Is Hydrofluoric Acid (HF) a Strong or Weak Acid?
Is Hydrofluoric Acid (HF) a strong or weak acid?
HF is surprisingly weak despite fluorine being the most electronegative element. The H-F bond is very strong (570 kJ/mol) due to the small size of fluorine, making it difficult to break.
| Formula | HF |
| Name | Hydrofluoric Acid |
| Category | Weak acid |
| pKa | 3.17 |
| Conjugate | Fluoride ion (F⁻) |
| Key Concept | Weak despite high electronegativity |
Definition
HF is a Bronsted-Lowry acid that donates its proton to water. Despite fluorine having the highest electronegativity, HF is a weak acid because the H-F bond is extremely strong.
Acidic Proton / Active Site
The single hydrogen bonded to fluorine is the acidic proton. Fluorine pulls electron density strongly, but the bond is so short and strong that it resists breaking.
Conjugate Pair
HF donates H+ to become F- (fluoride ion). Fluoride is a relatively strong conjugate base - it readily accepts protons back, which is why HF does not fully dissociate.
Strength Classification
HF has pKa = 3.17, making it a weak acid. Compare with HCl (pKa -7): the Cl atom is larger, so H-Cl is weaker and breaks more easily. Bond strength matters more than electronegativity here.
See acidic protons, conjugate base overlays, and pKa labels on interactive 3D molecules.
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