Question

Explain the apparent paradox. HBr is a stronger acid than HCl, but HOCl is a stronger acid than HOBr.

Short answer

HOCl is a stronger acid than HOBr as the electronegativity of chlorine is higher than that of bromine.

Step-by-step solution

Acidity and bond strength

The acid’s strength depends on the ease with which it can liberate hydrogen ions. The more easily the bond dissociates to generate ${\mathbf{H}}^{\mathbf{+}}$ions, the stronger the acid is.

The larger the atom's size bonded to hydrogen, the weaker the bond strength andthe stronger the acid.

Comparing the strengths of HBr and HCl

HBr is a stronger acid than HCl as the hydrogen-bromine (H-Br) bond is weaker than the hydrogen-chlorine (H-Cl) bond. It is because bromine is larger than chlorine.

Comparing the strength of the acids HOBr and HOCl

In the case of HOCl and HOBr,the hydrogen atom is attached to the oxygen atom.The strength of the acid depends on the strength of the oxygen-hydrogen (O-H) bond.

The stronger the oxygen-hydrogen (O-H) bond, the weaker the acid will be.

In the case of HOBr and HOCl, HOCl is a stronger acid as the electronegativity of chlorine is higher than that of bromine. Hence, the conjugate base of the acid${}^{-}\mathrm{OCl}$ is more stable than ${}^{-}\mathrm{OBr}$ .