The Experimental Determination of Reaction Orders

In earlier sections, we described various single-term rate expressions in which v = fc[A]n, where n has the values 0, j, 1, , and 2. One can decide which, if any, of these forms is correct by fitting the data with any of several approaches. 

This is the method emphasized in the preceding treatments. Data taken to only one half-life do not suffice Note in Fig. 2-2 how data fitting v = fc[A]2 appear nearly linear for one half-life when plotted as In [A]r versus time. Small errors in the end point reading may cause deviations from an otherwise linear form. Either the assumed order is 

The buildup of very low concentrations of product, if it can be monitored, will provide the initial rate. During this time the concentrations of the reactants remain effectively constant. A family of such determinations provides both the form of the kinetic expression and the value of the rate constant. 

The time required to convert a given fraction of the limiting reagent is a characteristic of the rate equation. A comparison of successive half-times, or any other convenient fractional time, reveals whether a reaction follows any simple-order rate law. Thus, the ratio of the time to reach 75 percent completion to that for 50 percent is characteristic of the reaction order. Values of this ratio for different orders are as follows  

The evaluation of the indicated ratio of times is easier than it looks. The display of Y, versus time allows the quick evaluation of f /2 and 73/4, as the times at which the recording of Y has progressed to 50 percent and 75 percent of its final value. 

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Before leaving the discussion of kinetics, two points concerning the experimental determination of reaction orders should be noted. First, the kinetics of surface reactions, in contrast to those of homogeneous systems, are temperature-dependent. This must be the case since the relative surface coverages of the reactants A and B are... 

Variations of this approach include the experimental determination of reaction orders. For a general rate law,... 

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