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Oxidation Number

I'm doing past papers for the IMS entrance exam and I'm really confused by one of the questions. If anyone can offer any help it would be much appreciated.

What is the oxidation number of nitrogen in H2N-COONH4?
A) +2 1. A and C are right
B) +3 2. A and D are right
C) +4 3. B and C are right
D) +5 4. B and D are right
E) +6 5. Just D is right
6. None are right

I'm not even sure what the question asking as the "-" symbol has thrown me a bit.


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Reply 1
Avatar for Borek
Borek
4
It is just a bond between C and N.
Reply 2
Avatar for bluemax
bluemax
16
Let's give it a try. H2N-COONH4.

Break it into two parts. COO(-) negative ion and NH4 (+) ....

so N + (1x4)= +1
N= -3

I'm sort of confused after this. I would probably mark it as none are right
Reply 3
Avatar for Borek
Borek
4
-3 looks OK to me. There is also another nitrogen and it most likely has another oxidation number, but to be honest, I have no idea what it is.

In general, assigning oxidation numbers to carbon and nitrogen in organic compounds is a waste of time IMHO. Quite often they are fractional, unless you do some tricks.
Reply 4
Avatar for Pigster
Pigster
20
By my recoking, the other N is also -3
C = +4
2xH = 2x+1
2xO = 2x-2
summing to 1- for the ion
Reply 5
Avatar for Borek
Borek
4
Original post by Pigster
C = +4


Why +4?
Reply 6
Avatar for Pigster
Pigster
20
Since C is attached by four bonds all of which involve more electronegative atoms, so would be +4.
N has three single bonds all to less electronegative atoms.

http://www.youtube.com/watch?v=BGd5ty3ueQA
Reply 7
Avatar for Borek
Borek
4
Just because it is a "method" doesn't mean it gives the right answer. Especially when there is no such thing as the "right answer" - there is no way to check what the "real" oxidation number is, as there is no physical property that can be assigned to the oxidation number of the atom. It is just an electron accounting device, nothing more.

If I remember correctly if you reverse the electronegativity rule - that is, you assign the electron to the atom with a lower electronegativity, not the one with a higher electronegativity - you get a different set of numbers, but when used for equation balancing, they yield the same final result. It was many years ago when I played with these things, so my memory is sketchy on details.
Reply 8
Avatar for haydyb123
haydyb123
9
Original post by Borek
Just because it is a "method" doesn't mean it gives the right answer. Especially when there is no such thing as the "right answer" - there is no way to check what the "real" oxidation number is, as there is no physical property that can be assigned to the oxidation number of the atom. It is just an electron accounting device, nothing more.

If I remember correctly if you reverse the electronegativity rule - that is, you assign the electron to the atom with a lower electronegativity, not the one with a higher electronegativity - you get a different set of numbers, but when used for equation balancing, they yield the same final result. It was many years ago when I played with these things, so my memory is sketchy on details.


Absolutely right, completely trivial and pointless... have no idea why so much emphasis is placed on these. I guess it's an easy question to ask for A-level!
Reply 9
Avatar for Borek
Borek
4
Original post by haydyb123
Absolutely right, completely trivial and pointless... have no idea why so much emphasis is placed on these. I guess it's an easy question to ask for A-level!


Sometimes I think it is just like with significant digits - they are never used in a real world, but taught and defended religiously by teachers. Probably because that's the most difficult thing they are capable of mastering, so they stick with it! :biggrin:

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