Oxidation Numbers: Confusion with Electronegativity

In summary: 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.
  • #1
jgens
Gold Member
1,593
50
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?
 
Physics news on Phys.org
  • #2
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.
 
  • #3
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.
 
  • #4
alxm said:
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
 
  • #5
kashiark said:
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 ?
 
  • #6
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
 
  • #7
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.
 
  • #8
JJAMDUNK said:
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!
 
  • #9
chemisttree said:
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 :wink:
 
  • #10
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!
 
  • #11
There is nothing strange here once you consider that ON are just a bookkeeping tool, nothing real.
 

1. What are oxidation numbers?

Oxidation numbers, also known as oxidation states, are numbers assigned to atoms in a chemical compound to indicate the distribution of electrons among them. They are used to track the transfer of electrons during a chemical reaction.

2. How do you determine the oxidation number of an element?

The oxidation number of an element is determined by a set of rules that take into account the electronegativity and bonding patterns of the elements in a compound. In general, elements in their elemental form have an oxidation number of 0, while ions have oxidation numbers equal to their charge. For more complex compounds, the oxidation number can be calculated by assigning electrons to the more electronegative element in a bond and then subtracting them from the overall charge of the compound.

3. What is the relationship between oxidation numbers and electronegativity?

Oxidation numbers and electronegativity are related in that both are measures of an element's ability to attract electrons. However, oxidation numbers are specific to a particular compound or ion, while electronegativity is a general property of an element. In other words, the electronegativity of an element can influence its oxidation number in a compound, but it does not determine it.

4. Can an element have multiple oxidation numbers?

Yes, an element can have multiple oxidation numbers depending on the compound it is in. For example, iron can have oxidation numbers of +2 or +3 in different compounds. The oxidation number of an element can also change within a compound if it undergoes a chemical reaction.

5. Why is confusion between oxidation numbers and electronegativity common?

Confusion between oxidation numbers and electronegativity is common because they both involve the concept of electron distribution and are often used interchangeably in chemistry. Additionally, the rules for determining oxidation numbers can be complex and may lead to mistakes. It is important to understand the difference between the two and use them correctly in chemical calculations and analyses.

Similar threads

  • Biology and Chemistry Homework Help
Replies
5
Views
1K
  • Biology and Chemistry Homework Help
Replies
1
Views
614
  • Biology and Chemistry Homework Help
Replies
5
Views
5K
  • Biology and Chemistry Homework Help
Replies
1
Views
1K
  • Biology and Chemistry Homework Help
Replies
3
Views
1K
  • Biology and Chemistry Homework Help
Replies
4
Views
2K
  • Biology and Chemistry Homework Help
Replies
1
Views
2K
  • Biology and Chemistry Homework Help
Replies
8
Views
3K
  • Biology and Chemistry Homework Help
Replies
4
Views
6K
Replies
6
Views
4K
Back
Top