[tamubsa]

In class, my professor was talking about how all strong acids in water have the same strength, because they completely dissociate. I guess this makes sense, however in book they talk about how HI is stronger than HCl because of electron attachment enthalpy and the strength of the H-X bond etc. Since HI and HCl are both strong acids... I'm a little confused. Could anybody shed some light on what I'm missing here? Thanks in advance!

[Kate]

Your Professor is simplifying the question really. Acids/Bases are usually characterized by pKa values (-log of Ka) and as you can see here

http://evans.harvard.edu/pdf/evans_pka_table.pdf

in the inorganic acids column strong acids have different values of pKa.

If you check the pKa values for HBr and HCl, they are respectively -9 and -8 (in water). The Ka in both cases is extremely high (10^9 and 10^8), so both completely dissociate. HBr dissociates a little more than HCl but the difference is negligible compared to the magnitude of their Ka's.

I think it's wrong to say all strong acids have the same strength, even though they completely dissociate. If you're just doing an exercise then it's alright to think that way, because for practical purposes you don't distinguish between the strength of HCl and HBr, but conceptually it's not exactly right.

[Borek]

Perhaps it was a (rather poor) reference to the leveling effect - in water all acids stronger than H₃O⁺ will simply protonate water, leaving just H₃O⁺ in the solution (sure, that's a simplification, but often a useful one).