Question

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

Answer 1

The hydrogen halides are extremely soluble in water due to ionisation.

\[\ce{X + H2O -> H3O + X^-}\]

(X = F, Cl, Br or I)

Solutions of hydrogen halides are therefore acidic and known as hydrohalic acids. Hydrochloric, hydrobromic and hydroiodic acids are almost completely ionised and are therefore strong acids but HF is a weak acid. For HF,

\[\ce{HF + H2O ⇌ H3O^+ + F^-}\]

\[\ce{HF + F^- -> HF^-_2}\]

At high concentration, the equilibrium involves the removal of fluoride ions is important. Since it affects the dissociation of hydrogen fluoride, therefore it is a weak acid.