[CrimpJiggler]

Firstly, HF is a weak acid whereas HB and HI are very strong acids with pKas of -9 and -10 respectively. Why is HI so much more acidic than HF? Does the atoms large size allow it to delocalise charge better or something?

Secondly I notice that adding flourine atoms to acetic acid yields trifluoroacetic acid which drops acetic acids pKa from around 4.8 to -0.25. Fluorinating sulphuric acid yields super acids such as fluorosulphonic acid and trifluoromethanesulphonic acid with pKas of -14 and -16 respectively. What about the other halogen atoms? Would triiodoacetic acid be more acidic than trifluoroacetic acid?

[spirochete]

That is exactly correct, the larger size of iodine allows for a better delocalization of the negative charge on the conjugate base of HI, making HI the strongest acid in the H-X series.

In the case of halogenated acids, the negative charge on the conjugate base cannot be delocalized onto the halogen atoms through resonance.  The main factor increasing the strength of these acids is induction, ie the withdrawal of electron density through sigma bonds.  Now the size of the halogen can't help delocalize charge as well, and its electronegativity becomes the most important variable.  Thus Trifluoracetic acid is the strongest acid in that series.  Other factors besides electronegativity come into play as well, such as polarizability, but the trends can be rationalized thinking only about electronegativity.

Look up "Evan's pKa Table" for some numbers you might find useful.  pKa values are readily available through googling.  Also, most of the info I talk about above can be found in sophmore level organic texts.

[CrimpJiggler]

Informative reply, thanks. You forgot astatine:
https://en.wikipedia.org/wiki/Hydrogen_astatide
according to the wiki page its the strongest hydrohalic acid but its not very practical because it decomposes too readily.

That pretty much explains it. Halogens have filled p orbitals so they act as resonance donors. I wonder could you increase the acidity of these fluoro acids even further by attaching some kind of resonance acceptor to the molecule near the F atoms so that it can accepting fluorines p electrons and thus, reduce fluorines resonance donor strength. I'm sure thats been thought of though.

EDIT: Evans pKa table, thats exactly what I was looking for, thanks a lot!

[spirochete]

Yes I often neglect to think about elements that have half lives below 12 hours  .

I probably confused the issue a bit by mentioning resonance.  Actually in the case of the halogenated acetic acids (Ie CH₂FCO₂H) there is never any resonance donation by the lone pairs because the halogens are directly bonded to an SP³ carbon - this means that induction is really the only thing you need to think about in these cases.

Certainly if you tack a halogen onto an aromatic ring then the their resonance donating abilities come into play somewhat, although their inductive withdrawal still tends to "win" in terms of net effect.  

[orgopete]

@CrimpJiggler
If you want to repeat arguments as to why HI can be more acidic and less electronegative, you have provided one of the accepted rationales.

Re: the acidity of fluoro compounds, perhaps you may look at the thread posted here:
HF acidity.

I have been researching this topic for some time. As you may note, I am not a subscriber to Pauling's electronegativity theory. I do not think it was good science. A consequence of the electronegativity theory has resulted in the kinds of paradoxes being noted. I argue in the thread that if you went into the laboratory and did experiments, you should conclude that iodine is more electron withdrawing than fluorine. However, I could be wrong. If so, what experiments do you have that show fluorine is more electron withdrawing?

The other part of the electronegativity story is a lot more complicated. That is, it is easy to conclude that iodine should be more electron withdrawing that fluorine. The hard part, and this was what Pauling was trying to explain, why are the heats of formation of fluoro compounds higher than virtually all other elements?

This becomes a lot more complicated. So, Pauling suggested that fluoro compounds had ionic properties. This creates a theory of ionic attraction. Therefore if fluorine has strong bonds due to its ionic properties, then ionic properties should increase attraction for all compounds. Logically, I argue, one should conclude that ionic compounds should have a greater attraction than non-ionic compounds. If HCl, NaCl, and NaOH ionize into cations and anions, they should all have similar attractive properties and behavior. These compounds show little attraction for their oppositely charged ions. Ammonia is more basic than chloride which shows that local electron density is more important than the macro effect of differing numbers of protons and electrons. (Ionic attraction, or lack thereof, fits nicely with what you might expect from atomic structure if you look closely.)

I think the correct answer has to do with the heats of formation and the scale for measurements. What are you actually measuring and how do you compare one result to another? For temperature, we compare the state property with that of water's freezing and boiling point. For heats of formation, I am unaware of a scale that has or can be used to compare one compound to another as has been done with temperature. Heats of formation are calculated from elemental states, but are all elemental states equal? Does carbon have the same reactivity as chlorine or iodine?