NEET 2021 Chemistry Question 61 Solution

Hydrogen halides in aqueous solution dissociate to produce hydronium ions and halide ions: $HX + H_2O \rightleftharpoons H_3O^+ + X^-$. Their experimental acid dissociation constants ($K_a$) and $pK_a$ values demonstrate that $HF$ is a weak acid ($pK_a \approx 3.2$), whereas $HCl$ ($pK_a \approx -7$), $HBr$ ($pK_a \approx -9$), and $HI$ ($pK_a \approx -10$) are strong acids:

As we descend Group 17 from Fluorine to Iodine, the atomic radius increases significantly ($F: 72\text{ pm} \to I: 133\text{ pm}$). The overlap between the small $1s$ orbital of hydrogen and the increasingly diffuse valence $p$ orbitals of halogen weakens dramatically. The $H-X$ bond length increases and the bond dissociation enthalpy decreases sharply:

Because the $H-I$ bond is the longest and weakest, it cleaves most readily to furnish $H^+$ ions in solution. Hence, the decrease in bond strength is the primary physical driving force behind the increase in acid strength. Statement II is scientifically true and correctly explains Statement I.