9. GROUP 13 ELEMENTS (IIIA)

Size: px

Start display at page:

Download "9. GROUP 13 ELEMENTS (IIIA)"

Silvester Stanley
6 years ago
Views:

1 SYNOPSIS: 9. GROUP 1 ELEMENTS (IIIA) GENERAL CARACTERSTICS Boron, Aluminium, Gallium, Indium and belong to III A group of the Periodic table. The general outer electronic configuration of III A group elements is 1 ns np B and Al show similar properties due to similar outer electronic configuration. They show differences in some of the properties due to the difference in the number of electrons present in their penultimate shells. Electronic Configurations : Element Symbol At No E.C. Boron B 5 1 [ e] s p 1 Aluminum Al 1 [ Ne] s p 1 1 Gallium Ga 1 [ Ar] d s p 1 1 Indium In 9 [ Kr] d 5s 5p Thallium Tl 81 [ Xe] f 5d 6s 6p OCCURANCE : B does not occur in free state in nature. B occurs in the form of salts of boric acid Al is most abundant metal and third most abundant element (FirstO, second Si) by weight. Al forms nearly 7.8% of the earth's crust. Al occurs in combined state as oxide, fluoride and silicates. Ga, In, and Tl occur in trace quantities along with sulphides of Zinc and Lead Element Abundance (%) Boron 1 Aluminum 8.1 Gallium 1. 1 Indium Thallium VARIATION OF PROPERTIES Boron is a non metal. 1

2 Group 1 Elements (IIIA) Al, Ga, In, and are metals. Atomic radius increases suddenly from B to Al. This is due to the greater screening effect of electrons present in the penultimate shell. The atomic radii of Al and Ga are same. This is because of the poor shielding effect of d-electrons in Gallium Density increases from Boron to Thallium The m.p.'s and b.p's of III A group elements do not show any regular trend. This is due to the difference in their crystal structures. Melting points order B > Al > Tl > In > Ga Boiling points order B > Al > In > Ga > Tl Melting point of Gallium is very low because it contains Ga molecules. Boron has high m.p because it exists as a giant covalent polymer both in liquid and solid Gallium exits as a liquid between 9.8 C and C. So Gallium is used in high temperature thermometry. The order of B> Tl > In> Ga> Al. The most electropositive element in III A group is Al. The compounds of Al and Ga (Ex: AlCl and GaCl ) are covalent only in the anhydrous state and Ionic in aqueous solution because their hydration energies, compensate the ionaization energies. Boran forms an oxide and a nitride when burnt in air. But Al does not undergo any change and remains in tact because it forms an oxide layer At high temperature Al gives its nitrides. The common oxidation state of these elements is + Boron shows - and + oxidation states. Ga, In and Tl exhibits + and +1 oxidation states

3 Group 1 Elements (IIIA) Due to inert pair effect +1 oxidation state of Tl is more stable than its + oxidation state. ANOMALOUS PROPERTIES OF BORON Boron is non mental. It always forms covalent compounds Boron show diagonal relationship with Si. Boron does not displace hydrogen from acids Oxide of boron B O is acidic oxide ydroxide of boron is acidic ( ) ( BO orbo ) Maximum covalency of boron is Boron form stable covalent hydrides which are known as bornanes. Boron never appears as a cation Among halides of boron only BF undergoes complete hydrolysis. Boron Preparation : In the laboratory, it is prepared by reducing BO with metallic magnesium % pure boron is obtained on reducing BO with Mg or Na metal at high temperature. It is known as "Moissan Boron". This is amorphous Boron. Δ BO + Mg B+ MgO bright red hot BO is obtained from borax Borax is treating with hot conc. Cl gives BO Boric acid ( BO ) on fusion loses water and forms BO. Na B O + Cl + 5 O NaCl + BO (Orthoboric acid) 5k red heat BO B O 7 O 7 tetraboric acid BO BO 75k O Metaboricacid O red heat O BO Pyrolysis of BI (or) thermal decomposition of boranes also gives crystalline Boron. PYSICAL PROPERTIES : Boron exists in two allotropic form a) Amorphous B b) Crystalline B. Amorphous B dissolves in molten Al at 15 C which on cooling gives yellow colored crystalline solid. Al is eliminated by dissolving in Cl. This results in very hard crystalline B. B is a non conductor of electricity B sublimes at ordinary pressure CEMICAL PROPERTIES :

4 Action with air (or) O : Amorphous B, on heating in air to about B + O B O B+ N BN Group 1 Elements (IIIA) 7 C, form BO and BN With non metals : On heating amorphous B will react with N, O, S, F, Cl and Br directly forming binary compounds B + O B O 1 C Boron tri sulphide B+ S B S ardest compound of Boran is Boran carbide ( BC ) Reducing property : B is a powerful reducing agent for CO and SiO. SiO + B B O + Si With Conc. acids B gives boric acid B + NO BO+ NO B + SO BO+ SO When fused with alkalies, B liberates B+ 6NaO Na BO + USES : B is a good neutron absorber. B and BC are used in the shielding of atomic piles and in the control rods used for controling of chain reactions. igh purity B is used as a semi conductor in the place of Ge and Si Boron is essential minor element for the healthy growth of plants BORAX : The most common meta borate is borax. [ ] Borax available in nature as tincal or kerinite or Razorite ( Na B O. ) minarals decahydrate is obtained by extracting. BO : Structure of Metaborate ion ( 9 ) NaBO.1 OorNa BO( O).8O O. From these two PROPERTIES : Borax is sparingly soluble in cold water. Its aqueous solution is alkaline in nature due to hydrolysis Na B O + O.NaO + B O 7 7 Borax exists in three crystalline form Na B O.1 O a) Prismatic borax ( ) 7

5 b) Octahedral borax ( NaBO7.5 O ) c) anhydrous sodium tetraborate ( NaBO 7) Group 1 Elements (IIIA) BORAX BEAD TEST : This test is useful in the identification of basic radicals in qualitative analysis. On heating borax, swells into a white, opaque mass of anhydrons sodium tetra borate. When it is fused, borax glass is obtained B O. combined with metal oxides give coloured beads fused NaBO.1O NaBO NaBO + BO 7 7 ( ) BO + CoO Co BO (Cobalt metaborate). (Blue bead) S.NO 1 5 Basic radical Metal metaborate formed Cu + ( ) Fe + ( ) Cr + ( ) Mn + ( ) Ni + ( ) Colour of the bead Cu BO Green Fe BO Yellowish brown Cr BO Yellow Mn BO Violet Ni BO Violet USES OF BORAX Borax bead test is used in the identification of basic radicals. This test uses borax Borax is used as a flux in soldering, in welding and in certain metallurgies In making optical glasses Pyrex glass. In leather industry for cleaning hides and skin As preservative borax is useful BORIC ACIDS : BO in water gives boricacids Ortho boric acid BO or BO.O Meta boric acid BO or BO. O Tetra boric acid BO 7 or BO. O Pyro boric acid 6BO 7 or BO.O PREPARATION OF BORIC ACID : Ortho boric acid is prepared from Colemanite( CaB6O 11). Finally powdered colemanite is suspended in hot water and SO gas is passed to saturate it. The reaction is Ca B O + 9 O + SO CaSO + 6 BO 6 11 When hot solution of borax is mixed with SO gives boric acid N B O + SO + 5 O Na SO + BO 7 5

6 PROPERTIES It is a mono basic acid BO loses water on heating Group 1 Elements (IIIA) at1 C + BO BO O 16 C 7 + BO B O 5 O BO B O + O hightemp BORON YDRIDES : The ores of Boron are The hydrides of boron are called boranes. These are electron deficient molecules. They can be classified as boranes( ) n n n n + ( n n + ) type ( Bn + n + 6 ) type 6 Diborane 1 Tetra borane 5 9 Penta borane Penta borane exa borane exa borane -1. B1 1 Deca borane-1 B116 Deca borane -16. PREPARATION OF 6: Industrial method : BF on reduction with Li gives Diborane. 5 BF + 6Li K B + 6LiF 6 + and hydroboranes ( ) BCl on reduction with LiAl gives 6 ( Lab method) dryether BCl + LiAl B + LiCl + AlCl 6 A mixture of BCl and at low pressure when subjected to silent electric discharge gives 6 ( Lab method) PROPERTIES OF 6 : It is stable in the absence of grease and moisture. It reacts with water forming boric acid and 6 + 6O BO + 6 It reacts with alkalies with evolution of. + KO+ O KBO potassiummetaborate It reacts with Cl to give BCl. 6 6

7 B + 6Cl BCl + 6Cl 6 Group 1 Elements (IIIA) 6 + Br 5Br+ Br It burns in O gas to give BO 6 + O BO+ O In presence of, diborane reacts with Cl to give chloro diborane 6 + O BO+ O It reacts with CO at 1 C and atm pressure to give borane carbonyls. 6 + CO CO It reacts with sodium amalgam to give Na 6 It reacts with ammonia at 1 C to form diammoniate of diborane. Which is on strong heating gives borazole and hydrogen 1 C 6 + N B Δ 6.N C.N BN 6 Δ 6 borazole or inorganic benzene At high temperature Δ / at C nbn BN + n ( ) 6 n STRUCTURE OF 6: The hybridisation of Boron in Diborane is sp. In 6 two Boron atoms and four hydrogen atoms are in one plane. The four hydrogen atoms which lie in one plane are called terminal hydrogen atoms. The remaining two hydrogen atoms are called bridge hydrogen atoms. This is further confirmed by methylation of diborane to give Me One of the bridge hydrogen atom is above the plane of the molecule and the other bridge hydrogen atom is below the plane of the molecule The terminal atoms involve in covalent bonds. Bridge atoms are involved in three centred two electron bond. This is called Banana bond or ydrogen bridge bond or Tau bond. The bridge hydrogen atoms involve three centre two electron bonds. Diborane molecule has covalent bonds, de-localised bonds. There is no direct bond between Boron atoms. o 1.A 11.5 (B 97 (B o 1.19A 7

8 ALUMINIUM : Bauxite is the important mineral Bauxite contains impurities such as FeO (red Bauxite) and SiO (white Bauxite) Group 1 Elements (IIIA) CEMICAL PROPERTIES OF AL : Al conc.no 8 No reaction dil.no ( ) conc. SO ( ) dil. SO Al( SO) + dil / conc.cl Al NO + N NO + O Al SO + + O SO AlCl + dil.nao ( ) ( ) Fused NaO Al O O + AlO + Aluminium metal is used in Making electrical cables. Making trays and picture frames In metallurgy as deoxidiser for removing blow holes. Making light and strong alloys. These alloys are used in making parts of air crafts. Painting Iron surface to prevent its rusting. Thermite welding. Al foils are used for wrapping cigarettes, chacolates etc. ALLOYS OF ALUMINIUM : S.NO Alloy Composition Uses 1 Magnalium -15% Mg Balance beams, 85-98% Al Lab articles etc. Aluminum Bronze Duralumin γ alloy 1-1% Al 9-88% Cu 95% Al % Cu.5% Mg.5% Mn 9.5% Al.% Ni.% Cu 1.5% Mg Utensils, jewellery In making air ships In making aeroplane Parts

9 ALUMS: Alums are double sulphates. The general formula of an alum is X SO. Y( SO). O X is a mono valent ion like Na, K, Rb, Cs, N Y is a trivalent ion like Fe, Al, Cr K SO Al SO O Potash Alum -. ( ). ChromeAlum - ( ) FerricAlum - ( ) ( ) SodiumAlum - ( ) K SO. Cr SO. O N SO. Fe SO. O Na SO. Al SO. O Group 1 Elements (IIIA) The co-ordination number of any cation in the alum is 6. Every cation in any alum is co-ordinated with six water molecules. Lithium cannot form the alums because the maximum covalency of Lithium ion is only. Aqueous solution of any alum is acidic due to the hydrolysis of cation. PREPARATION OF POTAS ALUM By concentrating solution, containing equimolar quantities of potassium sulphate and Aluminium sulphate, to the point of crystallization alum separates out. { } KSO.Al SO.Al O is boiled with dilute SO and a calculated amount of potassium sulphate and crystallized to get alum. Alunite ( ) ( ) Alum shale or pyrites shale ( Al O. xsio FeS ) + is roasted in air and the mass is digested with dil SO. Ferrous sulphate formed in the roasting process is removed by fractional crystallization. PROPERTIES ALUM : It is an ionic compound and so does not exist in the molecular forms in solutions It is a crystalline substance, soluable in O. The aqueous solution is acidic in nature due to hydrolysis It loses its water of crystallization on heating. USES OF POTAS ALUM : Used in the purification of water As a mordant in dyeing industry. For tanning leather. As a styptic to arrest bleeding. 9

AP Chemistry A. Allan Chapter 18 - The Representative Elements: Groups 1A through 4A

AP Chemistry A. Allan Chapter 18 - The Representative Elements: Groups 1A through 4A 18.1 A Survey of the Representative Elements A. Basic Trends 1. Metals tend to lose electrons and form cations 2. Nonmetals