Silicon Tetrafluoride, SiF₄

Silicon tetrafluoride was discovered by Scheele in 1771, obtained independently by Priestley, and examined by Gay Lussac and Thenard (1808), and by J. Davy (1812), but more especially by Berzelius (1823). It is usually prepared by the interaction of hydrofluoric acid and silica:

SiO₂ + 4HF = SiF₄ + 2H₂O,

which is carried out by heating together sand, fluorspar, and excess of concentrated sulphuric acid in a flask:

2CaF₂ + 2H₂SO₄ + SiO₂ = SiF₄ + 2CaSO₄ + 2H₂O,

the excess of acid being needed to prevent the decomposition of the silicon tetrafluoride by water.

Silicon tetrafluoride, which is evolved as a gas, is then frequently passed into water, with which it undergoes a characteristic reaction, to which reference will be shortly made.

Silicon tetrafluoride was prepared synthetically by Moissan; and, according to Vigouroux amorphous silicon reacts with fluorine with incandescence, forming the tetrafluoride. The pure gas may be conveniently prepared by heating dry barium silicifluoride, BaSiF₆, 30-40 grams of which yield 2-3 litres of the gas, which may be collected over mercury. Silicon tetrafluoride may be separated from hydrogen fluoride, occurring with it as it is commonly prepared, by cooling the mixture to -60° C., which condenses the latter gas, or by passing the mixed gases through sodium fluoride, which retains hydrogen fluoride.

Silicon tetrafluoride is a colourless, pungent, fuming gas, which can be liquefied and solidified. The gas solidifies at -97° C. under atmospheric pressure, and the solid sublimes again without melting, but melts under 2 atmospheres pressure at -77° C., forming a transparent and very mobile liquid, which boils at -65° C. under 1810 mm. pressure. The critical temperature of silicon tetrafluoride is -1.5° C. and the critical pressure 50 atmospheres. The density of the gas was found by J. Davy to be 3.5735 and by Dumas 3.6, theory requiring 3.619. Jaquerod and Tourpaian found the weight of a normal litre to be 4.693 grams; but when the gas was passed over glass wool at a red heat the weight was increased to 4.820 grams, which increase the authors attribute to the formation of a subfluoride. The heat of formation of silicon tetrafluoride from crystallised silicon and gaseous fluorine is, according to Guntz, 239,800 calories. The gas is not decomposed by electric sparks or the silent electric discharge, but silicon is separated in the electric arc; sodium and potassium burn in the gas, forming fluorides, and certain metallic oxides, e.g. lime, likewise react with silicon tetrafluoride, forming silica and the fluoride of the metal.

The most characteristic reaction of silicon tetrafluoride is that with water, by which gelatinous silica and hydrofluosilicic acid (H₂SiF₆) are formed. The first reaction is one of hydrolysis:

SiF₄ + 3H₂O = H₂SiO₃ + 4HF;

but this hydrolysis is not perfect, for silicon tetrafluoride combines with hydrogen fluoride to form hydrofluosilicic acid, H₂SiF₆, which can exist in aqueous solution:

2SiF₄ + 4HF = 2H₂SiF₆;

consequently the complete reaction is

3SiF₄ + 3H₂O = H₂SiO₃ + 2H₂SiF₆.

Hence it will be understood that when silicon tetrafluoride is passed into a concentrated solution of hydrogen fluoride, hydrofluosilicic acid is formed without separation of silicic acid.

Hydrolytic decomposition of silicon tetrafluoride can take place with steam at high temperature, and under these, conditions anhydrous crystalline silica may be gradually deposited. Probably this kind of reaction, which is called pneumatolysis, has played some part in the formation of silica minerals.

Dry ammonia combines with silicon tetrafluoride, forming the crystalline compound SiF₄.2NH₃, which is decomposed by water; phosphine forms the compound 3SiF₄.2PH₃ at -22° C. under 50 atmospheres pressure, but this compound is unstable, dissociating into its constituents at ordinary temperature. Silicon tetrafluoride reacts with alcohol, forming ethyl orthosilicate and hydrofluosilicic acid; it also combines with acetone and with organic bases.