Q.Which of the following is strongest acid among all?

Acid strength of the hydrohalic acids increases down the group, $HF<HCl<HBr<HI$, because bond dissociation energy (not electronegativity) governs how easily the H-X bond ionises in water, and H-I is the weakest bond.

Although fluorine is the most electronegative halogen, HF is actually the weakest of the four hydrohalic acids because the H-F bond is exceptionally strong and short (high bond dissociation energy), making it difficult to break/ionise; HF is also known to form strong intermolecular hydrogen bonds, further reducing its effective acidity in water (it is a weak acid, $K_a\approx 6.6\times10^{-4}$). As the halogen atom gets larger going down the group (F$\to$Cl$\to$Br$\to$I), the H-X bond length increases and its bond dissociation energy steadily decreases, so the bond ionises more readily in water. Consequently acid strength incre …

Thermal stability of HX depends on H–X bond dissociation enthalpy, which falls steadily down Group 17 as atomic size increases, so stability decreases from HF to HI.

Down the halogen group, atomic (and hence bond) length of H–X increases (F < Cl < Br < I), so orbital overlap between H(1s) and the halogen's valence orbital becomes poorer and the H–X bond becomes weaker. Bond dissociation enthalpies (approx.): HF ≈ 570 kJ/mol, HCl ≈ 431 kJ/mol, HBr ≈ 368 kJ/mol, HI ≈ 297 kJ/mol.

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