The Acidity of Hydrogen Halides

Halogens are non-metals, gaseous elements with high reactivity. They rarely occur in their free state in nature; we mostly find them as compound structures with other elements. Halides are one such compound of halogens. 

In this article, we will explore the acidity of hydrogen halides and related concepts. 

A Brief Overview of the Acidity of Hydrogen Chloride

Hydrogen chloride (HCl) is indeed an acid, according to the Bronsted and Lowry theory. It fits the description of basic acid, as it donates protons to additional substances. Let us consider how it reacts with water.

Hydrogen chloride vapour is soluble with water; hydrochloric acid is its solvent state. Hydrogen chloride reacts with water vapour in the wind to form a mist of strong Hydrochloric acid vapours.

In this phenomenon, the water molecule receives a proton from hydrogen chloride. The reaction establishes a coordinating (dative covalent) connection between the oxygen and the transported proton.

The reaction’s formula is as follows:

H2O + HCl → H3O+ + Cl–

This H3O+ molecule represents the hydronium ion (also known as the hydronium or oxonium ion). We are referring to this particle when we say H+ (aq).

Whenever hydrogen chloride mixes with water to form hydrochloric acid, all hydrogen chloride particles interact in this manner. As a result, hydrochloric acid would be a powerful acid. This powerful acid denotes one that completely ionises into the solution.

Strong acids: Hydrobromic Acid and Hydriodic Acid

Hydrogen bromide and iodide mix with (and interact with) water, similar to hydrogen chloride. Hydrobromic acid is the result of the reaction between hydrogen bromide and water. Hydrogen iodide combines with water to form hydriodic acid. Both are powerful acids.

As Another Exception, Consider Hydrofluoric Acid

Hydrofluoric acid remains a weaker acid than hydrogen fluoride, which dissolves freely with water. Hydrofluoric acid is equal in potency to organic acids such as methanoic acid.

Previously, scientists attributed this behaviour to the extremely strong H-F bond that had to be destroyed whenever hydrogen fluoride formed ions. However, this hypothesis falls apart when we examine the thermodynamic properties of the process.

It is always risky to examine the thermodynamic properties of only one stage in the entire sequence of power changes that occur throughout a reaction. In this scenario, breaking that hydrogen-fluorine connection requires much energy. 

However, whenever the fluoride particle becomes encircled by water particles in solution, it produces a considerable amount of heat. This fluoride ion’s strong hydration enthalpy somewhat balances the strong H-F binding strength.

Why Does Hydrofluoric Acid Seem So ineffective?

There is excellent spectral proof showing hydrogen fluoride ionises thoroughly in solution with water. However, instead of forming unbound hydronium particles, H3O+, and fluoride particles, their interaction is potent in creating a tightly bonded ion pairing (H3O+.F–).

H2O + HF → H3O+ + F–

This equilibrium’s placement is much towards the right.

However, this hydronium ion must be unbound and not tightly bound with another fluoride ion to behave as a strong acid.

H3O+.F– → H3O+ + F–

This equilibrium has been located significantly further toward the left.

Thus, hydrofluoric acid appears weak, not just because ionisation has been weak, but because the ions generated bind too tightly together.

Conclusion

The acidity of hydrogen halides is among the most fundamental topics in chemistry. The acidity of hydrogen halides serves as a foundation for many subsequent chemistry concepts. However, some hydrogen halides are more acidic than others due to their internal properties.