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Name: Sourabh
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 4/1/2004


Question:
What is the correct order of acidic strength among oxyacids of halogens in which different halogen atoms are present in same oxidation state?


Replies:
You mean to compare various series:

H+ ClO4-, H+ BrO4-, H+ IO4- (per-chloric, per-bromic, iodic acids)
H+ ClO3-, H+ BrO3-, H+ IO3- (chloric, bromic, iodic acids)
H+ ClO2-, H+ BrO2-, H+ IO2- (chlorous, bromous, iodous acids)
H+ ClO- , H+ BrO- , H+ IO- (hyochlorous, hypobromous, hypoiodous acids)

Fluorine is not included, because it has no oxy-acids. Fluorine is so hard to oxidize that no H+ FOx- exists. I can't find in my CRC handbook HBrO4. I would think it exists, since Cl and I on either side of it exist, but some dissociation might happen instead. Also can't find much about HBrO2, HIO2. They might normally dissociate into higher and lower oxidation states.

Enough re-stating the problem.

I kind of thought perchloric acid was king. (H+ CLO4-). But I don't actually know that perbromic (H+ BrO4-) or iodic (H+ IO4-) aren't great and a little better than perchloric. In principle, Chlorine ought to be the best. Cl has highest electronegativity, which in turn will leave the surrounding oxygens hungrier for their full complement of electrons, which in turn will strip electrons from the hydrogen more strongly. This is called an inductive effect. It implies an acidity rankng order Cl>Br>I, opposite that for hydro-halogen acids. Plenty of college-instructors' web-sites describe this. Search Google for "pK"+"oxyacids".

From my CRC, pK's- {HClO, HBrO, HIO} = {7.5, 8.9, 10.6}, confirming my speculation for at least one oxidation state. The other oxidation states should have similar ranking orders, shifted to higher acidity (lower pKa).

Unfortunately, the others aren't listed. Admittedly, the dissociation of perChloric acid is so high that it requires other-than-simple-water methods to measure pK or rank it's acidty. And bromic and bromous acids aren't commonly used. But these things must have been looked at more than once, and published.

Political plug- Your question should have been self-evidently answered by full list found with a quick internet search. I definitely feel that reference data is strangled, unnecessarily scarcified, by copyrights. I don't even know what I need to buy to get a list of all known numbers in this class. I wish NIST was covering lists like this, on the public's behalf.

Jim Swenson



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