One of the most important foods used by humans is nitric acid. The formula of the substance is HNO 3 , it also has a variety of physical and chemical characteristics that distinguish it from other inorganic acids. In our article we will study the properties of nitric acid, get acquainted with the methods for its preparation, and also consider the scope of the substance in various industries, medicine and agriculture.
Features of physical properties
The nitric acid obtained in the laboratory, the structural formula of which is given below, is a colorless liquid with an unpleasant odor, heavier than water. It evaporates quickly and has a low boiling point of +83 ° C. The compound is easily mixed with water in any proportions, forming solutions of various concentrations. Moreover, nitrate acid can absorb moisture from the air, that is, it is a hygroscopic substance. The structural formula of nitric acid is ambiguous, and can take two forms.
In a molecular form, nitrate acid does not exist. In aqueous solutions of various concentrations, the substance has the form of the following particles: H 3 O + - hydroxonium ions and anions of the acid residue - NO 3 - .
Acid-base interaction
Nitric acid, which is one of the strongest acids, enters the reactions of substitution, exchange, and neutralization. So, with basic oxides, the compound is involved in metabolic processes, resulting in salt and water. The neutralization reaction is the main chemical property of all acids. The products of the interaction of bases and acids will always be the corresponding salts and water:
NaOH + HNO 3 → NaNO 3 + H 2 O
Metal Reactions
In a molecule of nitric acid, the formula of which is HNO 3 , nitrogen exhibits the highest oxidation state of +5, so the substance has pronounced oxidizing properties. As a strong acid, it is able to interact with metals in the order of activity of metals to hydrogen. However, it, unlike other acids, can also react with passive metal elements, for example, with copper or silver. Reagents and reaction products are determined both by the concentration of the acid itself and by the activity of the metal.
Diluted nitric acid and its properties
If the mass fraction of HNO 3 is 0.4-0.6, then the compound exhibits all the properties of a strong acid. For example, it dissociates into hydrogen cations and anions of the acid residue. Indicators in an acidic environment, for example, purple litmus, in the presence of an excess of H + ions, changes its color to red. The most important feature of the reactions of nitrate acid with metals is the inability to release hydrogen, which is oxidized to water. Instead, various compounds are formed - nitrogen oxides. For example, during the interaction of silver with molecules of nitric acid, the formula of which is HNO 3 , nitrogen monoxide is detected, water and salt - silver nitrate. The degree of nitrogen oxidation in the complex anion decreases, since three electrons are attached.
With active metal elements, such as magnesium, zinc, calcium, nitric acid reacts with the formation of nitric oxide, the valency of which is the smallest, it is 1. It also forms salt and water:
4Mg + 10HNO 3 = NH 4 NO 3 + 4Mg (NO 3 ) 2 + 3H 2 O
If nitric acid, the chemical formula of which is HNO 3 , is very diluted, in this case, the products of its interaction with active metals will be different. It can be ammonia, free nitrogen or nitric oxide (I). It all depends on external factors, which include the degree of grinding of the metal and the temperature of the reaction mixture. For example, the equation of its interaction with zinc will have the following form:
Zn + 4HNO 3 = Zn (NO 3 ) 2 + 2NO 2 + 2H 2 O
Concentrated HNO 3 (96-98%) acid in reactions with metals is reduced to nitrogen dioxide, moreover, this usually does not depend on the position of the metal in N. Beketov's series. This happens in most cases, for example, when interacting with silver.
Remember the exception to the rule: concentrated nitric acid under normal conditions does not react with iron, aluminum and chromium, but passivates them. This means that a protective oxide film is formed on the surface of metals, which prevents their further contact with acid molecules. A mixture of a substance with concentrated perchloric acid in a ratio of 3: 1 is called aqua regia. She has the ability to dissolve gold.
How nitrate acid reacts with non-metals
Strong oxidizing properties of the substance lead to the fact that in its reactions with nonmetallic elements, the latter transform into the form of the corresponding acids. For example, sulfur is oxidized to sulfate, boron to boric, and phosphorus to phosphate acids. The following reaction equations confirm this:
S 0 + 2HN V O 3 → H 2 S VI O 4 + 2N II O
Getting nitric acid
The most convenient laboratory method for producing a substance is the interaction of nitrates with concentrated sulfate acid. It is carried out with gentle heating, not allowing the temperature to rise, since in this case the resulting product decomposes.
In industry, nitric acid can be produced in several ways. For example, the oxidation of ammonia obtained from nitrogen in air and hydrogen. Acid production takes place in several stages. Intermediates are nitrogen oxides. First, nitrogen monoxide NO is formed, then it is oxidized with atmospheric oxygen to nitrogen dioxide. Finally, in a reaction with water and an excess of oxygen, dilute (40-60%) nitrate acid is produced from NO 2 . If it is distilled with concentrated sulfate acid, it is possible to increase the mass fraction of HNO 3 in solution to 98.
The above method for the production of nitrate acid was first proposed by the founder of the nitrogen industry in Russia I. Andreev in the early 20th century.
Application
As we recall, the chemical formula of nitric acid is HNO 3 . What feature of chemical properties causes its use if nitrate acid is a large-tonnage product of chemical production? This is a high oxidizing ability of a substance. It is used in the pharmaceutical industry to obtain drugs. The substance serves as a raw material for the synthesis of explosive compounds, plastics, dyes. Nitric acid is used in military technology as an oxidizing agent for rocket fuel. Its large volume is used in the production of the most important types of nitrogen fertilizers - nitrate. They help increase the yield of the most important crops and increase the content of protein and green mass in fruits.
Applications for nitrates
Having considered the basic properties, the preparation and use of nitric acid, let us dwell on the use of its most important compounds - salts. They are not only mineral fertilizers, some of them are of great importance in the military industry. For example, a mixture of 75% potassium nitrate, 15% fine coal and 5% sulfur is called black powder. Ammonium, an explosive, is obtained from ammonium nitrate, as well as from coal and aluminum powder. An interesting property of nitrate salts is their ability to decompose upon heating.
Moreover, the reaction products will depend on which metal ion is part of the salt. If a metal element is in a series of activities to the left of magnesium, nitrites and free oxygen are found in the products. If the metal, which is part of the nitrate, is located from magnesium to copper, inclusive, then when the salt is heated, the formation of nitrogen dioxide, oxygen and oxide of the metal element. Salts of silver, gold or platinum at high temperature form free metal, oxygen and nitrogen dioxide.
In our article, we found out what the chemical formula of nitric acid is in chemistry, and what features of its oxidizing properties are most important.