# 9.7A: Physical Properties

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Hydrofluoric acid is difficult to work with because is corrodes glass and silica giving gaseous $$\ce{SiF4}$$ or the $$\ce{[SiF6]2-}$$ so it must be stored in PTFE or dry Cu or Monel metalI.

$\ce{4HF + SiO2 -> SiF4 + 2H2O}$

Liquid range: 190 to 292.5 K
Relative permittivity 84 at 273 K; 175 at 200 K

Liquid HF undergoes self-ionization: Kself =2 x10-12 at 273 K

$\ce{ 3HF <=> [H2F]+ + [HF2]-}$

dihydridofluorine(1+) ion
difluorohydrogenate(1-) ion

Large electronegativity difference between H (xP = 2.2) and F (xP = 4.0) results in the presence of extensive intermolecular hydrogen bonding in the liquid.

• Hydrogen bonded molecules (~7 molecules on average) in liquid phase
• Cyclic (HF)x species are present in the gas phase
##### Warning

$$\ce{HF(l)}$$ causes burns that are slow to heal or even get progressively worse. Exercise extreme caution if using this solvent.

Very few compounds exist which are strong enough F- acceptors to be considered acids. An example is $$\ce{SbF5}$$ which leads to pure HF having superacid properties. Pure HF has a dielectric constant of 84 and is a good solvent like water which is surprisingly gentle. It can be used for example to remove Fe from metalloproteins without damaging the apoprotein primary structure.

9.7A: Physical Properties is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.