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HCl, under ordinary conditions, a colorless gas, with a pungent odor. Hydrogen chloride fumes upon the absorption of moisture, the vapor consisting of tiny droplets of hydrochloric acid. The mass of 1 liter (l) of hydrogen chloride at 0°C and a pressure of 0.1 meganewton/m2 (1 kilogram-force/cm2) is 1.6391 g. The density relative to air is 1.268; the density of liquid hydrogen chloride at –60°C is 1.12 g/cm3. The melting point is –114.2°C, and the boiling point, –85.1°C.
Hydrogen chloride is a stable compound, which at 1800°C only slightly dissociates into Cl2 and H2. It has low chemical reactivity in the absence of water. In gaseous form, it does not react with carbon, phosphorus, sulfur, and iron. Upon heating with oxygen to 400°–500°C in the presence of the catalyst CuCl2, hydrogen chloride is oxidized to chlorine: 4HCl + O2 = 2H2O + 2Cl2. With alkali metals, aluminum, and magnesium, it reacts with the liberation of hydrogen, especially readily upon heating; for example, 2Al + 6HCl = 2AlCl3 + 3H2. With ammonia, it reacts with the formation of a smoke consisting of solid particles of ammonium chloride, NH4Cl. In the presence of catalysts, it adds to unsaturated organic compounds; for example, C2H4 + HCl → C2H5Cl.
Hydrogen chloride is obtained by direct synthesis from its elements or by the reaction of sulfuric acid with sodium chloride. Large amounts of hydrogen chloride are produced as a by-product in the chlorination of organic compounds: RH + Cl2 = RCl + HCl, where R is a radical. Large amounts are also produced in the hydrolysis of magnesium chloride: MgCl2 + H2O = MgO + 2HCl. The world production of hydrogen chloride in 1975 was about 10 million tons.
Hydrogen chloride is used in the production of hydrochloric acid and the synthesis of organic compounds, such as vinyl chloride.
REFERENCESIakimenko, L. M. Proizvodstvo khlora, kausticheskoi sody i neorganicheskikh khlorproduktov. Moscow, 1974.
Iakimenko, L. M., and M. I. Pasmanik. Spravochnik po proizvodstvu khlora, kausticheskoi sody i osnovnykh khlorproduktov, 2nd ed. Moscow, 1976.
L. M. IAKIMENKO