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Ssilicon Dioxide Structure

Silicais widely distributed in nature and it is the common constituent of earth's crust . It exhibits allotropy . It is found in nature in different forms like crystalline and amorphous forms . Quartz , tridymite , and cristobalite are the crystalline allotropes of silicon dioxide to the crystal . Coloured crystals are used as gems .


Quartz , tridymite and cristobalite , the three crstalline polymorphs have a low temperature `alpha`  form and a high temperature `beta`  modification .


Low  temperature       α –quartz         α –Tridymite        α Cristobalite

                                      845 K                 400-435 K              475-550 K            ↕475-550 K

High temperature       β- quartz   ↔       β- Tridymite      ↔         β- Cristobalite

                                                          1140 K                            1740 K                  ↕1985 K

    The purest form of silica is  quartz or rock crystal.Sand is the crushed form of quartz and sandstone  is sand particles with iron oxide bound to them.  Flint consists of amorphous silica associated with quartz. Kiesulguhr is a siliceous rock composed of the remains of minutesea organisms.


Amorphous forms of silica:-


 Agate, onyx,jasper are the amorphous  forms of silica. Silica is found in plants and animals both.

Silica can be prepared in the laboratory by the following methods:

1.Burning of silicon in air or oxygen gives silicon dioxide

Si  + O2→SiO2

2.  Silica is formedby the hydrolysis of silicon tetrafluoride or tetrachloride.

3SiF+4H2O→H4SiO4 +2H2SiF6

SiCl4 + 4H2O→ H4SiO4 +4HCl

OrthoSilicic acid obtained by the above reaction loses water at 1000⁰C and gives pure silica.


  H4SiO4 → SiO2+ H2O

PROPERTIES:-

3.  It is insoluble in water.

4.       It does not react with HCl,HBr,HI.  It react with HF  and gives silicon tetra  fluoride.

SiO2 +4HF →SiF4+ 2H2O

It reacts with alkalies to form silicates.

SiO2 +2NaOH →Na2Sio3 + H2O

At high temperatures , it reacts with metal oxides and gives silicates.

SiO2 +Na2 ONa2Sio3

SiO2 +CaO →CaSio3

When silica is heated with carbon in an electric furnace, it is reduced to carborundam,SiC.

SiO2 +3C→SiC+2CO

Whenheated to 1600°C , silica melts to form quartz glass. Red hot quartz glass when pluged into water does not break into pieces. Due to this  nature , it is used in making optical instruments.


STRUCTURE OF SILICA:-


 

 

                                           STRUCTURE  OF SILICA


Likediamond silica also exists as giant molecules and has got a three dimensional structure. In this giant molecule ,each silicon atom  is linked to four oxygen atoms by covelent bonds and they are arranged tetrahedrally around silicon. The crystal structure of silica is  compared with the C-C  bonds of diamond  the Si-O-Si bonds in silica areweak.Hence, it is not as hard as diamond and melts at lower temperature. It exists in the solid state due to its giants molecular structure

Inthe giant molecular structure of silicaon dioxide each silicon atom hasonly half the share of each of  the four oxygen atoms linked with it. Therefore silicon dioxide is given the formula SiO2. The ratio of silicon and oxygen atoms in silicon dioxide is 1:2. 


uses of silica dioxide:

  1.  SiO2 is used as a building material.
  2.  Bricks made from a mixture of powdered sand, clay and lime are used for lining the furnace used in the manufacture of steel.
  3. It is used as an acid flux in metallurgy.
  4. Colouredvarieties of quartz are used as gems while the transparent  varieties of quarz are used for the manufacture of lenses,optical instruments.
  5. Quartz glass used for making special glassware suitable for working with u.v radiation as quartz is transparent.