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Author: Subject: Chemistry 104 Help

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[*] posted on 16-3-2010 at 11:50
Chemistry 104 Help

Hey I'm in Chem 104 right now which is basically a general chemistry class in college and I had a couple questions regarding kinetics:

Under standard conditions, which of the following operations results in a spontaneous
chemical reaction taking place?

a) A piece of zinc metal is placed in an aqueous solution of potassium nitrate.
b) Iodine crystals are added to an aqueous solution of sodium chloride.
c) A piece of silver metal is placed in an aqueous solutions of copper(II) nitrate.
d) Chlorine gas is bubbled through an aqueous solution of sodium bromide.
e) At least two of the above (a-d) result in a spontaneous chemical reaction.

I was wondering how to set these equations up because I'm not really sure which ions I'm supposed to include and which ones I'm not.

And also:

Which of the following is the best proposed mechanism?

a) 2NO → N2O2 (fast equil.)
N2O2 + H2 → N2O + H2O (slow)
N2O + H2 → N2 + H2O (fast)

b) H2 + NO → H2O + N (slow)
N + NO → N2 + O (fast)
O + H2 → H2O (fast)

c) H2 + 2NO → N2O + H2O (slow)
N2O + H2 → N2 + H2O (fast)

I just have no idea how to judge reaction mechanisms on how valid or most appropriate they are.

Thanks for your help. Smiley
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International Hazard

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[*] posted on 16-3-2010 at 16:08

To the first lot of questions, I would say that only (b) (merely resulting in dissolution of some of the I2 into aqueous solution) and (d) (resulting in evolution of Br2 and possibly BrCl, some of which would remain in aqueous solution, along with NaCl), are the only things that would happen.

In the second lot, I presume that it is required to propose a mechanism for the reaction of H2 with NO. I think that (b) seems more plausible, provided that some sort of ignition e.g. by sparking is involved, because (a) requires initial dimerization of NO which only happens under great pressure, and because (c) requires in its first step a third-order reaction which is quite rare.
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Hazard to Others

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[*] posted on 16-3-2010 at 23:14

Heh, that class number looks quite familiar. I hope Madison is treating you well thus far (if I am thinking correctly).

As far as the reactions, it probably depends on how involved you want to get. Reaction A can happen, potentially reducing the nitrate to ammonia ultimately, and forming some sort of zincates. It is base mediated. An analogous reaction can happen with aluminum too. That being said, I don't know if it is considered spontaneous. I agree that D also happens as JohnWW describes.

Based on the mechanisms, something about A sticks out to me. I agree that C is pretty much ruled out due to a 3rd order rate law. There is just something I don't like about the formation of atomic nitrogen and oxygen in B. That second step where you react atomic nitrogen (a radical) with NO (also a radical species) seems like it would form N2O, not N2 and O.
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