Determining the reaction intermediates

[rwooduk]

Hi, I am working my way through the answers to the question sheet but am confused:

1. Homework Statement

Homework Equations

none

The Attempt at a Solution

Why isn't 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 C_NO 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.

 

[epenguin]

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.

 

[Borek]

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.

 

[rwooduk]

Many thanks for the helpful replies!

 

[epenguin]

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 realize. 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.