Hydrogen chloride - what is it? Hydrogen chloride is a colorless gas with a pungent odor. It easily dissolves in water, forming hydrochloric acid. The chemical formula of hydrogen chloride is HCl. It consists of a hydrogen atom and chlorine connected by a covalent polar bond. Hydrogen chloride easily dissociates in polar solvents, which ensures good acidic properties of this compound. The bond length is 127.4 nm.
Physical properties
As mentioned above, in the normal state, hydrogen chloride is a gas. It is slightly heavier than air, and also has hygroscopicity, that is, it attracts water vapor directly from the air, forming a thick cloud of steam. For this reason, it is said that hydrogen chloride "smokes" in air. If this gas is cooled, then at -85 ° C it liquefies, and at -114 ° C it becomes a solid. At a temperature of 1500 ° C it decomposes into simple substances (based on the formula of hydrogen chloride, into chlorine and hydrogen).
A solution of HCl in water is called hydrochloric acid. It is a colorless caustic liquid. Sometimes it has a yellowish tint due to impurities of chlorine or iron. Due to hygroscopicity, the maximum concentration at 20 ° C is 37-38% by weight. Other physical properties also depend on it: density, viscosity, melting and boiling points.
Chemical properties
Hydrogen chloride itself does not usually react. Only at high temperatures (more than 650 ° C) does it react with sulfides, carbides, nitrides and borides, as well as transition metal oxides. In the presence of Lewis acids, it can interact with hydrides of boron, silicon and germanium. But its aqueous solution is much more chemically active. According to its formula, hydrogen chloride is an acid, so it has some properties of acids:
- Interaction with metals (which are in the electrochemical series of voltages up to hydrogen):
Fe + 2HCl = FeCl 2 + H 2
- Interaction with amphoteric and basic oxides:
BaO + 2HCl = BaCl 2 + H 2 O
- Interaction with alkalis:
NaOH + HCl = NaCl + H 2 O
Interaction with some salts:
Na 2 CO 3 + 2HCl = 2NaCl + H 2 O + CO 2
- When interacting with ammonia, a salt of ammonium chloride is formed:
NH 3 + HCl = NH 4 Cl
But hydrochloric acid does not interact with lead due to passivation. This is due to the formation on the metal surface of a layer of lead chloride, which is insoluble in water. Thus, this layer protects the metal from further interaction with hydrochloric acid.
In organic reactions, it can join in multiple bonds (hydrohalogenation reaction). It can also react with proteins or amines, forming organic salts - hydrochlorides. Artificial fibers, such as paper, are destroyed by interaction with hydrochloric acid. In redox reactions with strong oxidizing agents, hydrogen chloride is reduced to chlorine.
A mixture of concentrated hydrochloric and nitric acid (3 to 1 by volume) is called "aqua regia." It is an extremely strong oxidizing agent. Due to the formation of free chlorine and nitrosyl in this mixture, aqua regia can dissolve even gold and platinum.
Getting
Previously in industry, hydrochloric acid was obtained by reacting sodium chloride with acids, usually sulfuric:
2NaCl + H 2 SO 4 = 2HCl + Na 2 SO 4
But this method is not effective enough, and the purity of the resulting product is low. Now another method is used to obtain (from simple substances) hydrogen chloride according to the formula:
H 2 + Cl 2 = 2HCl
To implement this method, there are special installations where both gases are fed in a continuous stream to the flame in which the interaction takes place. Hydrogen is supplied in a slight excess so that all chlorine reacts and does not contaminate the resulting product. Then, hydrogen chloride is dissolved in water and hydrochloric acid is obtained.
In the laboratory, more diverse methods of preparation are possible, for example, the hydrolysis of phosphorus halides:
PCl 5 + H 2 O = POCl 3 + 2HCl
Hydrochloric acid can also be obtained by hydrolysis of crystalline hydrates of certain metal chlorides at elevated temperatures:
AlCl 3 · 6H 2 O = Al (OH) 3 + 3HCl + 3H 2 O
Hydrogen chloride is also a by-product of the chlorination reactions of many organic compounds.
Application
Hydrogen chloride itself cannot be used in practice, as it absorbs water from the air very quickly. Almost all hydrogen chloride produced is used to produce hydrochloric acid.
It is used in metallurgy to clean the surface of metals, as well as to obtain pure metals from their ores. This occurs by converting them to chlorides, which are easily reduced. For example, titanium and zirconium are obtained. Acid has been widely used in organic synthesis (hydrohalogenation reactions). Also, pure chlorine is sometimes obtained from hydrochloric acid.
It is also used in medicine as a medicine mixed with pepsin. It is taken with insufficient acidity of the stomach. Hydrochloric acid is used in the food industry as an additive E507 (acidity regulator).
Safety precautions
At high concentrations, hydrochloric acid is a caustic substance. Getting on the skin, it causes chemical burns. Inhaling gaseous hydrogen chloride causes a cough, choking, and in severe cases, even pulmonary edema, which can lead to death.
According to GOST, it has a second hazard class. NFPA 704 hydrogen chloride has a third hazard category of four. Short-term exposure may cause serious temporary or moderate residual effects.
First aid
If hydrochloric acid gets on the skin, the wound should be thoroughly washed with water and a weak solution of alkali or its salt (for example, soda).
If vapors of hydrogen chloride enter the respiratory tract of the affected person, it is necessary to remove them to fresh air and make oxygen inhalation. After that, gargle, rinse your eyes and nose with 2% sodium bicarbonate solution. If hydrochloric acid gets into the eyes, then after that it is worth dripping them with a solution of novocaine and dicaine with adrenaline.