# Chemical Kinetics: Reaction Mechanism (My working attempt shown)

1. Feb 21, 2010

### tehaznboi

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. Feb 23, 2010

### epenguin

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