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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Kythulu on October 01, 2019, 04:20:38 PM

Title: Kinetics problem
Post by: Kythulu on October 01, 2019, 04:20:38 PM

Hello,

I'm working a post lab review problem for my Kinetics Gen Chem II lab where I'm provided experimental data from 3 trials and the general equation 2A+2C --> D and I'm asked to find the rate law. Only problem is that none of the concentrations are kept constant between any of the trials. How can I find the rate law with no constant concentrations in the experimental data?

Title: Re: Kinetics problem
Post by: Vidya on October 01, 2019, 09:35:48 PM

You have two variable x and y
Get two equations from the given date
assuming rate law
R=k[C]^x[D]^y
We can solve it for x and y --only thing mathematics is not so simple now in this question as compare to one in which one concentration is kept constant.

Title: Re: Kinetics problem
Post by: Kythulu on October 01, 2019, 09:47:10 PM

I suppose that's what I'm getting at. I know how to set the equations up, but I can't see how to possibly solve for either of the variables. I don't know what I'm missing. I've tried utilizing logarithms and just end up with 3=x+y or some variation which I already know but I still can't just randomly assign the variables numbers that just happen to add up to 3. Or I'll end up getting rid of one variable only to have the remaining variable being defined by itself in a way that I can't isolate it without canceling the second variable out.

Title: Re: Kinetics problem
Post by: Vidya on October 01, 2019, 09:56:32 PM

make two equations for two variable
You can not solve this question with one equation
you need two equations in x and y which you can get from data given the question

Title: Re: Kinetics problem
Post by: Kythulu on October 01, 2019, 10:03:34 PM

(https://i.imgur.com/4nBZKut.jpg)

This is what I've done so far (I know it's a bit messy, sorry) So I have 3 equations and regardless of which 2 I put together or whether I utilize logarithms on any of them I still can't seem to solve for one variable. I feel like I'm missing something simple but I can't for the life of me figure out what it is.

Title: Re: Kinetics problem
Post by: mjc123 on October 02, 2019, 04:37:00 AM

You cannot find the rate law from this data set. Not only are no concentrations held constant, but they are always varied in the same proportions. So if we said
Rate = k[ B]^x[C]^y
then we can write
k*0.2^x*0.3^y*1^x+y = 1.02e-3
k*0.2^x*0.3^y*2^x+y = 8.16e-3
k*0.2^x*0.3^y*3^x+y = 27.5e-3
from which we can deduce x+y = 3, but cannot determine x and y separately.

Title: Re: Kinetics problem
Post by: Vidya on October 03, 2019, 01:59:22 AM

Yes you are right every time we are getting
X + Y =3
This data will not give us Rate law