Carbon is capable of forming several allotropic modifications. These are diamond (the most inert allotropic modification), graphite, fullerene and carbine.
Charcoal and soot are amorphous carbon. Carbon in this state does not have an ordered structure and actually consists of the smallest fragments of graphite layers. Amorphous carbon treated with hot water vapor is called activated carbon. 1 gram of activated carbon, due to the presence of many pores in it, has a total surface of more than three hundred square meters! Due to its ability to absorb various substances Activated carbon is widely used as a filter filler, as well as an enterosorbent for various types poisoning.
From a chemical point of view, amorphous carbon is its most active form, graphite exhibits medium activity, and diamond is an extremely inert substance. For this reason, discussed below Chemical properties carbon should primarily be attributed to amorphous carbon.
Reducing properties of carbon
As a reducing agent, carbon reacts with non-metals such as oxygen, halogens, and sulfur.
Depending on the excess or lack of oxygen during the combustion of coal, the formation of carbon monoxide CO or carbon dioxide CO2:
When carbon reacts with fluorine, carbon tetrafluoride is formed:
When carbon is heated with sulfur, carbon disulfide CS 2 is formed:
Carbon is capable of reducing metals after aluminum in the activity series from their oxides. For example:
Carbon also reacts with oxides of active metals, however, in this case, as a rule, not the reduction of the metal is observed, but the formation of its carbide:
Interaction of carbon with non-metal oxides
Carbon enters into a co-proportionation reaction with carbon dioxide CO 2:
One of the most important processes from an industrial point of view is the so-called steam reforming of coal. The process is carried out by passing water vapor through hot coal. In this case, the following reaction takes place:
At high temperatures, carbon is able to reduce even such an inert compound as silicon dioxide. In this case, depending on the conditions, the formation of silicon or silicon carbide is possible ( carborundum):
Also, carbon as a reducing agent reacts with oxidizing acids, in particular, concentrated sulfuric and nitric acids:
Oxidizing properties of carbon
The chemical element carbon is not highly electronegative, so the simple substances it forms rarely exhibit oxidizing properties with respect to other non-metals.
An example of such reactions is the interaction of amorphous carbon with hydrogen when heated in the presence of a catalyst:
as well as with silicon at a temperature of 1200-1300 about C:
Carbon exhibits oxidizing properties in relation to metals. Carbon is able to react with active metals and some metals of intermediate activity. Reactions proceed when heated:
| Active metal carbides are hydrolyzed by water:
as well as solutions of non-oxidizing acids: In this case, hydrocarbons are formed containing carbon in the same oxidation state as in the original carbide. |
Chemical properties of silicon
Silicon can exist, as well as carbon in the crystalline and amorphous state, and, just as in the case of carbon, amorphous silicon is significantly more chemically active than crystalline silicon.
Sometimes amorphous and crystalline silicon is called its allotropic modifications, which, strictly speaking, is not entirely true. Amorphous silicon is essentially a conglomerate of the smallest particles of crystalline silicon randomly arranged relative to each other.
Interaction of silicon with simple substances
non-metals
At normal conditions Silicon, due to its inertness, reacts only with fluorine:
Silicon reacts with chlorine, bromine and iodine only when heated. It is characteristic that, depending on the activity of the halogen, a correspondingly different temperature is required:
So with chlorine, the reaction proceeds at 340-420 o C:
With bromine - 620-700 o C:
With iodine - 750-810 o C:
The reaction of silicon with oxygen proceeds, however, it requires very strong heating (1200-1300 ° C) due to the fact that a strong oxide film makes interaction difficult:
At a temperature of 1200-1500 ° C, silicon slowly interacts with carbon in the form of graphite to form carborundum SiC - a substance with an atomic crystal lattice similar to a diamond and almost not inferior to it in strength:
Silicon does not react with hydrogen.
metals
Due to its low electronegativity, silicon can exhibit oxidizing properties only with respect to metals. Of the metals, silicon reacts with active (alkaline and alkaline earth), as well as many metals of medium activity. As a result of this interaction, silicides are formed:
Interaction of silicon with complex substances
Silicon does not react with water even when boiling, however, amorphous silicon interacts with superheated water vapor at a temperature of about 400-500 ° C. This produces hydrogen and silicon dioxide:
Of all acids, silicon (in its amorphous state) reacts only with concentrated hydrofluoric acid:
Silicon dissolves in concentrated alkali solutions. The reaction is accompanied by the evolution of hydrogen.
Silicon in free form was isolated in 1811 by J. Gay-Lussac and L. Tenard by passing vapors of silicon fluoride over metallic potassium, but it was not described by them as an element. The Swedish chemist J. Berzelius in 1823 gave a description of the silicon he obtained by processing potassium salt K 2 SiF 6 with potassium metal at high temperature. The new element was given the name "silicon" (from the Latin silex - flint). The Russian name "silicon" was introduced in 1834 by the Russian chemist German Ivanovich Hess. Translated from other Greek. krhmnoz- "cliff, mountain".
Being in nature, getting:
In nature, silicon is found in the form of dioxide and silicates of various compositions. Natural silicon dioxide occurs mainly in the form of quartz, although other minerals exist - cristobalite, tridymite, kitite, cousite. Amorphous silica is found in diatom deposits at the bottom of the seas and oceans - these deposits were formed from SiO 2, which was part of diatoms and some ciliates.
Free silicon can be obtained by calcining fine white sand with magnesium, which chemical composition is almost pure silicon oxide, SiO 2 +2Mg=2MgO+Si. Industrial grade silicon is obtained by reducing the SiO 2 melt with coke at a temperature of about 1800°C in arc furnaces. The purity of silicon obtained in this way can reach 99.9% (the main impurities are carbon, metals).
Physical properties:
Amorphous silicon has the form of a brown powder, the density of which is 2.0 g/cm 3 . Crystalline silicon - dark gray, shiny crystalline substance, brittle and very hard, crystallizes in the diamond lattice. It is a typical semiconductor (conducts electricity better than a rubber-type insulator, and worse than a conductor - copper). Silicon is brittle, only when heated above 800 °C does it become plastic. Interestingly, silicon is transparent to infrared radiation starting at a wavelength of 1.1 micrometers.
Chemical properties:
Chemically, silicon is inactive. At room temperature, it reacts only with gaseous fluorine to form volatile silicon tetrafluoride SiF 4 . When heated to a temperature of 400-500 ° C, silicon reacts with oxygen to form dioxide, with chlorine, bromine and iodine - to form the corresponding easily volatile tetrahalides SiHal 4 . At a temperature of about 1000°C, silicon reacts with nitrogen to form nitride Si 3 N 4 , with boron - thermally and chemically stable borides SiB 3 , SiB 6 and SiB 12 . Silicon does not directly react with hydrogen.
For silicon etching, a mixture of hydrofluoric and nitric acids is most widely used.
Attitude towards alkalis ...
Silicon is characterized by compounds with an oxidation state of +4 or -4.
The most important connections:
Silicon dioxide, SiO 2- (silicic anhydride) ...
...
Silicic acids- weak, insoluble, formed by adding acid to a silicate solution in the form of a gel (gelatinous substance). H 4 SiO 4 (orthosilicon) and H 2 SiO 3 (metasilicon, or silicon) exist only in solution and irreversibly turn into SiO 2 when heated and dried. The resulting solid porous product - silica gel, has a developed surface and is used as a gas adsorbent, desiccant, catalyst and catalyst carrier.
silicates- salts of silicic acids for the most part (except for sodium and potassium silicates) are insoluble in water. Properties....
Hydrogen compounds- analogues of hydrocarbons, silanes, compounds in which silicon atoms are connected by a single bond, Silenes if the silicon atoms are double bonded. Like hydrocarbons, these compounds form chains and rings. All silanes are self-igniting, form explosive mixtures with air, and readily react with water.
Application:
Silicon finds the greatest use in the production of alloys for giving strength to aluminum, copper and magnesium and for the production of ferrosilicides, which are important in the production of steels and semiconductor technology. Silicon crystals are used in solar panels and semiconductor devices - transistors and diodes. Silicon also serves as a raw material for the production of organosilicon compounds, or siloxanes, obtained in the form of oils, lubricants, plastics and synthetic rubbers. inorganic compounds silicon is used in ceramic and glass technology, as an insulating material and piezocrystals
For some organisms, silicon is an important biogenic element. It is part of the supporting structures in plants and skeletal structures in animals. In large quantities, silicon is concentrated by marine organisms - diatoms, radiolarians, sponges. Large amounts of silicon are concentrated in horsetails and cereals, primarily in the Bamboo and Rice subfamilies, including common rice. Muscle human contains (1-2) 10 -2% silicon, bone- 17 10 -4%, blood - 3.9 mg / l. With food, up to 1 g of silicon enters the human body daily.
Antonov S.M., Tomilin K.G.
KhF Tyumen State University, 571 groups.
At normal conditions allotropic modifications of carbon - graphite and diamond - are rather inert. But as t increases, they actively enter into chemical reactions with simple and complex substances.
Chemical properties of carbon
Since the electronegativity of carbon is low, simple substances are good reducing agents. It is easier to oxidize fine-crystalline carbon, more difficult - graphite, even more difficult - diamond.
Allotropic modifications of carbon are oxidized by oxygen (burn) at certain ignition temperatures: graphite ignites at 600 °C, diamond at 850-1000 °C. If oxygen is in excess, carbon monoxide (IV) is formed, if there is a deficiency, carbon monoxide (II):
C + O2 = CO2
2C + O2 = 2CO
Carbon reduces metal oxides. In this case, metals are obtained in a free form. For example, when lead oxide is calcined with coke, lead is smelted:
PbO + C = Pb + CO
reducing agent: C0 - 2e => C+2
oxidizer: Pb+2 + 2e => Pb0
Carbon also exhibits oxidizing properties with respect to metals. At the same time, it forms various kinds of carbides. So, aluminum undergoes reactions at high temperatures:
3C + 4Al = Al4C3
C0 + 4e => C-4 3
Al0 – 3e => Al+3 4
Chemical properties of carbon compounds
1) Since the strength of carbon monoxide is high, it enters into chemical reactions when high temperatures. With significant heating, high reducing properties of carbon monoxide are manifested. So, it reacts with metal oxides:
CuO + CO => Cu + CO2
At an elevated temperature (700 °C), it ignites in oxygen and burns with a blue flame. From this flame, you can find out that carbon dioxide is formed as a result of the reaction:
CO + O2 => CO2
2) Double bonds in the carbon dioxide molecule are strong enough. Their rupture requires significant energy (525.6 kJ/mol). Therefore, carbon dioxide is rather inert. The reactions it enters into often occur at high temperatures.
Carbon dioxide exhibits acidic properties when it reacts with water. This forms a solution of carbonic acid. The reaction is reversible.
Carbon dioxide, as an acidic oxide, reacts with alkalis and basic oxides. When carbon dioxide is passed through an alkali solution, either an average or an acid salt can be formed.
3) Carbonic acid has all the properties of acids and interacts with alkalis and basic oxides.
Chemical properties of silicon
Silicon more active than carbon, and is oxidized by oxygen already at 400 °C. Other non-metals can oxidize silicon. These reactions usually take place at a higher temperature than with oxygen. Under such conditions, silicon interacts with carbon, in particular with graphite. In this case, carborundum SiC is formed - a very hard substance, inferior in hardness only to diamond.
Silicon can also be an oxidizing agent. This is manifested in reactions with active metals. For example:
Si + 2Mg = Mg2Si
The higher activity of silicon compared to carbon is manifested in the fact that, unlike carbon, it reacts with alkalis:
Si + NaOH + H2O => Na2SiO3 + H2
Chemical properties of silicon compounds
1) Strong bonds between atoms in the crystal lattice of silicon dioxide explain the low chemical activity. The reactions that this oxide enters into take place at high temperatures.
Silicon oxide is an acidic oxide. As you know, it does not react with water. Its acidic nature is manifested in the reaction with alkalis and basic oxides:
SiO2 + 2NaOH = Na2SiO3 + H2O
Reactions with basic oxides take place at high temperatures.
Silicon oxide exhibits weak oxidizing properties. It is reduced by some active metals.
In binary compounds of silicon with carbon, each silicon atom is directly bonded to four neighboring carbon atoms located at the vertices of a tetrahedron, the center of which is the silicon atom. At the same time, each carbon atom, in turn, is bonded to four neighboring silicon atoms located at the vertices of a tetrahedron, the center of which is a carbon atom. Such a mutual arrangement of silicon and carbon atoms is based on the Si-C- silicon-carbon bond and forms a dense and very strong crystalline structure.
Currently, only two binary compounds of silicon with carbon are known. This is a very rare mineral, moissanite, which is not yet found in nature. practical application, and artificially obtained carborundum SiC, which is sometimes called silund, refrax, carbofrax, kristolan, etc.
In laboratory practice and in technology, carborundum is obtained by reducing silica with carbon according to the reaction equation
SiO 2 + 3C \u003d 2CO + SiC
In addition to finely ground quartz or pure quartz fishing line and coke, table salt and sawdust. Sawdust during firing loosen the charge, and salt, reacting with ferruginous and aluminum impurities, turns them into le: opaque chlorides FeCl 3 and AlCl 3, which are removed from the reaction zone at 1000-1200 ° C. In fact, the reaction between silica and coke begins already at 1150 ° C, but proceeds extremely slowly. With an increase in temperature to 1220 ° C, its speed increases. In the temperature range from 1220 to 1340°C, it becomes exothermic and proceeds violently. As a result of the reaction, a mixture is first formed, consisting of the smallest crystals and an amorphous variety of carborundum. With an increase in temperature to 1800-2000 ° C, the mixture recrystallizes and turns into well-developed, tabular, rarely colorless, more often colored green, gray and even black with a diamond luster and iridescent play, hexagonal crystals containing about 98-99.5% carborundum. The process of obtaining carborundum from the charge is carried out in electric furnaces burning 2000-2200 ° C. To obtain chemically pure carborundum, the product obtained as a result of firing the charge is treated with alkali, which dissolves the silica that has not entered into the reaction.
Crystalline carborundum belongs to a very solids; its hardness is 9. The ohmic resistance of polycrystalline carborundum decreases with increasing temperature and becomes insignificant at 1500 0 C.
In air at temperatures above 1000 0 C, carborundum begins to oxidize slowly at first, and then vigorously with an increase in temperature above 1700 ° C. In this case, silica and carbon monoxide are formed:
2SiC + 3O 2 = 2SiO 2 + 2CO
Silicon dioxide formed on the carborundum surface is a protective film, somewhat slowing down the further oxidation of carborundum. In an environment of water vapor, the oxidation of carborundum under the same conditions proceeds more vigorously.
mineral acids, with the exception of orthophosphoric, carborundum is not affected, chlorine at 100 ° C decomposes it according to the reaction equation
SiC + 2Cl 2 = SiCl 4 + C
and at 1000 ° C, instead of carbon, CC1 4 is released:
SiC + 4C1 2 = SiCl + CC1 4
Molten metals, reacting with carborundum, form the corresponding silicides:
SiC + Fe = FeSl + C
At temperatures above 810 ° C, carborundum reduces oxides of alkaline earth metals to metal, above 1000 ° C it reduces iron oxide (III) Fe 2 O 3 and above 1300-1370 ° C iron oxide (II) FeO, nickel oxide (II) NiO and manganese oxide MnO.
Molten caustic alkalis and their carbonates in the presence of atmospheric oxygen completely decompose carborundum with the formation of the corresponding silicates:
SiC + 2KOH + 2O 2 \u003d K 2 SiO 3 + H 2 O + CO 2
SiC + Na 2 CO 3 + 2O 2 \u003d Na 2 SiO 3 + 2CO 2
Carborundum can also react with sodium peroxide, lead (II) oxide and phosphoric acid.
Due to the fact that carborundum has a high hardness, it is widely used as abrasive powders for grinding metal, as well as for the manufacture of carborundum abrasive wheels, bars and sanding paper from it. electrical conductivity carborundum at high temperatures makes it possible to use it as the main material in the manufacture of so-called silicate rods, which are resistance elements in electric furnaces. For this purpose, a mixture of carborundum with silicon is closed with glycerin or another organic cementing substance, and rods are formed from the resulting mass, which are fired at 1400-1500 ° C in an atmosphere of carbon monoxide or nitrogen. During firing, the cementitious organic matter decomposes, the released carbon, combining with silicon, turns it into carborundum and gives the rods the required strength.
Special refractory crucibles are made from carborundum
for melting metals that are obtained by hot pressing
carborundum at 2500 ° C under a pressure of 42-70 MPa. More famous
refractories made from mixtures of carborundum with nitrides
boron, steatite, molybdenum-containing bonds and other
entities.
SILICON HYDRIDES, OR SILANES
Hydrogen compounds of silicon are usually called silicon hydrides, or silanes. Like saturated hydrocarbons, silicon hydrides form a homologous series in which the silicon atoms are interconnected by a single bond
Si-Si-Si-Si-Si- etc.
The simplest.representative
of this homologous series is monosilane, or simply silane, SiH 4 , whose molecular structure is similar to that of methane, then follows
disilane H 3 Si-SiH 3, which is similar in molecular structure to ethane, then trisilane H 3 Si-SiH 2 -SiH 3,
tetrasilane H 3 Si-SiH 2 -SiH 2 -SiH 3,
pentasilane H 3 Si-SiH 2 -SiH 2 -SiH 2 ^--SiH 3 and the last of the obtained silanes of this homologous series
hexasilane H 3 Si-SiH 2 -SiH 2 -SiH 2 -SiH 2 -SiH 3 . Silanes in pure form do not occur in nature. Get them artificially:
1. Decomposition of metal silicides with acids or alkalis according to the reaction equation
Mg 2 Si + 4HCI \u003d 2MgCl 2 + SiH 4
this forms a mixture of silanes, which is then separated by fractional distillation at very low temperatures.
2. Reduction of halosilanes with lithium hydride or lithium aluminum hydride:
SiCl 4 + 4 LiH = 4LiCl + SiH 4
This method of obtaining silals was first described in 1947.
3. Reduction of halosilanes with hydrogen. The reaction proceeds at 300 - 400 ° C in reaction tubes filled with a contact mixture containing silicon, metallic copper and 1 - 2% aluminum halides as catalysts.
Despite the similarity in the molecular structure of sitanes and saturated hydrocarbons, physical properties they are different.
Compared to hydrocarbons, silanes are less stable. The most stable of them is monosilane SiH4, which decomposes into silicon and hydrogen only at red heat. Other silanes with a high silicon content form lower derivatives at much lower temperatures. For example, disilane Si 2 H 6 gives a silane and a solid polymer at 300 ° C, and hexasilane Si 6 H 14 decomposes slowly even at normal temperatures. When in contact with oxygen, silanes are easily oxidized, and some of them, for example, monosilane SiH 4, spontaneously ignite at -180 ° C. Silanes are easily hydrolyzed into silicon dioxide and hydrogen:
SiH 4 + 2H 2 0 \u003d SiO 2 + 4H 2
For higher silanes, this process occurs with splitting
bonds - Si - Si - Si - between silicon atoms. For example, three-
silane Si 3 H 8 gives three molecules of SiO 2 and ten molecules of hydrogen gas:
H 3 Si - SiH 2 - SiH 3 + 6H 3 O \u003d 3SiO 2 + 10H 2
In the presence of caustic alkalis, as a result of the hydrolysis of silanes, a silicate of the corresponding alkali metal and hydrogen are formed:
SiH 4 + 2NaOH + H 2 0 = Na 2 Si0 3 + 4H 2
SILICON HALIDES
Halosilanes also belong to binary silicon compounds. Like silicon hydrides - silanes - they form a homologous series chemical compounds, in which halide atoms are directly connected to silicon atoms linked by single bonds
etc. into chains of the appropriate length. Due to this similarity, halosilanes can be considered as products of substitution of hydrogen in silanes for the corresponding halogen. In this case, the replacement can be complete and incomplete. In the latter case, halogen derivatives of silanes are obtained. The highest known halogen silane is considered to be chlorosilane Si 25 Cl 52. Halogenosilanes and their halogen derivatives do not occur in nature in pure form and can only be obtained artificially.
1. Direct connection of elemental silicon with halogens. For example, SiCl 4 is obtained from ferrosilicon containing 35 to 50% silicon by treating it at 350-500°C with dry chlorine. In this case, SiCl 4 is obtained as the main product in a mixture with other more complex halosilanes Si 2 C1 6, Si 3 Cl 8, etc. according to the reaction equation
Si + 2Cl 2 \u003d SiCl 4
The same compound can be obtained by chlorinating a mixture of silica and coke at high temperatures. The reaction proceeds according to the scheme
SiO 2 + 2C \u003d Si + 2CO
Si + 2C1 2 \u003d SiС1 4
SiO 2 + 2C + 2Cl 2 \u003d 2CO + SiCl 4
Tetrabromosilane is obtained by bromination at red heat of elemental silicon with bromine vapor:
Si + 2Br 2 = SiBr 4
or mixtures of silica with coke:
SiO 2 + 2C \u003d Si + 2CO
Si + 2Br 3 = SiBi 4
SiO 2 + 2C + 2Br 2 \u003d 2CO + SiBr 4
In this case, the formation of silanes of higher degrees is possible simultaneously with tetrasilanes. For example, when chlorinating magnesium silicide, 80% SiCI 4 , 20% SiCl 6 and 0.5-1% Si 3 Cl 8 are obtained; during the chlorination of calcium silicide, the composition of the reaction products is expressed as follows: 65% SiCl 4 ; 30% Si 2 Cl 6 ; 4% Si 3 Cl 8 .
2. Halogenation of silanes with hydrogen halides in the presence of AlBr 3 catalysts at temperatures above 100°C. The reaction proceeds according to the scheme
SiH 4 + HBr = SiH 3 Br + H 2
SiН 4 + 2НВг = SiH 2 Br 2 + 2H 2
3. Halogenation of silanes with chloroform in the presence of AlCl 3 catalysts:
Si 3 H 8 + 4CHS1 3 \u003d Si 3 H 4 Cl 4 + 4CH 2 C1 3
Si 3 H 8 + 5CHCl 3 \u003d Si 3 H 3 C1 5 + 5CH 2 C1 2
4. Silicon tetrafluoride is obtained by the action of hydrofluoric acid on silica:
SiO 2 + 4HF \u003d SiF 4 + 2H 2 0
5. Some polyhalogenosilanes can be obtained from the simplest halosilanes by halogenation with their corresponding halide. For example, tetraiodosilane in a sealed tube at 200-300 ° C, reacting with silver, releases hexaiododisilane at
Iodosilanes can be obtained by reacting iodine with silanes in a medium carbon tetrachloride or chloroform, and in the presence of AlI 3 catalyst in the interaction of silane with hydrogen iodide
Halogenosilanes are less durable than halogen derivatives of hydrocarbons similar in structure. They are easily hydrolyzed, forming silica gel and hydrohalic acid:
SiCl 4 + 2H 2 O \u003d Si0 2 + 4HCl
The simplest representatives of halosilanes are SiF 4 , SiCl 4 , SiBr 4 and SiI 4 . Of these, tetrafluorosilane and tetrachlorosilane are mainly used in engineering. Tetrafluorosilane SiF 4 is a colorless gas with a pungent odor, smokes in air, hydrolyzes into fluorosilicic acid and silica gel. SiF 4 is obtained by the action of hydrofluoric acid on silica according to the reaction equation
SiO 2 + 4HF \u003d SlF 4 + 2H 2 0
For industrial production. SiF 4 use fluorspar CaF 2 , silica SiO 2 and sulfuric acid H 2 SO 4 . The reaction proceeds in two phases:
2CaF 2 + 2H 3 SO 4 \u003d 2СаSO 4 + 4HF
SiO 2 + 4HF \u003d 2H 2 O + SiF 4
2CaF 2 + 2H 2 S0 4 + SiO 2 = 2CaSO 4 + 2H 2 O + SiF 4
The gaseous state and volatility of tetrafluorosilane is used for etching sodium-lime silicate glasses with hydrogen fluoride. When hydrogen fluoride reacts with glass, tetrafluorosilane, calcium fluoride, sodium fluoride, and water are formed. Tetrafluorosilane, volatilizing, releases new deeper layers of glass for reaction with hydrogen fluoride. CaF 2 and NaF remain at the reaction site, which dissolve in water and thereby free access to hydrogen fluoride for further penetration to the freshly bare glass surface. The etched surface can be matte or transparent. Opaque etching is obtained by the action of gaseous hydrogen fluoride on glass, transparent - by etching with aqueous solutions of hydrofluoric acid. If tetrafluorosilane is passed into water, H 2 SiF 6 and silica are obtained in the form of a gel:
3SiF 4 + 2H 2 O \u003d 2H 2 SiF 6 + Si0 2
Hydrofluorosilicic acid is one of the strong dibasic acids; it has not been obtained in a free state; upon evaporation, it decomposes into SiF 4 and 2HF, which volatilize; with caustic alkalis forms acidic and normal salts:
H 2 SlF 6 + 2NaOH.= Na 2 SiF 6 + 2H 2 O
with an excess of alkalis gives alkali metal fluoride, silica and water:
H 2 SiF 6 + 6NaOH \u003d 6NaF + SiO 2 + 4H 2 O
The silica released in this reaction reacts with caustic
elbow and leads to the formation of silicate:
SiO 2 + 2NaOH \u003d Na 2 SiO 3 + H 2 O
Salts of fluorosilicic acid are called silicofluorides or fluates. At present, silicofluorides Na, H, Rb, Cs, NH 4 , Cu, Ag, Hg, Mg, Ca, Sr, Ba, Cd, Zn, Mn, Ni, Co, Al, Fe, Cr, Pb and etc.
Silicon fluorides of sodium Na 2 SiF 6 , magnesium MgSiF 6 * 6HgO, zinc ZnSiF 6 * 6H 2 O, aluminum Al 2 (SiF 6) 3 , lead PbSiF 6 , barium BaSiF 6 and others are used in technology for various purposes. Silicon fluorides have antiseptic and sealing properties; at the same time they are flame retardants. Due to this, they are used to impregnate wood in order to prevent premature decay and protect it from ignition in case of fires. Silicon fluorides are also impregnated with artificial and natural stones for building purposes to seal them. The essence of impregnation lies in the fact that the solution of silicon fluorides, penetrating into the pores and cracks of the stone, reacts with calcium carbonate and some other compounds and forms insoluble salts that are deposited in the pores and seal them. This greatly increases the resistance of the stone to weathering. Materials that do not contain calcium carbonate at all or contain little of it are pre-treated with prefluates, i.e. substances containing dissolved calcium salts, alkali metal silicates and other substances capable of forming insoluble precipitates with fluates. Magnesium, zinc and aluminum silicofluorides are used as fluates. The fluting process can be represented as follows:
MgSiF 6 + 2СаСО 3 = MgF 2 + 2CaF 2 + SiO 2 + 2СО 2
ZnSiF 6 + ZCaS0 3 \u003d 3CaF 6 + ZnCO 3 + SiO 2 + 2CO 2
Al 2 (SiF 6) 3 + 6CaCO 3 \u003d. 2A1F 3 + 6CaF 2 + 3SiO 2 + 6CO 2
Alkali metal silicofluorides are obtained by reacting fluorosilicic acid with solutions of salts of these metals:
2NaCl + H 2 SiF 6 = Na 2 SlF 6 + 2HC1
These are gelatinous precipitates, soluble in water and practically insoluble in absolute alcohol. Therefore, they are used in quantitative analysis in the determination of silica by the volumetric method. For technical purposes, sodium silicofluoride is used, which is obtained in the form of a white powder as a by-product in the production of superphosphate. From a mixture of Na 2 SiF 6 and A1 2 O 3 at 800 ° C, cryolite 3NaF٠AlF 3 is formed, which is widely used in the production of dental cements and is a good silencer both in glass making and in the manufacture of opaque glazes and enamels.
Sodium silicofluoride as one of the components is introduced into the composition of chemically resistant putties produced on liquid glass:
Na 2 SiF 6 + 2Na 2 SiO 3 \u003d 6NaF + 3SiO 2
The silica liberated from this reaction imparts chemical stability to the hardened putty. At the same time, Na 2 SiF 6 is a hardening accelerator. Sodium silicon fluoride is also introduced as a mineralizer into raw mixes in the production of cements.
Tetrachlorosilane SiCl 4 - colorless, fuming in air, easily hydrolyzed liquid obtained by chlorination of carborundum or ferrosilicon by the action of silanes at elevated temperatures
Tetrachlorosilane is the main starting product for the production of many organosilicon compounds.
Tetrabromosilane SiBr 4 is a colorless, fuming in air, easily hydrolyzed into SiO 2 and HBr liquid, obtained at a red-hot temperature, when bromine vapor is passed over hot elemental silicon.
Tetraiodsilane SiI 4 is a white crystalline substance obtained by passing a mixture of iodine vapor with carbon dioxide over hot elemental silicon.
Silicon borides and nitrides
Silicon borides are compounds of silicon with boron. Currently, two silicon bords are known: silicon triboride B 3 Si and silicon hexaboride B 6 Si. These are extremely hard, chemically resistant and refractory substances. They are obtained by fusing electric current finely ground mixture, consisting of 5 wt. hours of elemental silicon and 1 wt. hours of boron. The spilled mass is cleaned with molten potassium carbonate. G. M. Samsonov and V. P. Latyshev obtained silicon triboride by hot pressing at 1600-1800 0 C.
Silicon triboride with square. 2.52 g / cm 3 forms black plates -
chatty structure rhombic crystals, translucent
in a thin layer in yellow-brown tones. Silicon hexaboride with pl.
2.47 g / cm 3 is obtained in the form of opaque opaque grains of opaque
fork shape.
Silicon borides melt at about 2000°C, but oxidize very slowly even at high temperatures. This makes it possible to use them as special refractories. The hardness of silicon borides is very high, and in this respect they approach carborundum.
Silicon compounds with nitrogen are called silicon nitrides. The following nitrides are known: Si 3 N 4 , Si 2 N 3 and SIN. Silicon nitrides are obtained by calcining elemental silicon in an atmosphere of pure nitrogen in the temperature range from 1300 to 1500 ° C. Normal silicon nitride Si 3 N 4 can be obtained from a mixture of silica and coke, calcined in an atmosphere of pure nitrogen at 1400-1500 ° C:
6С + 3Si0 2 + 2N 3 ͢ Si 3 N 4 + 6CO
Si 3 N 4 is a grayish-white refractory and acid-resistant powder that volatilizes only above 1900 ° C. Silicon nitride hydrolyzes with the release of silica and ammonia:
Si 3 N 4 + 6H 2 O \u003d 3SiO 2 + 4NH3
concentrated sulphuric acid when heated, it slowly decomposes Si 3 N 4 , and dilute fluorosilicic acid decomposes it more vigorously.
Silicon nitride of the composition Si 2 N 3 is also obtained by the action of nitrogen at high temperatures on elemental silicon or on silicon carboxylic acid C 2 Si 2 N + N 2 \u003d 2C + Si2N 3 .
In addition to binary compounds of silicon with nitrogen, many other more complex ones are currently known, which are based on the direct bond between silicon atoms and nitrogen atoms, for example: 1) aminosilanes SiH 3 NH 2 , SiH 2 (NH 2) 2 , SiH (NH 2 ) 3, Si(NH 2) 4 ; 2) silylamines NH 2 (SiH 3), NH (SiH 3) 2, N(SiH 3) 3; 3) nitrogen-containing silicon compounds of a more complex composition.
GENERAL VIEWS
- Designation - Si (Silicon);
- Period - III;
- Group - 14 (IVa);
- Atomic mass - 28.0855;
- Atomic number - 14;
- Radius of an atom = 132 pm;
- Covalent radius = 111 pm;
- Electron distribution - 1s 2 2s 2 2p 6 3s 2 3p 2 ;
- t melting = 1412°C;
- boiling point = 2355°C;
- Electronegativity (according to Pauling / according to Alpred and Rochov) = 1.90 / 1.74;
- Oxidation state: +4, +2, 0, -4;
- Density (n.a.) \u003d 2.33 g / cm 3;
- Molar volume = 12.1 cm 3 / mol.
Silicon Compounds:
Silicon was first isolated in its pure form in 1811 (Frenchmen J. L. Gay-Lussac and L. J. Tenard). Pure elemental silicon was obtained in 1825 (the Swede J. Ya. Berzelius). The chemical element received its name "silicon" (translated from ancient Greek - mountain) in 1834 (Russian chemist G. I. Hess).
Silicon is the most abundant (after oxygen) chemical element on Earth (content in earth's crust 28-29% by weight). In nature, silicon is most often present in the form of silica (sand, quartz, flint, feldspars), as well as in silicates and aluminosilicates. Silicon is extremely rare in its pure form. Many natural silicates in their pure form are precious stones: emerald, topaz, aquamary - it's all silicon. Pure crystalline silicon(IV) oxide occurs as rock crystal and quartz. Silicon oxide, in which various impurities are present, forms precious and semiprecious stones- amethyst, agate, jasper.
Rice. The structure of the silicon atom.
The electronic configuration of silicon is 1s 2 2s 2 2p 6 3s 2 3p 2 (see Electronic structure of atoms). Silicon has 4 electrons in its outer energy level: 2 paired in the 3s sublevel + 2 unpaired in the p orbitals. When a silicon atom passes into an excited state, one electron from the s-sublevel "leaves" its pair and goes to the p-sublevel, where there is one free orbital. Thus, in the excited state, the electronic configuration of the silicon atom takes the following form: 1s 2 2s 2 2p 6 3s 1 3p 3 .
Rice. The transition of the silicon atom to an excited state.
Thus, silicon in compounds can exhibit valence 4 (most often) or 2 (see Valence). Silicon (as well as carbon), reacting with other elements, forms chemical bonds in which it can both give up its electrons and accept them, but the ability to accept electrons from silicon atoms is less pronounced than that of carbon atoms, due to bigger size silicon atom.
Silicon oxidation states:
- -4 : SiH 4 (silane), Ca 2 Si, Mg 2 Si (metal silicates);
- +4 - the most stable: SiO 2 (silicon oxide), H 2 SiO 3 (silicic acid), silicates and silicon halides;
- 0 : Si (simple substance)
Silicon as a simple substance
Silicon is a dark gray crystalline substance with a metallic sheen. Crystalline silicon is a semiconductor.
Silicon forms only one allotropic modification, similar to diamond, but not as strong, because the Si-Si bonds are not as strong as in the diamond carbon molecule (See Diamond).
Amorphous silicon- brown powder, melting point 1420°C.
Crystalline silicon is obtained from amorphous silicon by its recrystallization. Unlike amorphous silicon, which is quite active chemical, crystalline silicon is more inert in terms of interaction with other substances.
The structure of the crystal lattice of silicon repeats the structure of diamond - each atom is surrounded by four other atoms located at the vertices of the tetrahedron. The atoms bind to each other with covalent bonds, which are not as strong as the carbon bonds in diamond. For this reason, even at n.o.s. some of the covalent bonds in crystalline silicon are broken, as a result of which some of the electrons are released, due to which silicon has a small electrical conductivity. As silicon is heated, in the light or with the addition of some impurities, the number of destroyed covalent bonds increases, as a result of which the number of free electrons increases, therefore, the electrical conductivity of silicon also increases.
Chemical properties of silicon
Like carbon, silicon can be both a reducing agent and an oxidizing agent, depending on which substance it reacts with.
At n.o. silicon interacts only with fluorine, which is explained by the rather strong silicon crystal lattice.
Silicon reacts with chlorine and bromine at temperatures exceeding 400°C.
Silicon interacts with carbon and nitrogen only at very high temperatures.
- In reactions with non-metals, silicon acts as reducing agent:
- under normal conditions, from non-metals, silicon reacts only with fluorine, forming silicon halide:
Si + 2F 2 = SiF 4 - at high temperatures, silicon reacts with chlorine (400°C), oxygen (600°C), nitrogen (1000°C), carbon (2000°C):
- Si + 2Cl 2 = SiCl 4 - silicon halide;
- Si + O 2 \u003d SiO 2 - silicon oxide;
- 3Si + 2N 2 = Si 3 N 4 - silicon nitride;
- Si + C \u003d SiC - carborundum (silicon carbide)
- under normal conditions, from non-metals, silicon reacts only with fluorine, forming silicon halide:
- In reactions with metals, silicon is oxidizing agent(formed salicides:
Si + 2Mg = Mg 2 Si - In reactions with concentrated solutions of alkalis, silicon reacts with the release of hydrogen, forming soluble salts of silicic acid, called silicates:
Si + 2NaOH + H 2 O \u003d Na 2 SiO 3 + 2H 2 - Silicon does not react with acids (with the exception of HF).
Obtaining and using silicon
Getting silicon:
- in the laboratory - from silica (aluminum therapy):
3SiO 2 + 4Al = 3Si + 2Al 2 O 3 - in industry - by the reduction of silicon oxide with coke (commercially pure silicon) at high temperature:
SiO 2 + 2C \u003d Si + 2CO - the purest silicon is obtained by reducing silicon tetrachloride with hydrogen (zinc) at high temperature:
SiCl 4 + 2H 2 \u003d Si + 4HCl
Application of silicon:
- manufacturing of semiconductor radioelements;
- as metallurgical additives in the production of heat-resistant and acid-resistant compounds;
- in the production of photocells for solar batteries;
- as AC rectifiers.
