Concept of Rate-Determining Step rds

In a kinetics scheme involving more than one step, it may be that one change occurs much faster or much slower than the others (as determined by relative magnitudes of rate constants). In such a case, the overall rate, and hence the product distribution, may be determined almost entirely by this step, called the rate-determining step (rds). 

For reactions in parallel, it is the fast step that governs. Thus, if A B and A C are two competing reactions, and if kAB kAC, the rate of formation of B is much higher than that of C, and very little C is produced. Chemical rates can vary by very large factors, particularly when different catalysts are involved. For example, a metal catalyst favors dehydrogenation of an alcohol to an aldehyde, but an oxide catalyst often favors dehydration. 

For reactions in series, conversely, it is the slow step that governs. Thus, for the scheme A B - % C, if fcj fc2, the formation of B is relatively rapid, and the formation of C waits almost entirely on the rate at which B forms C. On the other hand, if k2 then B forms C as fast as B is formed, and the rate of formation of C is determined by the rate at which B is formed from A. These conclusions can be obtained quantitatively from equation 5.5-7. Thus, if kx 

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