CID 14917

Hydrogen fluoride

Q: Why is hydrofluoric acid a weak acid?

In water it ionises only partly, with a pKₐ of 3.19 on HSDB's figure, where lower numbers mean stronger acids, and it is the one weak member of the hydrohalogen acids. On Chem1's account, the energy released as its ions are hydrated, or surrounded by water molecules, plays a major part in making it the weak exception. Because the acid is weak, its salts, the fluorides, act as bases when dissolved in water.

Ball-and-stick Stick Space-filling

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2 atoms, 1 single bond.

IUPAC name: fluorane
Molar mass: 20.0060(12) g/mol
Atoms: 2
Polarity: polar
SMILES: F
InChIKey: KRHYYFGTRYWZRS-UHFFFAOYSA-N
PubChem CID: 14917

Made of

H F

Structural formula of hydrogen fluoride. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Below about 19 °C, hydrogen fluoride is a colourless fuming liquid; above it, a colourless gas. Its IUPAC name is fluorane, and each molecule joins one hydrogen atom to one fluorine atom, for a molar mass of 20.006 g/mol. With water it mixes readily to give hydrofluoric acid, which NIOSH, the US National Institute for Occupational Safety and Health, treats as the same chemical for practical purposes. Dilute solutions look just like water.

In water it is a weak acid, the only weak one among the acids hydrogen forms with the halogens, on Chem1’s account. Its pK_a, a scale on which smaller numbers mark stronger acids, is 3.19 (Hazardous Substances Data Bank, HSDB). Without water the picture turns over: the anhydrous liquid ranks among the most acidic substances known, on HSDB’s account. That pure liquid also works as a solvent, its molecules trading protons to give H₂F⁺ and F⁻ ions.

Structure and bonding

The model holds two atoms on one single bond. With no central atom, the molecule has no VSEPR (valence-shell electron-pair repulsion) shape and no bond angle. Its nuclei sit a computed 0.939 Å apart. Ball-and-stick shows two spheres on a short stick, Stick a single rod, and Labels marks its ends H and F. In Space-filling, the hydrogen sphere sinks most of the way into the slightly larger fluorine one.

Fluorine’s electronegativity, an atom’s pull on the electron pair it shares, is 3.98 on the Pauling scale against hydrogen’s 2.2. The pair is drawn toward fluorine, so the bond is polar with the hydrogen end slightly positive. The molecule’s dipole moment, the measured size of that separation of positive and negative charge, is 6.104 × 10⁻³⁰ C m (HSDB).

The model shows one free molecule. In the liquid and the solid, the molecules are strongly hydrogen-bonded, links the viewer does not draw. A weak acid, it ionises only slightly in most solutions, so in water the neutral molecule remains its main form.

Where it turns up

Volcanoes release hydrogen fluoride naturally, as does sea-salt aerosol, the fine salt particles carried off the sea into the air. Manufacturing and welding release it as well (NIOSH). Among human activities, coal-fired power plants are the largest source, on the US Environmental Protection Agency’s (EPA) account. Plants that make aluminium or phosphate fertiliser are major industrial emitters of fluoride, giving off the gas along with fluoride particles. Municipal incinerators release it too when the waste holds material containing fluoride.

Once in the air, the gas dissolves into rain, clouds and fog and falls back to the ground as hydrofluoric acid. Closer to home, it is an ingredient of commercial automotive cleaners, rust removers and water-spot removers. Enamelling and glassblowing are hobbies that carry a risk of exposure to it.

Why it matters

Fluorspar and sulfuric acid are the raw materials for nearly all the hydrogen fluoride made worldwide (HSDB). The US volume reported to the EPA for 2023 lay between 550 million and 700 million pounds. About 60 per cent of production goes into fluorocarbon chemicals, EPA reports: hydrofluorocarbons, hydrochlorofluorocarbons and fluoropolymers. One of these, the hydrofluorocarbon HFC-134a, circulates in car air conditioners.

In the manufacture of silicon chips the acid serves as a wet etchant, a liquid that eats material away. It frosts and etches glass and polishes crystal glass. On Los Alamos’s account it also etches the glass of light bulbs; why glass yields to it is told on the fluorine page.

Stainless steel is pickled, or acid-cleaned, in the aqueous acid, which also purifies quartz and helps extract exotic metals. Anhydrous hydrogen fluoride is a catalyst for petroleum alkylation, a refinery process taken up on the isobutane page. It helps refine uranium as well, and controls pH in oil wells.

Properties

Properties of hydrogen fluoride Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 14917.
Property	Value	Unit	Source
Molecular formula (Hill order)	FH
Molar mass	20.0060(12) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	1
Fluorine atoms	1
Bonds	1
PubChem molecular weight	20.0064 g/mol	g/mol	PubChem

Safety

Hydrogen fluoride is classed under the Globally Harmonized System (GHS) as fatal if swallowed, inhaled or in contact with skin, and as causing severe skin burns and eye damage. Its effects may be delayed, and NIOSH warns that its odour gives no reliable warning of exposure. Long-term exposure at work has caused skeletal fluorosis, a bone disease (EPA). The NIOSH Pocket Guide card sets out its target organs and workplace exposure limits, and the PubChem Laboratory Chemical Safety Summary gathers its classifications.

Questions

What shape is a hydrogen fluoride molecule?

It has no shape word at all in valence-shell electron-pair repulsion (VSEPR) theory: two atoms on a single bond leave no central atom and no angle. The model sets them 0.939 Å apart, a computed distance. Fluorine, 3.98 on the Pauling scale against hydrogen's 2.2, pulls the shared pair its way, so the bond is polar, and the molecule has a measured dipole moment (HSDB).

Why is hydrofluoric acid a weak acid?

In water it ionises only partly, with a pK_a of 3.19 on HSDB's figure, where lower numbers mean stronger acids, and it is the one weak member of the hydrohalogen acids. On Chem1's account, the energy released as its ions are hydrated, or surrounded by water molecules, plays a major part in making it the weak exception. Because the acid is weak, its salts, the fluorides, act as bases when dissolved in water.

Why is hydrofluoric acid so dangerous if it is a weak acid?

Its weakness is part of the danger. So little of it ionises that enough uncharged hydrogen fluoride remains to penetrate tissue, HSDB explains. In concentrated solutions and the anhydrous liquid, the hydrogen ion causes immediate injury. The fluoride ion does most of the harm, binding calcium and magnesium inside cells, and symptoms may not show for several days while that damage goes on. The NIOSH card sets out the hazards in full.

Why does hydrogen fluoride boil near room temperature?

Its boiling point is 19.51 °C (HSDB), though its molar mass is only about 20 g/mol. In the liquid and the solid, its molecules are strongly hydrogen-bonded to one another. That hydrogen bonding gives it a boiling point Chem1 calls anomalously high for so light a compound. No hydrogen bond yet known is stronger than the one inside the bifluoride ion, on Chem1's account.