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The p - Block Elements (Group - 14 Elements)

    Carbon (C), The king of elements; Silicon (Si) present in ceramics, glass, cement, computer chips; Germanium (Ge) used in transistors, semi-conductors; Tin (Sn), present in alloys bronze, bell metal, gun metal; Lead (Pb) used in alloys like type metal, wood metal, in nuclear reactions.
                  Carbon , Silicon, Germanium, Tin , Lead  belongs to 14th (or IV A) group in the periodic table. General electronic configuration of valency shell is ns2 np2. They exhibit +4, +2 (inert pair effect eg: Pb) oxidation states. C, Si are non metals, Ge, Sn are metalloids, Pb is a metal. Atomic radii, metallic nature, tendency of formation of ionic compounds, reducing character of hydrides are increasing trends in the group. I.P., E.N., E.A. non metallic character, thermal stability of hydrides, tendency of formation of covalent bonds are decreasing trends in the group. some of the exceptions are: I.P. of Pb > Sn.
                  C can form only pπ-pπ bonding, other elements can form dπ-pπ bonding. CO2, SiO2 are acidic, GeO2, SnO2, PbO2 are amphoteric. Due to filling of d-orbitals in Ge & Sn, f-orbitals in Pb in their valency shells, all the elements (except C) possess same electro negativities.


Order of EN:   C > Si= Ge =Sn = Pb 
        Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings. This property is called 'catenation'. This property is due to very high bond dissociation energy (for C-C bond).
        Important minerals of Si are Thorite (ThSiO4), Zircon (ZrSiO4), Quartz (SiO2), Feldspar (KAlSi3O8).  Sn are cassiterite (SnO2), Pb are Galena (PbS), Cerussite (Pb CO3), Anglesite (Pb SO4).
       These elements can form monoxides (MO) and dioxides (MO2). CO is neutral, GeO is acidic, SnO, PbO are amphoteric. C, Si, Ge are unaffected by water. Sn gives SnO2 & H2 when reacts with steam. Due to formation of protective oxide film on the surface, Pb is un reactive with water. Pb gives NO on reaction with dil. HNO3, Si gives Na2SiO3 (Sodium Silicate) & H2 on reaction with NaOH, Sn Gives Na2SnO3 (Sodium Stannate) and H2 on reaction with NaOH, Pb gives Na2PbO3 (Sodium plumbate) on reaction with NaOH. The order of reactivity with halogens is
F2 > > Cl2 > Br2 > I2.            
        Due to small atomic size, high E.N., I.P., un availability of d orbitals, C exhibit anomalous behaviour such as covalency of C is 4 (others is 6), it has high tendency of catenation (others low), can form multiple bonds (other can't), hydrides are stable (others less stable), Tetra halides of C does not hydrolyse (others hydrolyse), CCl4 is not lewis acid (other halides of Si, Ge are lewis acids).
                  Carbon is able to show allotropic forms due to catenation and Pπ-Pπ bond formation. C exhibits amorphous, fullerene, crystalline allotropic forms. Amorphous allotropes wood charcoal, animal charcoal are used as adsorbents to remove colouring matter in the manufacture of sugar, gas carbon, petroleum coke are used to prepare electrodes (as they are conductors of electricity), lamp black is used to prepare indian ink, coke is reducing agent and used to prepare fuel gases, sugar charcoal is the purest form of carbon. Crystalline allotropes of carbon such as diamond is used in cutting glass, marble, drilling rocks, sharpening instruments to make dies to draw thin wires diameter of hair) wires, in jewellry as Gem. Graphite is used in making lead pencils, as lubricant, to make electrodes, refractory crucibles, in electroplating and electrotyping, to paint stoves and furnaces, activated charcoal is used to absorb noble gases.    
 The existence of an element in different physical forms but possessing same chemical properties is known as allotropy.


Structure of Diamond:
      Each carbon in diamond undergoes sp3 hybridisation. Central carbon is bonded to 4 carbon atoms to form tetrahedron. Diamond has three dimensional polymeric tetrahedron network. So it is very hard solid, possesses high melting point of 42000K. C-C bond length is 154 Pm. The bond angle is 1090 28'. As all the 4 valence electrons of valence shell of carbon involves in strong covalent bonds, diamond is bad conductor of electricity. It has high refractive index. It is insoluble in any solvent. On heating up to 20000 C it changes to graphite. 

 
Graphite:
      It is prepared by Acheson's process by heating coke and sand up to 3500 ºC in electric furnace.
3 C + SiO2  SiC + 2CO
SiC Si ↑ + C (graphite)


Structure of Graphite:
      Each carbon atom in graphite undergoes sp2 hybridisation. By using three sp2 hybrid orbitals each C is going to form 3 covalent bonds with 3 other carbon atoms in the same plane to form two dimensional hexagonal layers. These layers are held together by weak vander waals force of attraction and makes graphite soft and can be used as lubricant. 4th valence e of C remains unpaired (free). All these unpaired e of C atoms makes graphite good conductor of electricity. C-C bond length is 141.50 Pm. Distance between 2 successive layers is 340 Pm. Bond angle is 1200 . Melting point is 3773K. 

 

Fullerenes:
Third important crystalline allotrope of Carbon is fullerene. It was discovered in 1985 by Kroto, Smalley and Curl. Fullerene is formed by heating of graphite in an electric arc in presence of inert gas like He or Ar. Fullerene has cage like structure, similar to soccer ball (C 60). Carbon undergoes sp2 hybridization. It is the purest form of carbon.
It contains 20 six membered rings and 12 five membered rings. Delocalised electrons make it aromatic. Δf H() values of graphite, diamond, fullerine are 0, 1.9, 38.1 K.J. mole-1 respectively.

C - C distance is 143.5 pm, whereas
C = C distance is 138.3 pm.

 

CO and CO2 :
CO is prepared as follows.

* 2 C (s) + O2 (g) 2 CO (g)

Water gas: It is prepared by passing steam over white hot coke.

Producer gas: It is prepared by passing air over red hot coke

CO is colourless gas, used in the extraction of metals.

   ZnO + CO  Zn + CO2

CO forms metal carbonyls. It is highly poisonous and forms carboxy haemoglobin, resulting in death.


CO2 is prepared as follows:

 CaCO3 + 2 HCl  CaCl2 + CO2 + H2O

It is colourless gas. CO2 is used in aerated drinks. It gives tangy taste due to formation of H2CO3. H2CO3/ HCO3 - buffer system helps to maintain pH of blood between 7.26 to 7.42. 0.03% (by volume) CO2 is present in atmosphere and is useful in photosynthesis.

Excess of CO2 in atmosphere causes "Green house effect". Solid CO2 (dry ice) used as a refrigerant. Hybridization of C in CO2 is 'sp'.

Silica (SiO2):
     It is widely distributed in earth's crust. It has two types of allotropes. Crystalline allotropes are Quartz, Tridymite and Crystobalite. Amorphous allotropes are Jaspar, Agate and Onyx.
             Quartz is the purest form of silica. It can be prepared by the following methods

Silica displaces CO2 from metal carbonates
                    Na2CO3 + SiO2

 CO2↑ + Na2SiO3


Structure of SiO2:
    Si in SiO2 undergoes sp3 hybridisation. Each Si forms 4 strong covalent bonds with 4 oxygen atoms present at tetrahedron.
    Like diamond SiO2 possess a giant 3 dimensional tetrahedral network.
    Each oxygen in SiO2 is shared by two Si atoms. So ratio of Si : O is 1 : 2 
    SiO2 is used to prepare sand paper, flux in metallurgy, to prepare glass, mortar, as building material.
 
    CO2 is gas, has linear structure has double bonds, Vander waals forces, acidic nature, low M.P., B.P. Where as SiO2 is solid, has 3-D polymeric tetrahedron structure, has single bonds, strong covalent bonds, weakly acidic nature and high M.P, B.P.

 

SILICONE: Due to presence of CH3 groups, silicones are hydrophobic. They are used as sealant, grease, electrical insulator and water proofing of fabrics.

SILICATES: Man made silicates are glass and cement. Naturally occuring silicates are feldspar, mica, zeolites, asbestos. The basic structural unit of silicates is SiO4-4 where Si is bonded to 4 oxygen atoms in tetrahedron fashion. When 1 or 2 or 3 or 4 oxygen atoms are shared by tetrahedron units, pyro or cyclic (or chain) or sheet or 3D (frame work) silicates are formed.

ZEOLITES: Zeolites are formed when few Si atoms are replaced with Al atoms. Feldspar and zeolites very useful. Zeolites are used as a catalyst in petrochemical industry and as ion exchangers in softening of hard water.
A type of zeolite ZSM - 5 (Zeolite Socony Mobil - 5) having formula.
           
                                                 

NanAlnSi96-nO192.16H2C is used in isomerisation of m-xylene to p-xylene in interconversion of hydrocarbon and alkylation of aromatic compounds. It is also useful in converting alcohols directly into gasoline.

Posted Date : 04-08-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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