Re: why does Cr2O7 only goes to a Cr+3 in a slightly acidic solution?

I’m not a chromium chemist, so all of my information comes from the 3rd 
edition of Cotton and Wilkinson’s "Advanced Inorganic Chemistry". You are 
unlikely to find this in a public library, but (I think) the later editions 
are still a standard upper-level college text, and you might be able to 
find a copy in a high school or college library.

In aqueous acid, Cr+3 exits as a hydrated ion: [Cr(H2O)6]3+.  (I hope you 
figure out the subscripts/superscripts that should be there!) This 
"hexacoordinate" octahedral structure is the most stable form of chromium. 
You can replace the coordinated water molecule with a variety of "ligands", 
such as Cl-, CN-, and NH3.

This begs the issue of WHY this is the most stable form of chromium--I 
don’t know the answer to that question!

The sulfuric acid/sulfurous acid questions are easier to answer. I’ll 
assume you are familiar with acid equilibria.

H2SO4 exists in water as H3O+ and SO4 2- because the first acidity constant 
is "very large", and the second acidity constant is still quite large 
(approx. 0.012).

On the other hand, H2SO3 is really just SO2 dissolved in water. It’s 
acidity constant is about 0.015, so it exists in water as HSO3 -. The 
dissociaion constant for HSO3 - to SO3 2- is about 1x10^-7, so very little 
SO3 2- exists in acidic solution.