Silicon tetrachloride
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Silicon tetrachloride (SiCl4) is a colourless volatile liquid compound of silicon and chlorine. It is most commonly prepared by reacting chlorine with hot silicon:
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- Si + 2 Cl2 → SiCl4
Silicon tetrachloride has a molar mass of 169.90 g/mol, a density of 1.483 g cm−3, a melting point of −70 °C, and a boiling point of 57.6 °C. It reacts violently with water, in contrast with carbon tetrachloride. This hydrolysis reaction occurs because the atomic radius of the silicon atom is such that the water molecules can attack it, whereas carbon has a smaller atomic radius so the chlorine atoms effectively shield the carbon from attack. In water, the following reaction occurs:
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- SiCl4 + 2 H2O → SiO2 + 4 HCl
Other oxidants, strong acids, alcohols, bases, ketones, and aldehydes can also react with SiCl4 to produce hydrogen chloride. It has corrosive effects on the skin, eyes and lungs.
Silicon tetrachloride is sometimes used as an intermediate in the manufacture of extremely pure silicon, since it has a boiling point convenient for purification by repeated fractional distillation; it can be reduced to silicon by hydrogen gas, or hydrolysed to SiO2 as a precursor for extremely pure synthetic fused silica. Very pure silicon is used in large amounts in the semiconductor industry, and also in the production of photovoltaic cells.
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