Ammonia

Related Category: Compounds and Elements

chemical compound, NH₃, colorless gas that is about one half as dense as air at ordinary temperatures and pressures. It has a characteristic pungent, penetrating odor. Ammonia forms a minute proportion of the atmosphere; it is found in volcanic gases and as a product of decomposition of animal and vegetable matter. Because ammonia was formerly obtained by destructive distillation of horns and hooves of animals, its water solution was called spirits of hartshorn. Ammonia has also been called alkaline air and volatile alkali.

Properties

Anhydrous (water-free) ammonia gas is easily liquefied under pressure (at 20°C; liquid ammonia has a vapor pressure of about 120 lb per sq in.) It is extremely soluble in water; one volume of water dissolves about 1,200 volumes of the gas at 0°C; (90 grams of ammonia in 100 cc of water), but only about 700 volumes at room temperature and still less at higher temperatures. The solution is alkaline because much of the dissolved ammonia reacts with water, H₂O, to form ammonium hydroxide, NH₄OH, a weak base. Liquid ammonia is used in the chemical laboratory as a solvent. It is a better solvent for ionic and polar compounds than ethanol, but not as good as water; it is a better solvent for nonpolar covalent compounds than water, but not as good as ethanol. It dissolves alkali metals and barium, calcium, and strontium by forming an unstable blue solution containing the metal ion and free electrons that slowly decomposes, releasing hydrogen and forming the metal amide. Compared to water, liquid ammonia is less likely to release protons (H⁺ ions), is more likely to take up protons (to form NH₄⁺ ions), and is a stronger reducing agent. Because strong acids react with it, it does not allow strongly acidic solutions, but it dissolves many alkalies to form strongly basic solutions.

Ammonia takes part in many chemical reactions. Ammonia reacts with strong acids to form stable ammonium salts: with hydrogen chloride it forms ammonium chloride; with nitric acid, ammonium nitrate; and with sulfuric acid, ammonium sulfate. Ammonium salts of weak acids are readily decomposed into the acid and ammonia. Ammonium carbonate, (NH₃)₂CO₃·H₂O, is a colorless-to-white crystalline solid commonly known as smelling salts; in water solution it is sometimes called aromatic spirits of ammonia. Ammonia reacts with certain metal ions to form complex ions called ammines. Ammonia also reacts with Lewis acids (electron acceptors), e.g., sulfur dioxide or trioxide or boron trifluoride.

Another kind of reaction, commonly called ammonolysis, occurs when one or more of the hydrogen atoms in the ammonia molecule is replaced by some other atom or radical. Chlorine gas, Cl₂, reacts directly with ammonia to form monochloramine, NH₂Cl, and hydrogen chloride, HCl. Products of such ammonolyses include amides, amines, imides, imines, and nitrides. Ammonia also takes part in oxidation and reduction reactions. It burns in oxygen to form nitrogen gas, N₂, and water. In the presence of a catalyst (e.g., platinum) it is oxidized in air to form water and nitric oxide, NO. It reduces hot-metal oxides to the metal (e.g., cupric oxide to copper).

Production

Ammonia is prepared commercially in vast quantities. The major method of production is the Haber process, in which nitrogen is combined directly with hydrogen at high temperatures and pressures in the presence of a catalyst. It is obtained as a byproduct of the destructive distillation of coal. Ammonia is also prepared synthetically by the cyanamide process: nitrogen gas combines with calcium carbide, CaC₂, at high temperatures to form calcium cyanamide, CaCN₂, and carbon; the calcium cyanamide reacts with steam to form calcium carbonate, CaCO₃, and ammonia. For use in the laboratory, ammonia is prepared by heating an ammonium salt with a strong base. It can also be prepared by reacting a metal nitride with water.

Uses

Ammonia solutions are used to clean, bleach, and deodorize; to etch aluminum; to saponify (hydolyze) oils and fats; and in chemical manufacture. The ammonia sold for household use is a dilute water solution of ammonia in which ammonium hydroxide is the active cleansing agent. It should be used with caution since it can attack the skin and eyes. The vapors are especially irritating—prolonged exposure and inhalation cause serious injury and may be fatal. Water solutions of ammonia are also called ammonium hydrate, aqua ammonia, or ammonia water; the solution may contain up to 30% ammonium hydroxide by weight at room temperature and pressure.

The major use of ammonia and its compounds is as fertilizers. Ammonia is also used in large amounts in the Ostwald process (see Ostwald, Wilhelm) for the synthesis of nitric acid; in the Solvay process for the synthesis of sodium carbonate; in the synthesis of numerous organic compounds used as dyes, drugs, and in plastics; and in various metallurgical processes.