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UNIT 6 ELEMENTS of GROUP 13 Structure 6.1 Introduction Objectives I 6.2 Oailcrence, Extraction and Uses Occurrec - Extraction Uses I 6.3 General Characteristics

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UNIT 6 ELEMENTS of GROUP 13 Structure 6.1 Introduction Objectives I 6.2 Oailcrence, Extraction and Uses Occurrec - Extraction Uses I 6.3 General Characteristics UNIT 6 ELEMENTS OF GROUP 13 structure 6.1 Introduction Objectives I 6.2 Oailcrence, Extraction and Uses Occurrec - Extraction uses i 6.3 General Characteristics I - 6.5 Halides of Bpron anel Aluminium Halides of Boron Halides of Aluminium 6.6 Oxides of Boron and Aluminium I Boric Oxide Aluminium Oxide 6.7 Oxoacids of Boron and Borates 6.8 Borazine 6.9 Complexation Behaviour 6.10 Anomalous Behaviour of Boron 6.11 Summary 6.12 Terminal Questions 6.13- Answers ' -- 6.1 INTRODUCTION In the previous two units, you studied the main features of the chemistry of Group 1 and Group 2 elements, i.e. the alkali and the alkaline earth metals. In this unit you - will study the elements of Group 13, namely, boron, aluminium, gallium, indium and, thallium. While studying the alkali and alkaline earth metals, you have seen that all Zhe elements of these two groups are highly reactive metals and the first element of each group shows some differences from the rest. In Group 13, the differences between the first element and the remaining elements become so pronounced that the first member of the group, i.e. boron is a nonmetal wheieas the rest of the elements are distinctly metallic in nature. In a way, this is the first group of the periodic table in which you observe a marked change in the hature of the elements . down the group. describe the chemistry of hydrides, halides and oxides of boron and aluminium, elucidate the structures of hydrides of boron and aluminium, 6.2 OCCURRENC~,EXTRACTION AND USES Elements bf Group 13 are sufficiently reactive. Therefore, none of them occurs in the native state. Some of these elements and their compounds find important uses in diverse,areas of modern science and technology and even in every day life. Let us, therefore, first study their occurrence, extraction and raes. 5 6.2.1 Occurrence Both boron and aluminium have a high affinity for oxygen, so neither of them is found native. Boron occurs principally in the earth's crust as boric acid, H3B03and as borates, such as, borax, Na2B407-10H20,kernite, Na2B407.4H20and colemanite, Ca2B,0,,.5H20. Aluminium is the most abundant metal (8.13%) in the earth's crust and is the third most abundant element, next only to oxygen (46.6%) and silicon (27.7%)). It occurs widely in the complex alumino-silicates, such as, clay from whicb, however, it cannot be extracted economically. The im ortant ores of aluminium are bauxite, A1,0,-xH20, where x = 1-3, cryolite, Na3!lF, and corundum, ~1~0;.Gallium (1.9 x lo--'%), indium\(2.1 x lod5%) and thallium (7.0 x lo"%) are much less abundant than aluminium. Gallium and indium are found in aluminium and zinc ores. But even the richest sources contain less than 1% galliuni and still less indium. Thallium is widely distributed in nature and occors in sulphide ores of zinc, lead, copper and iron. \ 6.2.2 Extraction Boron is-obiamed by the red-uction of B2O3wittimagnesium or sodium. B203is first prepared by strongly katingH,B03 which isobtained by the action of HCI or H2S04- on a concentrated solution of borax: Pure crystalline boron may be obtained in small quantities by the reduction of BBr, with H2 on a heated tantalum metal filament at 1275-1475 K. ' \ Bauxite approximately contains Aluminium cannot be extrpcted economically from the silicate minerals. Therefore, A~,o,=ss%. Fe203=159~, bauxite is the most important ore for the extraction of alumimum, but this has many Ti02=2%, sio2=39" and problems. A1203is a very stable compound. It is not reduced by heating in hydrogen. H20=25%. On strong heating with carbon, A120, gives the carbide, AI,C,. As A1203 does not India is fortunate in having large melt below 2300 K, it cannot be electrolysed conveniently. However, electrolysis of deposltsof bauxitein a reasonably a solution of A120, in fused cryolite occurs at a much lower temperature of 1100-1300 pure state. K. Thus, aluminium is extracted by electrolysis of purified alumina in fused cryolite. Pure alumina is obtained from bauxite ore. Powdered bauxite mineral is heated in a concentrated solution of sodium hydroxide under pressure when alumina and silica get dissolved: A1203 + 2NaOH + 3H20 2 NaAI(OH)4 Si02 + 2NaOH NazSi03 + H20 Iron oxide and titanium dioxide do not dissolve in the alkali and are filtered off as a sludge. The solution is cooled and most of the aluminium hydroxide is precipitated either by the passage of carbon dioxide or by seeding with some+reshlyprecipitated - aluminium hydroxide : 2NaAl(OH)4 + C02 ~A~(OH),4 + Na2C03 + H20 NaAl(OH)4 ~46~)~J + NaOH The silicates remain in solution, since silica is a more acidic oxide than alumina. Aluminium hydroxide is filtered, washed and heated to give pure alumina. '.4 Alumma is dissolved in fused cryolite to which calcium fluonfe is added to lower the vlt~ngpoint. The solution is then electrolysed at 1100-1300 K in an iron cell, lined For each kgof A1 produced. about Mth graphite, which acts as the cathode and carbon rods suspended in the electrolyte kg Of A'2°3, 0.150 kg Of atting as the anode (Fig. 6.1). Electrolysis of the solution gives aluminium at the NaOHpO.SOkgoEC and 6'0 lo' cathode and oxygen at the anode. The discharged aluminium sinks to the bottom of kJ of electricity are consumed the cell and is tapped off. Fresh alumina is added as required. The anode is slowly attacked by liberated oxygen to form carbon monoxide. Therefore, anode has to be continually replaced, adding substantially to the cost of the process. The temperature of the cell is maintained by the passage of electric current. Following reactions take place during electrolysis: 6 - 302- . 3/20, .+ 6e, at anode ~AI~++ 6e 2A1, at cathode Gallium, indium and thallium are usually obtained by electrolysing aqueous solutions of their salts. This method is not applicable in the case of aluminium salts as they are hyal G:, sed considerably by water. -Carbon electrodes ~nnnn~iy: I=-II - -- olten crvoltte-AL~mixture-: Molten Al Fig. 6.1 : man^^ d Al 6.2.3 Uses Boron is used to increase the hardness of steels. Crystalline boron is used in transistors. Boron is a good neutron absorber and is used as shields and control rods in nuclear reactors. Boranes are used as high energy fuels, for example, in rockets. Boric acid is used as' an antiseptic. Borax is used to make heat resistant borosilicate glass. It is also used for vitreous enamelling of baths, domestic appliances and for glazing tiles and pottery. Another use of borax is to make peroxyborates, e.g., NaBO2.H2O2-3H20,which are useful cleaning and bleaching agents. In the laboratory, borax is used for standardising acids and in the borax bead test in qualitative analysis. ~lu&%umexhibits useful properties of low density, high thermai and electricat conductivity, good corrosion resistance with non-toxic nature of the metal and its compounds. Due to these properties, it is the most widely used non-ferrous metal. Aluminium is used for making electrical conductors, cooking utensils and wrapping When an alum~niurna91cle is made the anode in the electrolysis materials. You must have seen milk bottles capped with aluminium foil. Aluminium of dil. sulphuric or chromic acid, a is extensively used for structural purposes, either alone or alloyed, in aircraft, ship thick hard film of AI2O3is formed and buildiig industries. Large amounts of aluminium are converted into alloys, such on the surface of the article. This as, duralumin and magnalium containing a few per cent of copper or magnesium. process is called anodising. A1203 These alloys are harder and stronger than pure aluminium but possess almost the layer adsorbs many dyes and takes a very high polish. You must have same properties of lightness and corrosion resistance which can be further increased seen many bright, colourful by anodising process. Aluminium beryllium alloys are harder and Iighter than other articles made of anodised alloys of aluminium and are extensively used in space-craft. However, toxic nature aluminium. of beryllium makes their handling difficult. Due to its strong affinity for oxygen, aluminium is a good reducing agent and is used in aluminothermic process for extraction of metals and in welding. Suspension of finely powdered aluminium in oil is used as paint. Anhydrous is an important catalyst, used in organic synthesis and in the cracking of petroleum. A12(S04)3as such or as potash alum, K2S04.A12(S04)3.24H20 is used for sizing paper, for tanning leather, for waterproofing cloth and as a mordant for dying cotton. It is extensively used for purification of water and in sewage treatment. Because of its extreme hardness, high m.p., non-volatility, chemical inertness and good electrical insulating properties, corundum (A120,) finds many applications in abrasives, refractories and ceramics. Large crystals of a-A1203 when coloured with metal-ion impurities are prized as gemstones, e.g., ruby (cr3+, red), sapphire (Fe2+"+ni4+ , blue), oriental emerald (cr3+/V3+, green), oriental topaz (~e~+, yellow), etc. Aluminates are important constituents of Portland cement. Gallium has the longest liquid range (303-2343 K) of any known substance and so finds use as a high temperature thermometer liquid. Gallium is mainly used in semicondyctor technology. It is used for doping othcr semicondl~ctorsand in solid state devices such as transistors. Compounds of Ga with P and As; such as, Gap and GaAs have semiconductor , properties similar to those of elemental Si and Ge. These are used as light emitting diodes (LEDs) familiar in pocket calculators, wrist watches, etc.
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