# Do hydrogen halides (HX) form strong or weak acids?

I know that when dissolved in water, hydrogen halides ($\ce{H-X}$), become acids. I know that $\ce{HF}$ forms a weak acid, but can it be generalized that all $\ce{HX}$ acids do so?

No, it can’t be generalised. Amongst the haloacids, only $\ce{HF}$ is weak.

Acid strength can be determined by the stability of the conjugate base (for most acids, the conjugate base is what you get when you remove $\ce{H+}$ from the acid). In this case the conjugate bases are the halides.

As the size of the halide ion increases, the charge is dispersed over a larger volume, and thus the ion is more stable. In this manner, one can say that the order of acid strengths is $\ce{HF<HCl<HBr<HI}$ (I’m not considering the radioactive ones here; they fit in the same trend though).

$\ce{HCl, HBr, HI}$ are all strong acids. $\ce{HCl}$ has a $\mathrm{p}K_\text{a}$ of −7 (the smaller the $\mathrm{p}K_\text{a}$ is, the more acidic it is), $\ce{HBr}$ is approximately −9, and $\ce{HI}$ is also approx −9.

On the other hand, $\ce{HF}$ is 3.2. The $\ce{H-F}$ bond is pretty strong, so it has a smaller tendency to dissociate—making it a weak acid (not to say that it isn’t dangerous).

• Ah, of course about HCl and everything. I originally asked if they were all weak, then remembered about HF and forgot about the others. Thanks for relating it with other things like pKa. – Maddy Byahoo Jun 21 '12 at 22:15
• I'd have said $\ce{HF \ll HCl<HBr<HI}$ with regards to the acidity myself. :) As another example of the tenacity of hydrogen bonding in $\ce{HF}$, consider ammonium bifluoride... – user95 Jul 5 '12 at 16:09

No: HCl is a very strong acid, and HBr even more so. HF is the only weak acid. This is because it binds hydrogen much more strongly then the other halides.