Oxidation states

You are missing that only some element have fixed oxidation states.

oxygen (o) is always -2 so in NO2 the O2 is -2x2=-4 and therefore to make zero the N is +4

same in NO

and H is always +1 so N in NH3 is -3

the others you should find on wiki like F=-1 and also oxidation of a molecule is 0 so the N is cancelling the O or H

Reply 2

mr Biomedicine

you are wanting to calculate the charges on each atom on the molecule. Oxygen is -2. So in the case of NO2 you have to oxygens therefore you have 2 x -2 charges. making -4. You know that NO2 is a neutral molecule so to balance the charge nitrogen must = +4

Reply 3

Edit: I think we all hit post together :tongue:

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The oxidation state of O is usually -2, and the oxidation state of H is usually +1. There's a couple of exceptions to each though:
- If oxygen is attached to flourine it becomes a +
- In peroxides, superoxies it's -1, -0.5 respectively (but you don't need to know that for A-levels)
- Hydrogen is +1 except when bonded to more electropositive elements where it's -1

- Group 1 elements = +1
- Group 2 elements = +2

Reply 4

OP

adie_raz

You are missing that only some element have fixed oxidation states.

oxygen (o) is always -2 so in NO2 the O2 is -2x2=-4 and therefore to make zero the N is +4

same in NO

and H is always +1 so N in NH3 is -3

the others you should find on wiki like F=-1 and also oxidation of a molecule is 0 so the N is cancelling the O or H

why?

is it because H = +1 so because there are 3 it's + 3 and therefore to get 0 you need -3. By the way how do we know to equal it to zero is it

-3++3 = 0 for example?

Reply 5

The molecule doesn't have a charge so it's oxidation states must balance for it to be equal to zero overall. If it had a charge of -1 for instance, the oxidation states would have to add to give -1.

And yeah there are 3 hydrogen's so you get: +1 +1 +1 = +3, so you need a -3 to balance it to make it 0.

Reply 6

OP

why is it that in CaCl2 the oxidation state of Cl2 is -1? is it because Ca is +2 and to equate to zero Cl has to be -1 and not -2 because there are two Cl's?

Reply 7

Correct - Ca is in group 2 so it's oxidation state is +2. Also, halogen's have oxidation states of -1 (except when attached to oxygen, nitrogen or other halogens) so each is -1. Cl cannot be -2.

Reply 8

waleedwal

because it gives you the oxidation state of an individual Cl atom.......each atom of Cl has a charge of -1 so since there are two Cl their combined charge is -1x2= -2 and Ca is +2 and so +2-2=0......abracadabra :P

Reply 9

OP

Thanks for all your help everyone. i think i've got it :biggrin:

Reply 10

OP

waleedwal

because it gives you the oxidation state of an individual Cl atom.......each atom of Cl has a charge of -1 so since there are two Cl their combined charge is -1x2= -2 and Ca is +2 and so +2-2=0......abracadabra :P

Reply 11

Try posting some more complex examples where you have more than 2 elements in the atom, or ones with charges just to double check you've got it :smile:

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Elements have oxidation states of 0 btw. Here's a useful article on oxidation states link - it's worth just taking a skim over if you get a chance.

Reply 12

OP

Yeah. I think I've lost it again:

in Na2O2 it's O2 is apparently -1. When working out such element is it just knowing the rules? Because I was going all complex by working out the oxidation state of Na...btw for future reference when working out for example Na2 do I just do Na? Because I thought I times by two. Argh confusion settled in again. :frown:

Reply 13

That's an exception to the rule because it's a peroxide (here I've quoted it from the link I've given above):

Oxygen in peroxides

Peroxides include hydrogen peroxide, H2O2. This is an electrically neutral compound and so the sum of the oxidation states of the hydrogen and oxygen must be zero.

Since each hydrogen has an oxidation state of +1, each oxygen must have an oxidation state of -1 to balance it.

It's the same principle with oxygen, because Na has to have an oxidation state of +1, each oxygen has to have a state of -1 to balance it.

Reply 14

OP

the same in MgH2 apparently H2 has an O.S of -1? is this because + 2 and -2 can cancel to one :s-smilie:

???

Reply 15

Yeah that's a metal hydride, another exception to the rule (quoted from the same article):

Hydrogen in the metal hydrides

Metal hydrides include compounds like sodium hydride, NaH. In this, the hydrogen is present as a hydride ion, H-. The oxidation state of a simple ion like hydride is equal to the charge on the ion - in this case, -1.

Alternatively, you can think of it that the sum of the oxidation states in a neutral compound is zero. Since Group 1 metals always have an oxidation state of +1 in their compounds, it follows that the hydrogen must have an oxidation state of -1 (+1 -1 = 0).

They've talked about group 1 metals above but it's the same principle for group 2. Group 2 metals always have oxidation states of +2, so in this case hydrogen has an oxidation state of -1. You have two hydrogens so you have -1 -1 to give -2. This balances the +2 charge on Mg.

Reply 16

OP

in MgBr2 why is the O.S of Br2 -1 surely because there are 2 Br's I say it's -2? Or do I not take that 2 into account?

I think I say -1 because there's two of them and then it balances!

Thanks for this lesson guys. :redface:

q

Reply 17

You consider each Br atom individually - so each Br atom has an oxidation state of -1.

Mg as you've correctly identified has an oxidation state of +2, being a group 2 metal.

Reply 18

OP

thank you. Does this mean I'm dumb because it took so long to grasp.:'(
:tongue:

I think I just need more practice and I should be ok.^^

Reply 19