Chemistry PREPARATION, PROPERTIES AND USES OF COMPOUNDS OF SILICON
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PREPARATION, PROPERTIES AND USES OF COMPOUNDS OF SILICON

Hydride :

Silane `Si_nH_(2n+2) `(`SiH_4` & `Si_2H_6`)

Only these two are found.

Higher molecules are not formed. `Si` can't show catenation property.

Hot `Mg + Si-text(vap) rightarrow Mg_2Si overset[text(dil.)H_2SO_4] rightarrow MgSO_4 + SiH_4 + Si_2H_6 +......`


`SiH_4` is more reactive than `CH_4` because of the following reasons :

(i) `Si^(delta+)- H^(delta-)`, `C^(delta-) - H^(delta+)`

`C` - electronegative than `H`

`Si` less electronegative than `H`

So bond polarity is reversed when `Nu^-` attacks, it faces repulsion in `C` but not in `Si`

(ii) Silicon is having vacant `d`-orbital which is not in case of carbon

(iii) Silicon is larger in size compared to `C`. By which the incoming `Nu^-` doesn't face any steric hindrance to attack at `Si` whereas `CH_4` is tightly held from all sides.

`SiC` (Carhorundom) :

Preparation :

`SiO_2 + 2Ctext(coke) underset(2500^oC) overset[2000 text(to)] rightarrow Si + 2COuparrow`

`Si + C underset(2500^oC) overset(2000^oC) rightarrow SiC` diamond like structure colourless to yellow solid in room temp. `rightarrow text(when impurity is present)`

Properties :

(i) It is very hard and is used in cutting tools and abrassive powder (polishing material).

(ii) It is very much inert.

(iii) It is not being affected by any acid except `H_3PO_4`.

Silicones :

It is organo silicon polymer.

`C Cl_4 + H_2O rightarrow text(no hydrolysis)`

but `C Cl_4 + undersettext(super heated)(H_2O) rightarrow COCl_2+2HCl`

`SiCl_4 + H_2 oversettext(steam) rightarrow Si(OH)_4+4HCl`

`Si(OH)_4 underset(-2H_2O) overset(Delta) rightarrow SiO_2 text(3-D silicate)`

`R_2SiCl_2 + H_2O underset(-2HCl) rightarrow R_2Si(OH)_2 underset(H_2O) overset(Delta) rightarrow undersettext(Linear silicone)(-O-R_2Si-O-R_2Si-O-R_2Si-O)`

`R_2C Cl_2 + H_2O undersettext[-2HCl text(looses) H_2O text(readily)] rightarrow R_2C(OH)_2 overset(-H_2O) rightarrow (R-C=O-R)`

Silicones may have the cyclic structure also having `3`, `4`, `5` and `6` numbers of silicon atoms within the ring. Alcohol analogue of silicon is known as silanol. See fig.1.

`R_3SiCl overset(H_2O) rightarrow undersettext(silanol)(R_3SiOH) underset(Delta) overset(-H_2O) rightarrow R_3Si-O-SiR_3`

`R_2SiCl_2 + R_3SiCl undersettext(hydrolysis) overset(H_2O) rightarrow undersettext(heating condensation) overset(-H_2O) rightarrow R_3Si-O-R_2Si-O-R_2Si--`

See fig.2.

Using `R_3SiCl` in a certain proportion we can control the chain length of the polymer.

See fig.3.

It provides the crosslinking among the chain making the polymer more hard and hence controlling the proportion of `RSiCl_3` we can control the hardness of polymer.

Uses :

(i) It can be used as electrical insulator (due to inertness of `Si-O- Si` bonds).

(ii) It is used as water repellant (because surface is covered) eg. car polish, shoe polish, massonary works in buildings.

(iii) It is used as anti foaming agent in sewage disposal, beer making and in cooking oil used to prepare potato chips.

(iv) As a lubricant in the gearboxes.

Silica `(SiO_2)` :

Occurrence :

Silica or silicon dioxide occurs in nature in the free state as sand, quartz and flint and in the combined state as silicates like, Feldspar : `K_2 O*Al_2O*6SiO_2`, Kaolinite : `Al_2O_3*2SiO_2*2H_2O` etc.

Properties :

(i) Pure silica is colourless, but sand is usually coloured yellow or brown due to the presence of ferric oxide as an impurity.

(ii) Silicon dioxide is insoluble in water and all acids except hydrofluoric acid.

`SiO_2(s) + 4HF(l) rightarrow SiF_4(l)+2H_2O(l)`

(iii) It also combines with metallic oxides at high temperature giving silicates e.g.

`SiO_2(s)+CaO(s) overset(Delta) rightarrow CaSiO_3(s)`

When silica is heated strongly with metallic salts, silicates are formed and the volatile oxides are driven off as vapours.

`SiO_2(s) + Na_2CO_3(s) oversettext(high temp.) rightarrow Na_2SiO_3(s)+CO_2(s)`

`SiO_2(s) + Na_2SO_4(s) oversettext(high temp.) rightarrow Na_2SiO_3(s)+SO_3(g)`

`3SiO_2(s)+Ca_3(PO_4)_2(s) oversettext(high temp.) rightarrow 3CaSiO_3(s)+P_2O_5(s)`

The first two examples quoted here are important in glass making.

Structures of Silica : Silica has a three-dimensional network structure. ln silica, silicon is `sp^3`-hybridized and is thus linked to four oxygen atoms and each oxygen atom is linked to two silicon atoms forming a three-dimensional giant molecule as shown in figure. This three dimensional network structure imparts stability to `SiO_2` crystal and hence a large amount of energy is required to break the crystal resulting in high melting point.

Uses :

(i) Sand is used in large quantities to make mortar and cement.

(ii) Being transparent to ultraviolet light, large crystal of quartz are used for making lenses for optical instruments and for controlling the frequency of radio-transmitters.

(iii) Powdered quartz is used for making silica bricks.

(iv) Silica gel `(SiO_2*xH_2O)` is used as a desiccant (for absorbing moisture) and as an adsorbent in chromatography.
 
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