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Why is hydrofluoric acid a weak acid?

Hydrofluoric acid, HF(aq) is a highly corrosive acid, however, it is considered a weak acid. To know why, it is necessary to understand what acids are, their characteristics, the types of acids that exist, and the particular qualities of hydrofluoric acid.

what are acids

Acids are substances that in an aqueous solution provide hydrogen ions. They are characterized by having a sour taste and being solid or liquid. They also neutralize bases and can dissolve metals.

In addition, they have a pH value of less than 7 and react in contact with pH indicators such as litmus paper or solutions, changing to different shades of red.

They are also good conductors of electricity and can form hydrogen salts when reacted with active metals. They are highly corrosive and can cause skin burns.

Acids are formed when a gas reacts with hydrogen or when a nonmetal oxide reacts with water.

strong acids and weak acids

Acids can be divided into strong and weak acids. Strong acids are those that completely dissociate in water, providing hydrogen ions, but do not accept them. Some common examples of strong acids are hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO 3 ).
Unlike strong acids, weak acids contribute hydrogen ions and can accept them, thus creating an acid-base balance. They do not completely dissociate in water. To this group belong most organic acids.

In addition to hydrofluoric acid, some common examples of weak acids are acetic acid, CH 3 -COOH (C2H 4 O 2 ), citric acid (C 6 H 8 O 7 ), boric acid (H 3 BO 3 ), carbonic acid (H 2 CO 3 ), phosphoric acid (H 3 PO 4 ), hydrogen sulfide (H 2 S) and the salt known as ammonium phosphate (NH₄)H₂PO₄.

What is hydrofluoric acid

Hydrofluoric acid, whose formula is HF (aq), is a chemical compound made up of fluorine, hydrogen and water. It is considered a hydracid and is transparent and colorless. It has a pungent odor and is extremely corrosive and dangerous. In fact, it can disintegrate glass, cement, metals, leather, enamels and other materials. Because of this, it is carefully stored or transported at very low temperatures and in plastic containers.

Why hydrofluoric acid is considered a weak acid

Although hydrofluoric acid can dissolve metals and corrode glass, it is considered a weak acid. To understand the reasons for this classification, it is necessary to take into account the previous concepts about weak acids and strong acids and the differences between them.

In the case of hydrofluoric acid, in an aqueous solution it will dissociate into hydrogen cations ( H + ) and fluorine anion ( F – ), forming an acid-base equilibrium, as shown by the following chemical equation:

HF ⇌ H + + F –

The bond strength of hydrofluoric acid is higher than that of the bonds formed by the acids of other similar compounds. This means that the HF bond is more difficult to break and therefore does not fully dissociate like strong acids.

The strength of a weak acid is closely related to its dissociation: the more it dissociates, the stronger the acid. To know the strength of weak acids and to determine if hydrofluoric acid is a weak acid, we must take into account the acid dissociation constant, K a . This measurement is also known as the “acidity constant” or “acid ionization constant”. It measures the strength of an acid in a solution.

Acids with pKa values ​​less than 2 are considered strong acids. A weak acid has a pKa value of 2 to 12 in water. Hydrofluoric acid has an acidity of 3.17 pKa, therefore it is a weak acid.

The strength of an acid can also be determined based on the following characteristics:

  • Electronegativity: The higher the electronegativity of an element, the greater the tendency to form acids. Conversely, elements with lower electronegativity tend to form more basic compounds.
  • Atomic radius: elements with a larger ionic radius have lower acidity.

Applications and uses of hydrofluoric acid

Although hydrofluoric acid is dangerous, it has many different applications in industry. It is used to etch and polish glass, aluminum, and stainless steel, and to make polishes for these items.

It is also used to purify quartz, treat titanium, separate uranium isotopes, and make organic and inorganic compounds. Another common use is the manufacture of fluorescent light bulbs, pesticides, fuels, and plastics.

Risks of using hydrofluoric acid

Hydrofluoric acid is a highly corrosive and toxic caustic chemical that can cause significant damage if it comes in contact with the skin, is swallowed, or inhaled. In the event of being exposed to this acid, immediate medical attention is necessary.