Proposing a reaction mechanism


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Lindsay H 2B
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Proposing a reaction mechanism

Postby Lindsay H 2B » Fri Mar 16, 2018 2:23 pm

Will we ever have to propose a reaction mechanism on our own on a test? In all the assigned homework problems about reaction mechanisms that I've found, the mechanism has already be given, but in some of the lecture examples it seemed like Dr. Lavelle wanted us to be able to propose a mechanism and come up with intermediates ourselves

Suchita 2I
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Re: Proposing a reaction mechanism

Postby Suchita 2I » Fri Mar 16, 2018 2:28 pm

If the rate law is given it is possible to determine the mechanism for the rate determining step. However, I do not think we will be asked to propose a mechanism and know the intermediates. I believe the question is more likely to focus on disproving a given mechanism or choosing the possible mechanism.

Jennifer Ho 1K
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Re: Proposing a reaction mechanism

Postby Jennifer Ho 1K » Sat Mar 17, 2018 2:12 am

You shouldn't have to propose a reaction mechanism on the test because this process mainly involves just guessing and checking to see if it matches the experimental rate constant expression, but you should be able to determine the rate law from the reaction mechanism (given a slow step), or choose the correct reaction mechanism based on the rate law by checking the overall rate law for each mechanism.

GabrielGarciaDiscussion1i
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Re: Proposing a reaction mechanism

Postby GabrielGarciaDiscussion1i » Sat Mar 17, 2018 2:41 am

I'm very close to 100% certain we will not have to propose a reaction mechanism on the test. Lavelle has mentioned it in his lectures when he says the majority of our work will be seen in the rate laws derived from mechanisms and seeing if the mechanism rate law matches the experimentally determined rate law.
Some of the UA's and TA's have had us question whether certain mechanisms make sense due to a unimolecular step being fast and termolecular step being slow, but I feel that covers the depth of most of what we will have to do with reaction mechanisms.

Clarissa Molina 1D
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Re: Proposing a reaction mechanism

Postby Clarissa Molina 1D » Sat Mar 17, 2018 3:28 am

Will we be told which step is the slow or fast step? If we are not told how do we find the slow step?

GabrielGarciaDiscussion1i
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Re: Proposing a reaction mechanism

Postby GabrielGarciaDiscussion1i » Sat Mar 17, 2018 11:28 am

Clarissa Molina 1D wrote:Will we be told which step is the slow or fast step? If we are not told how do we find the slow step?


Once again, almost positive that we will be told which step is the slow step or fast step. However, in the highly unlikely case we are not told, we can always look at factors such as Activation Energies. (A higher Ea means the reaction will be slower because a smaller percent of collisions will have the necessary amount of energy to overcome the Ea)
We can also look at how many molecules need to collide. Typically, a unimolecular reaction will be faster than a bimolecular, and way faster than a termolecular (THIS IS NOT A FOOL PROOF LAW, that's just something to look at and understand because termolecular requires 3 molecules to collide at the right energy and orientation versus 2 or 1.)


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