Hydrochloric acid

Hydrogen chloride is a gas about 1.3 times heavier than air. It is without color, but with a sharp suffocating and characteristic smell. At a temperature of minus 84 ° C, hydrogen chloride passes from gaseous to liquid state, and at minus 112 ° C solidifies. Hydrogen chloride is soluble in water. One liter of H2O can absorb up to 500 ml of gas. Its solution is called hydrochloric acid 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 (in the air it "smokes", and in the presence of moisture it forms an acid mist), it also has other names: hydrochloric acid, 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 class of danger.

In the "dry" form, hydrochloric acid can not interact even with active metals, but in the presence of moisture the reaction proceeds rather vigorously. This strong hydrochloric acid is able to react with all metals that stand to the left of hydrogen in a series of stresses. 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 concentrated form with various oxidants, releasing free chlorine. The salts of this acid are called chlorides. Almost all of them are readily soluble in water and completely dissociate into ions. Slightly soluble are: lead chloride PbCl2, silver chloride AgCl, chloride monovalent mercury Hg2Cl2 (calomel) and cupric chloride CuCl. Hydrogen chloride is capable of reacting with unsaturated hydrocarbons with a double or triple bond, thus forming chlorine-containing organic compounds.

Under laboratory conditions, hydrogen chloride is obtained when concentrated sulfuric acid is exposed to dry sodium chloride . 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 low heating, the second - at higher temperatures. Therefore, in the laboratory, hydrogen chloride is best obtained by the first method, for which the amount of sulfuric acid is recommended to be taken from the calculation of the acid salt NaHSO4. Then, hydrochloric acid is obtained by dissolving hydrogen chloride in water. In industry, it is produced by burning hydrogen in a chlorine atmosphere or by acting on dry sodium chloride (only by the second reaction equation) with concentrated sulfuric acid. Also, hydrogen chloride is obtained as a by-product in 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 the top to bottom, and the gas is fed from the bottom upwards, that is, according to the countercurrent principle.

Hydrochloric acid is transported in special rubber cisterns or containers, as well as in polyethylene casks with a capacity of 50 liters or glass bottles with a capacity of 20 liters. At interaction of acid with metals there is a risk of formation of explosive hydrogen-air mixtures. Therefore, contact of the hydrogen formed as a result of the reaction with air, and also (with the help of anticorrosive coatings), contact of the acid with metals should be completely excluded. Before the withdrawal of apparatus and pipelines, where it was stored or transported, for repair, it is necessary to carry out purging 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 etching metals, obtaining salts and as a reagent in analytical studies. Hydrochloric acid is produced according to GOST 857-95 (the text is identical to the international standard ISO 905-78), reagent - according to GOST 3118-77. The concentration of a technical product depends on the brand and variety and can be 31.5%, 33% or 35%, and the product is yellowish in appearance due to the content of impurities of iron, chlorine and other chemicals. Reactive acid should be a colorless and transparent liquid with a mass fraction of 35 to 38%.