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Topic: Rate Law/Rate Constant calculations  (Read 1271 times)

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Offline boodles

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Rate Law/Rate Constant calculations
« on: March 12, 2023, 06:28:12 AM »
Hello,

I am trying to complete a lab assignment for an organic chemistry course but I'm struggling because I feel like I understand the concept, but the data is not supporting my thoughts so unsure if I'm going wrong on calculations or in my understanding.

This is the data collected in the lab - ** Only one trial per concentration and poorly timed due to my lab partner not paying attention :(

Experiment 1  - Varying OH-
Run || [tBuCl] || [OH-] || Time(s) ||Reaction rate||Rate constant
1   0.03            0.003   36.09      8.3 X 10-5     0.0029
2   0.03            0.006   92.25      6.5 X 10-5     0.0024
3   0.03            0.009      183.78     5.0 X 10-5     0.0020

   
Experiment 2 Variation of [tBuCl]

Run||[tBuCl]||[OH] || Time||Reaction rate||Rate constant
4   0.06        0.003   34.84   8.6 X 10-5   0.0015
1   0.03        0.003   36.09   8.3 X 10-5   0.0029
5   0.015     0.003   46.40   6.5 X 10-5   0.0048


The reaction rates and rate constants were calculated from the concentrations and times. The OH- was meant to be used to neutralize the HCl produced and cause PH change/color change to time it. It was also assumed that the amount of OH that reacted would equal the amount of tBuCL that reacted at the measured time because it would stop neutralizing and cause color change at that point. I believe it's supposed to be an SN1 reaction where the Cl leaves first and H20 acts as the nucleophile, but I'm not 100% sure. If this is the case, shouldn't the rate constants all be the same for the varying tBuCl concentrations because rate constants don't change? I said the reaction is not dependent on OH because increasing the OH concentration did not increase the reaction rate. Increasing the tBuCl concentration did increase the reaction rate, which supports my SN1 hypothesis, but I don't understand why the rate constants differ for every experiment. For reference, nothing else about the reaction should have changed, they were carried out in the same room less than 2 minutes apart from each other so it's unlikely that temperature or any other factor is affecting the rate constant.

Please, any help to explain this would be great, or maybe I just made a math error and all the values are wrong? Any help is appreciated!

Offline Borek

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Re: Rate Law/Rate Constant calculations
« Reply #1 on: March 12, 2023, 02:27:13 PM »
Typically when you are getting different rate constants it means the order of the reaction (ie the mechanism assumed) is wrong.
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Offline boodles

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Re: Rate Law/Rate Constant calculations
« Reply #2 on: March 12, 2023, 02:51:59 PM »
Typically when you are getting different rate constants it means the order of the reaction (ie the mechanism assumed) is wrong.

We have only studied SN1 and SN2 reactions, so it has to be one of those. If it were SN2, wouldn’t the concentration of OH also be a factor?

Offline Hunter2

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Re: Rate Law/Rate Constant calculations
« Reply #3 on: March 12, 2023, 03:36:37 PM »
Tert. Butylchloride + OH- is SN1.

Offline boodles

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Re: Rate Law/Rate Constant calculations
« Reply #4 on: March 12, 2023, 04:09:34 PM »
Tert. Butylchloride + OH- is SN1.
Yes, this I understand, so my question is how can the calculated rate constants be different? Don’t they have to be the same, since it is literally a constant?

Offline Hunter2

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Re: Rate Law/Rate Constant calculations
« Reply #5 on: March 12, 2023, 04:12:07 PM »
I guess the measurements aren't accurate?

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