Hydrogen halides and their properties

General characteristics and physical properties

Under normal conditions, most hydrogen halides are gases with a pungent, suffocating odor. Upon contact with moist air, they instantly form a mist consisting of microscopic solution droplets. All of these compounds are distinguished by exceptionally high solubility in water.

Hydrogen fluoride stands out significantly from the other members of the group. Unlike hydrogen chloride, which turns into a gas already at −85 °C, hydrogen fluoride easily liquefies (its boiling point is only +20 °C). It is miscible with water in any proportion, and its aqueous solution is traditionally known as hydrofluoric acid.

Hydrochloric acid and its biological significance

The most well-known aqueous solution in this group is hydrochloric acid (a solution of hydrogen chloride). Industrially, it is produced as an almost saturated solution with a mass fraction of the active substance of about 37%. It is a strong and thermodynamically stable acid considered less toxic compared to other hydrogen halide analogues.

Nevertheless, concentrated hydrochloric acid requires strict safety precautions due to its pungent odor and ability to cause severe irritation of the respiratory tract. Interestingly, highly diluted acid (with a concentration of less than 4%) is virtually odorless, which is explained by the complete dissociation of molecules into ions. In the human body, hydrogen chloride performs a crucial biomedical function as an obligatory component of gastric juice.

Acidic and reducing properties

Aqueous solutions of hydrogen chloride, hydrogen bromide, and hydrogen iodide exhibit the properties of typical strong acids. Hydrogen fluoride is an exception here, being a weak acid with a low degree of dissociation (even at a concentration of 0.1 mol/L).

In addition to acidic properties, hydrogen halides possess pronounced reducing activity. The mechanism lies in the ability of halide anions to donate electrons.

• The acids themselves act as stronger reducing agents than their salts.
• This is because the acidic medium (the presence of hydrogen ions) significantly enhances the oxidizing capacity of other reactants. For example, potassium permanganate is actively reduced by hydrogen bromide. In a neutral medium with sodium chloride, the reaction is hindered, but upon acidification with sulfuric acid, the process is sharply activated.

Hydrogen halides also easily undergo addition reactions with molecules containing multiple bonds. They react with sulfur trioxide (VI) to form chlorosulfonic acid, as well as with alkenes (for example, converting ethylene into chloroethane).

Specifics of hydrogen fluoride

Hydrogen fluoride has unique chemical behavior. It shows absolutely no reducing properties characteristic of heavier hydrogen halides. Its main feature is the ability to effectively replace hydroxyl groups and oxygen ions with a fluoride anion.

The best-known property of hydrofluoric acid is its corrosive effect on glass (silicon dioxide). The visual effect depends on the state of aggregation of the reactant:

1. In hydrogen fluoride vapor: the glass surface becomes evenly frosted.
2. In liquid acid: the glass is corroded unevenly, and its surface becomes rough.

Preparation and hydrolysis of halides

The synthesis of hydrogen halides depends on the chemical nature of the halogen. Hydrogen chloride is obtained in the laboratory by treating dry sodium chloride with concentrated sulfuric acid upon heating. However, this method is not applicable for the synthesis of hydrogen bromide and hydrogen iodide, as sulfuric acid oxidizes these compounds. They are obtained differently—via the hydrolysis of phosphorus tribromide and triiodide.

The process of non-metal halide hydrolysis itself has clear regularities. Chlorides, bromides, and iodides undergo complete irreversible hydrolysis to form two acids (previously, such binary compounds were even called acid halides). The thermodynamic reason lies in the fact that bonds of non-metals (silicon, phosphorus) with oxygen are significantly stronger than with heavy halogens. In the case of non-metal fluorides (e.g., silicon tetrafluoride), hydrolysis proceeds only partially, since bonds with fluorine have a different nature and exceed oxygen bonds in strength.

Class and exam questions

What else to study on this topic

• Thermodynamics of hydrolysis: comparison of element-halogen and element-oxygen bond energies.
• Effect of medium pH on the redox potential of potassium permanganate.
• Mechanisms of electrophilic addition reactions of hydrogen halides to alkenes.
• Variation in polarizability and electronegativity in the halogen series from fluorine to iodine.

Other topics in the section "Chapter 9. Group VIIA Elements — Halogens"

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