[zsinger]

All,
I have come to notice that most, if not all, B/L A/B tables tables list HCl as a stronger mineral acid than H₂SO₄.  I was wondering what you all thought as to this, as the thermodynamic tables back this claim up.  However, using reasoning, why is this true when K_a1 is a full (100%) reaction, AND K_a2 is 1.2 X 10^-2.  Shouldn't that second K value, albeit not as big as the first, contribute also to [H₃O⁺].  Even only considering the first K, the Sulfuric acid has 4 oxygens, and a sulphur (all electronegative atoms) to stabilize negative charge density?  Thanks for the comments…..
                  -Zack

[hypervalent_iodine]

IIRC, H₂SO₄ is called a strong acid in reference only to the first deprotonation event, not the first and second. The second deprotonation event for sulphuric acid requires much more energy than the first and so the change in [H+] as a result of this is essentially negligible (i.e. you can consider it a monoprotic acid when comparing it to other strong acids).

In any case, it's difficult to really compare the relative strengths of strong acids in water since dissociation occurs at amounts approaching 100% (this is due to the levelling effect of water; see http://en.wikipedia.org/wiki/Leveling_effect). What this means is that in water, strong acids are all roughly the same strength as they are limited by the basic nature of water. When you compare the acidity of strong acids in other, differentiating solvents what you find is that HCl is noticeably stronger than H₂SO₄.

[zsinger]

Thanks.  Kinda what I was thinking.  Funny you should mention the leveling effect, as I threw that question on my bonus quiz yesterday when I asked my kids what the strongest acid they thought could exist in an ammonia solution!
             Z