[oneat]

What Will I got when I electrolyse Sulfur Acid ??
Will it be SO3 And H2 ??

[Grundalizer]

Are you trying to electrolyse H₂SO₄ (sulfuric acid) or H₂SO₃ (sulfurous acid) ?

Either way, what happens when acid is added to water?  
What two ions does it break into?  
Could those ions act as electrolytes for the electrolysis of water?
 What are the products of electrolysis of water?

So in conclusion, you are basically electrolyzing water, so if you can figure that out, that's your answer.

[oneat]

I want to electrolyse concentrated H2SO3.
Will I retrieve SO3 ?

[Grundalizer]

You aren't electrolysing H₂SO₃ you are electrolysing water, because H₂SO₃ dissociates in water into free H+ ions, HSO_3^- and the SO₃ ion, which act as electrolytes in the solution.  If you can figure out what the products are when you electrolyze water, that is your answer.

[typhoon2028]

You aren't electrolysing H₂SO₃ you are electrolysing water, because H₂SO₃ dissociates in water into free H+ ions, HSO_3^- and the SO₃ ion, which act as electrolytes in the solution.  If you can figure out what the products are when you electrolyze water, that is your answer.

Your explanation implies when a salt is added to water and electrolysis applied, you generate hydrogen and oxygen.  This will work for dilute solutions.  So I ask what happens when electrolysis is performed on concentrated sodium chloride solution?  The poster is asking a similar question, but substituting NaCl with sulfurous acid.

[Borek]

I want to electrolyse concentrated H2SO3.
Will I retrieve SO3 ?

No.

[Grundalizer]

You are definitely talking about sulfurous acid here?  Not sulfuric?

"There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase" according to Wikipedia.

Even concentrated sulfuric acid has water in it, as that is how you get it in the first place

SO3(g) + H2O(l) → H2SO4(l)

You can't electrolyse concentrated sulfurous (or ic) acid like you can molten Na Cl.  Well, I'm sure you CAN, but I don't think you'd get the products one might expect by looking at the molecular formula.  You'd still get hydrogen at the cathode, and at the anode I think you'd still get O2.  I get what you are saying though, one might think you could get back to SO₃(not very stable) or maybe SO₂ which is more stable, but the sulfate or sulfite ion will always stay in solution as ions floating around.  I'm guessing the reduction potential makes it not favorable in aqueous solution.

If you can find an article that shows any sulfate salts being electrolyzed back to sulfur oxides I'd be impressed and also learn something I didn't think was possible.  

I'm guessing the reduction potentials of those ions make them not take part.

So my answer would still be, H₂ and O₂

Sorry, best I can come up with.

[oneat]

But why does it stay in solution ?
Does it react with electrode ??
But what happens when electrode is made from coal for example.
It isn't reactive to much as I think.
So can you redraw it clearer ?

[Grundalizer]

Because reacting SO₃ (sulfur trioxide) with water gives you sulfuric acid.  Sulfurous acid, as was said above, has never even been detected in solution, but it has in the gas phase.  These ions ONLY exist in solution, so they do not exist outside of aqueous solution, and as long as there is water in something you are electrolyzing, you are going to get H2 and O2. 

Why are you asking about the electrolysis of sulfurous acid?  It seems to be just a "theoretical" compound.

The sulfate ion is an anion, so it does not exist by itself OUTSIDE of aqueous solution.  Sure it can be the anion in a salt, like Sodium Sulfate, or copper sulfate, but you can't ever have Sulfate ion by itself in a crystal compound, without a cation bonded to it. 

So, the sulfate or sulfite ion can ONLY exist alone in aqueous solution, which means that there is H2O (water) present.  The energy required to reduce/oxidize water into H2 and O2 takes less energy than it takes to reduce, say, Na+ from Na₂SO₄ into Na(solid metal).  That is why you can't get sodium metal from electrolyzing salt water, because 2H+ will get reduced first to H2.  So since the sulfate or sulfite ion can't ever exist by themselves in a pure form, you can't electrolyze them. 

I don't know how else I can explain it.  H2SO4 IMPLIES there is actually water present. 

Now, if you wanted to electrolyse sodium sulfate, I'm sure you might get different products. 

I understand your frustration, as there are questions like this I have that no book or internet site can give me a good answer too sometimes.

Na+ and Cl+ are ions of elements, but SO₄^2- and SO₃^2- are polyatomic ions with (I assume) reduction potentials that have not been measured. 

[jsmith613]

What Will I got when I electrolyse Sulfur Acid ??
Will it be SO3 And H2 ??

OK.

So H2SO3 (aq) will NOT give off SO3.

for a start SO3 is an anion (SO3-) and so cannot be given off
secondly only one product will be given of at each electrode (you can't have compounds)

Remember, electrolysis for water --> H2 and O2
in this case the Sulphur does not get given off (I think it remains in solution)

Products:
H2 + O2

[billnotgatez]

Was typhoon2028 trying to get us to discuss why there is a difference between salt in water and sulfuric acid in water?

[408]

Oy

Sulfuric acid does not have to contain water. The pure liquid simply would not conduct electricity significantly in an analogous manner to how pure water with no electrolyte has too high of a resistance for electrolysis.  That is not to say electrolysis needs water either, non-aqueous solvents can solvate compounds to their ions allowing conductivity and electrolysis, however the standard reduction potential tables become rather useless here as they are defined to be at a certain aqueous concentration (either 0.1 or 1M, I forget which).  The addition of some other acid to conc. sulfuric would allow ion formation via acid-base chemistry, and electrolysis would then be possible.  But this would have some really funky chemistry to it, and I am sure someone has tried some experiments regarding this, but I am not familiar with any literature in the field.  Adding say BF3 or anhydrous nitric acid to conc. sulfuric acid would definitely allow some electrolysis to occur, but I would not want to be the one to have to predict the electrolysis products

Now onto sulfurous acid, which to the best of my knowledge is simply aqueous SO2, wiki says it is only known in the gas phase but I have a redox table in front of me having aqueous sulfurous acid, but that may be the same as aq. SO2. 
S2O6 (2-)  + 4H+ +2e- --> 2H2SO3  0.564 V
H2SO3 +4H+ + 4e- ---> S + 3 H2O 0.449 V
SO4(2-) + 4H+ + 2e- --> H2SO3 +H2O 0.172V
2H2SO3 + H+ + 2e- --> HS2O4- + 2H2O -0.056V


Furthermore, SO3 (sulfur trioxide, not to be confused with sulfite, SO3(2-) ) is reactive with water, producing sulfuric acid.  We would encounter similar issues with reduction potentials, (if it is possible to oxidize sulfite to sulfur trioxide...) to that when one wonders why the electrolysis of aq. NaCl does not produce sodium metal.

So looking at a redox table, where the strongest oxidizing agent present is sulfurous acid, and the strongest reducing agent present is also sulfurous acid    The redox potentials indicate sulfurous acid would spontaneously react with itself, which in my limited experience usually means that one or more of these potentials was measured in reverse, and the half reaction only really works on one direction... but simply based on these potentials it would SEEM that electrolysis of sulfurous acid will produce sulfate and sulfur.  There is the chance I missed an important reduction potential in my table, it is 20 pages long, in descending reduction potential, so I could have easily missed a value.  Or there could be overpotential effects making the reaction take another pathway.  In any case, the electrolytic disporportionation of sulfurous acid to sulfate and sulfur makes chemical sense, but if you want to be sure, grab some sodium sulfite and some sulfuric acid, mix in low concentrations so you do not gas yourself with SO2 too badly and electrolyse with graphite or Pt or Au electrodes and look for sulfur and then test for sulfate.  If you get bubbles at either electrode, something other than what I predicted is occurring, and once you identify the gasses you can make a guess at what half reactions are occurring.  Like I said, half reactions cannot predict everything.  Experiment always trumps theory.

Now as for why sulfuric acid in water vs NaCl in water gives different products.  With sulfuric acid the strongest oxidizing agent and strongest reducing agent present is both water.  The acid and sulfate are merely helping increase conductivity.

With NaCl, just based on reduction potentials one would think that we would end up in the same boat as with sulfuric, after all chloride oxidation is less favorable than water oxidation based solely on reduction potentials, however there is an overpotential for the water oxidation (http://en.wikipedia.org/wiki/Overpotential) compared to chloride oxidation, so chloride oxidation actually takes place.

[Borek]

The pure liquid simply would not conduct electricity significantly in an analogous manner to how pure water with no electrolyte has too high of a resistance for electrolysis.

2H₂SO₄ <-> H₃SO₄⁺ + HSO₄^-

[H₃SO₄⁺][HSO₄^-] = 10^-2.9

So while to some extent you are right, to some extent you are not

[408]

Hmm I had no idea the auto dissociation was so much higher than that of water, I assumed it was on the same order of magnitude.  Thanks!

[typhoon2028]

Was typhoon2028 trying to get us to discuss why there is a difference between salt in water and sulfuric acid in water?

No