A monobasic strong acid, which is a colorless liquid under standard conditions, which turns yellow during storage, can be in the solid state, characterized by two crystalline modifications (monoclinic or rhombic lattices), at temperatures below minus 41.6 ° C. This substance with the chemical formula - HNO3 - is called nitric acid. 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 accompanied by decomposition (partial): 4HNO3 → 2H2O + 4NO2 + O2. An acid solution with a mass fraction of the main substance equal to 68% boils at a temperature of 121 ° C. The refractive index of a pure substance is 1.397. The acid is able to mix with water in any ratio and, being a strong electrolyte, almost completely decomposes into H + and NO3- ions. The solid forms - trihydrate and monohydrate have the formulas: HNO3 • 3H2O and HNO3 • H2O, respectively.
Nitric acid is a corrosive, toxic substance and a strong oxidizing agent. Since the Middle Ages, such a name as “strong water” (Aqua fortis) has been known. Alchemists who discovered acid in the 13th century gave this name, making sure of its extraordinary properties (corroded all metals except gold), exceeding the strength of acetic acid, which at that time was considered the most active, a million times. But another three centuries later it was found that even a gold can be corroded by a mixture of acids such as nitric and hydrochloric in a volume ratio of 1: 3, which for this reason was called “aqua regia”. The appearance of a yellow tint during storage is explained by the accumulation of nitrogen oxides in it. On sale, acid is more often with 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 it from dilute sulfuric or hydrochloric acids in that HNO3 is a stronger oxidizing agent, so hydrogen is never released in reactions with metals. Due to its oxidizing properties, it also reacts with many non-metals. In both cases, nitrogen dioxide NO2 is always formed. In redox reactions, nitrogen reduction occurs to various degrees: HNO3, NO2, N2O3, NO, N2O, N2, NH3, which is determined by the concentration of acid and metal activity. The molecules of the resulting compounds contain nitrogen with an oxidation state of +5, +4, +3, +2, +1, 0, +3, respectively. For example, copper is oxidized by 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 nitric acid interacts with non-metals. Using the example of a 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 with nitric acid with the release of chlorine, and then the metals form complex chlorides.
Nitric acid on an industrial scale is obtained in three main ways:
- The first is the interaction of salts with sulfuric acid: H2SO4 + NaNO3 → HNO3 + NaHSO4. Previously, this method was the only one, but, with the advent of other technologies, it is currently used in the laboratory to produce fuming acid.
- The second is the arc method. When air is blown through an electric arc with a temperature of 3000 to 3500 ° C, part of the nitrogen in the air reacts with oxygen to form nitrogen monoxide: N2 + O2 → 2NO, which, after cooling, oxidizes to nitrogen dioxide (at high temperature, the monoxide does not interact with oxygen) : O2 + 2NO → 2NO2. Then, practically, all nitrogen dioxide, with an excess of oxygen, dissolves in water: 2H2O + 4NO2 + O2 → 4HNO3.
- The third is the ammonia method. Ammonia is oxidized on a platinum catalyst to nitrogen monoxide: 4NH3 + 5O2 → 4NO + 6H2O. The resulting nitrous gases are cooled and nitrogen dioxide is formed, which is absorbed by water. In this way, an acid with a concentration of from 60 to 62% is obtained.
Nitric acid is widely used in industry for the production of drugs, dyes, explosives, nitrogen fertilizers and salts of nitric acid. In addition, it is used to dissolve metals (for example, copper, lead, silver) that do not react with other acids. In jewelry, it is used to determine gold in an alloy (this is the main method).