There is a big misunderstanding in student's minds that one can ignore the fast step while calculating the rate of a reaction. It is not always true. This article will teach you the accurate way to look at multi-step reaction.
First things first, rate is calculated with reference to time. Generally,
- If time taken for a reaction/step is low, rate is considered to be fast.
- If time taken for a reaction/step is high, rate is considered to be slow.
Consider a reaction occurring in two steps as shown below. Time taken by whole reaction and individual steps is given for two cases.
| Rxn time | 1st step | 2nd step | |
|---|---|---|---|
| Case 1 | 10 s | 0.1 s | 9.9 s |
| Case 2 | 10 s | 4 s | 6 s |
In first case, total reaction time is 10 seconds and the slower step is consuming almost all of it (9.9 seconds). Hence, in this case we can ignore the fast step while calculating the rate of reaction. In other words, slower step is rate determining.
However, in second case, both steps are consuming a significant amount of total time. If we want accurate calculations of the reaction rate, we have to take in account both the steps, the slow step as well as the faster one. In other words, slow step is no longer the rate determining.
In Chemical Kinetics, such reactions are called as the Consecutive reactions. Calculations for their rate accounts for both the steps.
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