Is hydrofluoric acid - HF - a strong or weak acid?

Answer 1

Hydrofluoric acid is a special case of the halogenic acids (HBr, HCl, HI). The other halogenic acids are considered strong acids not because they are more poisonous, more corrosive, or more dangerous, but because they fully dissociate in water (Ionize completely into their respective ions) For example the ionization of HCl in water looks like this. HCl(aq) + H2O(l) _>> H3O+(aq) + Cl-(aq)

The acid ionization for this reaction lies so far to the right that the reverse reaction happens in such small quantities, that we can say the reaction goes to completion at right.

Hydrofluoric acid on the other hand does not reacte in this way. The main reasons are the extreme reactivity of the fluoride ion and and small atomic radius it is not as stable in solution as its other halogenic counterparts. The reaction of HF looks like this.

HF(aq)+ H2O(l) <=> H3O+(aq) + F-(aq)

Because both the forward reaction and reverse reaction can occur at significant enough rates, this acid does not fully dissolve and is considered a weak acid.

The poisonous affects of HF vs HCl has nothing to do with the acidity, but with the toxicity of the fluoride ion produced by HF. If the fluoride ion gets into the bloodstream, it can killl. The fluoride ion is so reactive it pulls calcium out of the bones to form insoluble CaF2. Fluoride is so corrosive that it can even etch glass which is an extremely intert substance.

The chloride ion on the other hand goes through the human system everyday with almost no negative consequences. Common table salt, NaCl is a common source of chloride, even stomach acid consists of hydrochloric acid, it is vital to our digestive system.

Answer 2

HF is a weak acid even though F is a halogen and most halides form strong acids. Hydrogen bonding helps stabilize HF so it less apt to dissociate than the other hydrogen halides. For evidence and clarification see "Re: Why is hydrofluoric acid a weak acid?" http://www.madsci.org/posts/archives/1999-12/945201677.Ch.r.html.