Ammonia The most familiar compound composed ‘of the elements nitrogen and hydrogen, NH3. It is formed as a result of the decomposition of most nitrogenous organic material, and its presence is indicated by its pungent and irritating odor.
Ammonia has a wide range of industrial and agricultural applications. Examples of its use are the production of nitric acid and ammonium salts, particularly the sulfate, nitrate, carbonate, and chloride, and the synthesis of hundreds of organic compounds including many drugs, plastics, and dyes. Its dilute aqueous solution finds use as a household cleansing agent. Anhydrous ammonia and ammonium salts are used as fertilizers, and anhydrous ammonia also serves as a refrigerant, because of its high heat of vaporization and relative ease of liquefaction.
The physical properties of ammonia are analogous to those of water and hydrogen fluoride in that the physical constants are abnormal with respect to those of the binary hydrogen compounds of the other members of the respective periodic families. These abnormalities may be related to the association of molecules through intermolecular hydrogen bonding. Ammonia is highly mobile in the liquid state and has a high thermal coefficient of expansion.
Most of the chemical reactions of ammonia may be classified under three chief groups: (1) addition reactions, commonly called ammonation; (2) substitution reactions, commonly called ammonolysis; and (3) oxidation-reduction reactions.
Ammonation reactions include those in which ammonia molecules add to other molecules or ions. Most familiar of the ammonation reactions is the reaction with water to form ammonium hydroxide. The strong tendency of water and ammonia to combine is evidenced by the very high solubility of ammonia in water. Ammonia reacts readily with strong acids to form ammonium salts. Ammonium salts of weak acids in the solid state dissociate readily into ammonia and the free acid. Ammonation occurs with a variety of molecules capable of acting as electron acceptors (Lewis acids), such as sulfur trioxide, sulfur dioxide, silicon tetrafluoride, and boron trifluoride. Included among ammonation reactions is the formation of complexes (called ammines) with many metal ions, particularly transition metal ions. Ammonolytic reactions include reactions of ammonia in which an amide group (-NH2), an imide group ( NH), or a nitride group ( N) replaces one or more atoms or groups in the reacting molecule.
Oxidation-reduction reactions may be subdivided into those which involve a change in the oxidation state of the nitrogen atom and those in which elemental hydrogen is liberated. An example of the first group is the catalytic oxidation of ammonia in air to form nitric oxide. In the absence of a catalyst, ammonia burns in oxygen to yield nitrogen. Another example is the reduction with ammonia of hot metal oxides such as cupric oxide.
The physical and chemical properties of liquid ammonia make it appropriate for use as a solvent in certain types of chemical reactions. The solvent properties of liquid ammonia are, in many ways, qualitatively intermediate between those of water and of ethyl alcohol. This is particularly true with respect to dielectric constant; therefore, ammonia is generally superior to ethyl alcohol as a solvent for ionic substances but is inferior to water in this respect. On the other hand, ammonia is generally a better solvent for covalent substances than is water.
The Haber-Bosch synthesis is the major source of industrial ammonia. In a typical process, water gas (CO, H2, CO2) mixed with nitrogen is passed through a scrubber cooler to remove dust and undecomposed material. The CO2 and CO are removed by a CO2 purifier and ammoniacal cuprous solution, respectively. The remaining H2 and N2 gases are passed over a catalyst at high pressures (up to 1000 atrn or 100 megapascals) and high temperatures (approx. 1300°F or 700°C). Other industrial sources of ammonia include its formation as a by-product of the destructive distillation of coal, and its synthesis through the cyanamide process. In the laboratory, ammonia is usually formed by its displacement from ammonium salts (either dry or in solution) by strong bases. Another source is the hydrolysis of metal nitrides.
مقالات قد تفيدك :