OXYGEN CONTAINING COMPOUNDS

Silicon

Silicon dioxide. Si interacts with O vigorously when heated:

Si + O₂ = SiO₂ DH°₂₉₈ = -908 kJ / mol

SiO₂ structure.Natural SiO₂ occurs in crystalline (quartz) and amorphous forms. Crystalline SiO₂ exists as a set of several polymorphs, which are mutually transformed from one into another in accordance with the following scheme:

β-quartz ^{870 º} β-tridymite ^{1470 º} β-cristobalite ^1723º liquid

575 ^º 130 ^º 270 ^º <1650 ^º

α-quartz α-tridymite α-cristobalite glass

Quartz, tridymite, and crystobalite can have mutual transitions from one to another, but they are very slow. Consequently, tridymite, and crystobalite can be stored very long time at room temperature and exist as separate minerals despite their thermodynamic instability. Note that a-modifications are stable at room temperature and b-modifications — at high temperature. At the rapid cooling of the melt, crystallization has not time to occur and amorphous vitreous form, fused silica (quartz glass) appears.

Various modifications of SiO₂ are inorganic polymers with heterochains.

These giant molecular entities have strong covalent bonds Si—O—Si. Each Si atom is surrounded by tetrahedron of four O atoms and O atoms, in turn, connect to each other two such tetrahedrons.

These connected tetrahedrons twisted in a spiral like staircase of the type of screw. Spirals have parallel orientation in space. The crystalline form with rightscrew spirals is called a-quartz, and with leftscrew spirals is called b-quartz.

Properties. Polymeric SiO₂ is not soluble in water at STP. It is stable to attacks of acids. It dissolves only in hydrofluoric acid, HF, due to the formation of volatile SiF₄ with stronger Si—F than Si–O bonds:

SiO₂ + 4HF = SiF₄­ + 2H₂O

SiO₂ dissolves slowly in alkali solutions, and quickly in their melts forming silicates:

SiO₂ + 2NaOH = Na₂SiO₃ + H₂O

SiO₂ is a silicic acid anhydride. As a compound with extremely low volatility (fugitivity), it displaces other acid anhydrides when melting with salts that are used in glassmaking:

SiO₂ + Na₂SO₄ Na₂SiO₃ + SO₃

Silicic acids. The overwhelming majority of meta-silicic acid, H₂SiO₃, salts are insoluble in water. Na₂SiO₃ is one of the few soluble salts. It is called “soluble glass” and aqueous solutions of the salt – “liquid glass”.

H₂SiO₃ is a very weak acid. Its soluble in water fraction of molecules dissociates in negligible small degree (K₁ = 1 ∙ 10^-10). Liquid glass forms strong alkaline medium and high viscosity after hydrolysis due to the formation of polymorphs of orthosilicic acid (silicate glue):

Na₂SiO₃ + H₂O = Na₂H₂SiO₄

Na₂H₂SiO₄ + 2H₂O = H₄SiO₄ + 2NaOH

Some salt H₂SiO₃ with weak bases completely hydrolyze in a solution. Therefore, they cannot be obtained by the double replacement reaction:

Na₂SiO₃ + 2NH₄Cl + H₂O = H₄SiO₄ + NaCl + 2NH₃­

Metasilicic acid is displaced from its salts in aqueous solutions by other acids, including H₂CO₃. At first, water-soluble orthosilicic acid appears which can exist in very dilute solutions:

Na₂H₂SiO₄ + 2HCl = H₄SiO₄ + 2NaCl

The growth of concentration or just standing leads to polymerization:

The elementary block of such a chain polymer is H₂SiO₃, but the further condensation leads to branched, cross-linked, bulk polymers, and, eventually, polymeric SiO₂ due to transformations of OH-groups:

H – O O – H H - O O – H

Si + Si + -H₂O

H – O O – H H – O O – H ^{. . .}

HO OH

Si

OH OH H - O O O

H – O – Si – O – Si – OH -H₂O Si Si

OH OH H – O O O

Si

HO OH

Tetrahedral surrounding of silicon by oxygen atoms occurs in any polymeric form of silicic acid. These polymers are insoluble and have nonstoichiometric composition (SiO₂)_x∙(H₂O)_y. When x> 1 they correspond to polysilicic acids, which derivatives is a variety of natural silicates.

Silicate materials are produced artificially in large quantities. The maximal amount among synthetic silicates has glass, which composition expresses the formula Na₂CaSi₆O₁₄ or Na₂O∙CaO∙6SiO₂. The partial replacement of Na, Ca, Si atoms by the atoms of other elements gives a great number of different kinds of special glass depending on the nature of its application. For example, laboratory glassware contains significant content of B₂O₃ and Al₂O₃. It is characterized by high chemical stability (especially, in acidic medium) and low coefficient of thermal incandescences.

Silicon halides. Silicon forms halides of general formula SiX₄ with halogens:

SiO₂ + 2C + 2Cl₂ SiCl₄ + 2CO

Tetrahalides are very reactive substances. They hydrolyze easily as halogenanhydrides of ortho-silicic acid:

SiCl_4(aq) + 4H₂O = H₄SiO_4(aq) + 4HCl_(aq) DG°₂₉₈ = -238 kJ / mol

Hydrolysis is a result of consecutive H₂O molecules addition and HX molecules removal up to H₄SiO₄ formation.

SiX₄ fumes in air due to hydrolysis (e.g. SiCl₄ is used to create artificial fog). SiCl₄ is an active chlorinated agent that interacts with oxides of metals and nonmetals:

3 SiCl₄ + 2Al₂O₃ = 3SiO₂ + 4AlCl₃

SiF₄ interacts with HF acid forming hexafluorosilicic acid, H₂SiF₆:

SiF₄ + 2HF = H₂[SiF₆]

Si atom in H₂SiF₆ is in sp³d²-hybrid state and its coordination number is 6. H₂SiF₆ is not known in the free state, it is a strong acid (» H₂SO₄).

Other silicon tetrahalides do not form hexahalido compounds. Their resistance to various reagents and heating decreases toward SiI₄.

Silicon nitride. Amorphous Si interacts with N₂ only at t>1300 °C. Silicon nitride, Si₃N₄, formed is a solid stable compound that only decomposes slowly by molten alkali or hot concentrated HF:

Si₃N₄ + 12NaOH = 3Na₂SiO₃ + 4NH₃

Si₃N₄ + 16HF = 2(NH₄)₂SiF₆ + SiF₄

Silicon carbide. Si or SiO₂ react with carbon in electric furnaces at t » 2300 °C. SiC is a very hard (the second after diamond) refractory material. It is chemically stable at normal conditions, but decomposes when heated in molten alkali (in the presence of O₂), Cl₂, hot steam:

SiC + 4NaOH + 2O₂ = Na₂SiO₃ + Na₂CO₃ + 2H₂O

¯8e ­2e×2

SiC + 2Cl₂ SiCl₄ + C

SiC + 2H₂O SiO₂ + CH₄

CHEMICAL COMPOUNDS

Date: 2016-01-03; view: 989