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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

    \(\ce{HF(l)}\) 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.

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