{{distinguish|text = the element hafnium, symbol Hf}} {{chembox | verifiedrevid = 476992809 | Name = Hydrogen fluoride | ImageFileL1 = Hydrogen-fluoride-2D-dimensions.svg | ImageClassL1 = skin-invert | ImageFile = Hydrogen fluoride.svg | ImageClass = skin-invert | ImageSize = 120 | ImageFileR1 = Hydrogen-fluoride-3D-vdW.svg | ImageClassR1 = bg-transparent | OtherNames = Hydrogenfluoride | Section1 = {{Chembox Identifiers |UNII_Ref = {{fdacite|correct|FDA}} |UNII = RGL5YE86CZ |KEGG_Ref = {{keggcite|correct|kegg}} |KEGG = C16487 |InChI = 1/FH/h1H |ChEBI_Ref = {{ebicite|correct|EBI}} |ChEBI = 29228 |SMILES = F |InChIKey = KRHYYFGTRYWZRS-UHFFFAOYAC |StdInChI_Ref = {{stdinchicite|correct|chemspider}} |StdInChI = 1S/FH/h1H |StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} |StdInChIKey = KRHYYFGTRYWZRS-UHFFFAOYSA-N |CASNo = 7664-39-3 |CASNo_Ref = {{cascite|correct|CAS}} |ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} |ChemSpiderID = 14214 |UNNumber = 1052 |PubChem = 16211014 |RTECS = MW7875000 }} | Section2 = {{Chembox Properties |H=1 | F=1 |Formula = HF |Appearance = colourless gas or colourless liquid (below 19.5&nbsp;°C) |Odor = unpleasant |Density = 1.15 g/L, gas (25&nbsp;°C)<br />0.99 g/mL, liquid (19.5&nbsp;°C)<br />1.663 g/mL, solid (−125&nbsp;°C) |Solubility = miscible (liquid) |MeltingPtC = −83.6 |BoilingPtC = 19.5 |RefractIndex = 1.00001 |pKa = 3.17 (in water), 15 (in DMSO) <ref>{{Cite web|last=Evans|first=D. A.|title=pKa's of Inorganic and Oxo-Acids|url=http://ccc.chem.pitt.edu/wipf/MechOMs/evans_pKa_table.pdf|access-date=June 19, 2020}}</ref> |ConjugateAcid = Fluoronium |ConjugateBase = Fluoride |VaporPressure = 783 mmHg (20 °C)<ref name="PGCH" /> }} | Section3 = {{Chembox Structure |MolShape = Linear |Dipole = 1.86 D }} | Section4 = {{Chembox Thermochemistry |DeltaHf = −13.66 kJ/g (gas) <br /> −14.99 kJ/g (liquid) |Entropy = 8.687 J/g&nbsp;K (gas) }} | Section5 = {{Chembox Hazards |MainHazards = Highly toxic, corrosive, irritant |GHSPictograms = {{GHS corrosion}} {{GHS skull and crossbones}}{{GHS07}}{{GHS health hazard}} |GHSSignalWord = Danger |HPhrases = {{H-phrases|300+310+330|314}} |PPhrases = {{P-phrases|260|262|264|270|271|280|284|301+310|301+330+331|302+350|303+361+353|304+340|305+351+338|310|320|321|322|330|361|363|403+233|405|501}} |NFPA-H = 4 |NFPA-F = 0 |NFPA-R = 1 |NFPA-S = POI |FlashPt = none |IDLH = 30 ppm<ref name="PGCH">{{PGCH|0334}}</ref> |REL = TWA 3 ppm (2.5 mg/m<sup>3</sup>) C 6 ppm (5 mg/m<sup>3</sup>) [15&nbsp;min]<ref name="PGCH" /> |PEL = TWA 3 ppm<ref name="PGCH" /> |LD50 = 17 ppm (rat, oral) |LC50 = 1276 ppm (rat, 1&nbsp;hr)<br/>1774 ppm (monkey, 1&nbsp;hr)<br/>4327 ppm (guinea pig, 15&nbsp;min)<ref name="IDLH">{{IDLH|7664393|Hydrogen fluoride}}</ref> |LCLo = 313 ppm (rabbit, 7&nbsp;hr)<ref name="IDLH"/> }} | Section6 = {{Chembox Related |OtherAnions = Hydrogen chloride<br />Hydrogen bromide<br />Hydrogen iodide<br />Hydrogen astatide |OtherCations = Sodium fluoride<br />Potassium fluoride<br />Rubidium fluoride<br />Caesium fluoride |OtherCompounds = Water<br /> Ammonia }} }}

'''Hydrogen fluoride'''<ref>Registername 5: https://recherche.chemikalieninfo.de/public/stoff/21217?dv=18&sv=</ref> is an inorganic compound with chemical formula {{Chem2|HF|auto=yes}}. It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluorine, often in the form of hydrofluoric acid, and is an important feedstock in the preparation of many important compounds including pharmaceuticals and polymers such as polytetrafluoroethylene (PTFE). HF is also widely used in the petrochemical industry as a component of superacids. Due to strong and extensive hydrogen bonding, it boils near room temperature, a much higher temperature than other hydrogen halides.

Hydrogen fluoride is an extremely dangerous gas, forming corrosive and penetrating hydrofluoric acid upon contact with moisture. The gas can also cause blindness by rapid destruction of the corneas.

==History== In 1771 Carl Wilhelm Scheele prepared the aqueous solution, hydrofluoric acid, in large quantities, although hydrofluoric acid had been known in the glass industry before then. French chemist Edmond Frémy (1814–1894) is credited with discovering hydrogen fluoride while trying to isolate fluorine.

==Structure and reactions== thumb|class=skin-invert-image|left|The structure of chains of HF in crystalline hydrogen fluoride {{clear|left}} HF is diatomic in the gas phase. As a liquid, HF forms relatively strong hydrogen bonds, hence its relatively high boiling point. Solid HF consists of zigzag chains of HF molecules. The HF molecules, with a short covalent H–F bond of 95&nbsp;pm length, are linked to neighboring molecules by intermolecular H–F distances of 155&nbsp;pm.<ref>{{cite journal |last1=Johnson |first1=M. W. |last2=Sándor |first2=E. |last3=Arzi |first3=E. | title = The Crystal Structure of Deuterium Fluoride | journal = Acta Crystallographica | year = 1975 | volume = B31 | pages = 1998–2003 | doi = 10.1107/S0567740875006711 | issue = 8 }}</ref> Liquid HF also consists of chains of HF molecules, but the chains are shorter, consisting on average of only five or six molecules.<ref>{{cite journal | title = On the Structure of Liquid Hydrogen Fluoride | journal = Angewandte Chemie International Edition | year = 2004 | volume = 43 | pages = 1952–1955 | doi = 10.1002/anie.200353289 | last1 = McLain | first1 = Sylvia E. | pmid = 15065271 | last2 = Benmore | first2 = C. J. | last3 = Siewenie | first3 = J. E. | last4 = Urquidi | first4 = J. | last5 = Turner | first5 = J. F. | issue = 15 }}</ref>

===Comparison with other hydrogen halides=== Hydrogen fluoride does not boil until {{convert|20|°C|°F}} in contrast to the heavier hydrogen halides, which boil between {{convert|−85|and|−35|°C|°F}}.<ref name="Pauling HF hydrogen bonds">{{cite book |author-link=Linus Pauling |last=Pauling |first=Linus A. |title=The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry |year=1960 |publisher=Cornell University Press |isbn=978-0-8014-0333-0 |pages=454–464 |url=https://archive.org/details/natureofchemical00paul/page/454 |url-access=registration}}</ref><ref name="Atkins HF">{{cite book |last=Atkins |first=Peter |last2=Jones |first2=Loretta |title=Chemical principles: The quest for insight |year=2008 |publisher=W. H. Freeman & Co |isbn=978-1097774678 |pages=184–185 |url=https://books.google.com/books?id=4R6hb1OIMRUC&pg=PA184}}</ref><ref name="New Scientist HF">{{cite journal |last=Emsley |first=John |title=The hidden strength of hydrogen |journal=New Scientist |year=1981 |volume=91 |issue=1264 |pages=291–292 |url=https://books.google.com/books?id=ZbthaZCUXy4C&pg=PA292 |access-date=25 December 2012 |archive-date=22 July 2023 |archive-url=https://web.archive.org/web/20230722094703/https://books.google.com/books?id=ZbthaZCUXy4C&pg=PA292 |url-status=dead}}</ref> This hydrogen bonding between HF molecules gives rise to high viscosity in the liquid phase and lower than expected pressure in the gas phase.

===Aqueous solutions=== {{main|Hydrofluoric acid}} HF is miscible with water (dissolves in any proportion). In contrast, the other hydrogen halides exhibit limiting solubilities in water. Hydrogen fluoride forms a monohydrate {{nowrap|HF·H<sub>2</sub>O}} with melting point {{convert|−40|°C|°F}}, which is {{convert|44|C-change}} above the melting point of pure HF.<ref>{{cite book |last1=Greenwood |first1=N. N. |last2=Earnshaw |first2=A. |title=Chemistry of the Elements |date=1998 |edition=2nd |publisher=Butterworth Heinemann |isbn=0-7506-3365-4 |location=Oxford |pages=812–816}}</ref>

{| style="margin:1em auto 1em 1em; text-align:left; width:505px; float:center;" cellpadding="0" cellspacing="0" |- | colspan="2" style="text-align:center;"| '''HF and H<sub>2</sub>O similarities''' |- | style="width:285px; padding-right:5px;" | 285px|alt=graph showing trend-breaking water and HF boiling points: big jogs up versus a trend that is down with lower molecular weight for the other series members. | style="width:200px; padding-left:5px;" | 200px|alt=graph showing humps of melting temperature, most prominent is at HF 50% mole fraction |- style="vertical-align:top; font-size:.9em;" | style="width:285px; padding-right:5px; padding-left:25px; line-height:1.4em; "|Boiling points of the hydrogen halides (blue) and hydrogen chalcogenides (red): HF and H<sub>2</sub>O break trends. | style="width:200px; padding-left:25px; line-height:1.4em; "|Freezing point of HF/ H<sub>2</sub>O mixtures: arrows indicate compounds in the solid state. |}

Aqueous solutions of HF are called hydrofluoric acid. When dilute, hydrofluoric acid behaves like a weak acid, unlike the other hydrohalic acids, due to the formation of hydrogen-bonded ion pairs [{{H3O+}}·F<sup>−</sup>].<ref name=":0">{{Cite book |title=Dictionary of chemistry |date=2020 |publisher=Oxford University Press |isbn=978-0-19-884122-7 |editor-last=Rennie |editor-first=Richard |edition=8th |series=Oxford quick reference |location=Oxford, United Kingdom ; New York, NY}}</ref> However concentrated solutions are strong acids, because bifluoride anions are predominant, instead of ion pairs. In liquid anhydrous HF, self-ionization occurs:<ref>{{cite book|first1=C. E.|last1=Housecroft|first2=A. G.|last2=Sharpe|title=Inorganic Chemistry|page=221}}{{edition needed|date=April 2026}}{{ISBN?|date=April 2026}}</ref><ref>{{cite book|first1=F. A.|last1=Cotton|first2=G.|last2=Wilkinson|title=Advanced Inorganic Chemistry|page=111}}{{edition needed|date=April 2026}}{{ISBN?|date=April 2026}}</ref> : {{chem2|3 HF <-> H2F+ + HF2-}} which forms an extremely acidic liquid ({{nowrap|''H''<sub>0</sub>&nbsp;{{=}}&nbsp;−15.1}}).

===Reactions with Lewis acids=== Like water, HF can act as a weak base, reacting with Lewis acids to give superacids. A Hammett acidity function (''H''<sub>0</sub>) of −21 is obtained with antimony pentafluoride (SbF<sub>5</sub>), forming fluoroantimonic acid.<ref name="Jolly">{{cite book|first=W. L.|last=Jolly|title=Modern Inorganic Chemistry|publisher=McGraw-Hill|date=1984|page=203|ISBN=0-07-032768-8}}.</ref><ref name="Cotton 109">{{cite book|author1-link=F. Albert Cotton|first1=F. A.|last1=Cotton|first2=G.|last2=Wilkinson|title=Advanced Inorganic Chemistry|edition=5th|publisher=John Wiley and Sons|location=New York, NY|date=1988|isbn=0-471-84997-9|page=109}}</ref>

==Synthesis== Hydrogen fluoride is typically produced by the reaction between sulfuric acid and pure grades of the mineral fluorite (calcium fluoride):<ref name=":0" /><ref name="AigueperseMollard2000" />

:{{chem2|CaF2 + H2SO4 -> 2 HF + CaSO4}}

About 20% of manufactured HF is a byproduct of fertilizer production, which generates hexafluorosilicic acid. This acid can be degraded to release HF thermally and by hydrolysis:

:{{chem2|H2SiF6 -> 2 HF + SiF4}} :{{chem2|SiF4 + 2 H2O -> 4 HF + SiO2}}

==Use== In general, anhydrous hydrogen fluoride is more common industrially than its aqueous solution, hydrofluoric acid. Its main uses, on a tonnage basis, are as a precursor to organofluorine compounds and a precursor to synthetic cryolite for the electrolysis of aluminium.<ref name="AigueperseMollard2000" />

===Precursor to organofluorine compounds=== HF reacts with chlorocarbons to give fluorocarbons. An important application of this reaction is the production of tetrafluoroethylene (TFE), precursor to Teflon. Chloroform is fluorinated by HF to produce chlorodifluoromethane (R-22):<ref name="AigueperseMollard2000" />

:{{chem2|CHCl3 + 2 HF -> CHClF2 + 2 HCl}}

Pyrolysis of chlorodifluoromethane at 550–750&nbsp;°C yields TFE.

HF is a reactive solvent in the electrochemical fluorination of organic compounds. In this approach, HF is oxidized in the presence of a hydrocarbon and the fluorine replaces C–H bonds with C–F bonds. Perfluorinated carboxylic acids and sulfonic acids are produced in this way.<ref name="Ullmann">{{Ullmann|first1=G. |last1=Siegemund |first2=W. |last2=Schwertfeger |first3=A. |last3=Feiring |first4=B. |last4=Smart |first5=F. |last5=Behr |first6=H. |last6=Vogel |first7=B. |last7=McKusick |title=Fluorine Compounds, Organic |year=2005 |doi=10.1002/14356007.a11_349}}</ref>

1,1-Difluoroethane is produced by adding HF to acetylene using mercury as a catalyst.<ref name="Ullmann" /> :{{chem2|HC\tCH + 2 HF -> CH3CHF2}} The intermediate in this process is vinyl fluoride or fluoroethylene, the monomeric precursor to polyvinyl fluoride.

===Precursor to metal fluorides and fluorine=== The electrowinning of aluminium relies on the electrolysis of aluminium fluoride in molten cryolite. Several kilograms of HF are consumed per ton of aluminium produced. Other metal fluorides are produced using HF, including uranium tetrafluoride.<ref name="AigueperseMollard2000" />

HF is the precursor to elemental fluorine, F<sub>2</sub>, by electrolysis of a solution of HF and potassium bifluoride. The potassium bifluoride is needed because anhydrous HF does not conduct electricity. Several thousand tons of F<sub>2</sub> are produced annually.<ref>{{Ullmann|first1=M. |last1=Jaccaud |first2=R. |last2=Faron |first3=D. |last3=Devilliers |first4=R. |last4=Romano |title=Fluorine |year=2005 |doi=10.1002/14356007.a11_293}}.</ref>

===Catalyst=== HF serves as a catalyst in alkylation processes in refineries. It is used in the majority of the installed linear alkyl benzene production facilities in the world. The process involves dehydrogenation of ''n''-paraffins to olefins, and subsequent reaction with benzene using HF as catalyst. For example, in oil refineries "alkylate", a component of high-octane petrol (gasoline), is generated in alkylation units, which combine C<sub>3</sub> and C<sub>4</sub> olefins and isobutane.<ref name="AigueperseMollard2000">{{Ullmann|first1 = J. |last1=Aigueperse |first2=P. |last2=Mollard |first3=D. |last3=Devilliers |first4=M. |last4=Chemla |first5=R. |last5=Faron |first6=R. |last6=Romano |first7=J. P. |last7=Cuer|year=2000|doi=10.1002/14356007.a11_307|title=Fluorine Compounds, Inorganic|isbn = 3527306730}}</ref>

===Solvent=== Hydrogen fluoride is an excellent solvent. Reflecting the ability of HF to participate in hydrogen bonding, even proteins and carbohydrates dissolve in HF and can be recovered from it. In contrast, most non-fluoride inorganic chemicals react with HF rather than dissolving.<ref>{{cite book|last1=Greenwood |last2=Earnshaw |title=Chemistry of the Elements |pages=816–819}}{{edition needed|date=April 2026}}{{ISBN?|date=April 2026}}</ref>

==Health effects== thumb|250px|alt=left and right hands, two views, burned index fingers|HF burns, not evident until a day after {{Main|Hydrofluoric acid|Hydrofluoric acid burn}}

Hydrogen fluoride is highly corrosive and a powerful contact poison. Exposure requires immediate medical attention.<ref name="emergency.cdc.gov">{{cite web|url=http://emergency.cdc.gov/agent/hydrofluoricacid/basics/facts.asp |title=Facts About Hydrogen Fluoride (Hydrofluoric Acid)|website=Emergency Preparedness and Response|publisher=U.S. Centers for Disease Control and Prevention}}</ref> It can cause blindness by rapid destruction of the corneas. Breathing in hydrogen fluoride at high levels or in combination with skin contact can cause death from an irregular heartbeat or from pulmonary edema (fluid buildup in the lungs).<ref name="emergency.cdc.gov" /> Exposure of the intestinal system to HF solution is known to cause fulminant acute colitis requiring surgical intervention.<ref>{{cite journal |last1=Cappell |first1=M. S. |last2=Simon |first2=T. |title=Fulminant acute colitis following a self-administered hydrofluoric acid enema |journal=The American Journal of Gastroenterology |date=January 1993 |volume=88 |issue=1 |pages=122–126 |pmid=8420252 |issn=0002-9270}}</ref>

{{Clear}}

== References == {{Reflist|30em}}

==External links== {{Commons category|Hydrogen fluoride}} *[https://www.atsdr.cdc.gov/substances/toxsubstance.asp?toxid=38 Fluorides, Hydrogen Fluoride, and Fluorine] at ATSDR. Retrieved September 30, 2019 *[https://www.cdc.gov/niosh/npg/npgd0334.html CDC – NIOSH Pocket Guide to Chemical Hazards] *[https://web.archive.org/web/20191001001621/https://www.turi.org/TURI_Publications/TURI_Chemical_Fact_Sheets/Hydrogen_Fluoride_Fact_Sheet Hydrogen Fluoride Fact Sheet] at Toxics Use Reduction Institute

{{Hydrogen compounds}} {{Molecules detected in outer space}} {{fluorine compounds}} {{fluorides}} {{Hydrides by group}} {{Authority control}}

Category:Fluorides Category:Hydrogen compounds Category:Industrial gases Category:Inorganic solvents Category:Nonmetal halides Category:Diatomic molecules