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Hydrofluoric Acid Formula Uses and Properties

What is Hydrofluoric acid?

Hydrofluoric acid (HF) is an aqueous solution that is dissolved in hydrogen fluoride. This acid is obtained mainly from the reaction of concentrated sulfuric acid with the mineral fluorite (CaF2). The mineral is degraded by the action of the acid and the remaining water dissolves the hydrogen fluoride gases.

The pure product, that is, anhydrous hydrogen fluoride, can be distilled from this same acidic water. Depending on the amounts of the dissolved gas, different concentrations are obtained and, therefore, various available products of hydrofluoric acid are on the market.

At a concentration of less than 40%, it has a crystalline appearance indistinguishable from water, but at higher concentrations, it gives off white vapors of hydrogen fluoride. Hydrofluoric acid is known as one of the most aggressive and dangerous chemicals.

It is capable of “eating” almost any material with which it comes in contact: from glass, ceramics, and metals, to rocks and concrete. In what container then is it stored? In plastic bottles, synthetic polymers are inert to their action.

The formula of Hydrofluoric acid

The formula of hydrogen fluoride is HF, but that of hydrofluoric acid is represented in an aqueous medium, HF (aq), to differentiate itself from the former.

Thus, hydrofluoric acid can be considered as the hydrate of hydrogen fluoride, and this is its anhydride.

Structure Structure of hydrofluoric acid

Hydrofluoric acid 3D model

Every acid in water has the ability to generate ions in an equilibrium reaction. In the case of hydrofluoric acid, it is estimated that the pair of ions H 3 O + and F  exists in a solution.

The anion F  probably forms a very strong hydrogen bond with one of the hydrogens in the cation (F — HO + -H 2 ). This explains why hydrofluoric acid is a weak Bronsted acid (proton donor, H + ), despite its high and dangerous reactivity; that is, in water, it does not release as much H + compared to other acids (HCl, HBr, or HI).

However, in concentrated hydrofluoric acid, the interactions between the hydrogen fluoride molecules are sufficiently effective to allow them to escape in the gas phase.

That is, within the water they can interact as if they were in liquid anhydride, thus forming hydrogen bonds between them. These hydrogen bonds can be assimilated as almost linear chains (HF — HF — HF—…) surrounded by water.

Hydrofluoric acid

In the image above, the unshared pair of electrons oriented in the opposite direction of the bond (HF 🙂 interacts with another HF molecule to assemble the chain.

Properties of hydrofluoric acid

Since hydrofluoric acid is an aqueous solution, its properties depend on the concentration of the anhydride dissolved in water. HF is very soluble in water and is hygroscopic, being able to produce a variety of solutions: from very concentrated (smoky and with yellow tones) to very dilute.

As its concentration decreases, HF (ac) takes on properties more similar to pure water than anhydride. However, HF — H hydrogen bonds are stronger than those in water, H 2 O — HOH.

Both coexist in harmony in the solutions, raising the boiling points (up to 105ºC). Likewise, densities increase as more anhydride HF is dissolved. Otherwise, all HF (ac) solutions have strong, irritating odors and are colorless.

Reactivity

So what is the corrosive behavior of hydrofluoric acid due to? The answer lies in the HF bond and in the ability of the fluorine atom to form very stable covalent bonds.

Fluorine being a very small and electronegative atom is a powerful Lewis acid. That is, it separates from hydrogen to bind to species that offer it more electrons at a low energy cost. For example, these species can be metals, such as silicon present in glasses.

SiO 2  + 4 HF → SiF 4 (g) + 2 H 2 O

SiO 2  + 6 HF → H 2 SiF 6  + 2 H 2 O

If the dissociation energy of the HF band is high (574 kJ/mol), why does it break in the reactions? The answer has kinetic, structural, and energetic overtones. In general, the less reactive the resulting product, the more favored its formation.

What happens to the F  in the water? In concentrated solutions of hydrofluoric acid, another HF molecule can form a hydrogen bond with the F  of the pair [H 3 O + F  ].

This results in the generation of the difluoride ion [FHF] , which is extremely acidic. That is why any physical contact with it is extremely harmful. The slightest exposure can trigger infinite damage to the body.

There are many safety standards and protocols for its proper handling, thus avoiding potential accidents for those who operate with this acid.

Uses and Applications of Hydrofluoric Acid

It is a compound with numerous applications in industry, research, and in consumer work:

  • Hydrofluoric acid generates organic derivatives that intervene in the aluminum purification process.
  • It is used in the separation of isotopes from uranium, as in the case of uranium hexafluoride (UF 6 ). Likewise, it is used in the extraction, processing, and refining of metals, rocks, and oils, also being used for the inhibition of growth and removal of mold.
  • The corrosive properties of acid have been used to carve and engrave crystals, especially frosted ones, using the etching technique.
  • It is used in the manufacture of silicon semiconductors, with multiple uses in the development of computing and information technology, responsible for human development.
  • It is used in the automotive industry as a cleaner, being used as a mold remover on ceramics.
  • In addition to serving as an intermediate in some chemical reactions, hydrofluoric acid is used in some ion exchangers that are involved in the purification of metals and more complex substances.
  • It participates in the processing of oil and its derivatives, which has made it possible to obtain solvents for use in the manufacture of cleaning and grease removal products.
  • It is used in the generation of agents for plating and surface treatment.
  • Consumers use numerous products in which hydrofluoric acid has participated in their elaboration; for example, some necessary for car care, cleaning products for furniture, electrical and electronic components, and fuels, among other products.

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