CHAPTER # 4p-BLOCK ELEMENTS BORIC ACID There are three types of boric acids: Orthoboric acid (H3BO3) Metaboric acid (HBO2) Pyroboric acid (H2B4O7) ORTHOBORIC ACID It is a white needle like crystalline solid in which H3BO3 units are joined together by hydrogen bond to produce infinite layers of boric acid. These layers are 3.18 Ao apart. There exist weak Vander Waal forces between these layers. Boric acid is very soft because the layers of boric acid can slide over one another. PREPARATION FROM BORAX It is prepared by action of H2SO4 on aqueous solution of borax (Na2B4O7). The reselling solution on cooling gives crystals of Basic acid. Na2B4O7 + H2SO4 4H3BO3 + 5H2O + Na2SO4 FROM BORON NITRIDE When boron nitride reacts with hot water then boric acid is obtained. BN + 3H2O H3BO3 + NH3 PROPERTIES It is white odorless crystalline solid. It is sparingly soluble in cold water. It is fairly soluble in hot water. It is a very weak mono basic acid. (Lewis Acid) B(OH)3 + H20 B(OH)4- + H+ Melting point: 171 C o Boiling point: 300o C EFFECT OF HEAT ON BORIC ACID At 100oC H3BO3 HBO2 + H2O (metaboric acid) At 140oC 4HBO2 H2B4O7 + H2O (pyroboric acid) At 300oC H2B4O7 2B2O3 + H2O (boron oxide) USES It is used as Antiseptic in medicine (especially eye wash). In making glass. In making enamels, glazed potteries. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 1 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS BORAX Na2B4O7. 10H2O (Suhaga) It is a salt of boric acid. It is also called Sodium Tetraborate Decahydrate or Tincal. PREPARATION It can be prepared By neutralizing boric acid with strong base. 4H3BO3 + 2NaOH Na2 B4 O7 + 7H2O By adding Na2CO3 to hot solution of boric acid. 4H3BO3 + Na2CO3 Na2B4O7 + 6H2O + CO2 On recrystallization from aqueous solution yields Na2B4O7. 10H2O called sodium tetra borate decahydrate (Suhaga). PROPERTIES It is white crystalline solid, with composition Na2B4O7 .10H2O. It is soluble in water. Alkaline Nature Its solution is alkaline due to hydrolysis. Na2B4O7 + 7H2O 4H3BO3 + 2NaOH. Its aqueous solution on acidification gives boric acid. Na2 B4O7 + 5H2O + H2SO4 Na2SO4 + 4H3BO3 ACTION OF HEAT When heated looses water forming glossy anhydrous borax. Na2B4O7 . 10H2O Na2B4O7 + 10H2O heat (anhydrous borax) USES It is used for Softening of water. Manufacture of borax glass (Pyrex). Making tiles and glazed pottery. Soldering and welding as flux. Making optical fiber. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 2 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS ALUM Alum is a series of double sulphate salts of monovalent cations and trivalent cations containing 24 molecules of water of crystallization GENERAL FORMULA Family of alum can be represented by a simple formula: M2SO4.T2(SO4)3. 24H2O Where M = monovalent cation (Na+, K+,NH4+) T = trivalent cation (Al+3,Fe+3,Cr+3) NOMENCLATURE OF ALUM If alum contains aluminium then it is named with monovalent cation. For example: K2SO4.Al2(SO4)3.24H2O (potash alum) If alum does not contain aluminium then it is named with trivalent cation. For example: K2SO4.Cr2(SO4)3.24H2O (chrome alum) EXAMPLES K2SO4.Al2(SO4)3.24H2O (potash alum) (NH4)2SO4.Al2(SO4)3.24H2O (ammono alum) (NH4)2SO4.Fe2(SO4)3.24H2O (ferric alum) K2SO4.Cr2(SO4)3.24H2O (chrome alum) PREPARATION OF POTASH ALUM Potash alum is commonly known as "PHITKARI" Potash alum is prepared by mixing equi-molecular masses of potassium sulphate and aluminum sulphate in water followed by evaporation K2SO4 + Al2(SO4)3 + 24H2O K2SO4.Al2(SO4)3.24H2O PROPERTIES AND USES It is white crystalline solid. It is soluble in water. It is used for the purification of water. It is also used in leather industry and in paper industry. It is used in fire extinguisher. Melting point is 92oC PHUL PHITKARI When potash alum is heated at 200oC it loses all molecules of water of crystallization and become a porous mass known as phul phitkari or burned alum. HAZARDS Alum has a large molecular size and it has a property of blood clotting. Therefore, it must be used carefully in the purification of water otherwise you may suffer a great deal. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 3 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS LEAD PIGMENTS DEFINITION Pigments are the soluble inorganic colouring matters. They may be natural or synthetic. TYPES OF LEAD PIGMENTS Following are the lead pigments White lead pigment. Pb(OH)2 . 2PbCO3 Red lead pigment. Pb3O4 Chrome yellow pigment. PbCrO4 Chrome red pigment. Pb2CrO5 OR [PbCrO4 . PbO] Turner’s yellow pigment. PbCl2 . 4PbO 1.WHITE LEAD PIGMENT It is basic lead carbonate. It composition is Pb(OH)2 2PbCO3 OR Pb3 (OH)2 . (CO3)2. 1. PREPARATION BY CHAMBER OR DUTCH PROCESS FIRST STEP When a mixture of vapours of acetic acid and steam is passed into chambers containing lead sheets (buckets) for about 2 to 3 days until the corrosion of lead is complete and basic lead acetate is formed on its surface. 2Pb + 2CH3COOH + 2H2O [Pb(OH)2 Pb(CH3COO)2] + 2H2 SECOND STEP Now mixture of CO2 and vapours of acetic acid along with steam in passed through this corroded lead for about 3 to 4 weeks. As a result white lead is formed on the surface of lead sheets which is then scratched, dried and packed. 3[Pb(OH)2 . Pb(CH3COO)2] + 2H2O + 4CO2 [2PbCO3 . Pb(OH)2] + 6CH3COOH 2. PREPARATION FROM LEAD OXIDE When CO2 is passed through the suspension of lead oxide (PbO2) in water containing little amount of lead acetate. 2PbO2 + 2CO2 + 2H2O + Pb(CH3COO)2 [2PbCO3 . Pb(OH)2] + 2CH3COOH + O2 USES It is used as white pigments for paints. OBJECTION TO USES It blackens under the action of H2S. Due to its poisonous nature white lead has been replaced by titanium dioxide (TiO2). Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 4 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS 2.RED LED PIGMENT (SANDHUR) (Pb3O4 OR 2PbO . PbO2) PREPARATION It is prepared by heating litharge (PbO) in excess of air at about 450oC. 6PbO + O2 450 2Pb3O4. o C USES It is used as red pigment (its colour varies from orange – red to brick red). It is used in glass and match industry. 3.CHROME YELLOW PIGMENT (PbCrO4) PREPARATION By mixing dilute solution of Pb(NO3)2 and (K2 CrO4). Pb (NO3)2 + K2CrO4 PbCrO4 + 2KNO3 By action of K2CrO4 and Pb(CH3COO)2 Pb(CH3COO)2 + K2CrO4 PbCrO4 + 2CH3 COOH. USES It is used as yellow pigment. It is used in glass industry. 4. RED OR BASIC LEAD CHROMATE PREPARATION When yellow lead chromate is boiled with NaOH then the chrome red pigment is formed. 2PbCrO4 + 2NaOH PbCrO4 . PbO + Na2CrO4 USES It is used as red pigment in red coloured paints. 5.TURNER’S YELLOW PIGMENT [PbCl2 . 4PbO]. PREPARATION It is prepared by boiling the solution of common salt with litharge (PbO). 5PbO + H2O + 2NaCl PbCl2 . 4PbO + 2NaOH (Turner’s Yellow) USES It is used in yellow coloured paints. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 5 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS ALLOTROPY Existence of an element in more than one physical form in the same physical state is called ALLOTROPY and different physical forms are referred to as ALLOTROPES. Allotropes of an element are differing in physical properties but they have same chemical properties. ALLOTROPIC FORMS OF CARBON Carbon exists in two allotropic forms: Crystalline form Amorphous form CRYSTALLINE FORMS There are two crystalline forms of carbon. Diamond Graphite AMORPHOUS FORMS There are a number of amorphous forms of carbon such as: coal coke lampblack animal charcoal carbon black etc . 1.DIAMOND STRUCTURE In diamond, each carbon atom is covalently bonded to four other carbon atoms to give a tetrahedral unit. In diamond each carbon atom is sp3 hybridized. Therefore each carbon atom forms four sigma bonds with neighboring carbon atoms. In diamond C-C-C bond angle is 109.5o. These basic tetrahedral units unite with one another and produce a cubic unit cell. C-C bond length is 1.54Ao. C-C bond energy is 347 kJ/mole. In diamond crystal, basic units joined to form octahedral shape of diamond crystal.In diamond each C-atom utilizes its four unpaired electrons in bond formation. These bonding electrons are localized. Due to this reason diamond is a bad conductor of electricity. PROPERTIES Diamond is the hardest substance ever known. Pure diamond is colorless. Due to impurities it may be coloured. Its melting point is 3500oC. Pure diamond is transparent to X-rays It has high refractive index 2.45. Its density is 3.51 gm/cm3. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 6 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS 2.GRAPHITE STRUCTURE In graphite each Carbon is sp2 hybridized joined to three other carbon atoms by covalent bond at 120o is give basic hexagonal ring structure. The fourth electron of each carbon atom forms delocalized bonds. The hexagonal ring then forms different layers in graphite. The carbon – carbon bond length is 1.42oA. The bond energy is 17.0 KJ/mol or 3.99 Kcal/mol. The distance between the layers is 3.35oA therefore these layer slide over each other. Electricity is conducted parallel to plane of its layers and it is very soft. PROPERTIES Dark grey crystalline solid. Slippery to touch. Good conductor of current. USES It is used for making pencils. It is used as moderator in reactions. It is used as lubricants in machinery operating at very high temperature. Being good conductor of electricity it is used for the preparation of electrodes particularly for dry cells. ALLOTROPIC FORMS OF SULPHUR There are two allotropic forms of sulphur Crystalline Amorphous CRYSTALLINE FORM There are two crystalline forms of sulphur. Rhombic () sulphur or octahedral. Monoclinic () sulphur or prismatic. AMORPHOUS FORM Plastic () sulphur 1. RHOMBIC () SULPHUR OR OCTAHEDRAL It is the stable crystalline form at room temperature. It is prepared when solution of ordinary sulphur in carbon disulphide (CS2) is allowed to crystallize. STRUCTURE Rhombic sulphur molecule consists of eight atoms in S8 molecules in the form of puckered ring. In puckered ring four atoms lie in one plane. Crystals of Rhombic Sulphur are octahedral. The sulphur – sulphur bond length is 2.12oA and bond angle is 105o. A crystal of sulphur contains single covalent bond. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 7 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS PROPERTIES It is pale yellow in colour. It melts at 112.8oC changes to monoclinic sulphur at 95.6oC. It is soluble in petroleum, Benzene, Turpentine. It is bad conductor of heat & electricity. 2. MONOCLINIC () SULPHUR OR PRISMATIC This form of sulphur is obtained when molten sulphur is allowed to cool slowly and allowed to crystallize or when rhombic sulphur is heated, it changes to monoclinic sulphur at 95.6oC. PROPERTIES It is dark yellow but transparent crystal. It is stable at 95.6oC to 119oC. It is insoluble in water, but soluble in CS2 on standing it is converted into opaque brittle solid. 3. PLASTIC ( ) SULPHUR When sulphur is heated up to its boiling point & quickly poured into cold water then a rubber like plastic sulphur is obtained. STRUCTURE Its structure consist of long chains ‘S’ atoms coiled up its elasticity is due to uncoiling of long sulphur chains & then recoiling of on release of tension, hence it can be stretchable. PROPERTIES It is black rubber like material. It is non-crystalline forms of sulphur. It is insoluble in water but soluble in CS2. NITRIC ACID HNO3 OSTWALD’S METHOD Materials used in this process are: Ammonia Water Oxygen gas STEP NO. 1: OXIDATION OF NH3 TO NO In this method, a mixture of 1 volume of ammonia & 8 volumes of purified air is passed through Ammonia oxidation converter. It is cylindrical vessel which contains fine platinum gauze (as catalyst) initially heated up to 600oC to start the reaction. As the reaction is exothermic hence no further heating is required after once the reaction has started. In this converter oxygen of air oxidizes NH3 to NO in the presence of catalyst Pt. 4NH3 + 5 O2 4NO + 6H2O About 95% of NH3 is oxidized to No. Nitric oxide vapours along with uncombined N2 & O2 are cooled down to about 150oC by passing through boiler which absorbed heat from the gases & throw passed into the oxidation chamber. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 8 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS STEP NO. 2: OXIDATION OF NO TO NO2. In the oxidation chamber, nitric oxide (NO) is further oxidized to No2. 2NO + O2 2NO2 STEP NO. 3: PRODUCTION OF HNO3 IN THE ABSORPTION TOWER The NO2 along with NO un-reacted N2 & O2 etc. is introduced into the bottom of absorption tower & water is sprayed from the top NO2 is absorbed by water forming HNO3 of 60% strength. 3NO2 + H2O 2HNO3 + NO This nitric acid on distillation gives ordinary concentrated Nitric acid of 68% strength with (density of 1.4 gm/cm3). On further distillation in the presence of concentrated H2SO4 98% HNO3 is obtained having density of 1.5 gm/cm3. CHLORINE GAS NELSON’S CELL It consists of a rectangular tank made of steel the cathode (-ve) is a U-shaped vessel made of perforated steel plate and is mounted at the tank. The anode is a graphite rod in the middle. This cell is divided into two compartments. The anode is inside the U- shaped vessel and cathode compartment is outside the vessel. The two compartments are separated by the asbestos layer deposited in the inner wall of cathode to keep chlorine and NaOH apart from each other. First of all brine (concentrated aqueous NaCl solution) is fed into the anode compartment and electric current is passed through the solution as a result NaCl is decomposed. Chlorine is liberated at the graphite anode by the oxidation of it ions which is drown off by an out let pipe. It is then directed and packed in cylinders. At the same time Sodium ions after passing through the asbestos layer reach the cathode and reduced to sodium metal. This sodium metal on reaction with water produces Hydrogen gas and NaOH. H2 gas is led off through another outlet and is collected. NaOH is found around the cathode which is obtained as solution at the bottom of the cell. The NaOH solution is finally taken out. It is then evaporated. Following reaction takes place during the electrolysis. 2NaCl 2Na+ + 2Cl- Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 9 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS At graphite anode 2Cl- Cl2 + 2e- (Oxidation) At Cathode 2Na+ + 2e- 2Na (Reduction) 2Na + 2H2O 2NaOH + H2 2NaCl + 2H2O Cl2 + H2 + 2NaOH CASTNER-KELLNER’S CELL CONSTRUCTION: The cell consists of large rectangular tank with a layer of mercury at the bottom and is divided into three compartments by partition not touching the bottom. The partitions are dipping in mercury. Mercury is kept in circulation from one compartment to another by means of Eccentric wheels. Each of the side compartments is fitted with graphite anodes dipping in brine whereas a series of iron rods suspended in the middle compartment act as cathodes. The central compartment contains a very dilute solution of caustic soda. WORKING: When the electric current is passed, the electrolysis of brine takes place in the outer compartments. Chlorine is liberated at the anodes and is led away through an exit at the top. Sodium ions are dissolved with the liquid mercury layer which acts as the cathode and produces sodium amalgam. Now this sodium amalgam enters into the middle compartment due to the rotation of eccentric wheels where this mercury layer behaves as an anode. As a result of electrolysis of NaOH solution in the middle compartment OH- ions move to the mercury anode where it reacts with the sodium present in amalgam and forms sodium hydroxide while the H+ ions obtained by the dissociation of water produces hydrogen gas. SULPHURIC ACID H2SO4 CONTACT’S PROCESS Following steps are involved in the production of sulphuric acid. 1. PRODUCTION OF SO2 In this process first sulphur dioxide is prepared by burning sulphur or Iron pyrite in the presence of air in Pyrite burner. S + O2 SO2 2FeS2 + 11O2 2Fe2O3 + 8SO2 2. PURIFICATION OF SO2 Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 10 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS First he mixture of gases containing SO2 is passed through direct filter chamber, where the particles of arsenic oxide (A2SO3) it and dust particles are removed. The gases are then cooled to about 100oC in the cooling coils and then washed in a washing tower by a fire spray of water which washes all the impurities. The purified gases are finally dried in a drying tower packed with coke and concentrated sulphuric acid is sprayed from its top which being hygroscopic so the vapours are absorbed here and the purified gases are obtained. The purified gases containing SO2 and Oxygen are now perfectly clear, dry and free from moisture. The purified gases containing SO2 from drying tower are now passed into the Contact Tower. 3. OXIDATION OF SO2 TO SO3 IN CONTACT TOWER Contact tower contains iron pipes or trays containing catalyst V2O5 which is heated to 500oC at the start. As the reaction is exothermic, therefore no further heating is required after once the reaction has started. The temperature is kept between 450oC – 500oC (optimum temperature) so SO3 is obtained. At this temperature, SO2 gets oxidized to SO3 with evolution of much heat. 2SO2 + O2 2SO3 H = - 45 kcal 2vol 1vol 2vol 4. PREPARATION OF OLEUM (FUMING H2SO4) The SO3 coming out of the contact tower is absorbed first by concentrated H2SO4 to form Oleum. SO3 + H2SO4 H2S2O7 Oleum or pyrosulphuric acid or fuming sulphuric acid. 5. PRODUCTION OF H2SO4 Oleum is then changed in to 99% concentrated H2SO4 by adding calculated amount of H2O. H2S2O7 + H2O 2H2SO4 METALLURGY Metals are found in universe as combined form. These combined forms of metals are called minerals. The process by which metals can be extracted from their ores is known as metallurgy. OR The process involved in the production of pure metals from their ores is known as “metallurgy”. MINERALS When metals are found in nature in combined state with other elements, then such substances are called “MINERALS”. Naturally occurring substances which may obtain from earth crust are called minerals. ORES That mineral which is used to extract metals from its combined state (mineral) is called “ORE”. OR Ore is that mineral from which the metal can be extracted. Thus all ores are accordingly minerals but all the minerals are not ores. EXTRACTION OF ALUMINIUM Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 11 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS Aluminium is extracted from Bauxite ore. The metallurgy of aluminum involves following steps i) Purification of Bauxite to get pure Alumina. ii) Electrolysis of pure Alumina. iii) Electro-refining of Aluminum. STEP:1 PURIFICATION OF BAUXITE Bauxite usually contains impurities like Fe2O3 . SiO2 which makes aluminum brittle. The bauxite may be purified by following methods: Hall’s process. Baeyer’s process. Serpek’s process. 1. HALL’S PROCESS This method is used for the purification of bauxite containing the impurities i.e. Fe2O3 and SiO2. This method is carried out in three steps. In this method bauxite is mixed with Na2CO3 and heated to bright redness as a result sodium aluminate (NaAlO2) is formed while the impurities Fe2O3 and SiO2 remain unaffected. Al2O3 . nH2O + Na2CO3 2NaAlO2 + CO2 + nH2O NaAlO2 is soluble in water while the impurities Fe2O3, SiO2 remains insoluble. On filtration, sodium aluminate solution is obtained as filtrate and insoluble impurities are removed. CO2 is passed into a clear solution of sodium aluminate at 50o – 60oC and precipitates of Al(OH)3 are formed along with Na2CO3. 2NaAlO2 + 3H2O + CO2 50 2Al(OH)3 + Na2CO3 o o 60 C The precipitates of Al(OH)3 after filtration are washed with water to remove Na2CO3 and finally ignited at about 1500oC to get pure alumina (Al2O3). o 2Al(OH)3 1500 C Al2O3 + 3H2O Na2CO3 solution is evaporated to dryness and solid Na2CO3 is re-used. 2. BAEYER’S PROCESS This method is used for the purification of bauxite ore containing Fe2O3. This method is carried out in three steps. Bauxite is treated with concentrated solution of caustic soda (NaOH) at 150oC. The aluminum oxide present in bauxite dissolve in NaOH, while the impurities are left behind. Al2O3 . nH2O + 2NaOH 2NaAlO2 + nH2O This solution is filtered to remove Fe2O3 and other impurities. CO2 is passed into a clear solution of sodium aluminate at 50o – 60oC and precipitates of Al(OH)3 are formed along with Na2CO3. 2NaAlO2 + 3H2O + CO2 50 2Al(OH)3 + Na2CO3 o o 60 C The precipitates of Al(OH)3 after filtration are washed with water to remove Na2CO3 and finally ignited at about 1500oC to get pure alumina (Al2O3). o 2Al(OH)3 1500 C Al2O3 + 3H2O Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 12 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS 3. SERPEK’S PROCESS This method is used for the purification of bauxite containing SiO2. In this process, the powderedbauxite is mixed with carbon (coke) and heated up to 1800oC in the presence of nitrogen when AlN is formed. Al2O3 . nH2O + 3C + N2 2AlN + 3CO + nH2O SiO2 + 2C Si + 2CO Aluminum hydroxide on hydrolysis with hot water produces precipitates of Al(OH)3 and NH3. AlN + 3H2O Al(OH)3 + NH3 On filtration the precipitate of Aluminum oxide are obtained which are dried and finally ignited to pure alumina. 2Al(OH)3 heat Al2O3 + 3H2O STEP:2 ELECTROLYSIS OF PURE ALUMINA Pure alumina obtained from the purification of bauxite. It is then dissolved in fused cryolite (Na3AlF6) and fluorspar (CaF2) and electrolyzed in an electric furnace. Cryolite lowers the melting point of alumina from 2050oC to 950oC and also makes it a good conductor where as fluorspar (CaF2) increases the fluidity of the electrolyte. The electrolysis is carried out in a steel tank lined with carbon (Graphite) from inside this graphite lining serves as cathode. The anode consists of graphite rods hanging in the molten mass when the electric current is passed through the molten electrolyte the resistance of electrolyte gives sufficient heat to keep the mass in a molten state (around 950oC). During electrolysis aluminum is liberated at the cathode and being in liquid state it sinks to the bottom and forms a pool from when it is taken and periodically through tapping hole. At the same time oxygen is liberates at anode which reacts with graphite rods (and producing CO2 and CO. This destroys carbon rods which are replaced without stopping the electrolysis. Aluminum thus obtained is 99% pure. REACTIONS DURING ELECTROLYSIS Electrolytic solution contains pure alumina dissolved in fused cryolite (Na3AlF6) which gives the following reaction At Cathode 4Al+3 + 12e- 4Al At Anode 6O-2 3O2 + 12e- The oxygen liberated at anode reacts with carbon forming CO2. C + O2 CO2 In this way, the electrolysis of Al2O3 through the formation of AlF3 is continued. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 13 of 14 FROM THE DESK OF Muhammad Adeel CHAPTER # 4 p-BLOCK ELEMENTS STEP NO:3 ELECTRO-REFINING OF ALUMINUM (HOOPE’S PROCESS) 99.99% of pure aluminum is produced in this process. The fused bath consists of three layers. The bottom layer is fused alloy of Cu, Al and Si. The middle layer consists of electrolyte of a fused mixture of BaF2 and Cryolite, and the top layer is pure molten aluminum. These layers are maintained by the difference in their densities during electrolysis. The bottom layer is the anode and cathode consists of graphite rods dipped in pure molten Aluminum. On electrolysis, Aluminum from bottom layer goes into the middle layer leaving the impurities behind and at the same time equal amount of Al+3 ions passes from middle layer & go into the top layer and are reduced at cathode as a result top layer of Aluminum grows which is drown off periodically. 1000 C o o Na3AlF6 950 3NaF . AlF3 AlF3 Al+3 + 3F- At Cathode Al+3 + 3e- Alo (s) pure Al Al+3 + 3e- At Anode Al+3 + Al Al + Al+3 Impure Pure Aluminum as Reducing Agent (Thermite Process) Aluminum is a powerful reducing agent. It readily reduces the oxides of most of the other metals just like Fe, Mn, Cr etc. Fe2O3 + 2Al 2Fe + Al2O3 This is exothermic reaction and great amount of energy is released so that the temperature increase upto 3500OC. Due to this reason, this process is used for welding purposes. The reduction of metal oxides involving aluminum as reducing agent is termed as “Thermite Process”. Address: 31/216, Darakhshan Society, Malir Kalaboard, Behind Nehal Hospital, Karachi. Contact: 03323385161 Page 14 of 14 FROM THE DESK OF Muhammad Adeel