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The p - Block Elements (Group 13 Elements)

        Boron (B), Aluminium (Al), Gallium (Ga), Indium (In), Thallium (Tl) belong to 13th group (or IIIA) of the periodic table. General electronic configuration is ns2 np1. B exhibits - 3,  Tl exhibits +1 (due to inert pair effect) and others +3 oxidation states. B is non metal, others are reactive metals. Atomic radius, density, electropositive character, tendency to form ionic compounds, tendency to show +1 oxidation state, Lewis acid strength of trihalides of B (due to X-B Π bonding) are increasing trends in the group. I.P. E.N., E.A., B.P., M.P. (decrease gradually from B to Ga and increase to Tl), tendency to form covalent compounds are decreasing trends in the group. Some of the exceptions are: atomic size of Ga < Al, I.P. of Tl > In, I.P., of Ga ≈ Al.
       They form M2O3 type oxides, M(OH)3 type hydroxides, MX3 type halides. B2O3 is acidic, Al2O3, Ga2O3 are amphoteric, In2O3, Tl2O3 are basic oxides. B(OH)3 is mono basic acid,
Al (OH)3, Ga(OH)3 are amphoteric, In(OH)3, Tl(OH)3 are basic hydroxides. Trihalides of B are covalent, fluorides of other elements are ionic. Al is the 3rd most abundant element by weight. Important minerals of B are Borax (Na2B4O7.10H2O), Kernite (Na2B4O7.4H2O), Boric acid (H3BO3), Colemanite (Ca2B6O11.5H2O), Al are Corundum (Al2O3), Diaspore (Al2O3.H2O),
Bauxite (Al2O3.2H2O), Gibbsite (Al2O3.3H2O), Felspar (Al2O3.K2O.6SiO2), Cryolite (3NaF.AlF3)


Anomalous behaviour of B:
   Due to small atomic size, high I.P., 2 electrons in penultimate shell, B show anomalous properties. B is non-metal, Al is amphoteric, others are metals. B forms covalent compounds others form ionic compounds, B has diagonal relationship with Si, B does not displace H2 from acids, B2O3 is acidic, B(OH)3 is acidic, Covalency of B is 4 (eg: BF4-) and others is 6 (eg: AlF6-3), halides of B hydrolise in water (except BF3) where as others undergo either partial or no hydrolysis.


Boron, Boric acid:
   B can be prepared by reducing B2O3 with Mg. In this reaction 95-98% pure amorphous (Moissan) boron is obtained. Crystalline boron is obtained by the reduction of BCl3 with H2 at high temperature. The same can be obtained by pyrolysis of BI3. B is a non conductor of electricity and sublimes at high temperature. Borox on reaction with hot, conc. HCl gives orthoboric acid.
                       Na2B4O7 +   2 HCl + 5 H2O 4 H3BO3 + 2 NaCl

B is essential for the healthy growth of plants, used as a semi conductor, neutron absorber, used in shielding of atomic piles. H3BO3 is used as antiseptics, in glass, enamel industries.
BORAX
         Borax is most common meta borate. Impure borax is called 'TINCAL'. Its molecular formula is Na2B4O7. 10H2O (or Na2[B4O5(OH)4]. 8 H2O). Borax dissolves in water to give an alkaline solution due to anionic hydrolysis.
                              Na2B4O7 + 7 H2O 2 NaOH + 4 H3BO3
Borax exists in 3 crystalline forms:
       Octahedral borax (Na2B4O7. 5H2O),
       Prismatic borax (Na2B4O7. 10H2O) and
       Anhydrous sodium tetraborate (Na2B4O7).
Borax bead Test:  Borax on heating, losses water and swells up and forms opaque mass of anhydrous sodium tetraborate. On fusion, gives borax glass (sodium meta borate + B2O3). B2O3 with meta borates of Co gives blue bead, Cu gives green bead, Fe gives yellowish brown bead, Cr gives yellow bead, Mn, Ni give violet bead. Borax bead test is useful in the identification of basic radicals in qualitative analysis.


        CoO + B2O3 Co(BO2)2
                                 Cobalt meta borate (blue bead)
      Borax is used as preservative, for cleaning hides & skin, for making pyrex glass, as a flux in metallurgy in soldering, in welding, in the identification of basic radicals.


Boron hydrides
  Compounds of B & H are called boron hydrides. They are classified into BnHn+4 type, BnHn+6 type boranes. The Simplest boron hydride is diborane (B2H6). On large scale, it can be prepared by reducing BF3 by LiH
                        
B2H6 is colourless, highly toxic gas, has B.P. 180K. 1t catches fire in O2 with the evolution of high amount of heat energy.
                     B2H6 + 3 O2     B2O3  +  3 H2O                    ∆H = -1976 K.J.


Other reactions of B2H6:
             


         B2H6 on heating with ammonia at 120° C gives diammoniate of diborane, on heating upto 200°C gives borazine (or Borazole), called 'inorganic benzene' as it is similar to benzene ring.


                          Structure of Borazine
                          


Structure of Diborane:
B = 1S2 2S1 2Px1 2Py1 2Pz1 (1st excited state)
         In diborane there are 12 valence electrons (B2H6 = (2 × 3) + 6 × 1 = 12 e-) available to form only six bonds. When it is compared with C2H6 [(2 × 4) + 6 × 1 = 14 e- = 7 bonds] there is a shortage of 2e-to form 7 bonds. So B2H6 is called "electron deficient compound". In B2H6 both B atoms undergo sp3 hybridization and giving 3 sp3 orbitals with unpaired electrons and one is empty. 4 sp3 hybrid orbitals of 2B atoms overlap with 1s orbitals of H atoms to form 2 BH2 groups. One sp3 hybrid orbital with one unpaired electron of one B atom and one sp3 hybrid orbital without unpaired electron overlaps with 1s orbital of H atom giving 3 centred 2 electron B- H-B bridge (banana) bond. 2 such non planar banana bonds are formed. 4 terminal hydrogens and 2 boron atoms lie in one plane and 2 bridged hydrogens lie above and below the plane.

Uses of Boron and their compounds:
¤ Boron fibres are used in making bullet proof vest.
¤ Metal borides are used in nuclear industry.
¤ Borox, Boric acid are used in the manufacture of pyrex glass, fibre glass.
¤ Borox is used as a flux for soldering metals.
¤ Boric acid is used as mild antiseptic.

ALUMINIUM
        Al is widely used for making trays, utensils, electrical cables, as a deoxidizer in metallurgy, in alumino thermite process for instant welding, for making alloys.

Alloy

Composition

Uses

Magnalium

Mg (2-15%), Al (85-98%)

Cheap balances,
utensils in the laboratory

Gamma -alloy

Al (92.5%), Cu (4%)

Ni (2%), Mg (1.5%)

Making parts of

aeroplanes

Duralumin

A(95%), Cu (4%)

Mn (0.5%), Mg (0.5%)

Making aeroplanes

Aluminium

bronze

Cu (88-90%)

Al (10-12%)

Coins, photo frames,

utensils, cheap jewellry

Reactions of Al:
2 Al + 2 OH - + 8 H2 3 H+  2 [Al(OH)4 (H2O)2]-
                                           Meta Aluminate
2 Al + 6 OH  2 AlO3-3 (Aluminate) + 3 H2

2 A+ 6 HC(Dil./Conc.)  2 AlCl3 + 3 H2
2 A+ 3 H2SO4 (dil.)  Al2 (SO4)3 + 3 H2
2 A+ 6 H2SO4 (Conc.)  Al2 (SO4)3 + 6 H2O + 3 SO2
8 A+ 3 OHNO3 (dil.)  8 Al(NO3)+ 3 NH4NO3 + 9 H2O
Ais passive in Conc. HNO3 due formation of protective layer Al2O3.

 

Alums
Double salts (Sulphates) with general formula X2SO4. Y2(SO4)3. 24 H2O are called 'Alums'. Where X = Na+, K+, Rb+, Cs+, NH4+
Y = Al+3, Cr+3, Mn+3, Fe+3
Alums are isomorphous, soluble in water, each cation is surrounded with 6 H2O molecules. Due to small atomic size, Li+ cannot form alums. When alum is dissolved in water, solution becomes acidic due to cationic hydrolysis. Alum loses water of crystallization on heating.
Sodium alum        : Na2SO4. Al2(SO4)3. 24 H2O
Manganese alum : K2SO4. Mn2(SO4)3. 24 H2O
Ammonium alum : (NH4)2SO4. Al2(SO4)3. 24 H2O
Ferric alum : (NH4)2SO4. Fe2(SO4)3. 24 H2O
Potash alum : K2SO4. Al2(SO4)3. 24 H2O
Some alums appeared like alums but not alums are called "Pseudo alums".
           eg : MnSO4. Al2(SO4)3. 24 H2O
                  ( '.' Oxidation state of Mn = +2)
    Among alums, Potash alum is commonly used so it is called as 'Common alum'. It is used to stop bleeding of cuts (styptic), as mordant in dyeing fabrics in the purification of muddy water (loading). It can be prepared by the following methods.

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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