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Chemistry

Chemistry. Session Objectives. Physical properties of group 13 elements Halides of group 13 elements Hydrides of group 13 elements Oxides of group 13 elements Extraction of aluminum Compounds and uses of aluminum Physical properties of group 14 elements. Elements of group 13.

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Chemistry

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  1. Chemistry

  2. Session Objectives • Physical properties of group 13 elements • Halides of group 13 elements • Hydrides of group 13 elements • Oxides of group 13 elements • Extraction of aluminum • Compounds and uses of aluminum • Physical properties of group 14 elements

  3. Elements of group 13 • B-Boron = [He]2s22p1 • Al-Aluminium = [Ne]3s23p1 • Ga-Gallium = [Ar]3d104s24p1 • In-Indium = [Kr]4d105s25p1 • Tl-Thallium = [Xe]4f145d106s26p1

  4. Gallium M.P. = 29.78oC Semiconductor Applications Gallium Arsenide Abundance and Occurrence of group 13 elements Aluminium is the most abundant metal known and third most abundant element by mass in the earth’s crust.Boron is a rare element and occurs as concentrated deposits of Borax, Kernite . Gallium is twice as abundant but indium and thallium are less common.

  5. Atomic and ionic radii In case of Ga, In and Tl, there are 10-d electrons in the penultimate shell causing weaker screening of nuclear charge and thus the increase in size is not very large.

  6. Ionisation Energy The ionisation energy values do not decrease smoothly down the group. The decrease from B to Al is the usual trend on descending the group associated with increased size.The poor shielding by d-electrons affect the value for latter elements.

  7. Oxidation States Maximum oxidation number shown by group 13 elements is +3.Boron and aluminum generally have +3 oxidation state in its compounds.The stability of lower oxidation state (+1 ) increases as we move down the group due to inert pair effect. The decreasing stability of higher oxidation state is due to the fact that the bond energy decreases from Al to Tl,and the energy required to unpair the ns2 electrons is not compensated by the energy released in forming two additional bonds

  8. Illustrative Example What will be the order of stability of trivalent cations among Ga, In, Tl? Solution : The correct order will be Ga3+ > In3+ > Tl3+

  9. Illustrative Problem Why there is similarity in atomic radii of Al and Ga ? Solution : Due to poor shielding effect of 3d electrons in Ga,the atomic radii of Al and Ga is almost same.

  10. Hydrides of group 13 elements Forms hydrides of MH3 type. Thermal stability decreases as we move down the group. BH3 > AlH3 > GaH3 > InH3 > TlH3 Weak Lewis acids. Readily form adducts with strong Lewis bases. Also form tetrahydrido anions, eg. [MH4]-. Lithium tetrahydrido- aluminate(III), LiAlH4 is most important compound which is obtained by the reaction. LiAlH4 is a white crystalline solid and soluble in diethyl ether and is used as reducing agent in organic chemistry.

  11. The electronic structure of diborane Three centre two electron bonds.

  12. Halides of group 13 element With halogens,group 13 elements give binary halides. Fluorides are ionic and have high melting points. Chlorides, bromides and iodides are covalent with low melting point. Monohalides, GaX, InX and TlX are known for X= Cl, Br and I because stability of +1 oxidation state increases down the group. GaX and InX disproportionate in water

  13. Halides of group 13 element Trihalides are strong Lewis bases. Lewis acid character: BBr3 > BCl3 > BF3. BF3 is the weakest lewis acid because it is less electron deficient due to back donation of electrons from F atoms.

  14. Structure of AlCl3. Vapour density measurements show that AlCl3 is monomeric over 800°. Structure of AlCl3 is planar triangular and the bonding is covalent. Below 400°C it exists as a dimer. In solid state also it exists as a dimer and is non conducting. Its structure is AlCl3.6H2O is used as deodorant and antiperspirant.

  15. Illustrative Example Boronchloride exists as a monomer while in the same group anhydrous aluminium chloride exists as dimer, why? Solution BCl3 cannot form a dimer because B atom is too small that it is unable to attract four large sized halide ions while Al can do so because of the larger size.

  16. Illustrative Example What property of anhydrous AlCl3 makes it a very good preparative reagent in organic chemistry? Solution Anhydrous AlCl3 acts as a Lewis-acid and accepts lone pair of electrons, which makes it a useful reagent in organic chemistry.

  17. Oxides of group 13 elements Group 13 elements form oxides of the type M2O3. Basic character increases as we move from Aluminium to thallium. Only B2O3 shows the acidic nature. B2O3 + 2NaOH 2NaBO2 + H2O Al2O3 + 6HCl 2AlCl3 + 3H2O Al2O3 + 2NaOH 2NaAlO2 + H2O

  18. Illustrative Example An element X which occurs in the second period has an outer electronic structure s2p1. What are the formula and acid-base character of its oxides?

  19. Solution Al2O3 Ga3O2 amphoteric The element with outer electronic configuration s2p1 belongs to group III in the periodic table. Eg: B, Al, Ga, In and Tl. B2O3 acidic In2O3 basic Tl2O3 basic On moving down the group there is a gradual change from acidic to amphoteric. Therefore, answer is X2O3, acidic.

  20. Aluminium and gallium hydroxides show amphoteric behaviour. Hydroxides of group 13 elements Group 13 elements form hydroxides of the type M(OH)3 In contrast Tl(OH)3 is insoluble in water and Tl(OH) is strong base. Many of Tl(I) compounds are similar to corresponding alkali metal compounds.

  21. Reactant Reaction with group 13 elements (M) Oxygen Nitrogen Halogen (X2) Water Acid Base 4M(s) + 3O2(g)  2M2O3(s) 2M(s) + 3N2(g)  2MN(s), M=B, Al 2B(s) + 3X2(g,l,s)  2BX3(g) 2M(s) + 3X2(g,l,s)  M2X6(g), M=Al, Ga, In 2Tl(s) + 3X2(g,l,s)  2TlX(s) 2Tl(s) + 2H2O(l)  2Tl(OH)(aq)+H2(g) 2M(s)+6H3O+(aq)2E3+(aq)+6H2O(g)+3H2(g) M=Al, Ga, Tl 2M(s)+6H2O(l)+2OH- 2E3+(aq)+6H2O(g)+3H2(g) M=Al, Ga, Chemical properties of group 13 metals Note that thallium is the most reactive element in this family, and is the only one that reacts directly with liquid water. Note also that aluminum and gallium are amphoteric.

  22. Wöhler KAl(SO4)2•12H2O Aluminum: Al Identified in 1807 it was not isolated until 1828. AlCl3 + 3K  Al + 3KCl Third most abundant element; 8.3% in earth crust after oxygen. Important minerals are Bauxite, Al2O3.xH2O where x=1 to 3; Cryolite, Na3AlF6 Orthoclase, KAlSi3O8 Mica (muscovite), KAl2(Si3AlO10)(OH)2; Beryl, Be3Al2Si6O18; and Corundum, Al2O3 Only commercial source is bauxite, a hydrated impure oxide.

  23. Sapphire Ruby Topaz Cr3+ in place of some Al ions. Fe3+ and Ti4+ impurities. With Fe3+ impurities. Alumina Alumina exists in two crystalline forms a–Al2O3 or corundum and g-Al2O3 Alumina is white but it can be coloured by the addition of impurities

  24. Extraction of aluminium 1.Purification of bauxite 2.Electrolysis of alumina Purification can be done in three ways. 1.Baeyer’s process 2.Hall’s process 3.Serpeck’s process

  25. Al2O3 2H2O+2NaOH 2NaAlO2 + 3H2O Sodium aluminate Bauxite ore Baeyer’s process Roasted ore is digested with conc. NaOH under pressure at 150°C which forms sodium meta aluminate (NaAlO2). Ferric oxide and silica are removed by filtration.

  26. Al(OH)3 Al2O3 + 3H2O Precipitated Al(OH)3. ppt Fe(OH)3 Baeyer’s process The NaAlO2 solution is agitated with freshly precipitated Al(OH)3 NaAlO2 undergoes hydrolysis with the formation of Al(OH)3 precipitate.

  27. Al(OH)3 Al2O3 + 3H2O Hall’s process Bauxite, is fused with Na2CO3 to form NaAlO2. The fused mass is extracted with water where Fe2O3 and SiO2 remain as insoluble residue. NaAlO2 is warmed to 50°-60°C and CO2 is circulated through it. Al(OH)3 is separated as precipitate. The precipitate is removed by filteration,washed and ignited to ger alumina

  28. Serpeck’s process This process is used when silica is present in considerable amount in bauxite ore. The ore is mixed with coke and heated at 1800° C in presence of nitrogen, where AlN (aluminium nitride) is formed. Silica is reduced to silicon which volatilises off at this temperature AlN is hydrolysed with water into Al(OH)3 which when ignited ,gives alumina.

  29. 2Al2O3 + 3C  4Al(l) + 3CO2(g) He was only 23!! Hall-Heroult Process

  30. Cryolite Na3AlF6 Role of cryolite in electrolysis Cryolite improves the electrical conductivity of the cell as Al2O3 is a poor conductor.In addition , the cryolite serves as an added impurity and lowers the melting point of the mixture to about 1140K

  31. Properties of aluminum Metallic, strong, excellent electrical conductor, strongly reducing, low density. Aluminum reacts with both acids and bases. Al+ 6HCl  2AlCl3 + 3H2 Al + 2NaOH +H2ONaAlO2 + H2

  32. Chemical properties 1.Action of air Reactive metal but becomes unreactive due to formation of oxide film. 2. Reaction with water: Not attacked by pure water but easily corroded by saline water. 3. Reaction with metallic oxides: Thermite reaction Has strong affinity for oxygen, so reduces the oxide of metals. The reaction is exothermic and can be used for welding.

  33. aluminium is used for because aircraft light, strong, resists corrosion other transport such as ships' superstructures, container vehicle bodies, tube trains (metro trains) light, strong, resists corrosion overhead power cables (with a steel core to strengthen them) light, resists corrosion, good conductor of electricity saucepans light, resists corrosion, good appearance, good conductor of heat Uses of aluminum

  34. Alums Potash alum is used as a mordant in dyeing and printing of textiles. Alums are used for softening of hard water.

  35. Anomalous behaviour Anomalous behaviour of the first element of a group. • Small size • high electronegativity • non-availability of d-orbitals.

  36. Elements of group 14 C – Carbon = [He] 2s2 2p2 Si – Silicon = [Ne] 3s2 3p2 Ge – Germanium = [Ar] 3d10 4s2 4p2 Sn – Tin = [Kr] 4d10 5s2 5p2 Pb – Lead = [Xe] 4f14 5d10 6s2 6p2

  37. Atomic Radii and Atomic Volume Radii and volume increase gradually on moving down the group.

  38. Ionisation Energy The ionisation potential decreases gradually from carbon to lead but not systematically. The value of lead is slightly higher than expected due to lanthanide contraction.

  39. Oxidation state Oxidation state: +4 Due to inert pair effect Sn and Pb exhibit +2 state Order of stability for +2 oxidation state is

  40. Catenation Self linking property of an element to form chains of covalent bonds with atoms of the same elements. C >> Si >> Ge » Sn > Pb

  41. Allotropy Allotropy: Except Pb, all others show allotropy. Carbon: diamond, graphite, charcoal.

  42. Allotropy Known since 3000B.C. -Sn  -Sn “white” tin Metallic in appearance. Tetragonal structure. Highly crystalline. “grey” tin crumbly Cubic structure Si and Ge: Crystalline and amorphous forms. Sn:-Sn(Grey) , -Sn(white)

  43. Illustrative Problem What is the percentage of lead in lead pencil ? Solution : Zero

  44. Illustrative Example SiF62– is known but SiCl62- is not. Why is it so? Solution : SiCl62–is not known due to following reasons (i) due to smaller size of F,steric repulsions will be less in SiF62-. (ii) Interaction of lone pair of electrons of F with Si is stronger than that of lone pair electrons of Cl.

  45. Thank you

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