Silicates Notes for class 11 Chemistry

class 11 chemistry Notes Silicates

Binary compounds of silicon with oxygen are called silicates but they contain other metals also in their structures.

(i)             Since the electronegativity difference between O & Si is about 1.7, so Si–O bond can be considered 50% ionic & 50% covalent.

(ii)           If we calculate the radius ratio then,r_Si⁴⁺/r_O^2- = 0.29

It suggests that the coordination number of silicon must be 4 and from VBT point of view we can say that Si is sp3 hybridized. Therefore silicate structures must be based upon SiO44–  tetrahedral units.

(iii)      SiO44– tetrahedral units may exist as discrete units or may polymerise into larger units by sharing corners.

            Silicates have basic unit of SiO₄^4–, each silicon atom is bonded with four oxide ions tetrahedrally.

            Types of silicates

(i)    Orthosilicates( ):These silicates contain single discrete tetrahedral unit of

            e.g., Zircon                  ZrSiO₄;            Forsterite or Olivine   Mg₂SiO₄                     

                     Phenacite           Be₂SiO₄;          Willemite                    Zn₂SiO₄

      (ii) Pyrosilicates( ):  These silicates contain two units of  joined along a corner containing oxygen atom. These are also called as island silicate.

(iii) Cyclic structure: If two oxygen atoms per tetrahedron are shared to form closed rings such that the structure with general formula (SiO32–)n or (SiO3)n2n– is obtained, the silicates containing these anions are called cyclic silicates. Si3O96– and Si6O1812– anions are the typical examples of cyclic silicates.

      (iv) Chain silicates

(a)Chain silicates may be further classified into simple chain & double chain compounds. In case of simple chains two corners of each tetrahedron are shared & they form a long chain of tetrahedron. Their general formula is also same as the cyclic silicates i.e.  (SiO3)n2n–.

      (b) Similarly, double chain silicates can be drawn in which two simple chains are joined together by shared oxygen. Such compounds are also known as amphiboles. The asbestos mineral is a well known example of double chain silicates. The anions of double chain silicates have general formula (Si4O11)n6n

            e.g.,  Synthetic silicates          Li₂SiO₃, Na₂SiO₃

                     Spodumene                    LiAl(SiO₃)₂

                     Enstatite                         MgSiO₃;

                     Diopsite                         CaMg(SiO₃)₂

                     Tremolite                       Ca₂Mg₅(Si₄O₁₁)₂ (OH)₂

(v) Two dimensional sheet silicates

            In such silicates, three oxygen atoms of each tetrahedral are shared with adjacent tetrahedral units. Such sharing forms two-dimensional sheet structure with general formula .

(vi) Three dimensional sheet silicates:

             These silicates involve all four oxygen atoms in sharing with adjacent  tetrahedral units.  e.g. Quartz, tridymite, crystobalite, feldspar, zeolite and ultramarines.

 Glass

            Glass is an amorphous and transparent solid which is obtained by solidification of various silicates and borates of potassium and calcium.

(1)       Preparation : Ordinary glass is a mixture of sodium and calcium silicates and is produced by fusing  together a mixture of sodium carbonate, calcium oxide and silicon dioxide ( Silica) in a furnace at about 1700K

                       

                       

            On continuously heating the entire amount of is driven out and clear viscous fused mass is obtained. It is poured into moulds to get different types of articles, which are allowed to cool gradually.

            This type of glass is called soda glass or soft glass which has the approximate composition, .