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The following reaction exhibits the rate law: Rate = k[NO]2[Cl2].
|2NO(g) + Cl2(g) 2NOCl(g)|
Explain why the following mechanism is not plausible for this reaction.
||NO(g) + Cl2NOCl(g) + Cl(g)|
||NO(g) + Cl(g) NOCl(g)|
The rate law is determined by the slow step: Rate = kslow[NO][Cl]
However, this rate law includes and intermediate that cannot be part of the rate law for the overall reaction - only NO or Cl2 can be part of the rate law. The [Cl] term can be eliminated using the fast, reversible step where the rate of the forward reaction is the same as the rate of the reverse reaction:
Rateforward = Ratereverse
Rateforward = kforward[NO][Cl2]
Ratereverse = kreverse[NOCl][Cl]
so kforward[NO][Cl2] = kreverse[NOCl][Cl]
substituting for [Cl] in the slow step rate law gives:
This rate does not match the experimental rate law so this cannot be a plausible mechanism.