Reaction Mechanisms

The rate law for a reaction is not necessarily related to the stoichiometry of the balanced overall reaction. Instead it depends upon the rate of each step in the overall reaction. These individual steps are the mechanism for the reaction. The rate law for each of these steps is determined by the balanced stoichiometry for that step. If the overall reaction consists of several steps, the slowest step is the rate limiting step. The rate limiting step determines the overall rate of the reaction.

As an example the solution to Problem 41 in Chapter 21 from Kask and Rawn General Chemistry (W.C. Brown; Dubuque, Ia : W.C. Brown, 1993), p 808 is given below.

Nitrogen (II) oxide from jet engines reacts with ozone in the stratosphere to produce nitrogen (IV) oxide and oxygen:

NO (g) + O3 (g) --> NO2 (g) + O2 (g)

The rate law for this reaction is rate = k [NO][O3]. Which of the following mechanisms is consistent with this rate law?



  1. NO + NO --> N2O2 (slow)
    N2O2 + O3 --> N2O3 + O2 (fast)
    N2O3 --> NO + NO2 (fast)
    ---------------------------------------
    NO + O3 --> NO2 + O2

    In this mechanism the first step is rate limiting (slow). Because this reaction requires a collision between two NO molecules the rate law must depend upon the concentration of NO times the concentration of NO. The rate law would be rate = k [NO]2. Since this is not the same as the rate law given above, this mechanism is not consistent with the experimental data.


  2. NO --> N + O (slow)
    O3 + O --> 2 O2 (fast)
    O3 + N --> NO2 (fast)
    ---------------------------------------
    NO + O3 --> NO2 + O2

    In this mechanism the first step is rate limiting (slow). Because this reaction is a unimolecular decomposition of NO, the rate of this reaction will depend upon the concentration of NO. The rate law would be rate = k [NO]. Since this is not the same as the rate law given above, this mechanism is not consistent with the experimental data.


  3. NO + O3 --> NO3 + O (slow)
    NO3 + O --> NO2 + O2 (fast)
    ---------------------------------------
    NO + O3 --> NO2 + O2

    In this mechanism the first step is rate limiting (slow). Because this reaction requires an NO molecule to collide with an O3 molecule, the rate of the reaction will depend upon the concentration of NO and the concentration of O3. The rate law would be rate = k[NO][O3]. Since this is the same as the rate law given above, this mechanism is consistent with the experimental data. That does not mean that this IS the mechanism. It means that this COULD BE the mechanism.


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