The bifluoride ion is an inorganic anion with the chemical formula [HF2]−. The anion is colorless. Salts of bifluoride are commonly encountered in the reactions of fluoride salts with hydrofluoric acid. The commercial production of fluorine involves electrolysis of bifluoride salts.
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| Other names
Hydrogen(difluoride) anion
Bifluoride anion Hydrogen difluoride anion | |||
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3D model (JSmol)
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CompTox Dashboard (EPA)
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| Properties | |||
| [HF2]− | |||
| Molar mass | 39.005 g·mol−1 | ||
| Conjugate acid | Hydrogen fluoride | ||
| Conjugate base | Fluoride | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Structure and bonding
editThe bifluoride ion has a linear, centrosymmetric structure (D∞h symmetry), with an F−H bond length of 114 pm.[1] The bond strength is estimated to be greater than 155 kJ/mol.[2] In molecular orbital theory, the atoms are modeled to be held together by a 3-center 4-electron bond (symmetrical hydrogen bond),[3] in a sort of hybrid between a hydrogen bond and a covalent bond.[4]
Reactions
editSalts, such as potassium bifluoride and ammonium bifluoride are produced by treating fluoride salts with hydrofluoric acid:
- M+F− + HF → M+[HF2]−, where M+ = K+ or [NH4]+
Potassium bifluoride binds a second equivalent of HF:[5]
- K[HF2] + HF → K[H2F3]
Heating these salts releases anhydrous HF.
The bifluoride anion is present in solutions of HF and buffered oxide etch, used in microfabrication etching. In these processes, bifluoride breaks down silicon oxides, doing more effectively than HF (~4.5 times faster).[6]
- SiO2 + 3HF− + H+ → SiF2−6 + 2H2O
References
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- ^ Denne, W.A.; Mackay, M.F. (1971). "Crystal structure of p-toluidinium bifluoride". Journal of Crystal and Molecular Structure. 1 (5): 311–318. doi:10.1007/BF01200805. S2CID 101570657.
- ^ Emsley, J. (1980). "Very Strong Hydrogen Bonds". Chemical Society Reviews. 9: 91–124. doi:10.1039/CS9800900091.
- ^ 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
- ^ Dunning Jr., Thom H.; Xu, Lu T. (July 28, 2021). "Nature of the Bonding in the Bifluoride Anion, FHF–". The Journal of Physical Chemistry Letters. 12 (30). doi:10.1021/acs.jpclett.1c02123. Retrieved 17 September 2024.
- ^ Aigueperse, Jean; Mollard, Paul; Devilliers, Didier; Chemla, Marius; Faron, Robert; Romano, René; Cuer, Jean Pierre (2000). "Fluorine Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_307. ISBN 3527306730.
- ^ Williams, K.R.; Muller, R.S. (Dec 1996). "Etch rates for micromachining processing". Journal of Microelectromechanical Systems. 5 (4): 256–269. doi:10.1109/84.546406.