Q.Suggest a reason why HF is a weak acid, whereas binary acids of all the other halogens are strong acids.

Step 1. Compare bond dissociation enthalpies.

The H-X bond dissociation enthalpy is by far the highest for HF (562 kJ/mol) and falls steeply to HCl (431), HBr (366) and HI (299). A stronger H-X bond is harder to break heterolytically in water, directly opposing ionisation.

Step 2. Note fluorine's exceptionally strong hydrogen bonding.

Because fluorine is small and extremely electronegative, HF molecules (and F- ions in solution) engage in unusually strong hydrogen bonding, both with water molecules and with each other (even forming the stable bifluoride ion, HF2-, by association of F- with un-ionised HF). This hydrogen bonding effectively 'holds back' the proton, reducing the fraction of HF that exists as free, dissociated H3O+ and F- in dilute solution.

Step 3. Contrast with the other hydrogen halides.

HCl, HBr and HI have progressively weaker H-X bonds and, since Cl, Br and I are much less electronegative than F and do not form comparably strong hydrogen bonds, there is no equivalent restraining effect — their molecules ionise essentially completely in water, making them strong acids.

Step 4. Conclude. …

Compare bond dissociation enthalpies down the hydrogen-halide series and add the unique factor of fluorine's strong hydrogen bonding, showing both effects p …

Explaining HF's weakness using only 'fluorine is very electronegative' (which would actually suggest a MORE ionic, stronger acid) without identifying the two real reasons — …