Physical and chemical properties of silicon and carbon and their compounds. Chemical properties of carbon and silicon

Description and properties of silicon

Silicon is an element, the fourth group, the third period in the table of elements. Atomic number 14. silicon formula— 3s2 3p2. Defined as an element in 1811, and in 1834 received the Russian name "silicon", instead of the former "sicily". Melts at 1414º C, boils at 2349º C.

It resembles in molecular structure, but is inferior to it in hardness. Quite brittle, in a heated state (at least 800º C) acquires plasticity. Illuminated by infrared light. Monocrystalline type of silicon has semiconductor properties. According to some characteristics silicon atom similar to the atomic structure of carbon. silicon electrons have the same valence number as in the carbon structure.

workers silicon properties depend on the contents of certain contents in it. Silicon has a different type of conductivity. In particular, this is a "hole" and "electronic" type. To obtain the first, boron is added to silicon. If add phosphorus, silicon acquires the second type of conductivity. If silicon is heated together with other metals, specific compounds called "silicides" are formed, for example, in the reaction " magnesium-silicon«.

Silicon, which is used for the needs of electronics, is primarily evaluated by the characteristics of its upper layers. Therefore, it is necessary to pay attention to their quality, it is directly reflected in the overall performance. The operation of the manufactured device depends on them. To obtain the most acceptable performance of the upper layers of silicon, they are treated with various chemical methods or subjected to irradiation.

Compound "sulphur-silicon", forms silicon sulfide, which easily interacts with water and oxygen. When reacting with oxygen, under temperature conditions above 400º C, it turns out silica. At the same temperature, reactions with chlorine and iodine, as well as with bromine, become possible, during which volatile substances are formed - tetrahalides.

It will not work to combine silicon and hydrogen by direct contact; there are indirect methods for this. At 1000º C, a reaction with nitrogen is possible, as well as boron, resulting in silicon nitride and silicon boride. At the same temperature, by combining silicon with carbon, one can produce silicon carbide, the so-called "carborundum". This composition has a solid structure, chemical activity is sluggish. Used as an abrasive.

In conjunction with iron, silicon forms a special mixture, this allows the melting of these elements, which forms a ferrosilicon ceramic. Moreover, its melting point is much lower than if they are melted separately. At temperatures above 1200º C, the element begins to form silicon oxide, also under certain conditions it turns out silicon hydroxide. When etching silicon, alkaline water-based solutions are used. Their temperature must be at least 60º C.

Deposits and mining of silicon

The element is the second most common on the planet substance. Silicon makes up almost a third of the volume of the earth's crust. Only oxygen is more common. It is predominantly expressed by silica - a compound containing silicon dioxide at its core. The main derivatives of silicon dioxide are flint, various sands, quartz, and also field ones. They are followed by silicate compounds of silicon. Nativeness for silicon is a rare phenomenon.

Application of silicon

Silicon, chemical properties which determine the scope of its application, is divided into several types. Less pure silicon is used for metallurgical needs: for, for example, for additives in aluminium, silicon actively changes its properties, deoxidizers, etc. It actively modifies the properties of metals by adding to their compound. Silicon alloys them, changing the working characteristics, silicon quite a small amount is sufficient.

Also, higher-quality derivatives are produced from crude silicon, in particular, mono- and polycrystalline silicon, as well as silicon organics - these are silicones and various organic oils. It also found its application in the production of cement and the glass industry. He did not bypass brick production, factories producing porcelain and also cannot do without it.

Silicon is part of the well-known silicate adhesive, which is used for repair work, and earlier it was used in office needs, until more practical substitutes appeared. Some pyrotechnic products also contain silicon. Hydrogen can be obtained from it and its iron alloys in the open air.

What is better quality silicon? plates solar cells also include silicon, naturally not technical. For these needs, silicon of ideal purity is required, or at least technical silicon of the highest degree of purification.

So-called "electronic silicon", which contains almost 100% silicon, has much better performance. Therefore, it is preferred in the production of ultra-precise electronic devices and complex microcircuits. In their manufacture, high-quality production is required. circuit, silicon for which only the highest category should go. The operation of these devices depends on how much contains silicon unwanted impurities.

Silicon occupies an important place in nature, and most living beings constantly need it. For them, this is a kind of building compound, because it is extremely important for the health of the musculoskeletal system. Every day a person absorbs up to 1 g silicon compounds.

Can silicon be harmful?

Yes, for the reason that silicon dioxide is extremely prone to dusting. It has an irritating effect on the mucous surfaces of the body and can actively accumulate in the lungs, causing silicosis. To do this, in the production associated with the processing of silicon elements, the use of respirators is mandatory. Their presence is especially important when it comes to silicon monoxide.

silicon price

As you know, all modern electronic equipment, from telecommunications to computer technology, is based on the use of silicon, using its semiconductor properties. Its other counterparts are used to a much lesser extent. The unique properties of silicon and its derivatives are still out of competition for many years to come. Despite the decline in prices in 2001 for silicon, sales quickly bounced back. And already in 2003, the trade turnover amounted to 24 thousand tons per year.

For the latest technologies that require almost crystal-clear silicon, its technical counterparts are not suitable. And due to its complex cleaning system, the price accordingly increases significantly. The polycrystalline type of silicon is more common, its single-crystal prototype is somewhat less in demand. At the same time, the share of the use of silicon for semiconductors occupies the lion's share of the turnover.

Product prices vary depending on purity and purpose. silicon, buy which, you can start from 10 cents per kg of crude raw materials and up to $ 10 and above for "electronic" silicon.

Introduction

2.1.1 +2 oxidation state

2.1.2 +4 oxidation state

2.3 Metal carbides

Chapter 3. Silicon Compounds

Bibliography

Introduction

Chemistry is one of the branches of natural science, the subject of which is the chemical elements (atoms), the simple and complex substances (molecules) they form, their transformations and the laws that these transformations obey.

By definition, D.I. Mendeleev (1871), "chemistry in its present state can ... be called the doctrine of the elements."

The origin of the word "chemistry" is not completely clear. Many researchers believe that it comes from the ancient name of Egypt - Hemia (Greek Chemia, found in Plutarch), which is derived from "hem" or "hame" - black and means "science of the black earth" (Egypt), "Egyptian science".

Modern chemistry is closely connected both with other natural sciences and with all branches of the national economy.

The qualitative feature of the chemical form of the motion of matter, and its transitions to other forms of motion, determines the versatility of chemical science and its connection with areas of knowledge that study both lower and higher forms of motion. The knowledge of the chemical form of the motion of matter enriches the general doctrine of the development of nature, the evolution of matter in the Universe, and contributes to the formation of an integral materialistic picture of the world. The contact of chemistry with other sciences gives rise to specific areas of their mutual penetration. Thus, the areas of transition between chemistry and physics are represented by physical chemistry and chemical physics. Between chemistry and biology, chemistry and geology, special border areas arose - geochemistry, biochemistry, biogeochemistry, molecular biology. The most important laws of chemistry are formulated in mathematical language, and theoretical chemistry cannot develop without mathematics. Chemistry has exerted and is exerting an influence on the development of philosophy, and has itself experienced and is experiencing its influence.

Historically, two main branches of chemistry have developed: inorganic chemistry, which studies primarily the chemical elements and the simple and complex substances they form (except carbon compounds), and organic chemistry, the subject of which is the compounds of carbon with other elements (organic substances).

Until the end of the 18th century, the terms "inorganic chemistry" and "organic chemistry" indicated only from which "kingdom" of nature (mineral, plant or animal) certain compounds were obtained. Starting from the 19th century. these terms have come to indicate the presence or absence of carbon in a given substance. Then they acquired a new, broader meaning. Inorganic chemistry comes into contact primarily with geochemistry and then with mineralogy and geology, i.e. with the sciences of inorganic nature. Organic chemistry is a branch of chemistry that studies a variety of carbon compounds up to the most complex biopolymer substances. Through organic and bioorganic chemistry, chemistry borders on biochemistry and further on biology, i.e. with the totality of the sciences of living nature. At the junction between inorganic and organic chemistry is the area of organoelement compounds.

In chemistry, ideas about the structural levels of the organization of matter gradually formed. The complication of a substance, starting from the lowest, atomic, goes through the steps of molecular, macromolecular, or high-molecular compounds (polymer), then intermolecular (complex, clathrate, catenane), and finally, diverse macrostructures (crystal, micelle) up to indefinite non-stoichiometric formations. The corresponding disciplines gradually developed and became isolated: the chemistry of complex compounds, polymers, crystal chemistry, the study of dispersed systems and surface phenomena, alloys, etc.

The study of chemical objects and phenomena by physical methods, the establishment of patterns of chemical transformations, based on the general principles of physics, underlies physical chemistry. This area of chemistry includes a number of largely independent disciplines: chemical thermodynamics, chemical kinetics, electrochemistry, colloid chemistry, quantum chemistry and the study of the structure and properties of molecules, ions, radicals, radiation chemistry, photochemistry, the doctrine of catalysis, chemical equilibrium, solutions and others. Analytical chemistry acquired an independent character , whose methods are widely used in all areas of chemistry and the chemical industry. In the areas of practical application of chemistry, such sciences and scientific disciplines as chemical technology with its many branches, metallurgy, agricultural chemistry, medical chemistry, forensic chemistry, etc., arose.

As mentioned above, chemistry considers the chemical elements and the substances they form, as well as the laws that govern these transformations. One of these aspects (namely, chemical compounds based on silicon and carbon) will be considered by me in this paper.

Chapter 1. Silicon and carbon - chemical elements

1.1 Introduction to carbon and silicon

Carbon (C) and silicon (Si) are members of the IVA group.

Carbon is not a very common element. Despite this, its significance is enormous. Carbon is the basis of life on earth. It is part of carbonates (Ca, Zn, Mg, Fe, etc.) that are very common in nature, exists in the atmosphere in the form of CO 2, occurs in the form of natural coals (amorphous graphite), oil and natural gas, as well as simple substances ( diamond, graphite).

Silicon is the second most abundant element in the earth's crust (after oxygen). If carbon is the basis of life, then silicon is the basis of the earth's crust. It is found in a huge variety of silicates (Fig. 4) and aluminosilicates, sand.

Amorphous silicon is a brown powder. The latter is easy to obtain in the crystalline state in the form of gray hard, but rather brittle crystals. Crystalline silicon is a semiconductor.

Table 1. General chemical data on carbon and silicon.

The modification of carbon stable at ordinary temperature - graphite - is an opaque, gray greasy mass. Diamond - the hardest substance on earth - is colorless and transparent. The crystal structures of graphite and diamond are shown in Fig.1.

Figure 1. The structure of a diamond (a); graphite structure (b)

Carbon and silicon have their own specific derivatives.

Table 2. The most characteristic derivatives of carbon and silicon

1.2 Preparation, chemical properties and use of simple substances

Silicon is obtained by reduction of oxides with carbon; to obtain in an especially pure state after reduction, the substance is transferred to the tetrachloride and again reduced (with hydrogen). Then it is melted into ingots and subjected to cleaning by zone melting. An ingot of metal is heated from one end so that a zone of molten metal is formed in it. When the zone moves to the other end of the ingot, the impurity, dissolving in the molten metal better than in the solid one, is removed, and thus the metal is purified.

Carbon is inert, but at a very high temperature (in the amorphous state) it interacts with most metals to form solid solutions or carbides (CaC 2, Fe 3 C, etc.), as well as with many metalloids, for example:

2C + Ca \u003d CaC 2, C + 3Fe \u003d Fe 3 C,

Silicon is more reactive. It reacts with fluorine already at ordinary temperature: Si + 2F 2 \u003d SiF 4

Silicon has a very high affinity for oxygen as well:

The reaction with chlorine and sulfur proceeds at about 500 K. At very high temperatures, silicon interacts with nitrogen and carbon:

Silicon does not interact directly with hydrogen. Silicon dissolves in alkalis:

Si + 2NaOH + H 2 0 \u003d Na 2 Si0 3 + 2H 2.

Acids other than hydrofluoric do not affect it. With HF there is a reaction

Si+6HF=H 2 +2H 2 .

Carbon in the composition of various coals, oil, natural (mainly CH4), as well as artificially obtained gases is the most important fuel base of our planet

Graphite is widely used to make crucibles. Graphite rods are used as electrodes. A lot of graphite goes to the production of pencils. Carbon and silicon are used to produce various grades of cast iron. In metallurgy, carbon is used as a reducing agent, and silicon, due to its high affinity for oxygen, as a deoxidizer. Crystalline silicon in an especially pure state (no more than 10 -9 at.% impurity) is used as a semiconductor in various devices and devices, including as transistors and thermistors (devices for very fine temperature measurements), as well as in photocells, the operation of which It is based on the ability of a semiconductor to conduct current when illuminated.

Chapter 2. Chemical compounds of carbon

Carbon is characterized by strong covalent bonds between its own atoms (C-C) and with the hydrogen atom (C-H), which is reflected in the abundance of organic compounds (several hundred million). In addition to strong C-H, C-C bonds in various classes of organic and inorganic compounds, carbon bonds with nitrogen, sulfur, oxygen, halogens, and metals are widely represented (see Table 5). Such high possibilities of bond formation are due to the small size of the carbon atom, which allows its valence orbitals 2s 2 , 2p 2 to overlap as much as possible. The most important inorganic compounds are described in Table 3.

Among inorganic carbon compounds, nitrogen-containing derivatives are unique in composition and structure.

In inorganic chemistry, derivatives of acetic CH3COOH and oxalic H 2 C 2 O 4 acids are widely represented - acetates (type M "CH3COO) and oxalates (type M I 2 C 2 O 4).

Table 3. The most important inorganic compounds of carbon.

2.1 Oxygen derivatives of carbon

2.1.1 +2 oxidation state

Carbon monoxide CO (carbon monoxide): according to the structure of molecular orbitals (Table 4).

CO is similar to the N 2 molecule. Like nitrogen, CO has a high dissociation energy (1069 kJ/mol), has a low Tm (69 K) and Tbp (81.5 K), is poorly soluble in water, and is chemically inert. CO reacts only at high temperatures, including:

CO + Cl 2 \u003d COCl 2 (phosgene),

CO + Br 2 \u003d SOVg 2, Cr + 6CO \u003d Cr (CO) 6 -chromium carbonyl,

Ni + 4CO \u003d Ni (CO) 4 - nickel carbonyl

CO + H 2 0 pairs \u003d HCOOH (formic acid).

At the same time, the CO molecule has a high affinity for oxygen:

CO +1/202 \u003d C0 2 +282 kJ / mol.

Due to its high affinity for oxygen, carbon monoxide (II) is used as a reducing agent for the oxides of many heavy metals (Fe, Co, Pb, etc.). In the laboratory, CO oxide is obtained by dehydrating formic acid.

In technology, carbon monoxide (II) is obtained by reducing CO 2 with coal (C + CO 2 \u003d 2CO) or by oxidizing methane (2CH 4 + 3O 2 \u003d \u003d 4H 2 0 + 2CO).

Among CO derivatives, metal carbonyls are of great theoretical and certain practical interest (for obtaining pure metals).

Chemical bonds in carbonyls are formed mainly by the donor-acceptor mechanism due to free orbitals d- element and the electron pair of the CO molecule, there is also n-overlapping by the dative mechanism (metal CO). All metal carbonyls are diamagnetic substances characterized by low strength. Like carbon monoxide (II), metal carbonyls are toxic.

Table 4. Distribution of electrons over the orbitals of the CO molecule

2.1.2 +4 oxidation state

Carbon dioxide CO 2 (carbon dioxide). The CO 2 molecule is linear. The energy scheme for the formation of orbitals of the CO 2 molecule is shown in Fig. 2. Carbon monoxide (IV) can react with ammonia in a reaction.

When this salt is heated, a valuable fertilizer is obtained - carbamide CO (MH 2) 2:

Urea is decomposed by water

CO (NH 2) 2 + 2HaO \u003d (MH 4) 2COz.

Figure 2. Energy diagram of the formation of CO 2 molecular orbitals.

In technology, CO 2 oxide is obtained by decomposition of calcium carbonate or sodium bicarbonate:

In laboratory conditions, it is usually obtained by reaction (in the Kipp apparatus)

CaCO3 + 2HC1 = CaC12 + CO2 + H20.

The most important derivatives of CO 2 are weak carbonic acid H 2 CO s and its salts: M I 2 CO 3 and M I HC 3 (carbonates and bicarbonates, respectively).

Most carbonates are insoluble in water. Water-soluble carbonates undergo significant hydrolysis:

COz 2- + H 2 0 COz- + OH - (I stage).

Due to complete hydrolysis, carbonates Cr 3+ , ai 3 + , Ti 4+ , Zr 4+ and others cannot be isolated from aqueous solutions.

Practically important are Ka 2 CO3 (soda), K 2 CO3 (potash) and CaCO3 (chalk, marble, limestone). Bicarbonates, unlike carbonates, are soluble in water. Of the bicarbonates, NaHCO 3 (baking soda) finds practical application. Important basic carbonates are 2CuCO3-Cu (OH) 2 , PbCO 3 X XPb (OH) 2 .

The properties of carbon halides are given in Table 6. Of the carbon halides, the most important is a colorless, rather toxic liquid. Under normal conditions, CCI 4 is chemically inert. It is used as a non-flammable and non-flammable solvent for resins, varnishes, fats, as well as for obtaining freon CF 2 CI 2 (T bp = 303 K):

Another organic solvent used in practice is carbon disulfide CSa (colorless, volatile liquid with Tbp = 319 K) - a reactive substance:

CS 2 +30 2 \u003d C0 2 + 2S0 2 +258 kcal / mol,

CS 2 + 3Cl 2 \u003d CCl 4 -S 2 Cl 2, CS 2 + 2H 2 0 \u003d\u003d C0 2 + 2H 2 S, CS 2 + K 2 S \u003d K 2 CS 3 (salt of thiocarbonic acid H 2 CSz).

Vapors of carbon disulfide are poisonous.

Hydrocyanic (hydrocyanic) acid HCN (H-C \u003d N) is a colorless, easily mobile liquid, boiling at 299.5 K. At 283 K, it solidifies. HCN and its derivatives are extremely poisonous. HCN can be obtained by the reaction

Hydrocyanic acid dissolves in water; at the same time, it weakly dissociates

HCN=H++CN-, K=6.2.10-10.

Hydrocyanic acid salts (cyanides) in some reactions resemble chlorides. For example, CH - -ion with Ag + ions gives a white precipitate of silver cyanide AgCN, poorly soluble in mineral acids. Cyanides of alkali and alkaline earth metals are soluble in water. Due to hydrolysis, their solutions smell of hydrocyanic acid (the smell of bitter almonds). Heavy metal cyanides are poorly soluble in water. CN is a strong ligand, the most important complex compounds are K 4 and Kz [Re (CN) 6].

Cyanides are fragile compounds, with prolonged exposure to CO 2 contained in the air, cyanides decompose

2KCN+C0 2 +H 2 0=K 2 C0 3 +2HCN.

(CN) 2 - cyanogen (N=C-C=N) -

colorless poisonous gas; interacts with water to form cyanic (HOCN) and hydrocyanic (HCN) acids:

(HCN) acids:

(CN) 2 + H 2 0 \u003d\u003d HOCN + HCN.

In this, as in the reaction below, (CN) 2 is similar to a halogen:

CO + (CN) 2 \u003d CO (CN) 2 (analogue of phosgene).

Cyanic acid is known in two tautomeric forms:

H-N=C=O==H-0-C=N.

The isomer is the acid H-0=N=C (explosive acid). HONC salts explode (used as detonators). Rhodohydrogen acid HSCN is a colorless, oily, volatile, easily solidifying liquid (Tm=278 K). In the pure state, it is very unstable; when it decomposes, HCN is released. Unlike hydrocyanic acid, HSCN is a rather strong acid (K=0.14). HSCN is characterized by tautomeric equilibrium:

H-N \u003d C \u003d S \u003d H-S-C \u003d N.

SCN - blood-red ion (reagent for Fe 3+ ion). HSCN-derived rhodanide salts - easily obtained from cyanides by addition of sulfur:

Most thiocyanates are soluble in water. Salts of Hg, Au, Ag, Cu are insoluble in water. The SCN- ion, like CN-, tends to give complexes of the type M3 1 M "(SCN) 6, where M" "Cu, Mg and some others. Dirodan (SCN) 2 - light yellow crystals, melting - 271 K. Get (SCN) 2 by reaction

2AgSCN+Br 2 ==2AgBr+ (SCN) 2 .

Of the other nitrogen-containing compounds, cyanamide should be indicated.

and its derivative - calcium cyanamide CaCN 2 (Ca=N-C=N), which is used as a fertilizer.

2.3 Metal carbides

Carbides are the products of the interaction of carbon with metals, silicon and boron. By solubility, carbides are divided into two classes: carbides that are soluble in water (or dilute acids) and carbides that are insoluble in water (or dilute acids).

2.3.1 Carbides soluble in water and dilute acids

A. Carbides forming C 2 H 2 when dissolved This group includes the carbides of the metals of the first two main groups; close to them are the carbides Zn, Cd, La, Ce, Th of the composition MC 2 (LaC 2 , CeC 2 , ТhC 2 .)

CaC 2 + 2H 2 0 \u003d Ca (OH) 2 + C 2 H 2, ThC 2 + 4H 2 0 \u003d Th (OH) 4 + H 2 C 2 + H 2.

ANSz + 12H 2 0 \u003d 4Al (OH) s + ZSN 4, Be 2 C + 4H 2 0 \u003d 2Be (OH) 2 + CH 4. According to their properties, Mn z C is close to them:

Mn s C + 6H 2 0 \u003d ZMn (OH) 2 + CH 4 + H 2.

B. Carbides, which, when dissolved, form a mixture of hydrocarbons and hydrogen. These include most rare earth metal carbides.

2.3.2 Carbides insoluble in water and in dilute acids

This group includes most transition metal carbides (W, Mo, Ta, etc.), as well as SiC, B 4 C.

They dissolve in oxidizing environments, for example:

VC + 3HN0 3 + 6HF \u003d HVF 6 + CO 2 + 3NO + 4H 2 0, SiC + 4KOH + 2C0 2 \u003d K 2 Si0 3 + K 2 C0 3 + 2H 2 0.

Figure 3. Icosahedron B 12

Practically important are transition metal carbides, as well as silicon carbides SiC and boron B 4 C. SiC - carborundum - colorless crystals with a diamond lattice, approaching diamond in hardness (technical SiC has a dark color due to impurities). SiC is highly refractory, thermally conductive and electrically conductive at high temperature, extremely chemically inert; it can only be destroyed by fusion in air with alkalis.

B 4 C - polymer. The boron carbide lattice is built from linearly arranged three carbon atoms and groups containing 12 B atoms arranged in the form of an icosahedron (Fig. 3); the hardness of B4C is higher than that of SiC.

Chapter 3. Silicon Compounds

The difference between the chemistry of silicon and carbon is mainly due to the large size of its atom and the possibility of using free 3d orbitals. Due to additional binding (according to the donor-acceptor mechanism), silicon bonds with oxygen Si-O-Si and fluorine Si-F (Table 17.23) are stronger than those of carbon, and due to the larger size of the Si atom compared to the atom The Si-H and Si-Si bonds are less strong than those of carbon. Silicon atoms are practically incapable of forming chains. The homologous series of silicon hydrogens SinH2n+2 (silanes) analogous to hydrocarbons was obtained only up to the composition Si4Hio. Due to the larger size of the Si atom, the ability to n-overlapping is also weakly expressed, therefore, not only triple, but also double bonds are of little character for it.

When silicon interacts with metals, silicides are formed (Ca 2 Si, Mg 2 Si, BaSi 2, Cr 3 Si, CrSi 2, etc.), similar in many respects to carbides. Silicides are not characteristic of group I elements (except for Li). Silicon halides (Table 5) are stronger compounds than carbon halides; however, they are decomposed by water.

Table 5. Strength of some bonds of carbon and silicon

The most durable silicon halide is SiF 4 (it decomposes only under the action of an electric discharge), but, like other halides, it undergoes hydrolysis. When SiF 4 interacts with HF, hexafluorosilicic acid is formed:

SiF 4 +2HF=H 2 .

H 2 SiF 6 is close in strength to H 2 S0 4 . Derivatives of this acid - fluorosilicates, as a rule, are soluble in water. Alkali metal fluorosilicates (except for Li and NH 4) are poorly soluble. Fluorosilicates are used as pesticides (insecticides).

Practically important halide is SiCO 4 . It is used to obtain organosilicon compounds. So, SiCL 4 easily interacts with alcohols to form silicic acid esters HaSiO 3:

SiCl 4 + 4C 2 H 5 OH \u003d Si (OC 2 H 5) 4 + 4HCl 4

Table 6. Carbon and silicon halides

Silicic acid esters, hydrolyzing, form silicones - polymeric substances of a chain structure:

(R-organic radical), which have found application in the production of rubbers, oils and lubricants.

Silicon sulfide (SiS 2) n-polymer substance; stable at normal temperature; decomposed by water:

SiS 2 + ZN 2 O \u003d 2H 2 S + H 2 SiO 3.

3.1 Oxygen silicon compounds

The most important oxygen compound of silicon is silicon dioxide SiO 2 (silica), which has several crystalline modifications.

Low-temperature modification (up to 1143 K) is called quartz. Quartz has piezoelectric properties. Natural varieties of quartz: rock crystal, topaz, amethyst. Varieties of silica are chalcedony, opal, agate,. jasper, sand.

Silica is chemically resistant; only fluorine, hydrofluoric acid and alkali solutions act on it. It easily passes into a glassy state (quartz glass). Quartz glass is brittle, chemically and thermally quite resistant. Silicic acid corresponding to SiO 2 does not have a definite composition. Silicic acid is usually written as xH 2 O-ySiO 2 . Silicic acids have been isolated: H 2 SiO 3 (H 2 O-SiO 2) - metasilicon (tri-oxosilicon), H 4 Si0 4 (2H 2 0-Si0 2) - orthosilicon (tetra-oxosilicon), H 2 Si2O 5 (H 2 O * SiO 2) - dimethosilicon.

Silicic acids are poorly soluble substances. In accordance with the less metalloid nature of silicon compared to carbon, H 2 SiO 3 as an electrolyte is weaker than H 2 CO3.

The silicate salts corresponding to silicic acids are insoluble in water (except alkali metal silicates). Soluble silicates are hydrolyzed according to the equation

2SiOz 2 - + H 2 0 \u003d Si 2 O 5 2 - + 20H-.

Concentrated solutions of soluble silicates are called liquid glass. Ordinary window glass, sodium and calcium silicate, has the composition Na 2 0-CaO-6Si0 2 . It is obtained from the reaction

A wide variety of silicates (more precisely, oxosilicates) is known. A certain regularity is observed in the structure of oxosilicates: they all consist of Si0 4 tetrahedra, which are connected to each other through an oxygen atom. The most common combinations of tetrahedra are (Si 2 O 7 6 -), (Si 3 O 9) 6 -, (Si 4 0 l2) 8-, (Si 6 O 18 12 -), which, as structural units, can be combined into chains, tapes, meshes and frames (Fig. 4).

The most important natural silicates are, for example, talc (3MgO * H 2 0-4Si0 2) and asbestos (SmgO*H 2 O*SiO 2). Like SiO 2 , silicates are characterized by a glassy (amorphous) state. With controlled crystallization of glass, it is possible to obtain a finely crystalline state (sitalls). Sitalls are characterized by increased strength.

In addition to silicates, aluminosilicates are widely distributed in nature. Aluminosilicates - frame oxosilicates, in which some of the silicon atoms are replaced by trivalent Al; for example Na 12 [(Si, Al) 0 4] 12.

For silicic acid, a colloidal state is characteristic when exposed to its salts of acids H 2 SiO 3 does not precipitate immediately. Colloidal solutions of silicic acid (sols) under certain conditions (for example, when heated) can be converted into a transparent, homogeneous gelatinous mass-gel of silicic acid. Gels are high-molecular compounds with a spatial, very loose structure formed by Si0 2 molecules, the voids of which are filled with H 2 O molecules. When silicic acid gels are dehydrated, silica gel is obtained - a porous product with a high adsorption capacity.

Figure 4. The structure of silicates.

findings

Having examined chemical compounds based on silicon and carbon in my work, I came to the conclusion that carbon, being a quantitatively not very common element, is the most important component of earthly life, its compounds exist in air, oil, and also in such simple substances as diamond and graphite. One of the most important characteristics of carbon is strong covalent bonds between atoms, as well as the hydrogen atom. The most important inorganic compounds of carbon are: oxides, acids, salts, halides, nitrogen-containing derivatives, sulfides, carbides.

Speaking of silicon, it is necessary to note the large amounts of its reserves on earth, it is the basis of the earth's crust and is found in a huge variety of silicates, sand, etc. At present, the use of silicon due to its semiconductor properties is on the rise. It is used in electronics in the manufacture of computer processors, microcircuits and chips. Silicon compounds with metals form silicides, the most important oxygen compound of silicon is silicon oxide SiO 2 (silica). In nature, there is a wide variety of silicates - talc, asbestos, aluminosilicates are also common.

Bibliography

1. Great Soviet Encyclopedia. Third edition. T.28. - M.: Soviet Encyclopedia, 1970.

2. Zhiryakov V.G. Organic chemistry. 4th ed. - M., "Chemistry", 1971.

3. Brief chemical encyclopedia. - M. "Soviet Encyclopedia", 1967.

4. General chemistry / Ed. EAT. Sokolovskaya, L.S. Guzeya. 3rd ed. - M.: Publishing House of Moscow. un-ta, 1989.

5. The world of inanimate nature. - M., "Science", 1983.

6. Potapov V.M., Tatarinchik S.N. Organic chemistry. Textbook.4th ed. - M.: "Chemistry", 1989.

One of the most common elements in nature is silicium, or silicon. Such a wide distribution speaks of the importance and significance of this substance. This was quickly understood and adopted by people who learned how to properly use silicon for their own purposes. Its application is based on special properties, which we will talk about later.

Silicon - chemical element

If we characterize this element by position in the periodic system, then we can identify the following important points:

The serial number is 14.
The period is the third small.
Group - IV.
The subgroup is the main one.
The structure of the outer electron shell is expressed by the formula 3s 2 3p 2 .
The element silicon is represented by the chemical symbol Si, which is pronounced "silicium".
The oxidation states it exhibits are: -4; +2; +4.
The valence of an atom is IV.
The atomic mass of silicon is 28.086.
In nature, there are three stable isotopes of this element with mass numbers 28, 29 and 30.

Thus, from a chemical point of view, the silicon atom is a sufficiently studied element, many of its various properties have been described.

Discovery history

Since various compounds of the element under consideration are very popular and massive in content in nature, from ancient times people used and knew about the properties of just many of them. Pure silicon for a long time remained beyond the knowledge of man in chemistry.

The most popular compounds used in everyday life and industry by the peoples of ancient cultures (Egyptians, Romans, Chinese, Russians, Persians and others) were precious and ornamental stones based on silicon oxide. These include:

opal;
rhinestone;
topaz;
chrysoprase;
onyx;
chalcedony and others.

Since ancient times, it has been customary to use quartz in the construction business. However, elemental silicon itself remained undiscovered until the 19th century, although many scientists tried in vain to isolate it from various compounds, using catalysts, high temperatures, and even electric current. These are such bright minds as:

Carl Scheele;
Gay-Lussac;
Thenar;
Humphrey Davy;
Antoine Lavoisier.

Jens Jacobs Berzelius succeeded in obtaining pure silicon in 1823. To do this, he conducted an experiment on the fusion of vapors of silicon fluoride and metallic potassium. As a result, he received an amorphous modification of the element in question. The same scientist proposed a Latin name for the discovered atom.

A little later, in 1855, another scientist - Saint Clair-Deville - managed to synthesize another allotropic variety - crystalline silicon. Since then, knowledge about this element and its properties began to grow very quickly. People realized that it has unique features that can be very intelligently used to meet their own needs. Therefore, today one of the most demanded elements in electronics and technology is silicon. Its use only expands its boundaries every year.

The Russian name for the atom was given by the scientist Hess in 1831. That is what has stuck to this day.

Silicon is the second most abundant in nature after oxygen. Its percentage in comparison with other atoms in the composition of the earth's crust is 29.5%. In addition, carbon and silicon are two special elements that can form chains by connecting with each other. That is why more than 400 different natural minerals are known for the latter, in the composition of which it is contained in the lithosphere, hydrosphere and biomass.

Where exactly is silicon found?

In deep layers of soil.
In rocks, deposits and massifs.
At the bottom of water bodies, especially seas and oceans.
In plants and marine inhabitants of the animal kingdom.
In humans and land animals.

It is possible to designate several of the most common minerals and rocks, in which silicon is present in large quantities. Their chemistry is such that the mass content of a pure element in them reaches 75%. However, the specific figure depends on the type of material. So, rocks and minerals containing silicon:

feldspars;
mica;
amphiboles;
opals;
chalcedony;
silicates;
sandstones;
aluminosilicates;
clay and others.

Accumulating in the shells and external skeletons of marine animals, silicon eventually forms powerful deposits of silica at the bottom of water bodies. This is one of the natural sources of this element.

In addition, it was found that silicium can exist in a pure native form - in the form of crystals. But such deposits are very rare.

Physical properties of silicon

If we characterize the element under consideration by a set of physicochemical properties, then first of all, it is the physical parameters that should be designated. Here are a few main ones:

It exists in the form of two allotropic modifications - amorphous and crystalline, which differ in all properties.
The crystal lattice is very similar to that of diamond, because carbon and silicon are almost the same in this respect. However, the distance between the atoms is different (silicon has more), so the diamond is much harder and stronger. Lattice type - cubic face-centered.
The substance is very brittle, at high temperatures it becomes plastic.
The melting point is 1415˚С.
Boiling point - 3250˚С.
The density of the substance is 2.33 g / cm 3.
The color of the compound is silver-gray, a characteristic metallic sheen is expressed.
It has good semiconductor properties, which can vary with the addition of certain agents.
Insoluble in water, organic solvents and acids.
Specifically soluble in alkalis.

The designated physical properties of silicon allow people to control it and use it to create various products. For example, the use of pure silicon in electronics is based on the properties of semiconductivity.

Chemical properties

The chemical properties of silicon are highly dependent on the reaction conditions. If we talk about at standard parameters, then we need to designate a very low activity. Both crystalline and amorphous silicon are very inert. They do not interact with strong oxidizing agents (except fluorine) or with strong reducing agents.

This is due to the fact that an oxide film of SiO 2 is instantly formed on the surface of the substance, which prevents further interactions. It can be formed under the influence of water, air, vapors.

If, however, the standard conditions are changed and silicon is heated to a temperature above 400˚С, then its chemical activity will greatly increase. In this case, it will react with:

oxygen;
all kinds of halogens;
hydrogen.

With a further increase in temperature, the formation of products in the interaction with boron, nitrogen, and carbon is possible. Of particular importance is carborundum - SiC, as it is a good abrasive material.

Also, the chemical properties of silicon are clearly seen in reactions with metals. In relation to them, it is an oxidizing agent, therefore the products are called silicides. Similar compounds are known for:

alkaline;
alkaline earth;
transition metals.

The compound obtained by fusing iron and silicon has unusual properties. It is called ferrosilicon ceramics and is successfully used in industry.

Silicon does not interact with complex substances, therefore, of all their varieties, it can dissolve only in:

aqua regia (a mixture of nitric and hydrochloric acids);
caustic alkalis.

In this case, the temperature of the solution should be at least 60 ° C. All this once again confirms the physical basis of the substance - a diamond-like stable crystal lattice, which gives it strength and inertness.

How to get

Obtaining silicon in its pure form is a rather costly process economically. In addition, due to its properties, any method gives only 90-99% pure product, while impurities in the form of metals and carbon remain the same. So just getting the substance is not enough. It should also be qualitatively cleaned of foreign elements.

In general, the production of silicon is carried out in two main ways:

From white sand, which is pure silicon oxide SiO 2 . When it is calcined with active metals (most often with magnesium), a free element is formed in the form of an amorphous modification. The purity of this method is high, the product is obtained with a 99.9 percent yield.
A more widespread method on an industrial scale is the sintering of molten sand with coke in specialized thermal kilns. This method was developed by the Russian scientist N. N. Beketov.

Further processing consists in subjecting the products to purification methods. For this, acids or halogens (chlorine, fluorine) are used.

Amorphous silicon

The characterization of silicon will be incomplete if each of its allotropic modifications is not considered separately. The first one is amorphous. In this state, the substance we are considering is a brown-brown powder, finely dispersed. It has a high degree of hygroscopicity, exhibits a sufficiently high chemical activity when heated. Under standard conditions, it is able to interact only with the strongest oxidizing agent - fluorine.

Calling amorphous silicon just a kind of crystalline is not entirely correct. Its lattice shows that this substance is only a form of finely dispersed silicon that exists in the form of crystals. Therefore, as such, these modifications are one and the same compound.

However, their properties differ, and therefore it is customary to speak of allotropy. By itself, amorphous silicon has a high light absorption capacity. In addition, under certain conditions, this indicator is several times higher than that of the crystalline form. Therefore, it is used for technical purposes. In the considered form (powder), the compound is easily applied to any surface, be it plastic or glass. Therefore, it is amorphous silicon that is so convenient for use. The application is based on different sizes.

Although the wear of batteries of this type is quite fast, which is associated with abrasion of a thin film of the substance, however, the use and demand is only growing. Indeed, even in a short service life, solar cells based on amorphous silicon are able to provide energy to entire enterprises. In addition, the production of such a substance is waste-free, which makes it very economical.

This modification is obtained by reducing compounds with active metals, for example, sodium or magnesium.

Crystalline silicon

Silver-gray shiny modification of the element in question. It is this form that is the most common and most in demand. This is due to the set of qualitative properties that this substance possesses.

The characteristic of silicon with a crystal lattice includes a classification of its types, since there are several of them:

Electronic quality - the purest and highest quality. It is this type that is used in electronics to create especially sensitive devices.
Solar quality. The name itself defines the area of use. It is also a high-purity silicon, the use of which is necessary to create high-quality and long-lasting solar cells. Photovoltaic converters created on the basis of a crystalline structure are of higher quality and wear resistance than those created using an amorphous modification by deposition on various types of substrates.
Technical silicon. This variety includes those samples of a substance that contain about 98% of the pure element. Everything else goes to various kinds of impurities:

aluminum;
chlorine;
carbon;
phosphorus and others.

The last variety of the substance under consideration is used to obtain silicon polycrystals. For this, recrystallization processes are carried out. As a result, in terms of purity, products are obtained that can be attributed to the groups of solar and electronic quality.

By its nature, polysilicon is an intermediate product between the amorphous modification and the crystalline one. This option is easier to work with, it is better processed and cleaned with fluorine and chlorine.

The resulting products can be classified as follows:

multisilicon;
monocrystalline;
profiled crystals;
silicon scrap;
technical silicon;
production waste in the form of fragments and scraps of matter.

Each of them finds application in industry and is used by a person completely. Therefore, those related to silicon are considered waste-free. This significantly reduces its economic cost, without affecting the quality.

The use of pure silicon

Silicon production in the industry is established quite well, and its scale is quite voluminous. This is due to the fact that this element, both pure and in the form of various compounds, is widespread and in demand in various branches of science and technology.

Where is crystalline and amorphous silicon used in its pure form?

In metallurgy as an alloying additive capable of changing the properties of metals and their alloys. So, it is used in the smelting of steel and iron.
Different types of substances are used to produce a cleaner version - polysilicon.
Silicon compounds with are a whole chemical industry that has gained particular popularity today. Silicone materials are used in medicine, in the manufacture of dishes, tools and much more.
Manufacture of various solar panels. This method of obtaining energy is one of the most promising in the future. Environmentally friendly, cost-effective and durable - the main advantages of such electricity generation.
Silicon for lighters has been used for a very long time. Even in ancient times, people used flint to create a spark when lighting a fire. This principle is the basis for the production of lighters of various kinds. Today there are species in which flint is replaced by an alloy of a certain composition, which gives an even faster result (sparking).
Electronics and solar energy.
Manufacture of mirrors in gas laser devices.

Thus, pure silicon has a lot of advantageous and special properties that allow it to be used to create important and necessary products.

The use of silicon compounds

In addition to a simple substance, various silicon compounds are also used, and very widely. There is a whole branch of industry called silicate. It is she who is based on the use of various substances, which include this amazing element. What are these compounds and what is produced from them?

Quartz, or river sand - SiO 2. It is used for the manufacture of building and decorative materials such as cement and glass. Where these materials are used, everyone knows. No construction is complete without these components, which confirms the importance of silicon compounds.
Silicate ceramics, which includes materials such as faience, porcelain, brick and products based on them. These components are used in medicine, in the manufacture of dishes, decorative ornaments, household items, in construction and other household areas of human activity.
- silicones, silica gels, silicone oils.
Silicate glue - used as stationery, in pyrotechnics and construction.

Silicon, the price of which varies on the world market, but does not cross the mark of 100 Russian rubles per kilogram (per crystalline) from top to bottom, is a sought-after and valuable substance. Naturally, compounds of this element are also widespread and applicable.

The biological role of silicon

From the point of view of significance for the body, silicon is important. Its content and distribution in tissues is as follows:

0.002% - muscle;
0.000017% - bone;
blood - 3.9 mg / l.

Every day, about one gram of silicon should get inside, otherwise diseases will begin to develop. There are no deadly ones among them, however, prolonged silicon starvation leads to:

hair loss;
the appearance of acne and pimples;
fragility and fragility of bones;
easy capillary permeability;
fatigue and headaches;
the appearance of numerous bruises and bruises.

For plants, silicon is an important trace element necessary for normal growth and development. Animal experiments have shown that those individuals who consume a sufficient amount of silicon daily grow better.

A brief comparative description of the elements of carbon and silicon is presented in table 6.

Table 6

Comparative characteristics of carbon and silicon

Comparison criteria	Carbon - C	Silicon - Si
position in the periodic table of chemical elements	, 2nd period, IV group, main subgroup	, 3rd period, IV group, main subgroup
electronic configuration of atoms
valence possibilities	II - in a stationary state IV - in an excited state
possible oxidation states	, , , , ,	,
higher oxide	, acidic	, acidic
higher hydroxide	- weak unstable acid	() or - weak acid, has a polymeric structure
hydrogen bond	– methane (hydrocarbon)	– silane, unstable

Carbon. Allotropy is characteristic of the carbon element. Carbon exists in the form of the following simple substances: diamond, graphite, carbine, fullerene, of which only graphite is thermodynamically stable. Coal and soot can be thought of as amorphous varieties of graphite.

Graphite is refractory, slightly volatile, chemically inert at ordinary temperatures, is an opaque, soft substance that weakly conducts current. The structure of graphite is layered.

Alamaze is an extremely hard, chemically inert (up to 900 °C) substance that does not conduct current and conducts heat poorly. The structure of diamond is tetrahedral (each atom in a tetrahedron is surrounded by four atoms, etc.). Therefore, diamond is the simplest polymer, the macromolecule of which consists of only carbon atoms.

Carbyne has a linear structure (-carbine, polyyne) or (-carbine, polyene). It is a black powder, has semiconductor properties. Under the action of light, the electrical conductivity of carbine increases, and at temperature carbine turns into graphite. Chemically more active than graphite. It was synthesized in the early 1960s and was later found in some meteorites.

Fullerene is an allotropic modification of carbon formed by molecules having a “soccer ball” type structure. Molecules were synthesized, and other fullerenes. All fullerenes are closed structures of carbon atoms in the hybrid state. Unhybridized bond electrons are delocalized as in aromatic compounds. Fullerene crystals are of the molecular type.

Silicon. Silicon is not characterized by bonds, it is not typical to exist in a hybrid state. Therefore, there is only one stable allotropic modification of silicon, the crystal lattice of which is similar to that of diamond. Silicon is hard (on the Mohs scale, hardness is 7), refractory ( ), a very fragile substance of dark gray color with a metallic luster under standard conditions - a semiconductor. The chemical activity depends on the size of the crystals (coarse-crystalline is less active than amorphous).

The reactivity of carbon depends on the allotropic modification. Carbon in the form of diamond and graphite is quite inert, resistant to acids and alkalis, which makes it possible to manufacture crucibles, electrodes, etc. from graphite. Carbon exhibits higher reactivity in the form of coal and soot.

Crystalline silicon is rather inert, in amorphous form it is more active.

The main types of reactions that reflect the chemical properties of carbon and silicon are shown in Table 7.

Table 7

Basic chemical properties of carbon and silicon

reaction with	carbon	reaction with	silicon
simple substances	oxygen		oxygen
halogens		halogens
gray		carbon
hydrogen		hydrogen	does not react
metals		metals
complex substances	metal oxides		alkalis
steam		acids	does not react
acids

Binder materials

Binder materials – mineral or organic building materials used for the manufacture of concrete, fastening of individual elements of building structures, waterproofing, etc..

Mineral binders(MVM)– finely powdered materials (cements, gypsum, lime, etc.), which, when mixed with water (in some cases, with solutions of salts, acids, alkalis), form a plastic, workable mass that hardens into a strong stone-like body and binds particles of solid fillers and reinforcement into a monolithic whole.

Hardening of the MVM is carried out as a result of the processes of dissolution, the formation of a supersaturated solution and a colloidal mass; the latter partially or completely crystallizes.

MVM classification:

1. hydraulic binders:

When mixed with water (mixing), they harden and continue to maintain or increase their strength in water. These include various cements and hydraulic lime. During the hardening of hydraulic lime, CaO interacts with water and carbon dioxide in the air and the resulting product crystallizes. They are used in the construction of ground, underground and hydraulic structures that are constantly exposed to water.

2. air binders:

When mixed with water, they harden and retain their strength only in air. These include air lime, gypsum-anhydrite and magnesia air binders.

3. acid-resistant binders:

They consist mainly of acid-resistant cement containing a finely ground mixture of quartz sand and; they are closed, as a rule, with aqueous solutions of sodium or potassium silicate; they retain their strength for a long time when exposed to acids. During hardening, a reaction occurs. They are used for the production of acid-resistant putties, mortars and concretes in the construction of chemical enterprises.

4. binders for autoclave hardening:

They consist of lime-silica and lime-nepheline binders (lime, quartz sand, nepheline sludge) and harden during autoclaving (6-10 hours, steam pressure 0.9-1.3 MPa). They also include sandy Portland cements and other binders based on lime, ashes and low-level sludge. They are used in the production of products from silicate concrete (blocks, silicate bricks, etc.).

5. phosphate binders:

Consist of special cements; they are closed with phosphoric acid with the formation of a plastic mass, gradually hardening into a monolithic body, and retaining its strength at temperatures above 1000 ° C. Typically, titanium phosphate, zinc phosphate, aluminophosphate, and other cements are used. They are used for the manufacture of refractory lining mass and sealants for high-temperature protection of metal parts and structures in the production of refractory concrete, etc.

Organic binders(OBM)– substances of organic origin capable of changing from a plastic state to a solid or low-plastic state as a result of polymerization or polycondensation.

Compared to MVM, they are less brittle and have higher tensile strength. These include products formed during oil refining (asphalt, bitumen), products of thermal decomposition of wood (tar), as well as synthetic thermosetting polyester, epoxy, phenol-formaldehyde resins. They are used in the construction of roads, bridges, floors of industrial premises, rolled roofing materials, asphalt polymer concrete, etc.

The chemical sign of silicon is Si, the atomic weight is 28.086, the nuclear charge is +14. , as well as , is located in the main subgroup of group IV, in the third period. It is analogous to carbon. The electronic configuration of the electron layers of the silicon atom is ls 2 2s 2 2p 6 3s 2 3p 2 . The structure of the outer electron layer

The structure of the outer electron layer is similar to the structure of the carbon atom.
occurs in the form of two allotropic modifications - amorphous and crystalline.
Amorphous - a brownish powder with a slightly higher chemical activity than crystalline. At ordinary temperature, it reacts with fluorine:
Si + 2F2 = SiF4 at 400° - with oxygen
Si + O2 = SiO2
in melts - with metals:
2Mg + Si = Mg2Si
Crystalline silicon is a hard brittle substance with a metallic luster. It has good thermal and electrical conductivity, easily dissolves in molten metals, forming. An alloy of silicon with aluminum is called silumin, an alloy of silicon with iron is called ferrosilicon. Silicon density 2.4. Melting point 1415°, boiling point 2360°. Crystalline silicon is a rather inert substance and enters into chemical reactions with difficulty. Despite the well-marked metallic properties, silicon does not react with acids, but reacts with alkalis, forming salts of silicic acid and:
Si + 2KOH + H2O = K2SiO2 + 2H2

■ 36. What are the similarities and differences between the electronic structures of silicon and carbon atoms?
37. How to explain from the point of view of the electronic structure of the silicon atom why metallic properties are more characteristic of silicon than of carbon?
38. List the chemical properties of silicon.

Silicon in nature. Silica

Silicon is widely distributed in nature. Approximately 25% of the earth's crust is silicon. A significant part of natural silicon is represented by silicon dioxide SiO2. In a very pure crystalline state, silicon dioxide occurs as a mineral called rock crystal. Silicon dioxide and carbon dioxide are chemically analogous, however carbon dioxide is a gas and silicon dioxide is a solid. Unlike the CO2 molecular crystal lattice, silicon dioxide SiO2 crystallizes in the form of an atomic crystal lattice, each cell of which is a tetrahedron with a silicon atom in the center and oxygen atoms at the corners. This is explained by the fact that the silicon atom has a larger radius than the carbon atom, and not 2, but 4 oxygen atoms can be placed around it. The difference in the structure of the crystal lattice explains the difference in the properties of these substances. On fig. 69 shows the appearance of a natural quartz crystal composed of pure silicon dioxide and its structural formula.

Rice. 60. Structural formula of silicon dioxide (a) and natural quartz crystals (b)

Crystalline silica is most commonly found as sand, which is white unless contaminated with yellow clayey impurities. In addition to sand, silica is often found as a very hard mineral, silicon (hydrated silica). Crystalline silicon dioxide, colored in various impurities, forms precious and semi-precious stones - agate, amethyst, jasper. Almost pure silicon dioxide is also found in the form of quartz and quartzite. Free silicon dioxide in the earth's crust is 12%, in the composition of various rocks - about 43%. In total, more than 50% of the earth's crust is made up of silicon dioxide.
Silicon is a part of a wide variety of rocks and minerals - clay, granite, syenite, micas, feldspars, etc.

Solid carbon dioxide, without melting, sublimates at -78.5 °. The melting point of silicon dioxide is about 1.713°. She is very tough. Density 2.65. The expansion coefficient of silicon dioxide is very small. This is of great importance when using quartz glassware. Silicon dioxide does not dissolve in water and does not react with it, despite the fact that it is an acidic oxide and it corresponds to silicic acid H2SiO3. Carbon dioxide is known to be soluble in water. Silicon dioxide does not react with acids, except hydrofluoric acid HF, but gives salts with alkalis.

Rice. 69. Structural formula of silicon dioxide (a) and natural quartz crystals (b).
When silicon dioxide is heated with coal, silicon is reduced, and then it is combined with carbon and carborundum is formed according to the equation:
SiO2 + 2C = SiC + CO2. Carborundum has a high hardness, is resistant to acids, and is destroyed by alkalis.

■ 39. What properties of silicon dioxide can be used to judge its crystal lattice?
40. In the form of what minerals does silicon dioxide occur in nature?
41. What is carborundum?

Silicic acid. silicates

Silicic acid H2SiO3 is a very weak and unstable acid. When heated, it gradually decomposes into water and silicon dioxide:
H2SiO3 = H2O + SiO2

In water, silicic acid is practically insoluble, but can easily give.
Silicic acid forms salts called silicates. are widely found in nature. Natural ones are quite complex. Their composition is usually depicted as a combination of several oxides. If the composition of natural silicates includes alumina, they are called aluminosilicates. These are white clay, (kaolin) Al2O3 2SiO2 2H2O, feldspar K2O Al2O3 6SiO2, mica
K2O Al2O3 6SiO2 2H2O. Many natural gemstones in their purest form, such as aquamarine, emerald, etc.
Of the artificial silicates, sodium silicate Na2SiO3 should be noted - one of the few water-soluble silicates. It is called soluble glass, and the solution is called liquid glass.

Silicates are widely used in engineering. Soluble glass is impregnated with fabrics and wood to protect them from ignition. Liquid is part of refractory putties for bonding glass, porcelain, stone. Silicates are the basis in the production of glass, porcelain, faience, cement, concrete, brick and various ceramic products. In solution, silicates are easily hydrolyzed.

■ 42. What is it? How are they different from silicates?
43. What is liquid and for what purposes is it used?

Glass

The raw materials for glass production are Na2CO3 soda, CaCO3 limestone and SiO2 sand. All components of the glass mixture are carefully cleaned, mixed and fused at a temperature of about 1400 °. The following reactions take place during the melting process:
Na2CO3 + SiO2= Na2SiO3 + CO2

CaCO3 + SiO2 = CaSiO 3 + CO2
In fact, the composition of the glass includes sodium and calcium silicates, as well as an excess of SO2, so the composition of ordinary window glass is: Na2O · CaO · 6SiO2. The glass mixture is heated at a temperature of 1500° until the carbon dioxide is completely removed. Then cooled to a temperature of 1200 °, at which it becomes viscous. Like any amorphous substance, glass softens and hardens gradually, so it is a good plastic material. A viscous glass mass is passed through the slit, resulting in the formation of a glass sheet. A hot glass sheet is drawn in rolls, brought to a certain size and gradually cooled by air current. Then it is cut along the edges and cut into sheets of a certain format.

■ 44. Give the equations of the reactions that take place during the production of glass, and the composition of window glass.

Glass- the substance is amorphous, transparent, practically insoluble in water, but if it is crushed into fine dust and mixed with a small amount of water, alkali can be detected in the resulting mixture using phenolphthalein. During long-term storage of alkalis in glassware, the excess SiO2 in the glass reacts very slowly with alkali and the glass gradually loses its transparency.
Glass became known to people more than 3000 years before our era. In ancient times, glass was obtained with almost the same composition as at the present time, but the ancient masters were guided only by their own intuition. In 1750, M. V. managed to develop the scientific basis for glass production. For 4 years, M.V. collected many recipes for making various glasses, especially colored ones. At the glass factory he built, a large number of glass samples were made, which have survived to this day. Currently, glasses of different compositions with different properties are used.

Quartz glass is composed of almost pure silicon dioxide and is smelted from rock crystal. Its very important feature is that its coefficient of expansion is insignificant, almost 15 times less than that of ordinary glass. Dishes made of such glass can be red-hot in the flame of a burner and then lowered into cold water; there will be no change to the glass. Quartz glass does not retain ultraviolet rays, and if it is painted black with nickel salts, it will retain all visible rays of the spectrum, but remain transparent to ultraviolet rays.
Acids do not act on quartz glass, but alkalis noticeably corrode it. Quartz glass is more fragile than ordinary glass. Laboratory glass contains about 70% SiO2, 9% Na2O, 5% K2O 8% CaO, 5% Al2O3, 3% B2O3 (the composition of the glasses is not for memorization).

In industry, Jena and Pyrex glass are used. Jena glass contains about 65% Si02, 15% B2O3, 12% BaO, 4% ZnO, 4% Al2O3. It is durable, resistant to mechanical stress, has a low coefficient of expansion, resistant to alkalis.
Pyrex glass contains 81% SiO2, 12% B2O3, 4% Na2O, 2% Al2O3, 0.5% As2O3, 0.2% K2O, 0.3% CaO. It has the same properties as Jena glass, but to an even greater extent, especially after tempering, but is less resistant to alkalis. Pyrex glass is used to make household items that are heated, as well as parts of some industrial installations operating at low and high temperatures.

Some additives give different qualities to glass. For example, impurities of vanadium oxides give a glass that completely blocks ultraviolet rays.
Glass is also obtained, painted in various colors. M.V. also made several thousand samples of colored glass of different colors and shades for his mosaic paintings. At present, methods for coloring glass have been developed in detail. Manganese compounds color glass purple, cobalt blue. , sprayed in the mass of glass in the form of colloidal particles, gives it a ruby color, etc. Lead compounds give the glass a shine similar to that of rock crystal, which is why it is called crystal. Such glass can be easily processed and cut. Products from it refract light very beautifully. When coloring this glass with various additives, colored crystal glass is obtained.

If molten glass is mixed with substances that, when decomposed, form a large amount of gases, the latter, escaping, foam the glass, forming foam glass. Such glass is very light, well processed, and is an excellent electrical and thermal insulator. It was first received by Prof. I. I. Kitaygorodsky.
By drawing threads from glass, you can get the so-called fiberglass. If fiberglass laid in layers is impregnated with synthetic resins, then a very durable, rot-resistant, perfectly processed building material, the so-called fiberglass, is obtained. Interestingly, the thinner the fiberglass, the higher its strength. Fiberglass is also used to make workwear.
Glass wool is a valuable material through which strong acids and alkalis that are not filtered through paper can be filtered. In addition, glass wool is a good thermal insulator.

■ 44. What determines the properties of glasses of different types?

Ceramics

Of the aluminosilicates, white clay is especially important - kaolin, which is the basis for the production of porcelain and faience. Porcelain production is an extremely ancient branch of the economy. China is the birthplace of porcelain. In Russia, porcelain was obtained for the first time in the 18th century. D. I. Vinogradov.
The raw material for producing porcelain and faience, in addition to kaolin, are sand and. A mixture of kaolin, sand and water is subjected to thorough fine grinding in ball mills, then the excess water is filtered off and the well-mixed plastic mass is sent to the molding of products. After molding, the products are dried and fired in continuous tunnel kilns, where they are first heated, then fired and finally cooled. After this, the products undergo further processing - glazing, drawing a pattern with ceramic paints. After each stage, the products are fired. The result is porcelain that is white, smooth and shiny. In thin layers, it shines through. Faience is porous and does not shine through.

Bricks, tiles, earthenware, ceramic rings for fitting in absorption and washing towers of various chemical industries, flower pots are molded from red clay. They are also fired so that they do not soften with water and become mechanically strong.

Cement. Concrete

Silicon compounds serve as the basis for the production of cement, a binder material indispensable in construction. The raw materials for producing cement are clay and limestone. This mixture is fired in a huge inclined tubular rotary kiln, where raw materials are continuously loaded. After firing at 1200-1300 ° from the hole located at the other end of the furnace, the sintered mass - clinker - continuously exits. After grinding, the clinker turns into. Cement contains mainly silicates. If mixed with water until a thick slurry is formed, and then left for some time in air, it will react with cement substances, forming crystalline hydrates and other solid compounds, which leads to hardening (“setting”) of cement. This is no longer transferred to its previous state, therefore, before use, cement is tried to be protected from water. The hardening process of cement is long, and it acquires real strength only after a month. True, there are different types of cement. The ordinary cement we have considered is called silicate, or Portland cement. From alumina, limestone and silicon dioxide, a fast-hardening aluminous cement is made.

If you mix cement with crushed stone or gravel, you get concrete, which is already an independent building material. Crushed stone and gravel are called fillers. Concrete has high strength and can withstand heavy loads. It is waterproof and fire resistant. When heated, it almost does not lose strength, since its thermal conductivity is very low. Concrete is frost-resistant, weakens radioactive radiation, therefore it is used as a building material for hydraulic structures, for protective shells of nuclear reactors. Boilers are lined with concrete. If you mix cement with a foaming agent, then a foam concrete permeated with many cells is formed. Such concrete is a good sound insulator and conducts heat even less than ordinary concrete.