Oxidation Numbers: Confusion with Electronegativity

[jgens]

Just a brief question on oxidation numbers. My textbook says "the oxidation number of the more electronegative atom in a moledule or a complex ion is the same as the charge it would have if it were an ion." However, when considering compounds like hydrazine N₂H₄, Nitrogen is clearly the more electronegative element, however, it is assigned an oxidation number of -2. Am I misunderstanding something or is my textbook perhaps making large generalizations?

 

[alxm]

Nitrogen has a +2 oxidation number in hydrazine, hydrogen is acting as an anion here, which is why it's written with hydrogen last.

I doubt your textbook is over-generalizing, though. It's true in most circumstances and it's probably entirely true in the specific context. But it's not a general rule. If it doesn't say that something is, it shouldn't be interpreted as such outside the context it was mentioned.

 

[Borek]

Note that oxidation numbers don't reflect any real, measurable property of atoms. They are just artificially assigned numbers that help in the electron bookkeeping when balancing redox reactions.

 

[kashiark]

Nitrogen has a +2 oxidation number in hydrazine, hydrogen is acting as an anion here, which is why it's written with hydrogen last.

are you sure it's +2? I am not saying your wrong but i thought hydrogen only had negative oxidation numbers in metal compounds; ammonia is written as NH₃ and hydrogen has positive oxidation numbers and hydrogen is written last plus it's written last in all organic compounds

 

[symbolipoint]

are you sure it's +2? I am not saying your wrong but i thought hydrogen only had negative oxidation numbers in metal compounds; ammonia is written as NH₃ and hydrogen has positive oxidation numbers and hydrogen is written last plus it's written last in all organic compounds

What makes you think that way? Hydrogen atom has ONE proton so a hydrogen atom can have at its most extreme, a -1 charge. If this does not make sense, then somebody please explain, since I may be midjudging based on limited study.

If that is now acceptable, then if hydrazine molecular formula is examined, N₂H₄, I see 2*c + 4*(-1) = 0, in which I use "c" as the charge on the Nitrogen. Apparantly, c= +2.

Could a single proton of Hydrogen, be associated with enough electrons to have a NEGATIVE charge of of absolute value more than ONE ?

 

[kashiark]

or c*2 + 4(+1) = 0 where c = -2 (and is nitrogen's oxidation number) makes more sense because nitrogen is much much more electronegative than hydrogen; i don't think so otherwise BeH₂ would just be BeH

 

[JJAMDUNK]

Consider the Kjeldahl nitrogen digestion, which uses concentrated sulfuric acid, potassium sulfate, and heat to convert all organic nitrogen to ammonia. These compounds are strong oxidizers. Under these conditions the reaction can surely only oxidize nitrogen. The oxidation state of nitrogen in ammonia must be +3 and the hydrogens are -1. I cannot imagine the oxidation state of nitrogen becoming -3 under these conditions.

 

[chemisttree]

Consider the Kjeldahl nitrogen digestion, which uses concentrated sulfuric acid, potassium sulfate, and heat to convert all organic nitrogen to ammonia. These compounds are strong oxidizers. Under these conditions the reaction can surely only oxidize nitrogen. The oxidation state of nitrogen in ammonia must be +3 and the hydrogens are -1. I cannot imagine the oxidation state of nitrogen becoming -3 under these conditions.

Absolutely not! In ammonia, nitogen most definitely has an oxidation number of -3. Sulfuric acid and potassium sulfate are NOT OXIDIZERS!

 

[Borek]

Sulfuric acid and potassium sulfate are NOT OXIDIZERS!

Well, concentrated sulfuric acid can dissolve metallic copper, so it has some oxidizing power.

Which doesn't change ON of nitrogen in ammonia from +3

 

[JJAMDUNK]

Upon reading further it does appear the nitrogen gets reduced in a Kjeldahl digestion, but it gets done by the organic material. The hot H2SO4 oxidizes the organic C. So N probably does have a -3 oxidation number in NH3. That does make more sense when you substitute an N-C bond with an N-H bond. Strange brew!

 

[Borek]

There is nothing strange here once you consider that ON are just a bookkeeping tool, nothing real.