Jide? Stop (and listen). People's exam grades are at stake here.
The molecule HF *dissolves* freely in water (even more than 70%!), but it is a weak acid (does not *dissociate* well in water) for two reasons: 1) because hydrogen bonding is FON, and 2) because delG = delH - TdelS.
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N, O and F atoms have 1, 2 and 3 lone pairs whose electrons are concentrated enough to be attractive to H atoms in other molecules. So N, O and F hydrogen bond. No other lone pairs on the periodic table can do that.
Not Cl, not Br, not I (think HCl, HBr, HI). Hydrogen bonding is FON.
Yes, the HF bond is very strong (covalency has to do with proper orbital overlap, and the sizes of H and F are very appropriate for each other). But when you turn one mole of HX into H+(aq) and X-(aq), it actually won't matter whether X is F, Cl, Br or I: they will each have about the same heat of formation (delH). And it will be slightly negative (favored). Darn. So HF bond-breaking is not the reason that it doesn't dissociate in water.
DeltaG is equal to a couple of things. Firstly, delG = RTlnK, where K let's you know how well something dissociates. But that's the frosting on the cake, so save it for later. Secondly, delG also = delH - TdelS. DelG is free energy (how well a reaction will go forward), and it is helped by heat (enthalpy) and hindered by order (entropy) at a given temperature.
When you put HCl, HBr or HI in water, the lone pairs on the O of a nearby water kindly take the H+ away from the Cl- or Br- or I- because... hydrogen bonding is FON. Or, at least it's more FON than Cl-. Now the Cl- kinda hangs out with the H3O+ to keep things neutralized. And some other waters weakly hang out with both the Cl- and the H3O+ so that there's no jealousy.
But here's the punch. When you put HF in water, it starts taking hostages. Remember those 3 lone pairs on F that can hydrogen bond? Well, when a water wanders on over to take away the H+ from HF, the F- starts shoving other waters in its back pocket. That is, it hydrogen bonds with H's of other waters behind its back. This creates a real frenzy of strong order around the F-. Then the lifeguard blows the whistle. Everyone in the pool stands still. The head count (delG) reveals that there is too much order for that reaction (dissociation) to go forward, and water is commanded to give back the H+ to the F- that has 3 waters sticking out of its back pocket.
That is, when HCl dives in the pool, delH (from a few paragraphs above) is medium negative, delS is not too big negative, and so delG is overall negative, and the dissociation goes forward. But when HF dives in, everything gets real tense and there's lots of order. So delH is negative, but delS is really negative, and so delG is overall positive, and the dissociation does not go forward well.
delG = RTlnK. Solve for K with a negative delG (HCl) and you have a large dissociation constant. Solve for K with a positive delG (HF) and you have a small dissociation constant.
So, HF is a weak acid because hydrogen bonding is FON, and because delG = delH - TdelS.
(
http://www.chemguide.co.uk/inorganic/group7/acidityhx.html)