Re: Why is hydrofluoric acid a weak acid?

     You are correct!  Hydroflouric acid is a weak acid partly because of 
hydrogen bonding.  It can not only hydrogen bond to itself, but also to 
water if it is in aqueous solution.  Since the protons will be involved in 
hydrogen bonding, they are not free to move about in solution and affect 
pH (a measure of acid strength) - the protons will be attached through 
weak hydrogen bonds to flourine and the lone pairs of electrons on oxygen 
in a water molecule.  We know that hydrogen bonding occurs within 
compounds that have a hydrogen atom bonded to either nitrogen, oxygen, or 
flourine.  In fact, if we look at other halogen acids that cannot hydrogen 
bond (specifically HCl and HBr), we notice that both of these have boiling 
points in the negative range (HCl = -85.06 C, and HBr = -66.7 C), but HF 
has a boiling point of 19.5 C!  We can use hydrogen bonding to explain the 
unusually high boiling point in addition to the relatively weak acidity.

     But as you said in your question, we can also look at the strength of 
the HF bond to show why it is not such a strong acid.  HF has a bond 
dissociation energy of 569 kilojoules per mole (kJ/mol), meaning that it 
would take 569 kJ to break the H-F bond in one mole of HF molecules.  
That's alot of energy!  But lets look at the other halogen acids.  HCl 
only needs 431 kJ/mol, HBr requires 368 kJ/mol, and HI requires only 297 
kJ/mol.  Since HF requires more energy to break the H-F bond, it will not 
dissociate as easily as the others, and we know that stronger acids 
dissociate more easily, so HF cannot be as strong as the other halogen 
acids.

     When you ask why flourine doesn't just take an electron from hydrogen 
and ionize, things get a bit more complicated.  First, there is really no 
bond that can be characterized as 100% ionic where one atom will completly 
take electrons from another.  There will always be some electron sharing 
between elements involved in a chemical bond and hence some covalent 
character.  The amount of sharing depends on a few things including the 
molecule's dipole moment and the average bond length.  For HF, the ionic 
character is about 41% compared to 18% in HCl and 12% in HBr.  We also 
have to remember that everything tries to reach a point of equilibrium, 
and while the equilibrium may tend towards the products (such as H+ and Cl-
 in the dissociation of HCl), there will almost always be some reactants 
present.

     Strong acids that produce lots of H+ in solution (like HCl, HBr, and 
some organic acids) burn the skin because H+ is such a reactive species.  
It will react with the carbon-carbon double bonds in your cell membranes 
and thus causes a burn.  I'm not a biology expert, but i'm reasonably sure 
that after the cell membrane is damaged like that, the cell will probably 
burst.  Carbon-carbon double bonds are just one example, but there are 
other bonds present in cells that will easily react with H+.  The anions 
resulting from dissociation are usually so stable and unreactive (Cl- for 
example is very stable) that they will not damage the skin.