Determining oxidation state

[reising1]

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Determining oxidation state is usually pretty straight forward for me. But there's a few cases I've encountered which throw me off.

Here's an example: H2O2. Here, my approach was "well, oxygen is always -2, so that means that the H must be +2" but then the other approach is "well, hydrogen is always +1 so that means oxygen is -1".

Usually, the goal is to figure out the oxidation state of an element that's not known which is bonded to one that is known. For example, in MnO2, to figure out the oxidation state of Mn, I would say that it is +4 because oxygen is always -2.

But what if it's a formula, like H2O2, where both elements are supposed to be "known" so there seems to be two alternative answers? What is the approach in that case?

 

[Jepstein30]

Oxidation number rules are 'ranked'.

1) Elemental form = 0
2) Ionic charge = oxidation number
3) H is +1
4) O is (usually) -2
5) etc. (halogens, alkalai earth metals, alkalai metals)

The first three will always apply. When you start getting to the fourth and fifth+ rules though, you have to make sure you're fulfilling the rules above it first.

For your example, H would be +1 so the oxygens would be -1 each.

 

[reising1]

Oxidation number rules are 'ranked'.

1) Elemental form = 0
2) Ionic charge = oxidation number
3) H is +1
4) O is (usually) -2
5) etc. (halogens, alkalai earth metals, alkalai metals)

The first three will always apply. When you start getting to the fourth and fifth+ rules though, you have to make sure you're fulfilling the rules above it first.

For your example, H would be +1 so the oxygens would be -1 each.

I see. Okay, thanks!

 

[reising1]

Actually, one follow up. What would be the oxidation state of an element if it's elemental form is diatomic. For example, H in H2(g)? Or O in O2(g)?

 

[Jepstein30]

Actually, one follow up. What would be the oxidation state of an element if it's elemental form is diatomic. For example, H in H2(g)? Or O in O2(g)?

That IS the elemental form for those elements. Oxidation state would be zero.

Pretty much.. if you see the element alone and it has no charge... the oxidation state is zero.

 

[dsoz]

Oxidation number rules are 'ranked'.

1) Elemental form = 0
2) Ionic charge = oxidation number
3) H is +1
4) O is (usually) -2
5) etc. (halogens, alkalai earth metals, alkalai metals)

The first three will always apply. When you start getting to the fourth and fifth+ rules though, you have to make sure you're fulfilling the rules above it first.

For your example, H would be +1 so the oxygens would be -1 each.

there are EXCEPTIONS to most of those rules, for example rule 4 has the exceptions of in a peroxide (as in hydrogen peroxide) the oxygen is -1. Also, when oxygen is with fluorine (a more electronegative element), then oxygen has a +2.

Hydrogen is almost always +1, EXCEPT when it is a hydride (as in NaH = sodium hydride), then it is a -1.

Since your example of H2O2 is called hydrogen PEROXIDE, it is the exception for oxygen, and is a -1.

You not only need to know the rules, but also the most common exceptions.

dsoz

 

[Jepstein30]

there are EXCEPTIONS to most of those rules, for example rule 4 has the exceptions of in a peroxide (as in hydrogen peroxide) the oxygen is -1. Also, when oxygen is with fluorine (a more electronegative element), then oxygen has a +2.

Hydrogen is almost always +1, EXCEPT when it is a hydride (as in NaH = sodium hydride), then it is a -1.

Since your example of H2O2 is called hydrogen PEROXIDE, it is the exception for oxygen, and is a -1.

You not only need to know the rules, but also the most common exceptions.

dsoz

Thinking about the ranking of the rules will get you through all the exceptions in my experience. I actually left out the first rule which says "cations (positive oxidation number) on left, anions (negative oxidation number) on right" which would make hydrogen have to be negative, offsetting rule 3. Then you get down to rule 5 or whatever and see Na is + 1.. so H must be -1.

To be honest, if you are ever unsure.. count oxidation state the traditional way.
Bonded to a more EN atom? +1
Bonded to a less EN atom? -1

H-O-O-H?
H is bonded to O which is more EN = +1
O is bonded to H which is less EN = -1

Na-H?
Na is bonded to H which is more EN = +1
H is bonded to Na which is less EN = -1

F-O-F?
O is bonded to two F, which are more EN = +2
F is bonded to O, which is less EN = -1 (each)

of course this requires you know the structure..

 

[Patassa]

Since everyone here should also be fresh on their organic, for organic compounds like H2O2, just use the structure and electronegativities. For ionic use the common rules. You probably can't go wrong doing that.