Silicon tetrafluoride

BaCl2 + H2SiF - BaSiF6 + 2HC1 BaSiF6 A SiF4 + BaF2 

Silicon tetrafluoride has been prepared by the reaction1 of concentrated sulfuric acid with a mixture of sand and calcium fluoride, at room temperature or upon mild heating. However, the product is contaminated by hydrofluoric and fluorosilicic acids. J The thermal decomposition of barium hexafluorosilicate2 is a convenient method, requiring simple apparatus and capable of producing silicon tetrafluoride of high purity. 

Preparation of Barium Hexafluorosilicate, BaSiF 6. Twenty-five grams of barium chloride 2-hydrate (0.10 mol) is dissolved in 70 ml. of water. Aqueous fluorosilicic acid (30%) is added until precipitation is complete this requires about 40 ml. of the solution. The freshly precipitated barium hexafluorosilicate is filtered and washed until free from chloride. The product is dried in an oven at 110 to 120° and then dried further in a desiccator over phosphorus ) oxide or porous barium oxide. The yield is nearly quantitative. 

Thermal Decomposition of BaSiF6. The apparatus is dried thoroughly. The barium hexafluorosilicate is placed University of Illinois, Urbana, 111. 

J Booth and Swinehart6 discuss the difficulties encountered in this reaction and describe two additional methods of preparation. 

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Silicon tetrafluoride is formed when hydrogen fluoride reacts with silica or a silicate ... 

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule having, like the tetrahalides of carbon, a tetrahedral covalent structure. It reacts with water to form hydrated silica (silica gel, see p. 186) and hexafluorosilicic acid, the latter product being obtained by a reaction between the hydrogen fluoride produced and excess silicon tetrafluoride ... 

Silicon difluoride is obtained as a very reactive gas when silicon tetrafluoride and silicon are heated together. It polymerises rapidly to give (SiF2) , a solid. 

The action of concentrated sulphuric acid liberates hydrogen fluoride, which attacks glass, forming silicon tetrafluoride the latter is hydrolysed to silicic acid by water, which therefore becomes turbid,... 

L. silex, silicis, flint) Davy in 1800 thought silica to be a compound and not an element later in 1811, Gay Lussac and Thenard probably prepared impure amorphous silicon by heating potassium with silicon tetrafluoride. 

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

Halogens, halides. Hydrogen fluoride, hydrogen chloride, chlorine, fluorine, silicon tetrafluoride... 

Fluorides and dust are emitted to the air from the fertilizer plant. All aspects of phosphate rock processing and finished product handling generate dust, from grinders and pulverizers, pneumatic conveyors, and screens. The mixer/reactors and dens produce fumes that contain silicon tetrafluoride and hydrogen fluoride. A sulfuric acid plant has two principal air emissions sulfur dioxide and acid mist. If pyrite ore is roasted, there will also be particulates in air emissions that may contain heavy metals such as cadmium, mercury, and lead. 

The phosphoric acid plant generates dust and fumes, both of which contain hydrofluoric acid, silicon tetrafluoride, or both. 

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

Various highly fluorinated alkyltrifluorosilanes can be pyrolyzed at 140-200 °C with the loss of silicon tetrafluoride without affecting the carbon chain The resulting carbenes react with olefins m situ [103, 104] (equations 70-72)... 

It is recognised that vitreous enamel possesses good acid resistance, but an exception occurs with hydrofluoric acid. This is due to the relative ease of reaction between this acid and the silica (which is the largest constituent in the frit) to form silicon tetrafluoride. This reaction is made use of in some de-enamelling plants. 

Another example is the determination of pure silica in an impure ignited silica residue. The latter is treated in a platinum crucible with a mixture of sulphuric and hydrofluoric acids the silica is converted into the volatile silicon tetrafluoride ... 

H.13 Hydrofluoric acid is used to etch grooves in glass because it reacts with the silica, Si02(s), in glass. The products of the reaction are aqueous silicon tetrafluoride and water. Write a balanced equation for the reaction. 

Fig. 4.—Radial distribution curves for carbon and silicon tetrafluorides and silicon, germanium and tin tetrachloride.

It is not affected by halogens or acids, except for phosphoric and hydrofluoric acids. Phosphoric acid attacks fused silica at temperatures of 300-400°C, and hydrofluoric acid attacks it at room temperature, forming silicon tetrafluoride and water. At high temperatures silica reacts with caustic alkalis, certain metallic oxides, and some basic salts, and cannot be used for incinerating these materials. Over 1600°C, fused silica is reduced to silicon by carbon. It can also be reduced at high temperature by hydrogen. It is unaffected by water under normal conditions but is attacked by strong solutions of alkalis. 

Sodium beryllium fluoride (Na2BeF4) is water-soluble and sodium aluminum fluoride (Na,AlF6) is water-insoluble. A part of the silicon volatilizes off as silicon tetrafluoride (SiF4), while the other part remains in the residue as silicon dioxide (Si02). Fluorination of silicon is unnecessary and it would be economical to recover all of it as silica. This is accomplished by using soda ash, i.e., sodium carbonate (Na2C03) in the reaction mixture ... 

Silicone tetrafluoride C T Most common metals for the dry gas. Steel, Monel and copper for moist gas... 

Calcium disilicide ignites in close contact with alkali metal fluorides, forming silicon tetrafluoride. 

When heated in contact it attacks glass violently, silicon tetrafluoride being evolved. It is a powerful fluorinating agent. 

Hydrofluoric acid is used to etch glass by reacting with the silicates in glass to produce a very volatile and thermodynamically stable compound, silicon tetrafluoride, SiF4. For example,... 

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