Silicon Dioxide, SiO2

Silicon Dioxide, SiO₂

95% of the earth’s crust is made up of silica and silicates. Silicon dioxide, commonly known as silica, occurs in several crystallographic forms. Quarts, cristobalite and tridymite are some of the crystalline forms of silica, and they are interconvertable at suitable temperature. Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms. Each oxygen atom in turn covalently bonded to another silicon atom as shown in below diagram. Each corner is shared with another tetrahedron. The entire crystal may be considered as giant molecule in which eight member rings are formed with alternate silicon and oxygen atoms.

Silica in its normal form is almost non-reactive because of very high Si-O bond enthalpy. It resists the attack by halogens, dihydrogen and most of the acids and metals even at elevated temperatures. However, it is attacked by HF and NaOH.

SiO₂   +   2NaOH    →    Na₂SiO₃   +   H₂O

SiO₂   +   4HF   →   SiF₄   +   2H₂O

Used

Quartz is extensively used as a piezoelectric material; it has made possible to develop extremely accurate clocks, modern radio and television broadcasting and mobile radio communications. Silica gel is used as a drying agent and as a support for chromatographic materials and catalysts. Kieselghur, an amorphous form of silica is used infiltration plants