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Determining the reaction intermediates

  1. Nov 18, 2015 #1
    Hi, I am working my way through the answers to the question sheet but am confused:

    1. The problem statement, all variables and given/known data

    kMa7o8Q.jpg

    2. Relevant equations
    none

    3. The attempt at a solution
    vssbIvl.jpg

    Why isnt NO2 a reaction intermediate? i.e. I thought that if when the equations were added, the ones that cancel are the reaction intermediates??

    Also why isn't CNO in the rate law? I thought that the reaction intermediates were included in the rate law?

    Thanks for any help with this, it's not for any assigned work, I have to help with the tutorial itself so need to be able to explain this stuff to students.
     
  2. jcsd
  3. Nov 18, 2015 #2

    epenguin

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    Question b about NO2 either has no right and wrong answer or yours is right. Seems the NO2 has to collide with or interact somehow with an NO3 molecule in order for this to react. And difficult offhand to imagine how it could be determined whether the NO2 molecule emerging from the second one is the same one that entered it!

    c. If you are going to explain this to students you certainly need to state the equilibrium equation for the first step and between that and your kinetic equation eliminate to get hopefully only [N2O5] in the final rate equation.
    No [NO] should not come into it - this substance appears only after the irreversible rate-determining step.
    No, concentrations of intermediates which are often enough not easily observable like starting and end products are, usually do not appear in the rate equation, and iany mechanism gives enough equations to eliminate them.
     
    Last edited: Nov 18, 2015
  4. Nov 18, 2015 #3

    Borek

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    Staff: Mentor

    I can only guess that NO2 was excluded as it is the final product, but I see nothing wrong with NO2 being both the final product and the intermediate.
     
  5. Nov 19, 2015 #4
    Many thanks for the helpful replies!
     
  6. Nov 19, 2015 #5

    epenguin

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    I just took a quick google look and you can find this reaction much discussed on the web. It seems to be a didactic favorite. I think there are even you-tube demonstrations. And N2O5 I learned there is environmentally important.

    But downside of something being made an academic example and excercise, I did not find looking at a lot of documents in a hurry, the equilibrium constant for that first reaction, maybe someone else can easily find it . If I don't know that I don't know what it means to say x is the molarity of N2O5 I mean in terms of actually doing an experiment. If someone tells me the equilibrium is very much to the left, then it is simple, the gas can be assumed to be practically all N2O5. At that density or pressure though - there will be some lower density or pressure where that can't be true you realise. I guess this is eq is to the left, otherwise N2O5 couldn't be environmentally significant, but something that needs to be questioned, borne in mind IMO.
     
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