The hydrofluoric acid (HF) is an aqueous solution in which is dissolved hydrogen fluoride. This acid is obtained mainly from the reaction of concentrated sulfuric acid with the mineral fluorite (CaF 2). The mineral is degraded by the action of the acid and the remaining water dissolves the hydrogen fluoride gases.
The pure product, that is, the 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 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 hydrogen fluoride vapors. 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 inert to their action.
Formula
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
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 (FHO + -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 effective enough 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 (HFHFHF-…) surrounded by water.
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
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 capable of producing 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 those of anhydride. However, HFH 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, it 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 bond 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 hydrogen bond with the F - of the pair.
This results in the generation of the difluoride ion -, which is extremely acidic. That is why any physical contact with it is extremely harmful. The slightest exposure can trigger endless damage to the body.
There are many safety standards and protocols for its proper handling, and thus avoid potential accidents to those who operate with this acid.
Applications
It is a compound with numerous applications in industry, in research and in consumer affairs.
- Hydrofluoric acid generates organic derivatives that intervene in the aluminum purification process.
- It is used in the separation of uranium isotopes, 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 growth inhibition and mold removal.
- 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 silicone 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.
- Participates in the processing of oil and its derivatives, which has allowed the obtaining of 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.
References
- PubChem. (2018). Hydrofluoric Acid. Retrieved on April 3, 2018, from: pubchem.ncbi.nlm.nih.gov.
- Kat Day. (April 16, 2013). The acid that really does eat through everything. Retrieved on April 3, 2018, from: chronicleflask.com
- Wikipedia. (March 28, 2018). Hydrofluoric acid. Retrieved on April 3, 2018, from: en.wikipedia.org.
- Shiver & Atkins. (2008). Inorganic chemistry. (4th ed., pp. 129, 207-249, 349, 407). Mc Graw Hill.
- Hydrofluoric Acid. Musc. Medical University of South Carolina. Retrieved on April 3, 2018, from: academicdepartments.musc.edu