9.7A: Physical Properties
- Page ID
- 33840
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
\(\ce{HF(l)}\) 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.