Silicon Tetrahydride, SiH₄

Silicon tetra-hydride, SiH₄, was first obtained, in an impure state, by Buff and Wohler, in 1857, during the electrolysis of a solution of common salt by means of aluminium electrodes containing silicon, and also by the action of dilute hydrochloric acid on magnesium silicide, prepared by fusing together anhydrous magnesium chloride, sodium silicifluoride, common salt, and sodium. The magnesium silicide was more simply prepared by Gattermann, by heating magnesium with sand. The gas obtained in this way, by the reaction

Mg₂Si + 4HCl = 2MgCl₂ + SiH₄,

is impure, being mixed with hydrogen and another product, probably another hydride of silicon, which renders it spontaneously inflammable. According to Adwentowski and Drozdowski, however, pure silicane is spontaneously inflammable. The same spontaneously inflammable gas is sometimes observed when a porcelain crucible in which magnesium ribbon has been burnt is cleansed with acid, since the stain on the crucible is magnesium silicide.

According to Moissan and Smiles the gas evolved by the action of hydrochloric acid on the product formed by heating magnesium and silicon together in the proportion Mg:2Si is hydrogen mixed with about 5 per cent, of silicon hydrides. These may be separated from hydrogen by cooling them in liquid air, so that they solidify. If the solid hydrides are then allowed to liquefy and volatilise fractionally, silicane is obtained as a gas and silico-ethane remains behind as a liquid.

Silicane can also be obtained mixed with silico-ethane, when silicon tetrafluoride is passed over heated magnesium, and the solid product is decomposed by acid.

Pure silicane can be prepared by the method of Friedel and Ladenburg, by heating ethyl orthosilicoformate with sodium, though the function of the sodium is unknown.

Ethyl orthosilicoformate is prepared by the interaction of silico- chloroform and sodium ethoxide thus:

HSiCl₃ + 3NaOC₂H₅ = HSi(OC₂H₅)₃ + 3NaCl,

and yields silicane together with ethly orthosilicate, by a reaction which recalls the behaviour of phosphorous and hypophosphorous acids and their salts when heated, thus:

4SiH(OC₂H₅)₃ = SiH₄ + 3Si(OC₂H₅)₄.

Silicane, SiH₄, is a colourless gas which when pure is not spontaneously inflammable, but catches fire in contact with a heated knife-blade or hot mercury. It is condensed to a liquid under the following conditions:

Temperature ° C	-1°	-7°	-11°
Pressure (atmospheres).	100	70	50

its critical temperature being about 0° C. and its boiling-point -115° C. to -116° C. under 730 mm. pressure. According to more recent work, however, its critical temperature is -3.5° C. and critical pressure 47.8 atmospheres; it solidifies at -200° C.

Silicane is decomposed into its elements above 400° C., yielding twice its volume of hydrogen, and when it is passed through a red-hot tube a mirror of finely divided amorphous silicon is deposited. Its thermochemistry is as follows:

SiH₄ = Si + 2H₂ - 8700 calories.

SiH₄ = Si (vapour) + 2H₂ - 44,000 calories.

It is also decomposed by electric sparks, but in this case only a part of the hydrogen is set free, and the yellow solid hydride (Si₂H₃)_n is formed, which burns when heated in air or in chlorine.

Silicane takes fire in chlorine gas, producing the tetrachloride SiCl₄, and hydrogen chloride; chlorides which easily part with chlorine, such as carbonyl chloride, COCl₂, antimony pentachloride, SbCl₅, and tin tetrachloride, SnCl₄, react similarly with silicane. Silicane burns in oxygen or air, producing silica and water; if cold porcelain is depressed upon the flame, amorphous silicon is deposited upon it. Acids do not decompose silicane, but this gas reacts vigorously with caustic potash solution, producing potassium metasilicate and hydrogen, thus:

SiH₄ + H₂O + 2KOH = K₂SiO₃ + 4H₂.

By this means silicane may be estimated.

This reaction, as well as the ease with which silicane is decomposed by heat, shows the contrast in properties between this substance and methane, from which it is further distinguished by reacting with certain metallic salt solutions. Thus with copper sulphate solution silicane produces chiefly copper silicide, thus:

2CuSO₄ + SiH₄ = Cu₂Si + 2H₂SO₄,

together with some metallic copper; but with the more reducible silver nitrate the product consists chiefly of metallic silver mixed with silicon:

4AgNO₃ + SiH₄ = Si + 4Ag + 4HNO₃.

In these reactions silicane is comparable with the hydrides of sulphur and arsenic.