Is HF (Hydrofluoric Acid) a Strong Acid or a Weak Acid?

Hydrofluoric acid molecule


Hydrofluoric acid or HF is an extremely corrosive acid. However, it is a weak acid and not a strong acid because it does not completely dissociate in water (which is the definition of a strong acid) or at least because the ions it forms upon dissociation are too strongly bound to each other for it to act as a strong acid.

Why Hydrofluoric Acid Is a Weak Acid

Hydrofluoric acid is the only hydrohalic acid (such as HCl, HI) that is not a strong acid. HF ionizes in ​an aqueous solution like other acids:​

HF + H2O ⇆ H3O+ + F-

Hydrogen fluoride does actually dissolve fairly freely in water, but the H3O+ and F- ions are strongly attracted to each other and form the strongly bound pair, H3O+ · F-. Because the hydroxonium ion is attached to the fluoride ion, it isn't free to function as an acid, thus limiting the strength of HF in water.

Hydrofluoric acid is a much stronger acid when it is concentrated than when it is diluted. As the concentration of hydrofluoric acid approaches 100 percent, it's acidity increases because of homoassociation, where a base and conjugate acid form a bond:

3 HF ⇆ H2F+ + HF2-

The FHF- bifluoride anion is stabilized by a strong hydrogen bond between hydrogen and fluorine. The stated ionization constant of hydrofluoric acid, 10-3.15, does not reflect the true acidity of concentrated HF solutions. Hydrogen bonding also accounts for the higher boiling point of HF compared to other hydrogen halides.

Is HF Polar?

Another common question about the chemistry of hydrofluoric acid is whether the HF molecule is polar. The chemical bond between hydrogen and fluorine is a polar covalent bond in which the covalent electrons are closer to the more electronegative fluorine.