Se oxidation state!

1. Jan 12, 2004

ShakeWell

Redox question:

12.53 ml of .05093 M selenium oxide (SeO2) reacted exactly with 25.52 ml of .1 M CrSO4. Cr+3 is formed. To what oxidation state is Selenium converted?

I think that the ml + M stuff gives you the ratio between moles of SeO2 and moles of CrSO4 (1m SeO2 = 4m CrSO4). But how am I supposed to figure out what happens to the Se?

2. Jan 13, 2004

thunderfvck

Okay, this is how I'm looking at it, based solely on the information alone and not by looking up the oxidation states of selenium. I don't know if it's right but...
Ratio is 1:4, we know that, and we also know that chromium goes from +2 to +3. So SeO2 needed 4 moles of CrSO4 and so it follows that it needed 4 moles of e- from the Cr+2. So one SeO2 recieves 4 e-'s and makes the reduction: Se(+4) + 4e- --> Se(0) which is the selenium metal.

Wow it looks so nice, we've solved the problem, and I don't know if it's right. Something came up though, when you write out the equation there's a problem with the sulfate ion floating around:

SeO2 + CrSO4 --> Se + Cr2O3 + SO4(-2)

so maybe it reacts with the water in the solution (assuming everything's aqueous and it probably is), and forms H2SO4.

2H+ + SeO2 + 4CrSO4 --> Se + 2Cr2O3 + H2SO4

3. Jan 18, 2004

thunderfvck

someone care to help? I'm sorry but I just really want to know the answer, it bugs me when I can't solve a problem.

4. Jan 18, 2004

GCT

I'm thinking its 2+, that is it has been reduced while the chromium has been oxidized. Nonmetalic elements usually have oxidation states at even intervals e.g. from 4+ to 2+. Tis an oxidation reduction reaction.

5. Jan 18, 2004

thunderfvck

Se has oxidation states of +6, 4, 1, and -2. +1 being very rare. http://www.ivstandards.net/extras/pertable/
Regardless though, based on the information that was given and my reasoning, was it a correct approach?
And selenium sulfate doesn't exist, I can't find it on chemfinder, so I don't think that that will even form. SO what the hell is going on...

6. Jan 19, 2004

GCT

Chromium is oxidized, which means it has lost some electrons. Therefore it is a reducing agent; something has been reduced. Selenium is reduced. If the original assumption was right-1 Se for every 4 Cr, than we can assume that 4 electrons are lost for every 1 Se. Assuming that the original oxidation state of Se was 4+ we can say that the oxidation state for Se is now 0. However, I am not completely sure about this. I'll check back later with other details, I am a bit busy right now.

7. Jan 20, 2004

GCT

Yeah, the above is correct.

8. Jan 21, 2004

thunderfvck

thanks.
what happens to SO4(2-)?