question about a half equation

Looking at



O₂(g) + 4H⁺(aq) + 4e^- -> 2H₂O(l)1.23
from http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/electpot.html

Is that only one half of a redox reaction?

And in that one half reaction there.

Are both Oxygen and Hydrogen being reduced?

Or is it only Oxygen that is being reduced?

what's happening to Hydrogen.. Is it being reduced or staying the same or being oxidised?

Thanks

It is indeed just half a redox reaction- after all, where do the electrons on the LHS come from?

Only the oxygen is reduced- look at the oxidation states. Oxygen goes from 0 in O2 to -2 in H2O, implying reduction. Hydrogen’s oxidation state is unchanged as it is +1 in both H^+ and H2O.

Thanks..

So how about

2H2O(l) + 2e- -> H2(g) + 2OH-(aq)

I understand then that it's a reduction reaction since the +e- is on the right.. Presumably no oxidation occurring anywhere?

The H on the left is 2- That's oxidised in the H2. That seems strange 'cos isn't it meant to be a reduction reaction?

In the case of OH- , I understand that's an overall negative (OH)- and the - is actually on the O, so more like HO-.. But to get an overall negative, given the - on the O. The H must be oxidation number of 0.

So isn't the H getting oxidised there too in the HO-. From oxidation number of -2 on the left,in the H2O, to being oxidation number 0 on the right in the HO-?

ah I see, thanks..

I don't know why I made the error of thinking H was -2 on the left..

Yeah so H is +1 on the left. That explains the H2.. , H getting reduced by one there.

OH- i'm not sure of though.. 'cos it's not a compound 'cos a compound is neutral by definition.. It's a polyatomic ion.

But if we say that the rules are the same for compounds as for polyatomic ions.. (though with polyatomic ions,, sum of charges add to overall charge of non-zero)..

So H has +1, and O has 2- So that makes sense. We get an overall charge of -1

I had in mind at one point HO- and the negative charge being on the oxygen but that was definitely wrong then. because a)O there has an oxidation state of 2- not 1- and b)the actual charge on O would be a "partial charge", it'd be less than 1, like with covalent compounds, 'cos polyatomic ions are covalently bound internally

Ia that all right?

Thanks

Indeed polyatomic ions are internally covalently bound. The oxygen both has a negative formal charge from the one electron it has picked up from another atom and a partial charge from drawing the electrons in the covalent bond closer to itself. So it’s perhaps a little more complex than at first glance.

Thanks..

So for OH-

Looking at oxidation states. The O has oxidation state of -2, The H has oxidation state of +1 . So overall oxidation state of -1

Looking at formal charges. The O has a formal charge of -1. The H has a formal charge of 0. The overall formal charge is -1

Is the minus after the OH, indicating that the overall formal charge is -1, or is it indicating that the overall oxidation state is -1?

thanks.. do we ever look at total/overall oxidation state, is there even such a thing?

and do we ever determine the overall formal charge by summing the oxidation states on each atom?

“Overall oxidation state” is a largely meaningless concept as it doesn’t really indicate a lot about the chemistry of a compound. Though I suppose it could strictly be taken to mean the overall charge on a species (which is useful when you are dealing with ions and deducing the formula of an ionic compound).
The oxidation states individually are much more useful to consider, as they may indicate the characteristic redox reactions of a particular compound.

Hi, i'm just revisiting this..
You also wrote ""The minus after the OH indicates the overall formal charge on the ion is -1."
Perhaps that should say "overall charge" not "overall formal charge".
Looking here
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Formal_Charges/Formal_Charge
I notice it says that "A formal charge (FC) is the charge assigned to an atom in a molecule, "
It seems to me that the overall charge isn't referred to as a formal charge.. (just like it's not referred to as an oxidation state), would you agree?
Thanks

Hi, i'm just revisiting this..
You also wrote ""The minus after the OH indicates the overall formal charge on the ion is -1."
Perhaps that should say "overall charge" not "overall formal charge".
Looking here
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Formal_Charges/Formal_Charge
I notice it says that "A formal charge (FC) is the charge assigned to an atom in a molecule, "
It seems to me that the overall charge isn't referred to as a formal charge.. (just like it's not referred to as an oxidation state), would you agree?
Thanks