I just don't get oxidation numbers

[delco714]

I sit here, trying to do a dry lab, never took chem before, and now I'm premed in College... awesome.. so as the other students seem to have a nice grasp b/c this took this course in HS already (or failed once before), I'm stuck with the assumption that I should "know" this already, and I'm far from understanding it.. My lab has questions such as this: "indicate the oxidation number of carbon and sulfur following the compounds" a) Na2C2O4 b) SO2 ...etc.. is this right?.. Na has +1 oxid#, and there's 2, so it's 2+; O4 is 2-, there's 4 of them so.. 8-.. 8- + 2+ = 6-... there's 2 Carbon.. so each carbon must = 3+ to balance this...i have no idea what I'm doing.. i'm just trying to figure it out.... and for SO2.. oxygen is a peroxide.. so it's 1-...but S never has a charge of 1+ according to the PToElements... WTF.. if you can help.. thanks a mil.

[Cap'n Refsmmat]

You've the first one right, but for the second, peroxide has a -2 charge, not a -1 charge. (You'd get that number from a table of polyatomic ions.)

[delco714]

waitttt.. SO2 is a peroxide... so it is 2-... but only 1- when paired with H (H2O2 >>hydrogen peroxide).. is that right? ..so if it is 2-.. than the S will be 2+ to balance?

[Cap'n Refsmmat]

It's still 2- when paired with hydrogen peroxide.

 

Each H in [ce]H2O2[/ce] has a 1+ charge, for a total of 2+. The peroxide balances it out with 2-.

 

And yes, the S would be 2+.

[delco714]

shweeet.. im getting the hang of it.. thanks for the help.

[jdurg]

SO2 is NOT a peroxide! A peroxide is a species where the O-O bond exists. Thereby each oxygen has a single bond to the other oxygen. In SO2, the oxygens are bound to the sulfur atom, and each oxygen has an oxidation number of -2. So the S in SO2 has an oxidation number of +4. (SO2 is a structure like H2O where the O is in the middle with two hydrogens attached. In SO2, the S is in the middle with the two oxygens attached).

 

Whenever you do oxidation numbers, it is CRUCIAL to know the structure of the compound you're looking at.

[Cap'n Refsmmat]

Crap. I take it I should have gotten the hint from that being a covalently, and not ionically, bonded molecule?

[delco714]

jdurg, awesome point, you're right.. S2O2 would be a peroxide right? And yes, it's covalent, that much i know! 2 Non-Metals!

[big314mp]

I would think that it would be a structure similar to SO3, with one oxygen swapped for a sulfur.

 

I don't think that compound exists though.