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Topic: Mechanisms Consistent with the Rate Law  (Read 9259 times)

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Offline LHM

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Mechanisms Consistent with the Rate Law
« on: April 03, 2011, 12:22:57 PM »
Consider this reaction:
2 NO2(g) + O3(g) :rarrow: N2O5(g) + O2(g)
The reaction of nitrogen dioxide and ozone represented is first order in NO2(g) and in O3(g). Which of these possible reaction mechanisms is consistent with the rate law?
Mechanism I:
NO2 + O3 :rarrow: NO3 + O2 slow
NO3 + NO3 :rarrow: N2O5 fast
Mechanism II:
O3  ::equil::  O2 + O fast
NO2 + O :rarrow: NO3 slow
NO3 + NO2 :rarrow: N2O5 fast

I thought it would just be mechanism I, but apparently both work. Isn't the rate law for mechanism II rate=k[NO2][O3][O2]-1? How come it fits mechanism II too?
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Offline bakerbg

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Re: Mechanisms Consistent with the Rate Law
« Reply #1 on: April 04, 2011, 02:04:13 AM »
the reaction mechanism may very well be as you stated ; it can only be determined experimentally. In that case mechanism 2 make sense - although i am learning for the first time that ' order of reaction' could be negative as with the case of [O2]! :o

Mechanism 1, However is not balanced and does not follow a logical sequence. what happened to the other molecule of NO2? Where did the extra molecule of NO3 come from in the second equation?

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Offline LHM

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Re: Mechanisms Consistent with the Rate Law
« Reply #2 on: April 04, 2011, 04:08:59 PM »
Quote from: bakerbg on April 04, 2011, 02:04:13 AM
Mechanism 1, However is not balanced and does not follow a logical sequence. what happened to the other molecule of NO2? Where did the extra molecule of NO3 come from in the second equation?

Sorry, I made a typo, it should actually be this:
Mechanism I:
NO2 + O3 :rarrow: NO3 + O2 slow
NO2 + NO3 :rarrow: N2O5 fast

Quote from: bakerbg on April 04, 2011, 02:04:13 AM
the reaction mechanism may very well be as you stated ; it can only be determined experimentally. In that case mechanism 2 make sense - although i am learning for the first time that ' order of reaction' could be negative as with the case of [O2]! :o

Well that's the reaction mechanism that was given, but I still don't see how mechanism II fits the rate law since there's still the extra [O2]-1 in the rate law for mechanism II. Can anyone explain it?
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Offline DevaDevil

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Re: Mechanisms Consistent with the Rate Law
« Reply #3 on: April 04, 2011, 04:52:10 PM »
it is given that the rate is first order in nitrate and ozone.

which reaction pathway has the rate-determining step that is first order in there two species?
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Offline LHM

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Re: Mechanisms Consistent with the Rate Law
« Reply #4 on: April 04, 2011, 08:48:38 PM »
Mechanism I...?
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Offline DevaDevil

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Re: Mechanisms Consistent with the Rate Law
« Reply #5 on: April 05, 2011, 11:43:29 AM »
yup
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Offline LHM

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Re: Mechanisms Consistent with the Rate Law
« Reply #6 on: April 05, 2011, 04:24:35 PM »
But the answer said that mechanism II is consistent with the rate law too, how does that work?
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Offline DevaDevil

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Re: Mechanisms Consistent with the Rate Law
« Reply #7 on: April 05, 2011, 04:27:46 PM »
because O is made from O3 in a fast way, but only when the reaction with nitrate takes place you could say that the 1st order in O that mechanism II proposes extends to 1st order in O3, the origin of O
and of course in the r.d.s. of mechanism II, NO2 also exhibits 1st order behavior.
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Offline LHM

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Re: Mechanisms Consistent with the Rate Law
« Reply #8 on: April 05, 2011, 10:46:31 PM »
Quote from: DevaDevil on April 05, 2011, 04:27:46 PM
you could say that the 1st order in O that mechanism II proposes extends to 1st order in O3, the origin of O
and of course in the r.d.s. of mechanism II, NO2 also exhibits 1st order behavior.

Sorry, but I'm a bit lost. What do you mean by could? Why are you simply allowed to take out the [O2]-1? And in that case, how do you know when you're allowed to take out something with a -1st order, like [O2]-1 and when you're supposed to keep it?
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Offline DevaDevil

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Re: Mechanisms Consistent with the Rate Law
« Reply #9 on: April 06, 2011, 11:11:22 AM »
they are not taking anything out. They do not say k = [NO2]*[O3], they only say it is first order in NO2 and ozone.
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