Oxidation number confusion of nitrogen.

in HNO₃, why does nitrogen have the oxidation state of +5 and not +3?

Why does this occur and how do you know when it is in a +3 or +5 state?

+rep too :smile:

Thanks

Reply 1

Kyri

The rules you need to know are oxygen nearly always has an oxidation state of -2 (if not a peroxide or superoxide). Hydrogen nearly always has an oxidation state of +1 (if not a hydride). In order to have an overall oxidation state of 0 (which all neutral molecules have) the nitrogen must be +5.

Reply 2

Big-Daddy

Original post by Kyri

The rules you need to know are oxygen nearly always has an oxidation state of -2 (if not a peroxide or superoxide).

Or if O is bonded with F, in which case it can have an oxidation number of +1 or +2. So all in all: O normally has -2, but this takes priority behind F having -1, H having +1 and the Group 1 metals having +1, so that in cases where you have only O and one of these 3 (F, H or a Group 1 metal) O can have a -1(peroxide) or -1/2 (superoxide) oxidation number.

Reply 3

Hi, How are you ?

Original post by Big-Daddy

hi, so because in HNO₃, the addition of oxygen is -6 (-2*3). but because hydrogen is always (or most times) +1, it goes down to -5, right?, therefore N would have to be +5 to get to 0, right.

Note, is the oxidation state of oxygen in in HNO₃ -6, or is it -2 (-6/3)?

Reply 4

Hi, How are you ?

Original post by Kyri

What do you mean by not a hydride?

Reply 5

charco

Study Forum Helper

Original post by Hi, How are you ?

You don't take the sum of the atoms, you take the average. Oxygen is -2, as usual...

Reply 6

Swizzel

Original post by Hi, How are you ?

in HNO₃, why does nitrogen have the oxidation state of +5 and not +3?

Why does this occur and how do you know when it is in a +3 or +5 state?

+rep too :smile:

Thanks

In HNO3 the oxygen has a state of -2. There are 3 oxygens so you multiply the state by the three so oxygen is -6. The hydrogen has a +1 charge. When adding both states together you get -5. The overall charge on the molecule is 0 so you then know that the nitrogen is +5. Hope this makes sense!

Reply 7

Kyri

Original post by Hi, How are you ?

What do you mean by not a hydride?

You can get compounds where the H is bonded directly to an alkali metal like NaH (sodium hydride) where the oxidation state of H is -1. I don't know if you'd encounter them in your course but I mentioned it as I predicted someone would come along and tell me that H isn't always +1.

Reply 8

Big-Daddy

Original post by Hi, How are you ?

The oxidation number of an element in the compound is the average. So it's -2. That's what we meant when we said O usually has an oxidation number of -2: it doesn't matter how many there are, it'll always (usually) come out the same.

As for thinking, yes, it's spot on. Short of calculating it properly, that's the same way I tend to think about it. e.g. Here's a tough example:

[CuYAs₃O₆]^2- - given that Cu has an oxidation number of +2 and Y has +4, what is the As oxidation number?

Spoiler

BTW, don't ask me anything about that ion's properties, I completely made it up just for this example! :tongue:

(edited 11 years ago)

Reply 9

upthegunners

Original post by Hi, How are you ?

in HNO₃, why does nitrogen have the oxidation state of +5 and not +3?

Why does this occur and how do you know when it is in a +3 or +5 state?

+rep too :smile:

Thanks

use algebra to find oxidation numbers

H always has an ox number of +1 and O nearly always has one of -2

Let N = x

There

Easy as that. Just gotta remember that Oxygen always has an Oxidation number of -2 (except in a few cases) and Hydrogen +1 and Group 1 metals +1 and group 2 metals +2!