In the IV period of the system of chemical elements at number 25 is metal - manganese, along with technetium and rhenium, it forms a side subgroup of the seventh group. The element is widely distributed in the earth's crust and is part of various rocks: pyrolyusite, brownite, gausmanite. Manganese, the valency of which can be equal to II, III, IV, VI, VII, forms a large number of compounds. These are, for example, oxides, acids and salts, which play an important role in the metallurgical industry. In our article we will find out their properties, methods of production and application in various industries, in medicine and in everyday life.
Physical characteristic
According to the position in the periodic system of chemical elements of D. I. Mendeleev, metal refers to d-elements, in which not only two electrons of the last energy layer, but also negative particles located at the third level are valence. As is known, all d-elements exhibit the physicochemical properties of typical metals. Manganese is no exception. It has a silver-white color, hard and heavy (density is 7.44 g / cm 3 ), at a temperature below 727 ° it becomes brittle. The crystal lattice can form four modifications: α, β, ϒ and a cubic body-centered shape. All of them are persistent formations in only a certain temperature range.
Position in a row of activity of metals
The physicochemical properties of manganese depend not only on its place in the periodic system of chemical elements, but also on its position in the series of metal activity created by the Russian scientist N. N. Beketov. In it, manganese has a standard electrode potential of -1.179 V. The element is located between aluminum and zinc. This indicates the ability of Mn atoms to displace hydrogen ions from aqueous solutions of acids in redox processes. The valency of manganese in such reactions is usually equal to II. The element actively reacts with dilute solutions of nitrate and perchloric acids. Also, the metal is involved in the substitution reaction with hot concentrated sulfate acid. Sulfur dioxide and water are found in products. In addition to them, salt is formed - manganese sulfate. In it, the valency of a metal atom is two. But manganese does not dissolve in cold sulfuric acid due to the formation of a protective oxide film on its surface (metal passivation).
Characterization of divalent manganese compounds
Under the action of acids on natural compounds containing Mn 2+ ions or in the process of dissolving a simple substance - a metal in dilute solutions of inorganic acids, salts of divalent manganese are formed. For example, pink manganese chloride crystals can be obtained by the action of a solution of hydrochloric acid on manganese oxide, the valency of which is equal to IV. In the laboratory, acting on the corresponding salts with alkalis, a white precipitate of Mn (OH) 2 hydroxide is obtained. In an open test tube in air, it oxidizes, turning into a brown substance. Binary compound - divalent manganese oxide, is a product of hydrogen reduction of metal oxides with an oxidation state of +4 or +7.
Amphoteric properties of manganese dioxide
With an increase in valency in compounds of the metal with oxygen, weakening of the basic and strengthening of acidic properties is observed. So, the compound with the formula Mn 2 O 3 and manganese monoxide, in which the valency of the metal atom is II, are of the main character, the dioxide is amphoteric, and Mn 2 O 7 is manganese acid anhydride. In an environment where the pH is less than 7, the dioxide behaves as a strong oxidizing agent. It is in this quality that it is used in galvanic cells and in the industrial production of chlorine from hydrochloric acid. The reducing ability of manganese dioxide in reactions is very weak.
The most important compounds of hexavalent and hevalent manganese
Alloying metal dioxide with its salts - nitrates and carbonates, you can get a solid green color. It is highly soluble in water. Evaporating the solution, we obtain a dark green crystalline compound - potassium manganate, in which the valence of manganese is equal to VI. In reactions with strong oxidizing agents - fluorine, chlorine, the substance turns into a salt of manganese acid - potassium permanganate. Its dark violet crystals are well known, and in everyday life it is called potassium permanganate. Its solution has a raspberry color and in low concentrations is used as an excellent disinfectant for external use. The school chemistry course studies in detail redox processes in which potassium permanganate is used as an oxidizing agent. Various compounds are found in the products depending on the reaction of the solution (neutral, acidic or alkaline).
For example, in an acidic medium, when interacting with potassium sulfite, Mn +7 ions are reduced to Mn +2 , and manganese sulfate is obtained. A brown precipitate is observed in a neutral medium, and the valence of manganese in the oxide that has formed will be equal to IV. In an alkaline medium, the reaction of potassium sulfite and a salt of manganese acid ends with the appearance of potassium manganate ions in the solution.
Production and use of metal
The purest samples of manganese can be obtained during the electrolysis of a solution of its sulfate, which is formed during the reduction of pyrolusite, brownite, or gausmanite. As we said earlier, the valency of manganese in the compounds that make up the ores is equal to II. A cheaper way, in comparison with energy-intensive electrolysis, is the silicothermic process. It consists in the reaction of reduction of manganese oxides with silicon, which is carried out in electric furnaces. However, the purity of the obtained metal is reduced. Manganese is most in demand in the production of alloyed steel grades containing it up to 15%.
Such alloys are particularly durable and resistant to shock and vibration, so they are used in the manufacture of railway rails, parts of stone crushing machines and perforators. If the alloy contains not only manganese, but also magnesium, then it acquires the ability to withstand factors that cause various types of corrosion. Structural alloys - nickel silver and nickel silver, used in shipbuilding, production of pipelines and radiators, also contain a small part of manganese. In the composition of electrical alloys, for example, in manganin, the metal content reaches 12%, they are characterized by a low temperature index of electrical resistance and are widely used in technology.