[CHM 112] Predicted Rate Law

[Andrew J. Pounds]

On 07/07/13 16:38, wrote:
> Hey Dr. Pounds,
> Can you go over how to come up with a predicted rate law? (With particular reference/example of number 13 in additional problems for chapter 14).
>
>
I really can't go over ALL of this again, but I can give you the basic 
pieces as it relates to the problem you asked about.  Remember that we 
are looking for a mechanism that is consistent with the experimentally 
determined rate law.  In coming up with a mechanism we have to specify 
not only the reactions for the individual steps and if they come to 
equilibrium, but also the relative speed of that step (typically shown 
off to the right). Once we have all of these mechanism pieces in place 
we can derive a rate law for that mechanism.  What I did in the class 
notes was demonstrate how we do this.  With a mechanism in place the 
procedure is:

1. Find the rate determining step
2. Write a rate law based on the rate determining step ONLY
3. If the rate determining step is not the first step and involves 
intermediates, I have to work backwards up the train of individual steps 
to replace intermediates with components that are found in the overall 
reaction (again, I did this in the example I did in class)
4. Once I have a "derived" rate law from my mechanism that only involves 
components from the overall chemical reaction, then I am done with the 
"derivation".

Now -- in problem 13 all I wanted you to do was PICK which of the 
individual steps you thought was the rate limiting step and then derive 
the rate law based on your choice and see if it matched up with the 
experimentally determined rate law.  What I tried to demonstrate in my 
solution was that it could not be the first step but rather had to be 
the second step.  This is something you would have to do using "trial 
and error".  Problem 61 in Burdge (for which you have solutions in the 
student solutions manual) is similar to this.



-- 
Andrew J. Pounds, Ph.D.  (pounds_aj at mercer.edu)
Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627
http://faculty.mercer.edu/pounds_aj

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