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Homework Help: Chemical Kinetics: Reaction Mechanism (My working attempt shown)

  1. Feb 21, 2010 #1
    1. The problem statement, all variables and given/known data

    1. According to the mechanism below, what is the correct expression for the rate of consumption of A? In the mechanism, M is a molecule that collides with A and A* is a highly-energized form of A.

    Step 1 (reversible): A + M <---> A* + M rate constants k1 and k-1

    Step 2 (irreversible): A* ---> C rate constant k2

    Rate = k1 [A] [M] / (k-1[M] + k2)
    Rate = [A] / [C]
    Rate = k1k2 [A]
    Rate = (k1 [A] [M] + k-2[C] ) / (k-1[M] + k2)
    Rate=k1k2 / (k-1+k-2)
    Rate = (k1+k2-k-1-k-2) [A]
    Rate = k1k2 [A] / (k-1+k2[M])
    Rate=k1+k2-k-1-k-2
    Rate = [C] / [A]


    2. Relevant equations

    3. The attempt at a solution

    Sorry, I didn't show my work before. But anyways, I eliminated some choices because I know they're wrong. I tried submitting them, but it kept rejecting it, and I only have 1 more try.

    Anyways here is my though process.

    k1 is the rate constant for the forward reaction and k-1 is the rate of the backwards reaction. I'm assuming that k-2 is the rate for the backs reaction of step 2. However, it's irreversible, so I do not believe that rate constant should even exist.

    In step 1, M seems to be a catalyst, since the catalyst is regenerated, I assumed it won't have anything to do with the rate of consumption, as in, it just has to be present.

    Since step 2 is irreversible, I assumed that [C] does not play a role in the consumption of [A] as it's not an equilibrium reaction.

    After eliminating all choices that involve any of the above, I get k1k2[A] as my only possible choice. I don't want to submit this however cause I'm not 100% sure if my logic is correct.
     
  2. jcsd
  3. Feb 23, 2010 #2

    epenguin

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    Homework Helper
    Gold Member

    However, apart from anything else, your choice would be saying the rate is independent of [M] - contradicting the fact that M is a catalyst!

    I think you cannot do it just by looking at the given formulation of the mechanism. You have to make some assumption, try the simplest you can find, about the mechamism. M cannot affect the reaction by so to speak grandstanding like your scheme shows, it has to interact with A in some way then disengage from it. Try to elaborate a hypothetical simple model, it will be much more useful to you than guesswork (although many of the answers are in fact unreasonable as you have recognised).
     
    Last edited: Feb 24, 2010
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