Comparing the hydrogen halides with each other (HF, HCl, HBr, HI), which one is the strongest acid? Explain your reason briefly.

It only makes sense to discuss an aqueous solution of each. A strong acid is one which is completely ionized in aqueous solution. HCl, HBr and HI are all strong acids, they are all completely ionized in water. Therefore, you can’t say that one is “stronger” than another when they are all at the same level. Only HF is a “weak” acid, and that is because of the strong attraction between H and F. The attraction of water molecules is not great enough to pull apart all of the HF molecules in solution.

That being said, some might argue that HI is the strongest acid because it has the greatest ability to “lose” hydrogen as the H+ ion. This is illustrated by the values of pKa assigned to the acids.

HF …………… pKa = 3.1 ………… weakest
HCl ………….. pKa = -6.0
HBr…………….pKa = -9.0
HI ……….…… pKa = -9.5 ………… strongest

Well, of those listed [math]H-I[/math] has the WEAKEST halogen-hydrogen bond … And in addition, the iodide conjugate base, [math]I^{-}[/math], is LEAST efficiently solvated by water molecules (and the entropy component is especially important given the weakness of [math]HF(aq)[/math] as an acid). And thus BOTH enthalpy, and entropy conspire to make [math]HI[/math] the STRONGEST acid in aqueous solution… Note that for [math]HCl[/math], [math]HBr[/math], [math]HI[/math], in AQUEOUS solution, protonolysis of the solvent, i.e.

[math]underbrace{HX(aq) + H_{2}O(l) longrightarrow H_{3}O^{+}+X^{-}}_{X e F}[/math]

…would be more or less complete (fluoride anion is more basic)… And to order the acidities of the hydrogen halides, we would have to use a more acidic solvent such as acetic acid…

Do I win five pounds?