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PROFESSOR CIMA: OK, so what determines acid strength?
Bond energies. So here I've tabulated some interesting data.
So here's some K_a's, here's a bond energy, and what do I have here?
I have bond length in nanometers. And this one's in kilojoules per mole.
And I've listed HF, HCl, HBr, and HI. So this one ends up being 7.2 x 10^ -4.
This is 1 x 10^ 6, as we discussed. This is 1 x 10^ 9.
And this is 3 x 10^ 9. This is 568, this is 432, this is 366, and
this is 298. So the bond energy between the hydrogen and
the fluorine, the hydrogen and the chlorine and so forth, decreases
as I go down the chart, as you remember.
And, the bond lengths-- and just like you might expect, as the bond
energy decreases the bond gets longer.
Now what really looks-- if you set aside HF for a moment, you begin
to understand why these K_a's are going up.
Well, because it's just getting easier and easier to separate the hydrogen
from the anion. So that's not too surprising.
What's really surprising to me is just how small this is.
HF satisfies our definition of a weak acid, in all its members of the rest
of the column-- its analogs, I guess, in the rest of the column--
are way up in the strong acid category. So can anybody tell me why that might be?
What's unique about HF in this series? STUDENT: Hyrogen bonds?
PROFESSOR CIMA: Exactly. Hydrogen bonding.
This is the only one in this series that can hydrogen bond.
It could only hydrogen bond when it's in this form.
So in order to separate HF, or hydrogen from fluorine, I have to do
two things. I have to break that bond--
which by the way is strong-- and the second is I have to destroy all the
hydrogen bonds that this HF molecule is enjoying with all the water molecules
around it. So that's because of hydrogen bonding.
Very important.