[xstrae]

Hi, I have been asked to determine the oxidation states of

S in H₂S₂O₈
 
2(1) + 2x + 8(-2) = 0 ; x =+7 which is wrong according to my book.
So I thoughy it depends on the structure and came up with this:
                 O   O
                  i     i
  H - O - O - S - S - O - O - H
                  i     i
                 O   O

where the 'i' represents a coordinate bond from S to O. from this I got the oxid state of each S atom is 2(2) + 1 = 5.  (is my structure wrong?)

My book says the answer is +6 for each S atom.  Where am I going wrong?

By the way, when should I just find the oxidation state of the atom asked using usual algebraic methods and when should I opt for the "structure" method? I ask this because for certain questions, the structure fails to get me the answer. (H₄P₂O₇ - I didnt get the answer drawing the structure for this one)

thanks for your time





 

[Dan]

Your H₂S₂O₈ structure is incorrect.

http://images.google.co.uk/images?hl=en&rls=GGLJ,GGLJ:2006-34,GGLJ:en&q=h2s2o8&sa=N&tab=wi

[xstrae]

ok thanks
I seem to get the structure wrong all the time.Any tips?

[AWK]

H-O-SO2-O-O-SO2-OH

[Dan]

ok thanks
I seem to get the structure wrong all the time.Any tips?

Think about the oxidation states of the atoms in your proposed structure.
For example, S has 6 valence electrons, so do you think an oxidation state of +7 is feasable for S?

Google image search is pretty good for simple compounds.
Failing that, try "Chemistry of the Elements" by Greenwood and Earnshaw (check your local library)

[Will]

Think about the oxidation states of the atoms in your proposed structure.
For example, S has 6 valence electrons, so do you think an oxidation state of +7 is feasible for S?

Good point, but the thing with oxidation numbers is that some can be a little weird, and as I think Borek has mentioned many times, the oxidation number has no true relation to the properties of what you're looking at, and are not always useful... something along those lines anyway. There are a few oxidation numbers for elements in some species that are really awkward.

[Borek]

the thing with oxidation numbers is that some can be a little weird, and as I think Borek has mentioned many times, the oxidation number has no true relation to the properties of what you're looking at, and are not always useful... something along those lines anyway. There are a few oxidation numbers for elements in some species that are really awkward.

Thank you for bringing it

[AWK]

I am also completely in agreement with Borek that oxidation numbers are quite artificial ones.
This concept comes from simple ionic compounds and was expanded to all redox reactions. But this concept is very useful on quite elementary levels of chemistry.
I know Borek is a votary of pure mathematical methods (me to) but some compromise is needed
in order that not to teach redox reactions at the end of chemistry. I am opponent in teaching pushing buttons of keyboard  instead of solving chemical problems though on a very simplified way (but on the other hand very effective one)
In my scientific work I use much math in X-ray structure analysis. I even can start with completely unknown compound, but in this case I should derive some model of the structure to start calculations.This model can be completely wrong but at the end of calculations I will get a correct structure.

[Borek]

I know Borek is a votary of pure mathematical methods (me to) but some compromise is needed

To be precise: I have nothing against approximate and simplified methods, they are very often quite usefull. What I am against is giving them more weight that they deserve. Too often these simplified methods are becoming known as the real thing - and that's dangerous.