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Topic: reaction rate at concentrations >1M  (Read 4139 times)

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Offline AVG

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reaction rate at concentrations >1M
« on: December 14, 2008, 09:27:20 PM »
Dear colleagues,
I would really appreciate if someone could give me at least a hint...

Let's imagine two competitive reactions: one has the 1st order on a substrate A (e.g. intramolecular rearrangement), and the other one has the 2nd order (e.g. dimerization).

V1=k1[A]
V2=k2[A]2

Let say, k1 is 2 times higher than k2. Now, if we have concentrations [A] > 2M, this will mean that the intermolecular rxn with the lower rate constant will be faster than the intramolecular rxn with the higher rate constant. For example, if [A]=3M, dimerization will be 1.5 times faster...

I thought may be all these kinetics laws are derived with an assumption of concentrations <1M, but I have not found any restrictions...

Is there a mistake? It is contrary to my intuition.

Thanks in advance.


« Last Edit: December 14, 2008, 09:53:11 PM by AVG »

Offline macman104

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Re: reaction rate at concentrations >1M
« Reply #1 on: December 14, 2008, 10:30:53 PM »
You can reason it like this.  If you are in a very high concentration environment, then while the dimerization is slower, there are so many around that it doesn't even matter.  A molecules are colliding so much that the rate is faster than an intramolecular rearrangement.  If you move to a more dilute solution, the A molecules don't collide as much, and therefore the intramolecular reaction will take precedence.

Offline AVG

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Re: reaction rate at concentrations >1M
« Reply #2 on: December 16, 2008, 01:17:27 PM »
thanks a lot for your reply macman104, I will think about it.
But if somebody will have another opinion I will be thankful to read it as well.

Offline AVG

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Re: reaction rate at concentrations >1M
« Reply #3 on: December 16, 2008, 05:49:16 PM »
macman104, I think you are right, thanks once again.
The question is closed.

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