A monobasic strong acid representingunder normal conditions, a colorless liquid that turns yellow when stored, can be in a solid state, characterized by two crystalline modifications (monoclinic or rhombic lattice), at temperatures below minus 41.6 ° C. This substance with the chemical formula - HNO3 - is called nitric acid. It has a molar mass of 63.0 g / mol, and its density corresponds to 1.51 g / cm³. The boiling point of the acid is 82.6 ° C, the process is followed by decomposition (partial): 4HNO3 → 2H2O + 4NO2 + O2. A solution of acid with a mass fraction of the basic substance equal to 68% boils at a temperature of 121 ° C. The refractive index of a pure substance corresponds to 1.397. The acid is able to mix with water in any ratio and, being a strong electrolyte, almost completely break up into H + and NO3- ions. Solid forms - trihydrate and monohydrate have the formulas: HNO3 • 3H2O and HNO3 • H2O, respectively.
Nitric acid is a corrosive,toxic substance and strong oxidizer. From the Middle Ages, the name "strong water" (Aqua fortis) is known. The alchemists, who discovered acid in the 13th century, gave such a name, being convinced of its extraordinary properties (corroded all metals except gold), exceeding in a million times the strength of acetic acid, which in those days was considered the most active. But three centuries later it was found that a mixture of such acids as nitric and hydrochloric in a volume ratio of 1: 3, which for this reason was called "royal vodka", could be eaten up, even gold. The appearance of a yellow shade during storage is explained by the accumulation of nitrogen oxides in it. On sale, acid is more often at a concentration of 68%, and when the content of the main substance is more than 89% it is called "fuming".
The chemical properties of nitric acid distinguish itfrom dilute sulfuric or hydrochloric acids in that HNO3 is a stronger oxidant, so hydrogen is never released in reactions with metals. Due to its oxidizing properties, it also reacts with many non-metals. In both cases, NO2 is always formed. In redox reactions, the reduction of nitrogen occurs to different degrees: HNO3, NO2, N2O3, NO, N2O, N2, NH3, which is determined by the acid concentration and metal activity. The molecules of the compounds formed contain nitrogen with an oxidation state of +5, +4, +3, +2, +1, 0, +3, respectively. For example, copper is oxidized with concentrated acid to copper (II) nitrate: Cu + 4HNO3 → 2NO2 + Cu (NO3) 2 + 2H2O, and phosphorus to metaphosphoric acid: P + 5HNO3 → 5NO2 + HPO3 + 2H2O.
Otherwise, dilute nitrogenacid with nonmetals. Using the example of the reaction with phosphorus: 3P + 5HNO3 + 2H2O → 3H3PO4 + 5NO, it can be seen that nitrogen is reduced to a divalent state. As a result, nitrogen monoxide is formed, and phosphorus is oxidized to phosphoric acid. Concentrated nitric acid in a mixture with hydrochloric acid dissolves gold: Au + 4HCl + HNO3 → NO + H [AuCl4] + 2H2O and platinum: 3Pt + 18HCl + 4HNO3 → 4NO + 3H2 [PtCl6] + 8H2O. In these reactions, at the initial stage, hydrochloric acid is oxidized by nitrogen with the liberation of chlorine, and then the metals form complex chlorides.
Nitric acid on an industrial scale is obtained in three main ways:
Nitric acid in the industry is widelyis used for the production of drugs, dyes, explosives, nitrogen fertilizers and nitric acid salts. In addition, it is used to dissolve metals (for example, copper, lead, silver) that do not react with other acids. In the jewelry business is used to determine gold in the alloy (this method is the main one).
