Bifluoride

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Bifluoride
Hydrogendifluoride ion.svg Hydrogendifluoride-3D-vdW.png
Identifiers
CAS number 18130-74-0 YesY
Jmol-3D images Image 1
Properties
Molecular formula HF2
Molar mass 39.00 g mol−1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Bifluoride (systematically named difluorohydrogenuide and difluoridohydrogenate(1−)), also named hydrogen(difluoride) is an inorganic anion with the chemical formula HF
2
(also written [HF2]). It contributes no color to bifluoride salts.

Chemical properties[edit]

Basicity[edit]

The fluorohydrogenate group (-HF) in anions such as bifluoride can assimilate a proton by recombination:

HF
2
+ H+ → H2F2 → 2 HF

Because of this capture of a proton (H+), bifluoride has basic character. Its conjugate acid is the reactive intermediate, μ-fluoro-fluorodihydrogen (H2F2), which subsequently dissociates to become hydrogen fluoride. In solution, most bifluoride ions are dissociated.

HF
2
+ 3 H2O is in a favored equilibrium with 2 H3O+ + F + HO

Structure[edit]

This centrosymmetric triatomic anion features the strongest known hydrogen bond, with an FH length of 114 pm[1] and a bond strength of >155 kJ mol−1.[2] A molecular orbital diagram reveals the atoms to be held together by a 3-center 4-electron bond.[3] It is isoelectronic with the fluoroheliate anion, FHeO, whose existence is suspected[4] but not confirmed, and with the hypothetical compound helium difluoride, HeF2. "Hydrogen(difluoride)" is written as one word because it is an anion; "hydrogen difluoride" would instead imply the electrically neutral compound HF2 (CAS number 12528-21-1). It is classified as a weak base.

Chemical reactions[edit]

Bifluoride undergoes the typical chemical reactions of a fluorohydronate. Upon treatment with a standard acid, it converts to hydrofluoric acid and a metal salt. Oxidation of bifluoride gives fluorine. When heated to a high temperature, bifluoride salts decompose to produce fluoride salts and hydrogen fluoride:

[HF2] → HF + F

Production[edit]

Bifluoride is produced by hydrogen fluoride fluorination. In this process hydrogen fluoride and fluoride anions react to produce bifluoride according to the following reaction:

HF + F → [HF2]

This process also involves μ-fluoro-fluorodihydrogen as an intermediate, and occurs in two steps. No catalyst is needed for fluorination (step 2).

  1. 2 HF → [H2F2]
  2. [H2F2] + F → [HF2] + HF

Catalytic amounts of hydrogen fluoride are used for the process.

Salts[edit]

Some HF2 salts are common, examples include potassium bifluoride (KHF2, also called potassium hydrogen fluoride) and ammonium bifluoride ([NH4][HF2]). Many salts claimed to be anhydrous sources of simple fluoride (F) ions, for example, tetra-n-butylammonium fluoride, can decompose to yield bifluoride instead.[citation needed]

Autodissociation of pure HF[edit]

The bifluoride ion also contributes to the unusually high auto-protolysis constant of liquid anhydrous hydrogen fluoride, which autodissociates in a manner similar to the self-ionization of water. This equilibrium can be denoted as

HF \rightleftharpoons H+ + F

However, both the H+ and F ions are solvated by HF, so a better descriptive equation is

3HF \rightleftharpoons H2F+(HF) + HF2(HF)

References[edit]

  1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
  2. ^ Emsley, J., "Very Strong Hydrogen Bonds", Chemical Society Reviews, 1980, 9, 91-124.
  3. ^ Pimentel, G. C. The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method. J. Chem. Phys. 1951, 19, 446-448. doi:10.1063/1.1748245
  4. ^ "Collapse of helium’s chemical nobility predicted by Polish chemist". Retrieved 2009-05-15.