Silicon compounds are indispensable. Silicon and its compounds - Knowledge Hypermarket. Distribution of silicon in nature

Silicon is a solid non-metal that is part of rocks. Under normal conditions it is inert, but when heated it exhibits oxidizing and reducing properties. The chemical properties of silicon are used by the silicate industry for the manufacture of glass, mirrors, electronics, and building materials.

General description of the element

Silicon is located in the fourth group and the third period of the periodic table. The nucleus of a silicon atom has a positive charge of +14. 14 negatively charged electrons move around the nucleus.

An atom can pass into an excited state due to the free d-sublevel. Therefore, the element exhibits two positive oxidation states (+2 and +4) and one negative (-4). Electronic configuration - 1s 2 2s 2 2p 6 3s 2 3p 2.

Rice. 1. The structure of the silicon atom.

Silicon is a fragile semiconductor with high boiling and board temperatures. A relatively light non-metal: the density is 2.33 g / cm 3.

Silicon is not found in its pure form. It is part of sand, quartz, agate, amethyst and other rocks.

Rice. 2. Agate.

Reactions with non-metals

When interacting with non-metals, silicon exhibits reducing properties - it gives up electrons. Reactions are possible only with strong heating. Under normal conditions, silicon only reacts with fluorine. Reactions with basic non-metals are shown in the table.

Silicon hydride - silane (SiH 4) - can be obtained by the decomposition of silicides with acid. For example, Mg 2 Si + 2H 2 SO 4 → SiH 4 - + 2MgSO 4.

Interaction with metals

Silicon exhibits oxidizing properties only in reactions with metals. During fusion, salts are formed - silicides:

• Si + 2Mg → Mg 2 Si;
• Si + 2Ca → Ca 2 Si;
• Si + Na → NaSi;
• 2Si + Fe → FeSi 2.

Silicides are used industrially for the production of alloys and materials. Vanadium silicide (V 3 Si) is used as a superconductor, and rhenium silicide (ReSi) is used as a semiconductor.

Reactions with complex substances

In addition to simple substances, silicon reacts with complex compounds - acids and alkalis. The main reactions are described in the table.

At 1200 ° C, silicon reacts with dioxide to form a monoxide: Si + SiO 2 → SiO.

Rice. 3. The use of silicon.

What have we learned?

Silicon is a brittle non-metal that interacts with metals, non-metals and complex substances. In reactions with metals, it exhibits the properties of an oxidizing agent, with non-metals - a reducing agent. Reacts under normal conditions only with fluorine, hydrofluoric acid (including together with nitric acid), with alkalis. The rest of the reactions take place at elevated temperatures.

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Silicon is a chemical element of group IV of the Periodic Table of the Elements of D.I. Mendeleev. Opened in 1811 by J. Gay-Lusac and L. Ternard. Its serial number is 14, atomic mass 28.08, atomic volume 12.04 10 -6 m 3 / mol. Silicon is a metalloid, belongs to the carbon subgroup. Its oxygen valence is +2 and +4. In terms of prevalence in nature, silicon is second only to oxygen. Its mass fraction in the earth's crust is 27.6%. The earth's crust, according to V.I. Vernadsky, more than 97% consists of silica and silicates. Oxygen and organic silicon compounds are also found in plants and animals.

Artificially produced silicon can be either amorphous or crystalline. Amorphous silicon is a brown, finely dispersed, highly hygroscopic powder; according to X-ray diffraction data, it consists of the smallest silicon crystals. It can be obtained by reduction at high temperatures of SiCl 4 with zinc vapor.

Crystalline silicon has a steel gray color and a metallic luster. The density of crystalline silicon at 20 ° C is 2.33 g / cm 3, liquid silicon at 1723 - 2.51, and at 1903K - 2.445 g / cm 3. The melting point of silicon is 1690 K, the boiling point is 3513 K. According to the data, the vapor pressure of silicon at T = 2500 ÷ 4000 K is described by the equation log p Si = -20130 / T + 7.736, kPa. Heat of sublimation of silicon 452610, melting 49790, evaporation 385020 J / mol.

Silicon polycrystals are characterized by high hardness (at 20 ° С HRC = 106). However, silicon is very fragile, therefore it has high compressive strength (σ SJ B ≈ 690 MPa) and very low tensile strength (σ B ≈ 16.7 MPa).

At room temperature, silicon is inert, reacts only with fluorine, forming volatile 81P4. Of the acids, it reacts only with nitric acid mixed with hydrofluoric acid. However, silicon reacts fairly easily with alkalis. One of his reactions with alkalis

Si + NaOH + H 2 O = Na 2 SiO 3 + 2H 2

used to produce hydrogen. Along with this, silicon is capable of producing a large number of chemically strong compounds with non-metals. Of these compounds, it is necessary to note halides (from SiX 4 to Si n X 2n + 2, where X is halogen, and n ≤ 25), their mixed compounds SiCl 3 B, SiFCl 3, etc., oxychlorides Si 2 OCl 3, Si 3 O 2 Cl 3 et al., nitrides Si 3 N 4, Si 2 N 3, SiN and hydrides with the general formula Si n H 2n + 2, and of the compounds found in the production of ferroalloys - volatile sulfides SiS and SiS 2 and refractory carbide SiC.

Silicon is also capable of producing compounds with metals - silicides, the most important of which are silicides of iron, chromium, manganese, molybdenum, zirconium, as well as rare earth metals and alkali metals. This property of silicon - the ability to give chemically very strong compounds and solutions with metals - is widely used in the technology for the production of low-carbon ferroalloys, as well as in the reduction of low-boiling alkaline earth (Ca, Mg, Ba) and hard-to-recover metals (Zr, Al, etc.).

Silicon-iron alloys were studied by P.V. Geld and his school, special attention was paid to the part of the Fe-Si system related to alloys with its high content. This is due to the fact that, as can be seen from the Fe-Si diagram (Figure 1), a number of transformations occur in alloys of this composition, which significantly affect the quality of ferrosilicon of various grades. Thus, FeSi 2 disilicide is stable only at low temperatures (< 918 или 968 °С, см. рисунок 1). При высоких температурах устойчива его высокотемпературная модификация - лебоит. Содержание кремния в этой фазе колеблется в пределах 53-56 %. В дальнейшем лебоит будем обозначать химической формулой Fe 2 Si 5 , что практически соответствует максимальной концентрации кремния в лебоите.

When cooling alloys with a Si content> 55.5%, leboite at T< 1213 К разлагается по эвтектоидной реакции

Fe 2 Si 5 → FeSi 2 + Si (2)

and alloys 33.86-50.07% Si at T< 1255 К - по перитектоидной реакции

Fe 2 Si 5 + FeSi = ЗFeSi 2 (3)

Alloys of intermediate composition (50.15-55.5% Si) first undergo peritectoid (3) at 1255 K and then, at 1213 K, eutectoid (2) transformations. These transformations of Fe 2 Si 5 by reactions (2) and (3) are accompanied by changes in the volume of the silicide. Such a change is especially large in the course of reaction (2) - about 14%; therefore, alloys containing leboite lose their continuity, crack, and even disintegrate. With slow, equilibrium crystallization (see Figure 1), leboite can precipitate during crystallization of both FS75 and FS45 alloys.

However, cracking associated with eutectoid decay of leboite is only one of the causes of disintegration. The second reason, apparently the main one, is that the formation of cracks along the grain boundaries creates an opportunity for liquates released along these boundaries - phosphorus, arsenic, aluminum sulfides and carbides, etc. - to react with air moisture by reactions, as a result of which in the atmosphere is released H 2, PH 3, PH 4, AsH 4, etc., and in the cracks - loose oxides Al 2 O 3, SiO 2 and other compounds, bursting them. Spreading of alloys can be prevented by modifying them with magnesium, alloying with additives of elements that refine the grain (V, Ti, Zg, etc.) or make it more plastic. Refinement of the grain reduces the concentration of impurities and their compounds at its boundaries and affects the properties of alloys in the same way as a general decrease in the concentration of impurities (P, Al, Ca) in the alloy, which promote disintegration. The thermodynamic properties of Fe-Si alloys (heat of mixing, activity, carbon solubility) have been studied in detail, they can be found in works. Information about the solubility of carbon in Fe-Si alloys is shown in Figure 2, about the activity of silicon - in Table 1.

The physicochemical properties of oxygen silicon compounds were studied by P.V. Geld with staff. Despite the importance of the Si-O system, its diagram has not yet been built. Currently, two oxygen compounds of silicon are known - silica SiO 2 and monoxide SiO. There are also indications in the literature about the existence of other oxygen compounds of silicon - Si 2 O 3 and Si 3 O 4, but there is no information on their chemical and physical properties.

In nature, silicon is represented only by silica SiO 2. This silicon compound is different:

1) high hardness (on the Mohs scale 7) and refractory (T pl = 1996 K);

2) high boiling point (T KIP = 3532 K). The vapor pressure of silica can be described by the equations (Pa):

3) the formation of a large number of modifications:

A feature of allotropic transformations of SiO 2 is that they are accompanied by significant changes in the density and volume of the substance, which can cause cracking and crushing of the rock;

4) a high tendency to hypothermia. Therefore, it is possible, as a result of rapid cooling, to fix the structure of both liquid melt (glass) and high-temperature modifications of β-cristobalite and tridymite. On the contrary, with rapid heating, quartz can be melted, bypassing the structures of tridymite and cristobalite. The melting point of SiO 2 in this case decreases by about 100 ° C;

5) high electrical resistance. For example, at 293 K it is 1 10 12 Ohm * m. However, as the temperature rises, the electrical resistance of SiO 2 decreases, and in the liquid state, silica is a good conductor;

6) high viscosity. So, at 2073 K the viscosity is 1 10 4 Pa ​​s, and at 2273 K - 280 Pa s.

The latter, according to N.V. Solomin, is explained by the fact that SiO 2, like organic polymers, is capable of forming chains, which at 2073 K consist of 700, and at 2273 K - of 590 SiO 2 molecules;

7) high thermal stability. The Gibbs energy of formation of SiO 2 from elements, taking into account their aggregate state, in accordance with the data, is described with high accuracy by the equations:

These data, as can be seen from Table 2, slightly differ from the authors' data. Two-term equations can also be used for thermodynamic calculations:

Silicon monoxide SiO was discovered in 1895 by Potter in the gas phase of electric furnaces. It has now been reliably established that SiO also exists in condensed phases. According to the research of P.V. Gelda, oxide is characterized by low density (2.15 g / cm 3), high electrical resistance (10 5 -10 6 Ohm * m). The condensed oxide is brittle, its hardness according to the Mohs scale is ~ 5. The melting point, due to its high volatility, could not be determined experimentally. According to O. Kubashevsky, it is equal to 1875 K, according to Berezhny, - 1883 K. The heat of fusion of SiO is several times higher than ΔH 0 SiO2 according to the data it is equal to 50242 J / mol. Apparently, owing to volatility, it is overestimated. It has a glassy fracture, its color changes from white to chocolate, which is probably due to its oxidation with atmospheric oxygen. Fresh fracture SiO usually has a pea color with a greasy sheen. The oxide is thermodynamically stable only at high temperatures in the form of SiO (G). When cooled, the oxide disproportionates according to the reaction

2SiO (G) = SiO (L) + SiO 2 (6)

The boiling point of SiO can be roughly estimated from the equation:

Silicon oxide gas is thermodynamically very stable. Gibbs energy of its formation can be described by equations (see table 2):

from which it can be seen that the chemical strength of SiO, like CO, increases with increasing temperature, which makes it an excellent reducing agent for many substances.

Two-term equations can also be used for thermodynamic analysis:

The composition of gases over SiO 2 was estimated by I.S. Kulikov. Depending on the temperature, the content of SiO over SiO 2 is described by the equations:

Silicon carbide, like SiO, is one of the intermediate compounds formed during the reduction of SiO 2. Carbide has a high melting point.

Depending on the pressure, it is resistant up to 3033-3103 K (Figure 3). At high temperatures, silicon carbide sublimates. However, the vapor pressure of Si (G), Si 2 C (G), SiC 2 (G) over carbide at T< 2800К невелико, что следует из уравнения

Carbide exists in the form of two modifications - cubic low-temperature β-SiC and hexagonal high-temperature α-SiC. In ferroalloy furnaces, only β-SiC is usually found. Calculations using the data have shown that the Gibbs energy of formation is described by the equations:

which are markedly different from the data. It follows from these equations that carbide is thermally stable up to 3194 K. According to its physical properties, carbide is distinguished by high hardness (~ 10), high electrical resistance (at 1273K p≈0.13 ⋅ 10 4 μOhm ⋅ m), increased density (3.22 g / cm 3) and high resistance both in a reducing and oxidizing atmosphere.

In appearance, pure carbide is colorless and has semiconducting properties that are retained even at high temperatures. Technical silicon carbide contains impurities and is therefore colored green or black. So, green carbide contains 0.5-1.3% impurities (0.1-0.3% C, 0.2-1.2% Si + SiO 2, 0.05-0.20% Fe 2 O 3 , 0.01-0.08% Al 2 O 3, etc.). In black carbide, the content of impurities is higher (1-2%).

Carbon is used as a reducing agent in the production of silicon alloys. It is also the main substance from which electrodes and lining of electric furnaces are made, melting silicon and its alloys. Carbon is quite common in nature, its content in the earth's crust is 0.14%. In nature, it occurs both in a free state and in the form of organic and inorganic compounds (mainly carbonates).

Carbon (graphite) has a hexagonal cubic lattice. The X-ray density of graphite is 2.666 g / cm 3, the pycnometric density is 2.253 g / cm 3. It is distinguished by high melting (~ 4000 ° C) and boiling points (~ 4200 ° C), electrical resistance increasing with increasing temperature (at 873 K p≈9.6 μΩ⋅m, at 2273 K p≈ 15.0 μΩ⋅m) is quite durable. Its temporary resistance on the whiskers can be 480-500 MPa. However, electrode graphite has σ b = 3.4 ÷ 17.2 MPa. The hardness of graphite on the Mohs scale is ~ 1.

Carbon is an excellent reducing agent. This is due to the fact that the strength of one of its oxygen compounds (CO) increases with increasing temperature. This can be seen from the Gibbs energy of its formation, which, as shown by our calculations using the data, is well described as a three-term

and two-term equations:

Carbon dioxide CO 2 is thermodynamically strong only up to 1300 K. Gibbs energy of CO 2 formation is described by the equations:

As an independent chemical element, silicon became known to mankind only in 1825. Which, of course, did not prevent the use of silicon compounds in such a number of spheres that it is easier to list those where the element is not used. This article will shed light on the physical, mechanical and useful chemical properties of silicon and its compounds, applications, and we will also talk about how silicon affects the properties of steel and other metals.

First, let's focus on the general characteristics of silicon. From 27.6 to 29.5% of the mass of the earth's crust is silicon. In seawater, the concentration of the element is also hefty - up to 3 mg / l.

Silicon occupies the second place of honor in the lithosphere after oxygen. However, its most famous form, silica, is dioxide, and it is its properties that have become the basis for such widespread use.

This video will tell you what silicon is:

Concept and features

Silicon is a non-metal, but under different conditions it can exhibit both acidic and basic properties. It is a typical semiconductor and is widely used in electrical engineering. Its physical and chemical properties are largely determined by the allotropic state. Most often, they deal with the crystalline form, since its qualities are more in demand in the national economy.

• Silicon is one of the basic macronutrients in the human body. Its lack has a detrimental effect on the condition of bone tissue, hair, skin, nails. In addition, silicon affects the performance of the immune system.
• In medicine, the element, or rather, its compounds, found their first application in this capacity. Water from wells lined with silicon differed not only in purity, but also had a positive effect on resistance to infectious diseases. Today, a compound with silicon serves as the basis for drugs against tuberculosis, atherosclerosis, and arthritis.
• In general, the non-metal is inactive, however, it is difficult to find it in its pure form. This is due to the fact that in air it is quickly passivated by a layer of dioxide and stops reacting. When heated, the chemical activity increases. As a result, humanity is much more familiar with the compounds of the substance, and not with itself.

So, silicon forms alloys with almost all metals - silicides. All of them differ in their refractoriness and hardness and are used in the respective areas: gas turbines, furnace heaters.

The non-metal is located in the table of D.I. Mendeleev in group 6 together with carbon, germanium, which indicates a certain commonality with these substances. So, with carbon it is "related" by the ability to form compounds of the type of organic. At the same time, silicon, like germanium, can exhibit the properties of a metal in some chemical reactions, which is used in synthesis.

Pros and cons

Like any other substance from the point of view of use in the national economy, silicon has certain useful or not very good qualities. They are important precisely for determining the scope of use.

• A considerable advantage of the substance is its availability... In nature, it is true, it is not in a free form, but nevertheless, the technology for producing silicon is not so complicated, although it is energy-consuming.
• The second most important advantage is formation of many compounds with unusually useful properties. These are silanes, silicides, dioxide, and, of course, various silicates. The ability of silicon and its compounds to form complex solid solutions is practically endless, which allows endless production of the most varied variations of glass, stone and ceramics.
• Semiconductor properties non-metal provides it with a place as a base material in electrical and radio engineering.
• Non-metal is non-toxic, which allows application in any industry, and at the same time does not turn the technological process into a potentially hazardous one.

The disadvantages of the material include only relative fragility with good hardness. Silicon is not used for supporting structures, but this combination allows the crystal surface to be properly processed, which is important for instrumentation.

Let's now talk about the basic properties of silicon.

Properties and characteristics

Since crystalline silicon is most often used in industry, it is its properties that are more important, and it is they that are given in the technical specifications. The physical properties of the substance are as follows:

• melting point - 1417 C;
• boiling point - 2600 C;
• the density is 2.33 g / cc. cm, which indicates fragility;
• the heat capacity, as well as the thermal conductivity, are not constant even in the purest samples: 800 J / (kg K), or 0.191 cal / (g deg) and 84-126 W / (m K), or 0.20-0, 30 cal / (cm · sec · deg), respectively;
• transparent to long-wave infrared radiation, which is used in infrared optics;
• dielectric constant - 1.17;
• hardness on the Mohs scale - 7.

The electrical properties of a non-metal are highly dependent on impurities. In industry, this feature is used by modulating the desired type of semiconductor. At normal temperatures, silicon is brittle, but when heated above 800 C, plastic deformation is possible.

The properties of amorphous silicon are strikingly different: it is highly hygroscopic, and reacts much more actively even at normal temperatures.

The structure and chemical composition, as well as the properties of silicon, are discussed in the video below:

Composition and structure

Silicon exists in two allotropic forms, which are equally stable at normal temperatures.

• Crystal looks like a dark gray powder. The substance, although it has a diamond-like crystal lattice, is fragile due to the excessively long bond between atoms. Its properties of a semiconductor are of interest.
• At very high pressures, you can get hexagonal modification with a density of 2.55 g / cc. see. However, this phase has not yet found practical significance.
• Amorphous- brown-brown powder. In contrast to the crystalline form, it reacts much more actively. This is due not so much to the inertness of the first form as to the fact that in air the substance is covered with a layer of dioxide.

In addition, it is necessary to take into account another type of classification associated with the size of the silicon crystal, which together form a substance. The crystal lattice, as is known, presupposes the ordering not only of atoms, but also of the structures that these atoms form - the so-called long-range order. The larger it is, the more homogeneous in properties the substance will be.

• Monocrystalline- the sample is one crystal. Its structure is maximally ordered, its properties are uniform and well predictable. It is this material that is most in demand in electrical engineering. However, it also belongs to the most expensive species, since the process of obtaining it is complicated, and the growth rate is low.
• Multicrystalline- the sample is a certain amount of large crystalline grains. The boundaries between them form additional defect levels, which reduces the performance of the sample as a semiconductor and leads to faster wear. The technology for growing a multicrystal is simpler, and therefore the material is cheaper.
• Polycrystalline- consists of a large number of grains located randomly relative to each other. It is the purest type of industrial silicon used in microelectronics and solar energy. Quite often it is used as a raw material for growing multi- and single crystals.
• Amorphous silicon also occupies a separate position in this classification. Here, the order of arrangement of atoms is maintained only at the shortest distances. However, in electrical engineering, it is still used in the form of thin films.

Non-metal production

It is not so easy to obtain pure silicon, given the inertness of its compounds and the high melting points of most of them. In industry, carbon reduction from dioxide is most often resorted to. The reaction is carried out in arc furnaces at a temperature of 1800 C. Thus, a non-metal with a purity of 99.9% is obtained, which is not enough for its use.

The resulting material is chlorinated in order to obtain chlorides and hydrochlorides. Then the compounds are purified from impurities by all possible methods and reduced with hydrogen.

The substance can also be purified by obtaining magnesium silicide. The silicide is exposed to hydrochloric or acetic acid. Silane is obtained, and the latter is purified by various methods - sorption, rectification, and so on. Then the silane is decomposed into hydrogen and silicon at a temperature of 1000 C. In this case, a substance is obtained with an impurity fraction of 10 -8 -10 -6%.

Application of the substance

For industry, the most interesting are the electrophysical characteristics of a nonmetal. Its monocrystalline form is an indirect-gap semiconductor. Its properties are determined by impurities, which makes it possible to obtain silicon crystals with desired properties. So, the addition of boron, indium makes it possible to grow a crystal with hole conductivity, and the introduction of phosphorus or arsenic - a crystal with electronic conductivity.

• Silicon is literally the foundation of modern electrical engineering. Transistors, photocells, integrated circuits, diodes and so on are made from it. Moreover, the functionality of the device is almost always determined only by the near-surface layer of the crystal, which determines very specific requirements for the surface treatment.
• In metallurgy, technical silicon is used both as a modifier of alloys - it imparts greater strength, and as a component - in, for example, and as a deoxidizer - in the production of cast iron.
• Ultra-pure and refined metallurgical forms the basis of solar energy.
• Non-metal dioxide occurs naturally in very different forms. Its crystalline varieties - opal, agate, carnelian, amethyst, rock crystal - have found their place in jewelry. Not so attractive outwardly modifications - flint, quartz, are used in metallurgy, and in construction, and in radio-electrical engineering.
• The compound of a non-metal with carbon - carbide, is used in metallurgy, and in instrument making, and in the chemical industry. It is a wide-gap semiconductor, has a high hardness - 7 on the Mohs scale, and strength, which allows it to be used as an abrasive material.
• Silicates - that is, silicic acid salts. Unstable, easily decomposed under the influence of temperature. They are remarkable for the fact that they form numerous and varied salts. But the latter are the basis for the production of glass, ceramics, earthenware, crystal, etc. We can safely say that modern construction is based on a variety of silicates.
• Glass is the most interesting case here. It is based on aluminosilicates, but negligible impurities of other substances - usually oxides, give the material a lot of different properties, including color. -, faience, porcelain, in fact, has the same formula, albeit with a different ratio of components, and its diversity is also amazing.
• Non-metal has another ability: it forms compounds like carbon, in the form of a long chain of silicon atoms. Such compounds are called organosilicon compounds. The scope of their application is no less well-known - these are silicones, sealants, lubricants, and so on.

Silicon is a very widespread element and is of extraordinary importance in very many areas of the national economy. Moreover, not only the substance itself is actively used, but all its various and numerous compounds.

This video will tell you about the properties and uses of silicon:

silicates. Among them, the most common are aluminosilicates (it is clear that these silicates contain aluminum). Aluminosilicates include granite, various types of clays, mica. The non-aluminum silicate is, for example, asbestos.

SiO2 oxide is essential for plant and animal life. It gives strength to plant stems and animal protective covers. Fish scales, insect shells, butterfly wings, bird feathers and animal hair are durable because they contain silica.

3) Rhinestone

Rock crystal is a colorless, transparent, usually chemically pure, almost free of impurities type of low-temperature modification of quartz - SiO2, crystallizing in a trigonal system. It is found in the form of single crystals or crystals of a prismatic-hexagonal shape, collected in druses, with a mass sometimes reaching a ton or more.

Quartz is one of the most abundant minerals in the earth's crust, the rock-forming mineral of most igneous and metamorphic rocks. Chemical formula: SiO2.

Varieties of quartz: colorless, rose quartz, "hairy", carnelian, agate, "tiger's eye", polished pebbles.

5) Carnelian Formula - SiO2, a kind of chalcedony. Chemical composition - SiO2 content - 90-99%; impurities of Fe2O3, Al2O3, MgO, CaO, H2O are noted. Carnelian, like agates, are aggregates of an essentially chalcedony composition of a complex structure.

Jasper is an opaque variety of quartz - silicon dioxide SiO2 - with a fibrous structure that includes a wide variety of minerals: garnets, hematite, pyrite, etc. Therefore, jasper is distinguished by a wide variety of its color, including all tones except pure blue.

7) Amethyst

Amethysts are violet or reddish quartz crystals, which are silicon dioxide and belong to the trigonal crystal system.

Opal is an amorphous type of quartz SiO2 with a variable water content (6-10%). The chemical name for opal is silicon dioxide polyhydrate. The main advantage of opal is the ability to emit successively different rays under the influence of sunlight, to cause a varied play of colors. There are three types of opal: black opal, which has a very dark blue color with "flashes" of colors; fire opal orange-red and white opal.

7) Citrine The name of the stone, derived from the word citreus - "lemon", indicates the yellow tint of this kind of quartz, which gives citrine ferric impurities. Citrine is good for concentration and concentration.

Jade is a translucent white and green mineral. From a mineralogical point of view, jade is a silica compound.

9) Agate is a type of translucent quartz. Chemical formula: SiO2.

Application of silicon compounds:

Silicon is used in the silicate industry:

Natural silicon compounds - sand (SiO2) and silicates are used for the production of ceramics, glass and cement.

Silicate glue is widely known, used in construction as a drier, and in pyrotechnics and in everyday life for gluing paper.

Silicone oils and silicones have become widespread - materials based on organosilicon compounds.

54) Physical and chemical bases of corrosion of concrete and mineral materials.

Corrosion of concrete is the main enemy of all mineral building materials and structures (concrete, reinforced concrete, brick, asbestos cement, silicate, foam concrete and aerated concrete blocks). The most serious problem is the influence of the atmospheric-chemical factor - the impact of aggressive atmospheric substances (carbonates, sulfates, chlorides), as well as frequent freeze-thaw cycles.

Mineral-based building materials are capillary-porous. As a result of aggressive atmospheric action, crystals are formed inside the porous structure, the growth of which leads to the appearance of cracks. As a result of exposure to water, salts and carbon dioxide - corrosion of concrete and destruction of building structures.

Mineral surface protection is a global challenge in the design, construction and operation of any facility. It is relevant for all types of buildings, structures and structures used in modern construction.

Element characteristic

14 Si 1s 2 2s 2 2p 6 3s 2 3p 2

Isotopes: 28 Si (92.27%); 29 Si (4.68%); 30 Si (3.05%)

Silicon is the second most abundant element in the earth's crust after oxygen (27.6% by weight). It does not occur in the free state in nature, it is found mainly in the form of SiO 2 or silicates.

Si compounds are toxic; inhalation of the smallest particles of SiO 2 and other silicon compounds (for example, asbestos) causes a dangerous disease - silicosis

In the ground state, the silicon atom has valence = II, and in the excited state = IV.

The most stable oxidation state for Si is +4. In compounds with metals (silicides) S.O. -4.

Methods for obtaining silicon

The most common natural silicon compound is silica (silicon dioxide) SiO 2. It is the main raw material for silicon production.

1) Reduction of SiO 2 with carbon in arc furnaces at 1800 "C: SiO 2 + 2C = Si + 2CO

2) High-purity Si from a technical product is obtained according to the scheme:

a) Si → SiCl 2 → Si

b) Si → Mg 2 Si → SiH 4 → Si

Physical properties of silicon. Allotropic modifications of silicon

1) Crystalline silicon is a silvery-gray substance with a metallic luster, a crystal lattice like a diamond; t. pl. 1415 "C, bp 3249" C, density 2.33 g / cc; is a semiconductor.

2) Amorphous silicon is a brown powder.

Silicon chemical properties

In most reactions, Si acts as a reducing agent:

At low temperatures, silicon is chemically inert; when heated, its reactivity increases sharply.

1.It interacts with oxygen at T above 400 ° C:

Si + O 2 = SiO 2 silicon oxide

2.Reacts with fluorine already at room temperature:

Si + 2F 2 = SiF 4 silicon tetrafluoride

3.With the rest of the halogens, reactions take place at a temperature of = 300 - 500 ° С

Si + 2Hal 2 = SiHal 4

4.With sulfur vapor at 600 ° C forms a disulfide:

5. The reaction with nitrogen occurs above 1000 ° C:

3Si + 2N 2 = Si 3 N 4 silicon nitride

6. At a temperature = 1150 ° C, it reacts with carbon:

SiO 2 + 3C = SiC + 2CO

Carborundum is close to diamond in hardness.

7. Silicon does not directly react with hydrogen.

8. Silicon is resistant to acids. Interacts only with a mixture of nitric and hydrofluoric (hydrofluoric) acids:

3Si + 12HF + 4HNO 3 = 3SiF 4 + 4NO + 8H 2 O

9.reacts with alkali solutions to form silicates and release hydrogen:

Si + 2NaOH + H 2 O = Na 2 SiO 3 + 2H 2

10. The reducing properties of silicon are used to isolate metals from their oxides:

2MgO = Si = 2Mg + SiO 2

In reactions with metals, Si is an oxidizing agent:

Silicon forms silicides with s-metals and most d-metals.

The composition of the silicides of this metal can be different. (For example, FeSi and FeSi 2; Ni 2 Si and NiSi 2.) One of the most famous silicides is magnesium silicide, which can be obtained by direct interaction of simple substances:

2Mg + Si = Mg 2 Si

Silane (monosilane) SiH 4

Silanes (silicas) Si n H 2n + 2, (compare with alkanes), where n = 1-8. Silanes are analogs of alkanes, differing from them in the instability of the -Si-Si- chains.

Monosilane SiH 4 is a colorless gas with an unpleasant odor; dissolves in ethanol, gasoline.

Methods of obtaining:

1. Decomposition of magnesium silicide with hydrochloric acid: Mg 2 Si + 4HCI = 2MgCI 2 + SiH 4

2. Reduction of Si halides with lithium aluminum hydride: SiCl 4 + LiAlH 4 = SiH 4 + LiCl + AlCl 3

Chemical properties.

Silane is a powerful reducing agent.

1.SiH 4 is oxidized by oxygen even at very low temperatures:

SiH 4 + 2O 2 = SiO 2 + 2H 2 O

2. SiH 4 readily hydrolyzes, especially in an alkaline environment:

SiH 4 + 2H 2 O = SiO 2 + 4H 2

SiH 4 + 2NaOH + H 2 O = Na 2 SiO 3 + 4H 2

Silicon (IV) oxide (silica) SiO 2

Silica exists in various forms: crystalline, amorphous and glassy. The most common crystalline form is quartz. When quartz rocks are destroyed, quartz sands are formed. Quartz monocrystals are transparent, colorless (rock crystal) or colored with impurities in various colors (amethyst, agate, jasper, etc.).

Amorphous SiO 2 occurs in the form of the opal mineral: silica gel is artificially obtained, consisting of colloidal SiO 2 particles and is a very good adsorbent. Glassy SiO 2 is known as quartz glass.

Physical properties

SiO 2 dissolves very slightly in water, and practically does not dissolve in organic solvents. Silica is a dielectric.

Chemical properties

1. SiO 2 is an acidic oxide, therefore amorphous silica slowly dissolves in aqueous solutions of alkalis:

SiO 2 + 2NaOH = Na 2 SiO 3 + H 2 O

2. SiO 2 also interacts when heated with basic oxides:

SiO 2 + K 2 O = K 2 SiO 3;

SiO 2 + CaO = CaSiO 3

3. Being a non-volatile oxide, SiO 2 displaces carbon dioxide from Na 2 CO 3 (by fusion):

SiO 2 + Na 2 CO 3 = Na 2 SiO 3 + CO 2

4. Silica reacts with hydrofluoric acid, forming hydrofluorosilicic acid H 2 SiF 6:

SiO 2 + 6HF = H 2 SiF 6 + 2H 2 O

5. At 250 - 400 ° С SiO 2 interacts with gaseous HF and F 2, forming tetrafluorosilane (silicon tetrafluoride):

SiO 2 + 4HF (gas) = ​​SiF 4 + 2H 2 O

SiO 2 + 2F 2 = SiF 4 + O 2

Silicic acid

Known:

Orthosilicic acid H 4 SiO 4;

Metasilicic (silicic) acid H 2 SiO 3;

Di- and polysilicic acids.

All silicic acids are slightly soluble in water and easily form colloidal solutions.

Methods of obtaining

1. Precipitation with acids from solutions of alkali metal silicates:

Na 2 SiO 3 + 2HCl = H 2 SiO 3 ↓ + 2NaCl

2. Hydrolysis of chlorosilanes: SiCl 4 + 4H 2 O = H 4 SiO 4 + 4HCl

Chemical properties

Silicic acids are very weak acids (weaker than carbonic acid).

When heated, they dehydrate to form silica as the final product.

H 4 SiO 4 → H 2 SiO 3 → SiO 2

Silicates - silicic acid salts

Since silicic acids are extremely weak, their salts in aqueous solutions are highly hydrolyzed:

Na 2 SiO 3 + H 2 O = NaHSiO 3 + NaOH

SiO 3 2- + H 2 O = HSiO 3 - + OH - (alkaline medium)

For the same reason, when carbon dioxide is passed through silicate solutions, silicic acid is displaced from them:

K 2 SiO 3 + CO 2 + H 2 O = H 2 SiO 3 ↓ + K 2 CO 3

SiO 3 + CO 2 + H 2 O = H 2 SiO 3 ↓ + CO 3

This reaction can be considered as a qualitative reaction for silicate ions.

Among silicates, only Na 2 SiO 3 and K 2 SiO 3 are highly soluble, which are called soluble glass, and their aqueous solutions are called liquid glass.

Glass

Ordinary window glass has the composition Na 2 O CaO 6SiO 2, that is, it is a mixture of sodium and calcium silicates. It is obtained by fusing soda Na 2 CO 3, limestone CaCO 3 and sand SiO 2;

Na 2 CO 3 + CaCO 3 + 6SiO 2 = Na 2 O CaO 6SiO 2 + 2CO 2

Cement

A powdery binder that forms a plastic mass when interacting with water, which over time turns into a solid stone-like body; basic building material.

The chemical composition of the most common Portland cement (in% by weight) is 20 - 23% SiO 2; 62 - 76% CaO; 4 - 7% Al 2 O 3; 2-5% Fe 2 O 3; 1-5% MgO.