List the following compounds in order of increasing acidity in aqueous solution, giving reasons for your choices: HCl, HI, HBr, HF.

The acidity of a compound is measured by its pKa, which is defined as: pKa=-log([H⁺]) ([H⁺] is the concentration of H⁺ in solution). As [H⁺] increases, pKa decreases, meaning strong acids have low pKas while weaker acids have high pKas. Down the group, the bond enthalpies of the HX compounds (X=F,Cl,Br,I, i.e. the group 17 halogens) decrease, due to an increasing difference in size between H and X. As the size of the atoms increases, so does their orbital size. A large difference in orbital size leads to weak orbital overlap and bonding. Hence, it becomes increasingly easier for HX compounds to dissociate into H⁺ and X^- in solution as we go down the group, meaning acidity increases down the group as well. In terms of pKas, the compounds can be ordered as follows: (weakest acid) HF (pKa=3.2) < HCl (pKa~ -7) < HBr (pKa~ -9) < HI (pKa~ - 11) (strongest acid).