Initial Rates Iodine Clock

The several steps in a reaction are usually consecutive and tend to proceed at different speeds. Usually, when the overall rate is slow enough to measure at all, it is because one of the steps tends to proceed so much more slowly than all the others that it effectively controls the overall reaction rate and can be designated the rate-controlling step. A steady state is quickly reached in which the concentrations of the reaction intermediates are controlled by the intrinsic speeds of the reaction steps by which they are formed and consumed. A study of the rate of the overall reaction yields information of a certain kind regarding the nature of the rate-controlling step and closely associated steps. Usually, however, only 

When the mechanism is such that steady state is attained quickly, the rate law for Eq. (1) can be written in the form 

The most frequently encoimteied type of rate law is of the form [again using the reaction of Eq. (1) as an example] 

The order of a reaction is determined by the reaction mechanism. It is related to and is often (but not always) equal to the number of reactant molecules in the rate-controlling step—the molecularity of the reaction. Consider the following proposed mechanism for the hypothetical reaction 3A + 2B = products  

The overall reaction involves five reactant molecules, but it is by no means necessarily of fifth order. Indeed the rate-controlling step in this proposed mechanism is bimolecular, and the overall reaction order predicted by the mechanism is It is also important to note that this mechanism is not the only one that would predict the above - -order rate law for the given overall reaction thus experimental verification of the predicted rate law would by no means constitute proof of the validity of the above proposed mechanism. 

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The order of a reaction can also be determined by plotting the rate against different initial concentrations of a reactant, or the rate for reactions like the iodine clock and that between thiosulfate ions and dilute acid (see Chapter 6). 

In a closed system, the chlorite iodide reaction is a clock reaction if 1 < [I ]o/[C102 ]o < 4. Its kinetics were determined and the basic elements of a mechanism were proposed first for the batch reaction by Kern and Kim [22]. Figure 1 shows kinetic curves (iodine absorbance vs time) of the reaction with different initial ratios of the reactants. In the first part of each curve, iodine is produced at an accelerating rate. During this period, the stoichiometry is... 

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