Chlorosilanes are the most important reagents in the chemical industry, many of which are obtained by chlorination of the silicon-hydrogen (Si-H) bond. Such chlorination is typically achieved through the use of toxic and/or expensive metal-containing reagents. Researchers at Tel Aviv University have found a new, simple, selective and highly efficient catalytic method for chlorinating Si-H bonds without the use of metals. The boron compound tris(pentafluorophenyl)borane B(C 6 F 5) 3 is used as a catalyst, and hydrochloric acid HCl is used as a chlorinating agent. The reaction mechanism was proposed on the basis of competing reactions and quantum mechanical calculations. The work was published in Angewandte Chemie International Edition- one of the most influential chemical journals in the world.

Chlorosilanes - substances with a silicon-chlorine bond with the general formula R 3 Si-Cl (where R is any organic group, hydrogen or other chlorine) - are used in many branches of organic chemistry: the synthesis of drugs, polymers and many other substances. For example, almost no multistage organic synthesis can do without them, since many active groups are protected with their help (see also Protecting group). If there are several active groups on the molecule, one of them can be selectively (without affecting the others) blocked with a silicon shield (silyl ether) using the appropriate chlorosilane, then the desired reactions can be carried out with other reactive groups, and in the next step, the silicon protection can be removed, releasing the protected group for further reactions. The silicon protective group is removed quite easily, while other parts of the molecule are not affected, so this protection is very popular. Different groups require different conditions to be protected. Moreover, usually the same groups placed in different chemical environments will react differently. Therefore, chemists require chlorosilanes with different reactivity, or, in other words, with various groups on the silicon atom.

One of the most popular methods for obtaining chlorosilanes is the chlorination of the silicon-hydrogen (Si-H) bond. Classical (including commercial) methods of chlorination of these bonds can be conventionally divided into stoichiometric (for each mole of the chlorinated bond, the corresponding number of moles of the activating agent is needed) and catalytic (the catalyst activates the molecule and, after its chlorination, returns to its original state to activate the next molecule). Stoichiometric chlorination of Si-H bonds is carried out by means of metal salts in combination with dangerous sources of chlorine, such as toxic tin chlorides, poisonous elemental chlorine and carcinogenic carbon tetrachloride. Known methods of catalytic chlorination of these bonds with non-toxic chlorine sources (such as hydrochloric acid) involve the use of expensive transition metal catalysts such as palladium. Directly, without activation, silanes do not react with hydrochloric acid.

Despite the fact that silicon is located directly below carbon in the periodic table, their chemistry is very different (see, for example, Structures of the contact and solvate-separated ion pairs of the silenyl-lithium compound “Elements”, 09/23/2016) were obtained for the first time. In particular, the bond of hydrogen to silicon is weaker than that to carbon, and is polarized so that hydrogen is negatively charged and can behave like a pseudohalogen. This feature was used by scientists from Tel Aviv University to activate the Si-H bond with tris(pentafluorophenyl)borane B(C 6 F 5) 3 . B(C 6 F 5) 3 is a non-toxic and relatively inexpensive (compared to transition metals) boron compound with three pentafluorophenyl rings. Fluorophenyls pull electron density off the boron atom, so boron interacts with the negatively charged hydrogen atom on silicon and weakens the Si-H bond, allowing chlorine from hydrochloric acid (HCl) to replace the hydrogen. From two hydrogen atoms (H - from silicon and H + from hydrochloric acid), molecular hydrogen H 2 is obtained (Fig. 1).

A separate example of the triethylsilane chlorination reaction is shown in fig. 3. Hydrochloric acid is generated by dropping a concentrated solution of sulfuric acid onto common salt. Gaseous hydrochloric acid is formed, which is fed through a tube into a stirred toluene solution of chlorosilane and a catalyst. Using just one molecule of B(C 6 F 5) 3 to 100 molecules of Et 3 SiH (i.e. one mole percent, 1 mol%) with an excess of HCl, the reaction goes to completion in 15 minutes.

Using quantum mechanical calculations, the authors obtained a model of the structure of the transition state of the reaction (Fig. 4) and the energy required for this reaction to proceed in the gas phase (25.5 kcal/mol).

Just opening a new reaction is not enough to get published in a good journal. It is necessary to at least demonstrate the possibility of its wide application and confirm the proposed mechanism by additional experiments and/or theoretical calculations. But even this may not be enough. For a very good publication, it is desirable to demonstrate a feature of the reaction that is not present in already known and used reactions.

To begin with, the authors chlorinated by their own method, using both B(C 6 F 5) 3 and its etherate Et 2 O B(C 6 F 5) 3 , several silanes with various substituents R - from organosilicon (tBuMe 2 Si) to siloxide (Et 3 SiO): Me 2 (tBuMe 2 Si)SiH, Ph 2 (Et 3 SiO)SiH, Me 2 SiClH, Ph 2 SiClH, Ph 2 SiH 2 , PhMeSiH 2 . They also succeeded in demonstrating stepwise chlorination of silanes with two hydrogens Ph 2 SiH 2 , PhMeSiH 2 , using different catalyst concentrations (from 1 to 10 mol%) and varying the reaction time.

At this stage, apart from the reaction itself, no unusual results were found. Then the authors tested the chlorination of the more reactive three-hydrogen silane, PhSiH 3 . Here it is worth noting that the stepwise chlorination of PhSiH 3 is not an easy task, since the reaction can easily skip the stage of monochlorination (PhSiClH 2) to double chlorination (PhSiCl 2 H). Here the authors were in for a pleasant surprise. With 10 mol% B(C 6 F 5) 3 , the reaction jumped in 10 minutes, yielding 87% PhSiCl 2 H and 13% PhSiClH 2 . However, when Et 2 O B(C 6 F 5) 3 etherate was used as a catalyst under exactly the same conditions (10 mol%, 10 minutes), the ratio of products turned out to be almost opposite: 16% PhSiCl 2 H and 84% PhSiClH 2 ( reactions 1 and 2 in the table). By reducing the concentration of the catalyst by 10 times, it was possible to achieve the exclusive production of PhSiClH 2 in one stage (reaction 4 in the table). Double chlorination with etherate does not occur completely even after 1000 minutes (reaction 6 in the table).

Why is the reaction with etherat so different from the original? After all, etherate was used only because of convenience - it is easier to isolate, and it is more stable in air than its non-ether counterpart. In solution, the diethyl ether molecule (Et 2 O) is detached from boron, and it should, in theory, behave identically to the original catalyst. Perhaps the diethyl ether molecule itself somehow participates in the reaction? Confirmation of this hypothesis was obtained by analyzing the solution after the reaction - it turned out that ethane C 2 H 6 was present there, which could appear in the solution only through the decomposition of the diethyl ether molecule. Then the researchers carried out a stoichiometric (in a ratio of 1:1) reaction of PhSiH 3 with Et 2 O·B(C 6 F 5) without adding HCl and obtained phenyl(ethoxy)silane and ethane as products. The diethyl ether actually decomposed (Fig. 5).

Apparently, this is the first stage of all reactions catalyzed by etherate. At the second, HCl reacts with ethoxysilane and ethanol is released, which adds back to boron instead of diethyl ether, continuing the catalytic chain (Fig. 6). The authors suggested that the second chlorination slows down, since ethanol reacts with the already chlorinated molecule more slowly than with the non-chlorinated one. This assumption was proved by a separate experiment and using quantum mechanical calculations of the energies of all stages of the reaction with two types of catalysts.

The replacement of catalysts based on precious metals in the industry is very important due to the high cost of the latter, limited resources and toxicity. Tris(pentafluorophenyl)borane is gaining popularity among catalysis chemists, and we are likely to see many more interesting reactions involving it.

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 of 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, the chemical properties of carbon considered below 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 CO 2 is possible:

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

Under 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 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.

Under the action of hydrochloric acid on silicide, magnesium Mg 2 Si, silicon hydrogen SiH 4 is obtained, similar to methane:

Mg 2 Si + 4HCl \u003d 2MgCl 2 + SiH 4

Silicon hydrogen SiH 4 is a colorless gas that ignites spontaneously in air and burns to form silicon dioxide and water:

SiН 4 + 2O 2 = SiO 2 + 2Н 2 O

In addition to SiH 4 , a number of other silanes are known: Si 2 H 6 . Si 3 H 8, etc., which are collectively called sylane. Silanes are similar to hydrocarbons, but differ from them in instability. It is obvious that the bond between silicon atoms is much less strong than the bond between carbon atoms, as a result of which the chains -Si-Si-Si-, etc. are easily destroyed. The bond between silicon and hydrogen is also unstable, which indicates a significant weakening of the metalloid properties of silicon.

Chloride SiCl 4 is obtained by heating a mixture of silica and coal in a stream of chlorine:

SiO 2 + 2C + 2Cl 2 \u003d SiCl 4 + 2CO

or chlorination of technical silicon. It is a liquid boiling at 57°. Under the influence of water S1CI 4 undergoes complete hydrolysis with the formation of silicic and hydrochloric acids:

SiCl 4 + 3H 2 O \u003d H 2 SiO 3 + 4HCl

As a result of this reaction, when SiCl 4 evaporates in moist air, dense smoke is formed; therefore, SiCl 4 is used as a smoke generator.

fluorine SiF 4 is formed by the interaction of fluoro- hydrogen chloride with silica:

SiO 2 + 4HF \u003d SiF 4 + 2H 2 O

It is a colorless gas with a pungent odor.

If you pass fluoride into water, you get a solution of fluorosilic acid H 2 SiFe:

3SiF 4 + 3H 2 O \u003d 2H 2 SiF 6 + H 2 SiO 3

From a concentrated solution, upon cooling, crystals of the composition H 2 SiF6 2H 2 O stand out.

Fluorosilicic acid H 2 SiF 6 is one of the strong acids. The degree of its dissociation in 0.1 n. solution is 75%. Even at very low concentrations, it is strongdisinfectant. Salts of fluorosilicic acid - fluorosilicates are mostly soluble in water. Fluorosilicates of sodium and barium are widely used to control pests of agricultural crops. Sodium fluorosilicate is also used in the manufacture of various enamels. Magnesium and zinc fluorosilicates are used to waterproof cement.

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Element characteristic

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

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

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

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

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

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

Methods for obtaining silicon

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

1) Recovery of SiO 2 with carbon in arc furnaces at 1800 "C: SiO 2 + 2C \u003d Si + 2CO

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

a) Si → SiCl 2 → Si

b) Si → Mg 2 Si → SiH 4 → Si

Physical properties of silicon. Allotropic modifications of silicon

1) Crystalline silicon - a silvery-gray substance with a metallic sheen, a diamond-type crystal lattice; m.p. 1415 "C, b.p. 3249" C, density 2.33 g/cm3; is a semiconductor.

2) Amorphous silicon - brown powder.

Chemical properties of silicon

In most reactions, Si acts as a reducing agent:

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

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

Si + O 2 \u003d SiO 2 silicon oxide

2. Reacts with fluorine already at room temperature:

Si + 2F 2 = SiF 4 silicon tetrafluoride

3. Reactions with other halogens proceed at a temperature = 300 - 500 ° C

Si + 2Hal 2 = SiHal 4

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

5. Reaction with nitrogen occurs above 1000°C:

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

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

SiO 2 + 3C \u003d SiC + 2CO

Carborundum is close to diamond in hardness.

7. Silicon does not directly react with hydrogen.

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

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

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

Si + 2NaOH + H 2 O \u003d Na 2 SiO 3 + 2H 2

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

2MgO \u003d Si \u003d 2Mg + SiO 2

In reactions with metals, Si is an oxidizing agent:

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

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

2Mg + Si = Mg 2 Si

Silane (monosilane) SiH 4

Silanes (silicon hydrogens) Si n H 2n + 2, (compare with alkanes), where n \u003d 1-8. Silanes - analogues of alkanes, differ from them in the instability of -Si-Si- chains.

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

Ways to get:

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

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

Chemical properties.

Silane is a strong reducing agent.

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

SiH 4 + 2O 2 \u003d SiO 2 + 2H 2 O

2. SiH 4 is easily hydrolyzed, especially in an alkaline environment:

SiH 4 + 2H 2 O \u003d SiO 2 + 4H 2

SiH 4 + 2NaOH + H 2 O \u003d Na 2 SiO 3 + 4H 2

Silicon (IV) oxide (silica) SiO 2

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

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

Physical properties

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

Chemical properties

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

SiO 2 + 2NaOH \u003d Na 2 SiO 3 + H 2 O

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

SiO 2 + K 2 O \u003d K 2 SiO 3;

SiO 2 + CaO \u003d CaSiO 3

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

SiO 2 + Na 2 CO 3 \u003d Na 2 SiO 3 + CO 2

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

SiO 2 + 6HF \u003d H 2 SiF 6 + 2H 2 O

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

SiO 2 + 4HF (gas.) \u003d SiF 4 + 2H 2 O

SiO 2 + 2F 2 \u003d SiF 4 + O 2

Silicic acids

Known:

Orthosilicic acid H 4 SiO 4 ;

Metasilicic (silicic) acid H 2 SiO 3 ;

Di- and polysilicic acids.

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

Ways to receive

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

Na 2 SiO 3 + 2HCl \u003d H 2 SiO 3 ↓ + 2NaCl

2. Hydrolysis of chlorosilanes: SiCl 4 + 4H 2 O \u003d H 4 SiO 4 + 4HCl

Chemical properties

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

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

H 4 SiO 4 → H 2 SiO 3 → SiO 2

Silicates - salts of silicic acids

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

Na 2 SiO 3 + H 2 O \u003d NaHSiO 3 + NaOH

SiO 3 2- + H 2 O \u003d HSiO 3 - + OH - (alkaline medium)

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

K 2 SiO 3 + CO 2 + H 2 O \u003d H 2 SiO 3 ↓ + K 2 CO 3

SiO 3 + CO 2 + H 2 O \u003d H 2 SiO 3 ↓ + CO 3

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

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

Glass

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

Na 2 CO 3 + CaCO 3 + 6SiO 2 \u003d Na 2 O CaO 6SiO 2 + 2CO 2

Cement

A powdered binder material that, when interacting with water, forms a plastic mass, which eventually turns into a solid stone-like body; main building material.

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

Si is one of the most abundant elements in the earth's crust. The most common after O2. In nature, Si occurs only in the form of a compound: SiO2. The most important element of the plant and animal kingdom.

Receiving: Technical: SiO2 + 2C ==== Si + 2CO. Pure: SiCl4 + 2H2 = Si + 4HCl. SiH4 =(t)Si + 2H2. Used in metallurgy and semiconductor technology. To remove O2 from molten Me and serves as an integral part of the alloys. For the manufacture of photocells, amplifiers, rectifiers.

Physical properties aza. Silicon - gray-steel color. brittle, only when heated above 800 ° C, it becomes a plastic substance. Transparent to infrared radiation, semiconductor. The crystal lattice is cubic like diamond, but due to the greater bond length between Si-Si atoms compared to the C-C bond length, the hardness of silicon is much less than diamond. Allotropic Si-powder of gray color.

Chemical properties: At n. y. Si is inactive and reacts only with gaseous fluorine: Si + 2F2 = SiF4

Amorphous Si is more reactive, molten Si is very active.

When heated to a temperature of 400-500 °C, silicon reacts with O2, Cl2, Br2, S: Si + O2 = SiO2 . Si + 2 Cl2 = SiCl4

With nitrogen, silicon at a temperature of about 1000 ° C forms the nitride Si3N4,

with boron - thermally and chemically resistant borides SiB3, SiB6 and SiB12.,

with carbon - silicon carbide SiC (carborundum).

When silicon is heated with metals, silicides can form.

Si does not react with acids, only with a mixture of HNO3 and HF oxidizes it to hexafluorosilicic acid: 3Si + 8HNO3 + 18HF = 3H2 + 4NO + 8H2O

It dissolves vigorously in alkali solutions in the cold (non-metallic properties): Si + 2NaOH + H2O = Na2SiO3 + 2 H2

At high temperatures, it slowly interacts with water: Si + 3H2O = H2SiO3 + 2H2

Hydrogen compoundsSi.Silicon does not directly react with hydrogen, silicon compounds with hydrogen - silanes with the general formula SinH2n+2 is obtained indirectly. Monosilane SiH4 Ca2Si + 4HCl → 2CaCl2 + SiH4 admixture of other silanes, disilane Si2H6 and trisilane Si3H8.

Polysilanes Toxic, odorless, less thermally stable than СnH2n+2 Reducing agents SiH4 + O2 = SiO2 + 2 H2O

Hydrolyze in water SiH4 + 2H2O = SiO2 + 4H2

Silicon compounds with metals - SILICIDES

I.Ion-covalent: silicides of alkali, alkaline earth metals and magnesium Ca2Si, Mg2Si

Easily destroyed by water: Na2Si + 3H2O = Na2SiO3 + 3 H2

Decomposed by acids: Ca2Si + 2H2SO4 = 2CaSO4 + SiH4

II. Metal-like: transition metal silicides. Chemically stable and do not decompose under the action of acids, resistant to oxygen even at high temperatures. They have high Tm (up to 2000 °C). Many are metallically conductive. The most common MeSi, Me3Si2, Me2Si3, Me5Si3 and MeSi2.

Silicides of d-elements are used to obtain heat-resistant and acid-resistant alloys. Lanthanide silicides are used in nuclear power engineering as neutron absorbers.

SiC - carborundum Solid, refractory substance. The crystal lattice is similar to that of a diamond. It is a semiconductor. Used to make artificial gems

Silica easily reacts with F2 and HF: SiO2 + 4HF = SiF4 + 2 H2O. SiO2 + F2 = SiF4 + O2 Does not dissolve in water.

Dissolves in alkali solutions when heated: SiO2 + 2NaOH = Na2SiO3 + H2O

Sintered with salts: SiO2 + Na2CO3 = Na2SiO3 + CO2. SiO2 + PbO = PbSiO3

Silicic acids Very weak, slightly soluble acids in water. Silicic acids form colloidal solutions in water.

Salts of silicic acids are called silicates. SiO2 corresponds to silicic acid, which can be obtained by the action of a strong acid on silicateNa2SiO3 + HCl = H2SiO3 + NaCl

H2SiO3 - metasilicic, or silicic acid. H4SiO4 - orthosilicic acid exist only in solution and irreversibly turn into SiO2 if water is evaporated.

silicates-salts of silicic acids, each Si atom surrounds a tetrahedrally located O2 atom around it. Close relationship between Si and O2.