Hydrofluoric acid or HF is an extremely powerful, corrosive acid. However, it’s classified as a weak acid rather than a strong acid. This makes HF the only hydrohalic acid that isn’t classified as a strong acid (e.g., HCl, HBr, HI).
Why Hydrofluoric Acid Is a Weak Acid
The simple reason hydrofluoric acid is a weak acid is because it does not completely dissociate into its ions in water (the definition of a strong acid). Initially, HF actually does dissociate almost completely:
HF + H2O ⇆ H3O+ + F–
However, after the acid dissociates, it participates in other reactions with itself and with water. The behavior of hydrofluoric acid greatly depends on its concentration in water. The hydronium cation and fluoride anion are strongly attracted to one another and form H3O+ · F–. The chemical bond in this species is strong enough to limit acidity, so hydrofluoric acid is a very weak acid in dilute solution.
Concentrated HF solutions behave much like a strong acid. This is due to homoassociation, in which a chemical bond forms between a base and conjugate acid:
3 HF ⇆ H2F+ + HF2–
Hydrogen bonding between hydrogen and fluorine stabilizes the FHF– bifluoride ion. Hydrogen bonding also causes HF to have a higher boiling point than the other hydrogen halides.
Is HF Polar?
Two factors play into acid strength: atom size and the polarity of the H-A bond (where A is the acid). Fluorine is highly electronegative, so the bond in HF is a polar covalent bond. The more polar a bond is, the easier it is to remove the proton or hydrogen from the acid. So, hydrochloric acid (HCl) is a stronger acid than hydrobromic acid (HBr). Based only on its polarity, one might expect HF to be stronger than HCl. But, HF participates in reactions after dissociation that make it a weaker acid!
Weak but Dangerous
Although HF is a weak acid, it is highly corrosive. It attacks glass, even in dilute solution, so it must be stored in plastic containers. It’s hazardous to work with for a few reasons. First, HF interferes with nerve function, so exposure isn’t immediately painful like it would be with other acids. Second, it’s absorbed through tissue, so it does more than produce a chemical burn on skin or mucous membranes. Ultimately, the acid works its way deeper into tissues, causing bone damage and fluid buildup in the lungs. A hydrofluoric acid burn might not seem serious, but it requires immediate attention.
References
- Favre, Henri A.; Powell, Warren H., eds. (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. Cambridge: The Royal Society of Chemistry. ISBN 9781849733069.
- Giguère, Paul A.; Turrell, Sylvia (1980). “The nature of hydrofluoric acid. A spectroscopic study of the proton-transfer complex H3O+…F−“. J. Am. Chem. Soc. 102 (17): 5473. doi:10.1021/ja00537a008
- Prkić, Ante; Giljanović, Josipa; Bralić, Marija; Boban, Katarina (2012). “Direct Potentiometric Determination of Fluoride Species by Using Ion-Selective Fluoride Electrode”. Int. J. Electrochem. Sci. 7: 1170–1179.