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Topic: Reaction rates and mechanisms  (Read 4421 times)

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Offline cliverlong

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Reaction rates and mechanisms
« on: March 21, 2009, 04:23:39 AM »
Does anyone know of a website or book that gives a systematic treatment and quite a few examples of deducing reaction mechanisms from rate equations.

The nearest I have got is the treatment in chemguide
http://www.chemguide.co.uk/physical/basicrates/ordermech.html

but I don't feel I have grasped how to tackle such questions in general.

Thanks

Clive

Offline Las

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Re: Reaction rates and mechanisms
« Reply #1 on: March 22, 2009, 08:02:19 AM »
could you post an example of a question?

Offline cliverlong

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Re: Reaction rates and mechanisms
« Reply #2 on: March 22, 2009, 12:11:07 PM »
could you post an example of a question?
2NO2 --> 2NO + O2

Has a rate equation of, say,

k[NO2]

or

k[NO2]2

I'm not looking for a specific answer to these two cases - but some more general rules I can apply say when rate equations for

A + B + C -->  something

[A] or [A]2 or [A][C] or [A]2[C] or [A][ B][C]

and so on


Clive

Offline Las

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Re: Reaction rates and mechanisms
« Reply #3 on: March 23, 2009, 02:31:47 AM »
http://www.cartage.org.lb/en/themes/sciences/Chemistry/Miscellenous/Helpfile/Kinetics/Rateorder.htm


See if this helps...this is how we've done it in school..I dont know how you can get the order of a reaction by simply looking at it unless its an elementary reaction..
« Last Edit: March 23, 2009, 02:43:03 AM by Las »

Offline Las

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Re: Reaction rates and mechanisms
« Reply #4 on: March 23, 2009, 03:34:40 AM »
OK I spent over two hours searching for a decent explanation on the graphical representation as well, but this was all I could find..

http://www.tutorvista.com/content/chemistry/chemistry-iv/chemical-kinetics/graphical-order-determination.php

Try this to see if you've gotten the idea..I think it'll help..

http://www.mp-docker.demon.co.uk/as_a2/topics/reaction_kinetics/quiz_1.html

Offline cliverlong

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Re: Reaction rates and mechanisms
« Reply #5 on: March 23, 2009, 10:35:49 AM »

http://www.mp-docker.demon.co.uk/as_a2/topics/reaction_kinetics/quiz_1.html

That is the type of problem I was looking to solve and the example is fine.

There seem to be no generic notes on once one has calculated the rate equation on how to deduce the mechanism.


Clive

Offline Las

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Re: Reaction rates and mechanisms
« Reply #6 on: March 24, 2009, 04:07:16 AM »
Within a given time limit if the concentration doubles its in the first order and if it quadraples its in the 2nd order..(the concentration of the reactant is halved within the first hundred seconds and its halved again within the next hundred seconds...etc) that's how I figured it out..I dont know whether that's the way we're supposed to get the answer though..

As for the mechanism I dont think there's a way to find the exact mechanism, but you can suggest the reactants and the stoichiometry of the rate determining step..the fast steps are unfortunately guesswork in my case:( lol

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