by which the mechanism (one feature of which is the molecularity of the participating reaction steps) is determined will be presented in Chapter 6 these steps are not always either simple or unambiguous. It is not very useful to try to define a molecularity for reaction (1-13), although the molecularity of the several individual steps of which it is comprised can be defined. 

A formal definition of the reaction order with respect to the concentration of substance i, C,, is 

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