15. The Rates of Chemical Reactions   Previous PageNext Page
       Reduction of NO by H2
In an example discussed earlier we saw that the reaction of NO with H2 leads to a rate expression that is different from what might be expected from the equation of the reaction alone:

The reaction is first order in H2 concentration, not second order. One mechanism that will account for this rate behavior is

This mechanism is illustrated in fig 7 opposite. If all reactions except the k3 process are quite fast, then the rate equation is

You should be able to use the first equilibrium condition to eliminate N202 concentration, and show that the rate equation given previously is the result.

The last reaction (k4) occurs so fast that it scavenges any N20 as rapidly as it is formed, and has no effect on the overall rate of the process. In effect, N2 is produced as fast as N20 appears, so


In a series of reactions, the slowest one has the greatest influence on reaction rates. To invoke a painful analogy, if it takes ten days to to get a certified letter from Los Angeles to the White House, then rushing to get it into the one o'clock mail instead of the three o'clock will make little difference in the long run.

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