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

I learned in chem ap today that Cr at a +6 in Cr2O7 goes to just a Cr 
+3 after reacting in a slightly acidic solution. (ex: the equation, 
HSO3(-) + Cr2O7(-2)  --->  SO4(-2) + Cr(+3)   ).  Why are there no 
oxygens attached to the Cr?  CrO2(-) is also a Cr+3, but it's no good. 
 There's a lot of Cr+3 oxide compounds, but why only this?  Also, I 
want to verify, there's an H attached to the SO3 when it's in a 
slightly acidic solution because there's an excess amount of H+ (the 
definition of an acidic solution) and so it attaches itself to the 
SO3.  I have another question (now that I think of it), why wouldn't 
the HSO3(-) go to a more stable H2SO3?  and why aren't there any H's 
attached to the SO4 in the reaction?  Thanks.

Ben

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