[ritwik06]

For the reaction:
2NO+O₂ -----> 2NO₂
The experimental rate law is r=k [NO]²[O₂]. Propose a mechanism for the reaction.

I cant think of anything except all three molecules:
2 NO and 1 O₂ combine together in one step.


Is there any possibility of multi step mechanism? I have tried several ways, but I always end up with k[NO][O₂]
Is it possible to get this rate law with a multi step reaction???

[Vidya]

 What do you think for this mechanism which is fit for the rate law which you suggested

      NO + O2     NO3 (fast )
      NO3 + NO  2NO2 (SLOW)

[ritwik06]

What do you think for this mechanism which is fit for the rate law which you suggested

      NO + O2     NO3 (fast )
      NO3 + NO  2NO2 (SLOW)

The slow step is the rate determining step:
r=k₂[NO₃][NO]

NO₃ is the intermediate. By a healthy assumption, that it is consumed a soon as it is produced, we have:
k₁[NO][O₂]=k₂[NO₃][NO]

so the final rate is k1[NO][O₂]. Aint it???


MERRY CHRISTMAS!!

[Rabn]

When dealing with rate laws and mechanisms you need to consider some basic probabilities.  The probability that 2 things collide is pretty high (think about how often two cars collide). Now think about the probability of 3 things colliding at the same time (how often do three cars collide simultaneously?). 

Now onto the proposed mechanism/rate law:
              k₁             
NO + O₂  NO₃
              k₁^'             
              k₂   
NO₃ + NO 2 NO₂
              k₂^'

rate = k₁[NO][O₂] - k₁^'[NO₃] + k₂[NO₃][NO] - k₂^'[NO₂]²

Contrary to vidya's suggestion, if k₁ is slow and k₂ is fast and irreversible, k₁^' will be negligible because as soon as the NO₃ is produced it is consumed in the forward direction. This is basically the steady state approximation, http://en.wikipedia.org/wiki/Steady_state_(chemistry).

Rate = k₁[NO][O₂]

If k₁ is fast and k₂ is slow and irreversible, k₁^' becomes important.

Rate = k₂[NO₃][NO] - k₁^'[NO₃]

The question now becomes this, which reaction is slowest? Try to visualize what happens during each reaction. Come on back and let us know what you think.

[ritwik06]

I am sorry, as I could not reply promptly as I had not studied equilibria then. But now I have done so.
I understood the first case where k1 is slow.


But in the second case where k1 is fast and k2 is slow and irreversible. Yeah, I got your point, but what does it have to do with the situation in my question.
The slowest step in my opinion is the backward reaction of step 2.
Please help me further.