9.7A: Physical Properties

Last updated
Save as PDF

Page ID: 33840

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

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.