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August 29, 2014

Homework Help: Chem 1

Posted by Rossina on Thursday, November 16, 2006 at 8:28pm.

Given the bond energy data below, which is the best prediction for the bond energy
of the bond between the nitrogen and the oxygen atom in N2O?

bond energy (kJ/mol)
N-O single bond 201
N-O double bond 607

Indicate if it will be the single bond, double bond or an intermediate value.


I drew it... and I got N--N--O (according to google, N is in the middle, surrounded by double bonds)

With O not being in the middle, how do I do this? Maybe I don't get the question...?

Am I even doing it right?


The structures I have seen show it as
NtriplebondN-O with middle N delta + and O delta - formal charge.
This structure is in resonance with
NdoublebondNdoublebondO with the first N delta - and the middle N delta + formal charge. Based on this, the bond order between N and O is 1.61 and I would think it would be intermediate between double and single bond. I THINK I have a book that talks about that. I can look for it if need be.

Since the problem doesn't mention triple bonds, then I am assuming to eliminate N---O-N.

This leaves me with N--N--O.

What do you mean the bond order between N and O is 1.61?

Here are two references I found on the Internet. I found the book at home and it shows the standard NtriplebondN-O which you can draw with the 16 electrons available and that shows a +1 formal charge on the central N and a -1 formal charge on the oxygen. It does not show any resonance structures. However, the other two I list below do talk about resonance structures. Based on that, I still go with an intermediate bond between single and double. Check my thinking. One thing you might do is to look up bond energies for molecules and see if you can find that for the N-O bond in N2O. I know there are tables on the Internet.

http://en.wikipedia.org/wiki/Nitrous_oxide

http://chem241transcripts.blogspot.com/2006/04/lecture-004-more-lewis-structures-and.html

As an after thought, I would not eliminate the NtriplebondN-O for that is the ONLY one that shows eight electrons around N, N, and O (unless I counted wrong). Try it. Each N has 5 and 5x2=10 for N. O has 6 so we have 16 electrons. :N:::N:O: with two dots both above and below O gives 16. In fact, I lean toward this being THE structure except for the two references I found on the Internet that talk about resonance forms.

I think I somewhat get it now... I also missed the part about the "N-O" bond in the problem... I was trying to think of the "N-N" bond. Therefore you are right, I can not eliminate the N---N-O.

Eventhough you say that IS the structure, how it can it be intermediate it then? Is it intermediate because nitrogen can either have the double bond OR single bond with oxygen (depending the structure used)?

I really appreciate your time...

Also... what does "bond order" mean again?

I can see why you may be confused. I can try to help clear it up but since I don't KNOW the structure of N2O, I must go by what I read. IF we know nothing about the N2O molecule except that there are 16 electrons, I would draw it as NtriplebondN-O. That gets 8 electrons around N, N, and O. Try it to check me out on that. Its hard to draw with the computer but here goes.
       :
:N:::N:O:
       :<b/>

And the answer I would give to the question is I would expect a single bond for N-O. BUT, the two references I posted to your question PLUS the one you found show a N=N=O structure, also, as a possible resonance form. I don't know what course you are in or if you have studied resonance forms yet; however, I will assume you have not. Here is a quickie review all tied up into the explanation. When two or more structures can be drawn for a molecule, we call them resonance structures. The ACTUAL structure is not any of the ones we can draw but a hybrid of all of them. Therefore, if we attribute any credence to the N=N=O resonance structure which you and I found on the Internet, then that means that the N-O bond has SOME double bond character and it would be a little stronger than a single bond but not as strong as a double bond (in other words intermediate between single and double). The answer to your question lies with what we believe the structure to be; i.e., if we think it is :N:::N:O: (plus : above and below O)ONLY, the answer is we would expect the N-O to be a single bond. However, if we give some credibility to there being resonance forms that we could draw, then the correct answer is that we would expect the N-O bond to be intermediate between a single and double bond. (I think that pretty well explains where the confusion lies. Here is what I have done.)
The :N:::N-O has three bonds for the first N (good), four bonds for the middle N (not great), and one bond for O (I would rather see two). The N=N=O structure has two bonds for the first N (not good, I would rather see three), four bonds for the middle N (not great), and two for O (good). So what do we do? I have come down on the side that since neither structure we can draw satisfies all of the bond requirements (2 for O, three for N), that the resonance structures have credibility and that makes the correct answer that the N-O bond is intermediated between a single and a double bond. You may see it another way. I would be interested in knowing what your prof thinks. I hope this helps.

The bold didn't exactly do what I intended and the electrons aren't spaced properly on the structures, but I think you can see what I was trying to do. As for the 1.61 term, I simply picked that up from the wikipedia reference source. The idea from that source is that 1 would be a single bond, 2 would be a double bond, but their calculations lead them to believe that the bond is 1.61 (1.5 would be half way between a single and a double bond).

Thank you so much for the explanation, it cleared up my confusion...!! With all of your explanations, I believe it is intermediate.

Again, thank you for your time!

Got it... just making sure I understand the term :)

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