[xoannnnna]

Why is HI a stronger acid than HF? Does it have to do with electronegativity or the size of the atom? It would have to do with going up or down the columns, right?

Does something like that apply to going left to right? That something containing nitrogen is going to be less acidic than something containing oxygen?

Also, another thing my teacher said was that the higher the electronegativity of an atom on a molecule, the stronger the acidity of that molecule. I attached a picture of two molecules. The one one the left is the stronger acid since it has a fluorine atom compared to the less electronegative iodine. Well, going back to my first question, wouldn't I- be the weaker base and therefore make the molecule on the right the stronger acid?

I'm just so confused. I was thinking that maybe this has to do with the Lewis acids/base and the Bronstead Lowry theory. That the first question has to do with just the electrons moving around, and the second one is losing to proton attached to the oxygen. Will the higher electronegative atom make it easier to hold on to the hydrogen or make it easier to get rid of it?

Thanks.

[orgopete]

I have been working on this problem for a while now. The answer is not easy to give. It isn't necessarily difficult except for the paradox you noted. It is a bit complicated though. Which answer do you want, the one your professor expects or the one you are intuitively expecting to be correct?

Electronegativity is the source of the problem. This stemmed from Linus Pauling trying to explain why fluorine had higher than expected bond energies. The trends one must contend with is that as you go from C-C, N-N, and O-O, the bond strengths decrease, while from I-I, Br-Br, and Cl-Cl, the bond strengths increase. What is going to happen with F-F? It is neither as low as expected from the O-O bond nor as high as expected from the Cl-Cl bond. In 1932, Pauling suggested bonds contain ionic and covalent character. This supposition was brilliant in 1932, but not good science, especially as it has become taken as gospel. The Pauling electronegativity calculations do not follow the supposition of bonds being comprised of covalent and ionic components. This is to avoid a poor correlation, violation of opposites attracting, and the obvious argument that HI is covalent and HF is ionic.

If you wished, you could challenge your professor's suggestion that the fluoroalcohol is more acidic. I doubt he actually knows because if he did, the likelihood is that the iodoalcohol is more acidic. (If I am wrong, please post the pKa values. I am in constant search of examples (there are some) of fluorine being a better electron withdrawing group than Cl, Br, or I.) See my post here:
http://www.chemicalforums.com/index.php?topic=45100.0
You could ask him which is a stronger acid glycolic acid (hydroxyacetic acid) or iodoacetic acid? (The less e-neg iodoacetic.)

I have suggested to students to learn that I>Br>Cl>F>O>N,H>C as electron withdrawing properties. (HI will be a stronger acid, better leaving group, and carbon a better electron donor. This will generally be correct, but not 100%, see the acetic acids.) Use the electronegativity values when discussing electronegativity. Keep them separate. The first for chemistry, the second for exams.

[Cesium-137]

As to why HI is stronger than HF (or why HF is a weak acid in general), I believe it has to do with the fact that fluorine is so electronegative it forms hydrogen bonds, decreasing the acidity of hydronium to some extent. I may be wrong, but thats how I've always understood it!

[orgopete]

See the linked posts at http://www.chemicalforums.com/index.php?topic=45100.0