Silicates (SiO4)-4: salts of silicic acids:

(1) Silicates is the general term applied for the solids with silicon – oxygen bonds.

(2) Silicates are regarded as the salts of silicic acid, H₄SiO₄. All the silicates are comprised of SiO₄ units.

(3) The Silicates units have a tetrahedral structure formed as a result of sp3 hybridization. Silicon atom has its complete octet but each oxygen atom is still short of one electron to complete its octet. They can complete their octet by taking up 4 electrons from a metal, getting converted to an anion [SiO4]^-4

(4) All the solids Silicates contain silicate ion (SiO4)^4- as the basic structural unit.

(5)The silicate ion is tetrahedral in structure and when the one or more oxygen atoms between such tetrahedrons, a complex structure arise.

(6) All the silicates units are non planer

(7) The Si atom in Silicates unit is covalently bonded to 4 oxygen atoms. Each oxygen atom possesses a formal negative charge. Hence each tetrahedral unit has a formal charge of –4. When linked together, the extended units are also negatively charged. Presence of other metallic ions such as Ca²⁺or Mg²⁺ are necessary for electrical neutrality.

(8) The covalent Si-O bond, having a bond enthalpy of 466 kJ mol^-1 is particularly strong compared with the C-C bond which has a bond enthalpy of 347 kJ mol^-1. The linkage -Si-O-Si-O- is very stable and instead of existing as discrete units of (SiO4)^4- ions, the silicates tend to form chains, sheets or networks.

 CLASSIFACATION OF SILICATES:

The silicates may be classified in to following groups chain silicates, ring silicates, cyclic silicates, sheet silicates, three – dimensional silicates depends on the way in which the (SiO4)4- tetrahedral units are linked together.

(1) Ortho silicates or Neso Silicates (SiO4)^4-: 

Number of oxygen shared by tetrahedron is Zero(0).Orthosilicates are salts of orthisilicic  acid

Example; Zircon ( ZrSiO₄ ), Phenacite (Be₂SiO₄) , Willemite (Zn₂SiO₄),  

(2) Pyro silicates or Sorosilicates or Disisilicates (Si₂O₇^-6) :    

(1) Number of oxygen atoms shared per tetrahedron is 1(one).

(2)Total number of shared oxygen towards one Si atom is ½

(3)Total contribution of all oxygen towards one Si atom is 3.5

(4) In pyrosilicates, one tetrahedron shares its one oxygen atom with other tetrahedron on they are salts of pyrosilicic acids .

Example: Thortveitite (Se₂Si₂O₇); lanthanoids disilicates (Ln₂[Si₂O₇] and Hemimorphite  Zn₄ (OH)₂[Si₂O₇].

(3) Meta silicates: 

 (A) Linear chain Meta Silicates [(SiO₃)_n ^-2n  ] :

In chain silicates each tetrahedron shares its  two  oxygen atoms with other tetrahedron atom such that a linear endless chain of silicates is formed , however in terminating chain silicates (n) tetrahedron  shares one oxygen.

Example: Spodumene (LiAlSiO₃)₂ ; Wollastonite [Ca₃(SiO₃)₃] Enstatite [Mg₂SiO₃)₃] and Diopsite [ CaMg(SiO₃)₂]

(B) Cyclic meta silicates [(Si_nO_3n) ^-2n  ] :

(1) Number of oxygen atoms shared per tetrahedron is 2(Two).

(2) Total number of shared oxygen towards one Si atom is one (1)

(3) In cyclic silicates each tetrahedron shares its two oxygen atoms with other tetrahedron atom such that a cyclo silicates is formed.

(4) There are several cyclo silicate is known like n= 3, 4, 5 ,6 but  3 and 6 is most common.

Example: Wollasponite [ Ca₃(Si₃O₉)] , Benitoite [BaTi(Si₃O₉)] , Beryl [Be₃Al₂(Si₃O₉)₂]

(4) Double chain Silicates or Amphiboles  (Si₄O₁₁)^-6n :

(1) Number of Silicon atoms in basic units is 2 .

(2) Total number of oxygen atoms in basic unit is 5.5.

(3) Number of oxygen atoms shared per tetrahedron is 3+2/2=2.5

(4) In this type silicates two strands of chain silicates are linked to each other by sharing oxygen atoms, Amphiboles are an asbestos mineral which contains magnesium.

(5)  General formula of double chain silicates (Si₄O₁₁)^-6n

Example: Tremotites  [Ca₂Mg₅(Si₄O₁₁)₂(OH)₂] Crocidolite [Na₂Fe₃Fe₂(Si₄O₁₁)₂(OH)₂]

(5) Sheet Silicates or phyllo silicates [(Si₂O₅)_n]^-2n:

In Sheet Silicates, one tetrahedron shares its three (3) oxygen atom such that  a two dimensional non planer layer is formed , such type of silicates have greasy touch or lubricating action.

Example:  Clay [Al₂(Si₂O₅)(OH)_{4 ,}Talc or Soap Stone  [Mg₃(Si₂O₅)₂(OH)_{2 ,} Kaolinite  [Al₂(Si₂O₅)(OH)₄  Chrysotite (white Asbestos)  [Mg(Si₂O₅)(OH)₄

(6) 3D-Silicates:

Examples: Silica, Quarts, feldspar, Zeolite, Ultramarine

Related Questions:

(1) What is "Zeolite's" ?

(2) What are most common cyclic meta silicates?

(3) What are the Silicones? Give one example one cyclic Silicone?

SILICONES: (R2SiO WITH -Si-O-Si- LINKAGE):

(1) Silicones are organosilicon polymeric compounds containing repeated R₂SiO units and (-Si-O-Si-) linkage.

(2) The name is given silicone because their empirical formula is analogous to that of ketones (R₂CO).

(3)  The Silicones are form by hydrolysis of silicone tetra chloride (SiCl₄) .we know that CCl₄ do not hydrolyses by water at room temperature While SiCl₄  undergoes   water Hydrolysis  to corresponding OH group.  

                          CCl₄ +H₂O¾® no hydrolysis

                     But super heated steam gives phosgene gas.

                      CCl₄ + H₂O ¾®COCl₂ + 2HCl

(4) SiCl₄ under goes hydrolysis due to presence vacant d orbital and gives Silicic acid followed by dehydration gives 3D Silicates (SiO₂)

     

PREPARATION OF SILICONES:

It is two step process

Step: (1) Preparation of Organosilicon halides  as silicone intermediates.

Step: (2) hydrolysis of Organosilicon halides followed by condensation polymerisation.

Step: (1) [A] FROM FRIGNARD REAGENT:

Step: (1) [B] BY DIRECT HEATING PROCESS:

Note-The Yield of  above reaction is 50% R₂MgCl₂ and  50 % ( R₃MgCl + RMgCl₃), now these can seperated by fractional distilation.

Step: (2) HDROLYSIS FOLLOWED BY CONDENSATION:

(1) RMgCl₃ (R = Me or Ph): On hydrolysis  and followed by condensation give 3D network cross linked Silicoes It also provides the crosslinking among the chain making the polymer more hard and hence controlling the proportion of RSiCl₃ we can control the hardness of polymer. 

(2) R₂SiCl₂ (R = Me or Ph): On hydrolysis  and followed by condensation give linear as well as cyclic Silicones depending upon number of Silicon atomes.  Commercial silicon polymers are usually methyl derivatives and to a lesser extent phenyl derivatives. They are prepared by the hydrolysis of R₂SiCl₂ (R = Me or Ph).

Note-Silicones may have the cyclic structure also having 3, 4, 5 and 6 nos. of silicon atoms within the ring. Alcohol analogue of silicon is known as silanol.

Note-Cyclic Silicone have Sp3 oxygen and Silicon atoms so cyclic Silicones have following properties .they are non planer ,polar and have Back Bonding  and chair confermer.

(3) R₃SiCl (R = Me or Ph): On hydrolysis  and followed by condensation  so only dimmerisation take place due to presence of single OH group.

Note- R₃SiCl use in a certain proportion we can control the chain length of the polymer  due to this reason R₃SiCl is called as chain stopping unit.

INERTNESS OF SILICONES: DUE TO..

(1) Silicones are chemically inert due to back bonding between oxygen and Silicone atoms

(2) High bond energy of Si-C and Si-O bond also due to Back Bonding.

(3) Alkyl group constitute hydrophobic part which act as water repellent hence nucleophilic attack retarded .

USES OF SILICONES:

(1) Silicones are chemically inert, water repelling nature, heat resistance and having good electrical insulating properties.

(2) Silicones are used as sealants, greases, electrical insulators and for water proofing of fabrics, car polish, shoe polish and masonry works in buildings

(3) Silicones can be used as electrical insulator (due to inertness of Si-O-Si bonds)

(4) Silicones are used as antifoaming agent in sewage disposal, beer making and in cooking oil used to prepare potato chips.

(5) Silicones use as a lubricant in the gear boxes

Silicon and Its Compounds:

(1) SILICON (Si): 

(1) The Silicon was discovered by Berzelius in 1824.

(2) The silicon name is taken from Latin silver which means “flint”.

(3) Silicon is the second most abundant (27.2%) element after oxygen (45.5%) in the earth's crust.

(4) Silicon is a crystalline semi – metal or metalloid. One of its forms is shiny, grey and very brittle. In another allotropic form silicon is a brown amorphous powder most familiar in “dirty” beach sand.

(5) Silicon does not occur free in nature but in the combined state, it occurs widely in form of silica (SiO₂) and silicates (SiO₄ ^-4) .

 (6) All mineral rocks, clays and soils are built of silicates of magnesium, aluminium, potassium or iron.

(7) Aluminium silicate is however the most common constituent of rocks and clays.

(8) the most common compound of silicon is Silica and Silica (SiO₂) is found in the Free State in sand, flint and quartz and in the combined state as silicates like

(i) Feldspar   K₂O.Al2O₃. 6SiO₂

(ii) Kaolinite   Al₂O₃. 2SiO₂. 2H₂O

(iii) Asbestos CaO. 3MgO. 4SiO₂

(8) Silica (SiO_­2) is the most abundant chemical compound in the earth’s crust.

PREPARATION SILICON:

(i) From silica (sand): Elemental silicon is obtained by the reduction of silica (SiO2) with high purity coke in an electric furnace

(ii) From silicon tetrachloride (SiCl4) or silicon chloroform (SiHCl3): Silicon of very high purity required for making semiconductors is obtained by reduction of highly purified silicon tetrachloride or silicon chloroform with dihydrogen followed by purification by zone refining.

PHYSICAL PROPERTIES:

(i) Elemental silicon is very hard having diamond like structure.

(ii) It has shining luster with a melting point of 1793K and boiling point of about 3550K.

(iii) Silicon exists in three isotopes, i.e. ₂₈Si¹⁴, ₂₉Si¹⁴ and ₃₀Si¹⁴ but ₂₈Si¹⁴ is the most common isotope.

CHEMICAL PROPERTIES:

Silicon is particularly unreactive at room temperature towards most of the elements except fluorine.

Some important chemical reactions of silicon are discussed below.

(i) Action of air: Silicon reacts with oxygen of air at 1173K to form silicon dioxide and with nitrogen of air at 1673K to form silicon nitride,.

(ii) Action of steam: It is slowly attacked by steam when heated to redness liberating hydrogen gas.

(iii) Reaction with halogens: It burns spontaneously in fluorine gas at room temperature to form silicon tetra fluoride (SiF4).

However, with other halogens, it combines at high temperatures forming tetra halides.

(iv) Reaction with carbon: Silicon combines with carbon at 2500 °C forming silicon carbide (SiC) known as carborundum. Carborundum is an extremely hard substance next only to diamond. It is mainly used as an abrasive and as a refractory material.

USES OF SILICON:

(i) Silicon is added to steel as such or more usually in form of ferro silicon (an alloy of Fe and Si) to make it acid-resistant.

(ii) High purity silicon is used as semiconductors in electronic devices such as transistors.

(iii) It is used in the preparation of alloys such as silicon-bronze, magnesium silicon.

(2) SILICA (SiO2):

Silicon is unable to form pp - pp bond with oxygen atom due to its relatively large size. Thus it satisfies its all four valency with four oxygen atoms and constitutes three - dimensional network. In this structure each oxygen atom is shared by two silicon atoms. Three crystalline modification of SiO₂ are quartz, cristobalite and tridymite of which quartz and cristobalite are important.

Quartz (rock crystal) is the purest form of silica. It is used in preparation of costly glasses and lenses. It is also used as piezoelectric material (crystal oscillators and transducers).

Several amorphous forms of silica such as silica gel and fumed silica are known. Silica gel in made by acidification of sodium silicate and when dehydrates, is extensively used as a drying agent in chromatographic and catalyst support.

(3) Silicon Carbide (SiC): Carborundum:

Silicon carbide (SiC) is a compound of silicon and carbon. It is extremely rare on Earth in mineral form (moissanite) and it has semiconductor properties. It has a bluish-black appearance. It has a large number of crystalline forms.Silicon Carbide (SiC): Carborundum:

(4) SILICATES:

(1) Silicates is the general term applied for the solids with silicon – oxygen bonds.

(2) Silicates are regarded as the salts of silicic acid, H₄SiO₄.All the silicates are comprised of SiO₄ units.

(3) The Silicates units have a tetrahedral structure formed as a result of sp3 hybridization. Silicon atom has its complete octet but each oxygen atom is still short of one electron to complete its octet. They can complete their octet by taking up 4 electrons from a metal, getting converted to an anion [SiO4]^-4

(4) All the solids Silicates contain silicate ion (SiO₄)^4- as the basic structural unit.

(5)The silicate ion is tetrahedral in structure and when the one or more oxygen atoms between such tetrahedrons, a complex structure arise.

(6) All the silicates are non planer.

 CLASSIFACATION OF SILICATES:

The silicates may be classified in to following groups chain silicates, ring silicates, cyclic silicates, sheet silicates, three – dimensional silicates depends on the way in which the (SiO₄)^4- tetrahedral units are linked together

(1) Ortho silicates or Neso Silicates

(2) Pyro silicates or Sorosilicates or Disisilicates  

(3) Meta silicates (A) Cyclic meta silicates (B) Linear chain Meta Silicates

(4) Double chain Silicates or Amphiboles

(5) Sheet Silicates or phyllo silicates

(6) 3D-Silicates

(5) SILICONES (R2SiO WITH -Si-O-Si- LINKAGE):

(1) Silicones are organosilicon polymeric compounds containing repeated R₂SiO units and (-Si-O-Si-) linkage.

(2) The name is given silicone because their empirical formula is analogous to that of ketones (R₂CO).

(3)  The Silicones are form by hydrolysis of silicone tetra chloride (SiCl₄) 
.
USES OF SILICONES:

(1) Silicones are chemically inert, water repelling nature, heat resistance and having good electrical insulating properties.

(2) Silicones are used as sealants, greases, electrical insulators and for water proofing of fabrics
                           SILICONES: (R2SiO WITH -Si-O-Si- LINKAGE):

ALLOTROPES OF CARBON:

A characteristic property of the elements of carbon family is that these show allotropy. Example: carbon has two important allotropic forms i.e.  Crystalline and amorphous

(1) DIAMOND:

(1) Each carbon is linked to another atom and there is very closed packing in structure of Diamond.

(2) Density and hardness is very much greater for diamond because of closed packing in diamond due to sp3 hybrid and are tetrahedrally arranged around it. And C-C distance is 154 pm

(3) Diamond has sharp cutting edges that's why it is employed in cutting of glass.

(4) Diamond crystals are bad conductor of electricity because of absence of mobile electron.

(5) 1 carat of diamond = 200 mg.

(6) Diamond powder if consumed is fatal and causes death in minutes.

(2) GRAPHITE: 

(1) In Graphite Carbons are sp² hybridised out of the four valence electrons, three   involved in (sp²-sigma) covalent bonds form hexagonal layers and fourth unhybridised p– electron of each carbon forms an extended delocalized p-bonding with carbon atoms of adjacent layers

 (2) Each carbon is linked with 3 carbons and one carbon will be left and form a two dimensional shed like structure.

(3) Distance between two layers is very large so no regular bond is formed between two layers. The layers are attached with weak vander waal force of attraction.

(4) The carbon have unpaired electron so graphite is a good conductor of current.

(5) The C-C bond length within a layer is 141.5 pm while the inter layer distance is 335.4 pm shorter than that of Diamond (1.54 Å).

(6) Due to wide separation and weak interlayer bonds, graphite is sift , greasy and a lubricant character and low density.

(7) Graphite marks the paper black so it is called black lead or plumbago and so it is used in pencil lead.

(8) Composition of pencil lead is graphite plus clay .the percentage of lead in pencil is zero .

 (9) Graphite has high melting point so it is employed in manufacture of crucible.

(10) Graphite when heated with oxidizing agents like alkaline KMnO4 forms mellatic acid (Benzene hexa carboxylic acid).

(11) Graphite on oxidation with HNO₃ gives acid i.e. known as Graphite acid C₁₂H₆O₁₂

(3) FULLERENCES:

(1) A fascinating discovery was the synthesis of spherical carbon-cage molecules called fullerences. The discovery of fullerene was awarded the noble prize in chemistry (1996). Fullerenes   were first prepared by evaporation of graphite using laser.

(2) Fullerences are sooty material so formed consists of C₆₀ with small amount of C₇₀ and other fullerences containing an even number of carbon up to 350

(3) Fullerences have a smooth structure and unlike diamond and graphite, dissolved in organic solvent like toluene.

(4)  C₆₀ is the most stable fullerene. It has the shape of a football and called buckminsterfullerene

(5) C₆₀ consists of fused five and six membered carbon rings

(6) Six membered rings surrounded by alternatively by hexagons and pentagons of carbon.

(7) Five membered rings are surrounded by five hexagons carbon rings.

(8) There are 12 five –membered rings

(9) There are 20 Six –membered rings

(10) In fullerenes all the carbon sp2 hybridised each carbon formed three sigma bond and the fourth electron delocalized to formed pi bond .

(11) All the carbon atom are equivalent but all C-C bond are not equivalent.

(12)  In the structure C-C bonds of two different bond length occur at the fusion of two six membered rings the bond length is  C-C = 135.5 pm and at the fusion of  five and six membered rings C-C bond length is 146.7 pm.

(13) There are both single and double bonds

(14) The smallest fullerenes are C₂₀.

(15) Thermodynamically the most stable allotrope of carbon is considered to be graphite. This is due standard enthalpy of formation of graphite is taken zero .while enthalpy of formation of diamond and fullerenes are 1.90 KJ/Mole and 38.1 KJ/Mole respectively.