July 11, 2020, 10:23:01 AM
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Topic: Rate determining step (conceptual question)  (Read 525 times)

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Offline bebe_gege

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Rate determining step (conceptual question)
« on: April 07, 2020, 10:22:09 AM »
Hello,

In school I learned that the rate determining step is the slowest step in a reaction because it has the highest activation energy. I was wondering whether there is a situation where the slowest step does not have the highest activation energy due to some other factor, such as shape or size of the molecule. So far, I have not found any source that talks about factors other than the activation energy, and my teacher is not aware of such situations.

Best,
bebe_gege.

Offline Babcock_Hall

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Re: Rate determining step (conceptual question)
« Reply #1 on: April 07, 2020, 11:35:03 AM »
I am not sure that what you wrote is correct.  What about a mechanism with a fast rate constant but with a transition state that is the highest point in the free energy profile?

Offline Enthalpy

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Re: Rate determining step (conceptual question)
« Reply #2 on: April 09, 2020, 09:41:46 AM »
The activation energy is only one parameter in the reaction rate. There is a pre-exponential factor too. And concentrations matter as well.

Offline Corribus

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Re: Rate determining step (conceptual question)
« Reply #3 on: April 09, 2020, 11:53:47 AM »
In chemistry there are usually exceptions to everything. But because the activation energy has an exponential relationship to rate, it is usually the dominant factor in determining relative rates of different steps in a complex reaction pathway. The step with the highest activation energy also determines the overall activation energy for the multi-step reaction, which is why it is usually called the rate limiting step.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Babcock_Hall

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Re: Rate determining step (conceptual question)
« Reply #4 on: April 09, 2020, 10:45:32 PM »
Pages 26-28 in Organic and Bio-organic mechanisms by Page and Williams has a discussion of this topic.  My summary may not do the topic justice.  They point out that one needs to distinguish between rate constants and rates.  In a reaction in steady state, all of the rates are the same.  When the intermediates do not build up to significant levels, the highest point in the free energy profile is the rate-determining step.

In enzymatic reactions Northrop questioned the concept: Biochemistry 1981 20(14) 4056-61.  Keith Laidler wrote an article in J. Chem Ed. 1988 65(3) 250.  https://doi.org/10.1021/ed065p250.  He discusses some misconceptions about the concept.

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