Silicon Dioxide, Silica

Silicon dioxide is known as Silica , SiO₂.

95 % of the earth crust is made up of silica and silicates. It occurs in several crystallographic forms , namely , quartz , cristobalite and tridymite. These crystalline forms are interconvertible at suitable temperature.

Structure

Silicon dioxide is covalent , three- dimensional network solid in which each silicon atom is bonded to four oxygen which are arranged tetrahedrally around it and each oxygen atom is attached to two silicon atoms by covalent bonds.

Each corner is further shared by another tetrahedron.The entire crystal may be considered as a giant molecule in which eight- membered rings are formed with alternate silicon and oxygen atoms.

Properties

1) Since Si-O bonds are very strong , silica is inert and has high melting point.

2) It does not react with halogens , dihydrogen and most of the acids and metals even at high temperature.

3) It is attacked by F₂ , HF and NaOH.

SiO₂ + 2 F₂  ———-> SiF₄ + O₂

SiO₂ + 4 HF ———> SiF₄ + 2 H₂O

SiF₄ + 2 HF ——> H₂SiF₆

SiO₂ + 2 NaOH —–> Na₂SiO₃ + H₂O

Due to the formation of H₂SiF₆ , SiO₂ present in glass dissolves.There HF cannot be stored in glass bottles but is stored in containers made up of copper or metal alloy.

4) Quartz is used as piezoelectric material.

Amorphous form of Silica

2 Important amorphous forms of silica are : silica gel and Kieselguhr

Silica gel

Preparation

1) By the action of water on silicon tetrachloride or tetrafluoride

SiCl_{4 +} 4H₂O ————> 4 HCl + SiO₂.2H₂O

2) By the action of HCl on soluble silicate when gelatinous ppt. of silica gel are obtained.

Na₂SiO₃ + 2 HCl+ H₂O ——-> Si(OH)₄ + 2 NaCl

3) By dehydrating silicic acid

Si(OH)₄ ——–> SiO₂. 2 H₂O

Uses

1) It is used as a catalyst in petroleum industry and as an adsorbent in column chromatography.

2) When (NH₄)₂CoCl₄ is added to silica gel, it is used as a humidity detector since it is blue when dry but turns pink when hydrated.