Hydrogen chloride is a gas about 1.3 times heavier than air. It is colorless, but with a sharp, suffocating and characteristic odor. At a temperature of minus 84 ° C, hydrogen chloride passes from a gaseous state to a liquid state, and at minus 112 ° C it hardens. Hydrogen chloride dissolves in water. One liter of H2O can absorb up to 500 ml of gas. Its solution is called hydrochloric or hydrochloric acid. Concentrated hydrochloric acid at 20 ° C is characterized by the maximum possible mass fraction of the basic substance, equal to 38%. The solution is a strong monobasic acid (it “smokes” in air, and forms an acid fog in the presence of moisture), it also has other names: hydrochloric, and according to the Ukrainian nomenclature - chloride acid. The chemical formula can be represented as follows: HCl. The molar mass is 36.5 g / mol. The density of hydrochloric acid concentrated at 20 ° C is 1.19 g / cm³. This is a harmful substance that belongs to the second hazard class.
In the “dry” form, hydrogen chloride cannot interact even with active metals, but in the presence of moisture, the reaction proceeds quite vigorously. This strong hydrochloric acid is able to react with all metals that are to the left of hydrogen in a series of voltages. In addition, it interacts with basic and amphoteric oxides, bases, and also with salts:
- Fe + 2HCl → FeCl2 + H2 ↑;
- 2HCl + CuO → CuCl2 + H2O;
- 3HCl + Fe (OH) 3 → FeCl3 + 3H2O;
- 2HCl + Na2CO3 → 2NaCl + H2O + CO2 ↑;
- HCl + AgNO3 → AgCl ↓ + HNO3.
In addition to the general properties characteristic of each strong acid, hydrochloric acid has reducing properties: it reacts in a concentrated form with various oxidizing agents, releasing free chlorine. Salts of this acid are called chlorides. Almost all of them dissolve well in water and completely dissociate into ions. Weakly soluble are lead chloride PbCl2, silver chloride AgCl, monovalent mercuric chloride Hg2Cl2 (calomel) and monovalent chloride CuCl. Hydrogen chloride is capable of entering into the reaction of addition to unsaturated hydrocarbons with a double or triple bond, and chlorine derivatives of organic compounds are formed.
Under laboratory conditions, hydrogen chloride is obtained by exposure to dry sodium chloride with concentrated sulfuric acid. The reaction under different conditions can proceed with the formation of sodium salts (acidic or medium):
- H2SO4 + NaCl → NaHSO4 + HCl
- H2SO4 + 2NaCl → Na2SO4 + 2HCl.
The first reaction proceeds to the end with weak heating, the second at higher temperatures. Therefore, it is better to obtain hydrogen chloride in the laboratory in the first way, for which it is recommended to take the amount of sulfuric acid based on the production of the acid salt NaHSO4. Then, by dissolving hydrogen chloride in water, hydrochloric acid is obtained. In industry, it is obtained by burning hydrogen in an atmosphere of chlorine or by acting on dry sodium chloride (only according to the second reaction equation) with concentrated sulfuric acid. Also, hydrogen chloride is obtained as a by-product of the chlorination of saturated organic compounds. In industry, hydrogen chloride, obtained by one of the above methods, is dissolved in special towers in which liquid is passed from top to bottom, and gas is supplied from bottom to top, that is, according to the counterflow principle.
Hydrochloric acid is transported in special rubberized tanks or containers, as well as in plastic barrels with a capacity of 50 l or glass bottles with a capacity of 20 l. When acid and metals react, there is a risk of the formation of explosive hydrogen-air mixtures. Therefore, the contact of the hydrogen formed as a result of the reaction with air, and also (with the help of anti-corrosion coatings) the contact of acid with metals should be completely excluded. Before removing the apparatus and pipelines where it was stored or transported for repair, it is necessary to purge with nitrogen and monitor the state of the gas phase.
Hydrogen chloride is widely used in industrial production and in laboratory practice. It is used for pickling metals, producing salts and as a reagent in analytical studies. Technical hydrochloric acid is produced in accordance with GOST 857-95 (the text is identical to the international standard ISO 905-78), the reagent is in accordance with GOST 3118-77. The concentration of a technical product depends on the brand and grade and may be 31.5%, 33% or 35%, and externally the product is yellowish due to the content of impurities of iron, chlorine and other chemicals. Reactive acid should be a colorless and clear liquid with a mass fraction of 35 to 38%.