1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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 = [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


    User Avatar
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook