Lecture notes in Engineering Chemistry

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CY6251 ENGINEERING CHEMISTRY - II A Course Material on ENGINEERING CHEMISTRY II By Mrs. K. MABEL HEBSUBA/ ASSISTANT PROFESSOR Mr. G. DHINESH/ ASSISTANT PROFESSOR Mr. J. MUHAMED ALI/ ASSISTANT PROFESSOR & Mrs. S. VIJAYA LAKSHMI/ ASSISTANT PROFESSOR DEPARTMENT OF SCIENCE AND HUMANITIES SASURIE COLLEGE OF ENGINEERING VIJAYAMANGALAM – 638 056 1 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II 5.10 GLOSSARY 97 6 QUESTION BANK 98 7 UNIVERSITY QUESTIONS 158 CY6251 ENGINEERING CHEMISTRY-II L T P C 3 0 0 3 OBJECTIVES:  To make the students conversant with boiler feed water requirements, related problems and water treatment techniques.  Principles of electrochemical reactions, redox reactions in corrosiion of materials and methods for corrosion prevention and protection of materials.  Principles and generation of energy in batteries, nuclear reactors, solar cells, wind mills and fuel cells.  Preparation, properties and applications of engineering materials.  Types of fuels, calorific value calculations, manufacture of solid, liquid and gaseous fuels. UNIT I WATER TECHNOLOGY 9 Introduction to boiler feed water-requirements-formation of deposits in steam boilers and heat exchangers- disadvantages (wastage of fuels, decrease in efficiency, boiler explosion) prevention of scale formation -softening of hard water -external treatment zeolite and demineralization- internal treatment- boiler compounds (phosphate, calgon, carbonate, colloidal) - caustic embrittlement-boiler corrosion-priming and foaming- desalination of brackish water –reverse osmosis. UNIT II ELECTROCHEMISTRY AND CORROSION 9 Electrochemical cell - redox reaction, electrode potential- origin of electrode potential- oxidation potential- reduction potential, measurement and applications - electrochemical series and its significance - Nernst equation (derivation and problems). Corrosion- causes- factors- types-chemical electrochemical corrosion (galvanic, differential aeration), corrosion control - material selection and design aspects - electrochemical protection – sacrificial anode method and impressed current cathodic method. Paints- constituents and function. Electroplating of Copper and electroless plating of nickel. , UNIT III ENERGY SOURCES 9 Introduction- nuclear energy- nuclear fission- controlled nuclear fission- nuclear fusion- differences between nuclear fission and fusion- nuclear chain reactions- nuclear reactor power generatorclassification of nuclear reactor- light water reactor- breeder reactor- solar energy conversion- solar cells- wind energy. Batteries and fuel cells:Types of batteries- alkaline battery- lead storage batterynickel- cadmium battery- lithium battery- fuel cell H2 - O2 fuel cell- applications. UNIT IV ENGINEERING MATERIALS 9 6 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II Abrasives: definition, classification or types, grinding wheel, abrasive paper and cloth. Refractories: definition, characteristics, classification, properties – refractoriness and RUL, dimensional stability, thermal spalling, thermal expansion, porosity; Manufacture of alumina, magnesite and silicon carbide, Portland cement- manufacture and properties - setting and hardening of cement, special cement waterproof and white cement–properties and uses. Glass - manufacture, types, properties and uses. UNIT V FUELS AND COMBUSTION 9 Fuel: Introduction- classification of fuels- calorific value- higher and lower calorific values- coalanalysis of coal (proximate and ultimate)- carbonization- manufacture of metallurgical coke (Otto Hoffmann method) - petroleum- manufacture of synthetic petrol (Bergius process)- knocking- octane number - diesel oil- cetane number - natural gas- compressed natural gas(CNG)- liquefied petroleum gases(LPG)- producer gas- water gas. Power alcohol and bio diesel. Combustion of fuels: introduction- theoretical calculation of calorific value- calculation of stoichiometry of fuel and air ratioignition temperature- explosive range - flue gas analysis (ORSAT Method). TOTAL: 45 PERIODS OUTCOMES: The knowledge gained on engineering materials, fuels, energy sources and water treatment techniques will facilitate better understanding of engineering processes and applications for further learning. TEXT BOOKS: 1. Vairam S, Kalyani P and SubaRamesh.,“Engineering Chemistry”., Wiley India PvtLtd.,New Delhi., 2011 2. Dara S.S and Umare S.S. “Engineering Chemistry”, S. Chand & Company Ltd., New Delhi , 2010 REFERENCES: 1. Kannan P. and Ravikrishnan A., “Engineering Chemistry”, Sri Krishna Hi-tech Publishing Company Pvt. Ltd. Chennai, 2009. 2. AshimaSrivastava and Janhavi N N., “Concepts of Engineering Chemistry”, ACME Learning Private Limited., New Delhi., 2010. 3. RenuBapna and Renu Gupta., “Engineering Chemistry”, Macmillan India Publisher Ltd., 2010. 4. Pahari A and Chauhan B., “Engineering Chemistry”., Firewall Media., New Delhi., 2010 7 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II UNIT I WATER TECHNOLOGY 1.1 INTRODUCTION Water is essential for the existence of human beings, animals and plants. Though 80% of the earth‟s surface is occupied by water, less than 1% of the water is available for ready use. The main sources of water are  Rain  rivers and lakes (surface water)  wells and springs (ground water)  sea water Among the above sources of water, rain water is the purest form of water but it is very difficult to collect whereas sea water is the most impure form. Thus, surface and ground water are normally used for industrial and domestic purposes. Such water must be free from undesirable impurities. “The process of removing all types of impurities from water and making it fit for domestic or industrial purposes are called water treatment.” Before treating water one must know the nature as well as the amount of impurities. 1.2 HARD WATER AND SOFT WATER  Hard Water “Water which does not produce lather with soap solution, but produces white precipitate is called hard water”. This is due to the presence of dissolved Ca and Mg salts. ++ + 2C H COONa + Ca → (C H COO) Ca +2Na 17 35 17 35 2 Water soluble Water insoluble  Soft Water “Water which produces lather readily with soap solution is called soft water.” This is due to the absence of Ca and Mg salts.  1.3 BOILER FEED WATER In Industry, one of the chief uses of water is generation of steam by boilers. The water fed into the boiler for the production of steam is called boiler feed water. 8 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II Requirements of boiler feed water  It should have zero hardness.  It must be free from dissolved gases like O , CO , etc. 2 2  It should be free from suspended impurities.  It should be free from dissolved salts and alkalinity  It should be free from turbidity and oil.  It should be free from hardness causing and scale forming constituents like salts. Ca and Mg 1.3.1 Formation of deposits (Scales and Sludges) in boilers and heat exchangers In a boiler, water is continuously converted into steam. Due to this continuous evaporation of water, the concentration of soluble matters increases progressively. Then the salts separating out from the solution in the order of their solubility, the lease soluble ones separating out first. (i) Sludge  If the precipitate is loose and slimy it is called sludges.  Sludges are formed by substances like MgCl , MgCO , MgSO 2 3 4 and CaCl . 2  They have greater solubilities in hot water than cold water. (ii) Scale  If the precipitate forms hard and adherent coating on the inner walls of the boiler, it is called scale.  Scales are formed by substances like Ca (HCO ) , CaSO and 3 2 4 Mg(OH) . 2 9 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II Disadvantages of Scale Formation (i) Wastage of fuels Scales have a low thermal conductivity, so the rate of heat transfer from boiler to inside water is greatly decreased. In order to provide a supply of heat to water, excessive or over-heating is done. This causes increase in fuel consumption. The wastage of fuel depends upon the thickness and the nature of scale. (ii) Decrease in efficiency Scales sometimes deposit in the valves and condensers of the boiler and choke them partially. This results in decrease in efficiency of the boiler. (iii) Boiler explosion When thick scales crack due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes in formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler. Prevention of scale formation  At the initial stage, scales can be removed using scraper, wire brush etc.  If scales are brittle, they can be removed by thermal shocks.  If the scales are loosely adhering, they can be removed by frequent blow down operation. 1.3.2 Caustic Embrittlement Caustic embrittlement is a form of corrosion caused by a high concentration of sodium Hydroxide in the boiler feed water. It is characterized by the formation of irregular intercrystalline cracks on the boiler metal, particularly at places of high local stress such as bends and joints. Causes of caustic embrittlement Boiler water usually contains a small amount of Na CO . In high pressure boilers, 2 3 Na CO undergoes hydrolysis to produce NaOH. 2 3 Na CO + H O → 2NaOH 2 3 2 +CO 2 This NaOH flows into the minute hairline cracks present on the boiler material by capillary action and dissolves the surrounding area of iron as sodium ferroate, Na FeO . 2 2 Fe + 2NaOH → Na FeO 2 2 +H 2 10 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II This type of electrochemical corrosion occurs when the concentration of NaOH is above 100 ppm. This causes embrittlement of boiler parts, particularly the stressed parts like bends, joints, rivets, etc. Caustic embrittlement can be prevented by  Using sodium phosphate as the softening agent instead of sodium carbonate.  Adding chemicals such as tannin, lignin to the boiler water. They block the hairline cracks.  Adjusting the pH of the feed water carefully between 8 and 9. 1.3.3 Boiler Corrosion Corrosion in boilers is due to the presence of  Dissolved oxygen  Dissolved carbon dioxide  Dissolved salts like magnesium chloride. Dissolved oxygen The presence of dissolved oxygen is responsible for corrosion in boilers. Water containing dissolved oxygen when heated in a boiler, free oxygen is evolved, which corrodes the boiler material. 4Fe + 6H O + 3O → 4 Fe 2 2 (OH) 3 Dissolved carbon dioxide When water containing bicarbonates is heated, carbon dioxide is evolved which makes the water acidic. Carbon dioxide dissolved in water forms carbonic acid. This leads to intense local corrosion called pitting corrosion. Ca(HCO ) → CaCO + H O + 3 2 3 2 CO 2 CO + H O → H CO 2 2 2 3 Dissolved magnesium chloride When water containing dissolved magnesium chloride is used in a boiler, hydrochloric acid is produced. HCl attacks the boiler in a chain-like reaction producing hydrochloric acid again and again which corrodes boiler severely. MgCl + 2H O → 2HCl + Mg (OH) 2 2 2 11 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II Fe + 2 HCl → FeCl + H 2 2 FeCl + 2H O → Fe (OH) + 2 HCl 2 2 2 Corrosion by HCl can be avoided by the addition of alkali to the boiler water. Prevention of boiler corrosion Removal of dissolved oxygen and carbon dioxide can be done either chemically or mechanically. Chemical method For the removal of dissolved oxygen, sodium sulphite, hydrazines are used. 2Na SO + O → 2Na SO 2 3 2 2 4 N H + O → N + 2H O 2 4 2 2 2 Hydrazine is the ideal compound for the removal of dissolved O as it forms 2 only water and inert nitrogen gas during the reaction. Dissolved CO is removed by the addition of ammonium hydroxide. 2 2NH OH + CO → (NH ) CO + H O 4 2 4 2 3 2 Mechanical method Oxygen along with carbon dioxide can be removed mechanically by the de- aeration method In this method, water is allowed to fill in slowly on the perforated plates fitted inside the tower. To reduce the pressure inside the tower, the de-aerator is connected to a vacuum pump. The sides of the tower are heated by means of a steam jacket. This is based on the principle that the solubility of a gas in water is directly proportional to pressure and inversely proportional to temperature. 12 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II High temperature, low pressure and a large exposed surface, reduces the dissolved gases (O and CO ) in water. 2 2 The water flows down through a number of perforated plates and this arrangement exposes a large surface of water for de-aeration. 1.3.4 Priming and Foaming During the production of steam in the boiler, due to rapid boiling, some droplets of liquid water are carried along with steam. Steam containing droplets of liquid water is called wet steam. These droplets of liquid water carry with them some dissolved salts and suspended impurities. This phenomenon is called carry over. It occurs due to priming and foaming. Priming Priming is the process of production of wet steam. Priming is caused by  High steam velocity.  Very high water level in the boiler.  Sudden boiling of water.  Very poor boiler design. Prevention Priming can be controlled by  Controlling the velocity of steam.  Keeping the water level lower.  Good boiler design.  Using treated water. Foaming The formation of stable bubbles above the surface of water is called foaming. These bubbles are carried over by steam leading to excessive priming. Foaming is caused by the  Presence of oil and grease.  Presence of finely divided particles. Prevention Foaming can be prevented by  Adding coagulants like sodium aluminate, aluminium hydroxide.  Adding anti-foaming agents like synthetic polyamides. 13 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II 1.4 PREVENTION OF SCALE FORMATION (OR) SOFTENING OF HARD WATER The process of removing hardness – producing salts from water is known as softening or conditioning of water. Since water is a source for industrial purpose. It is mandatory to soften water to make it free from hardness producing substances, suspended impurities and dissolved gases, etc. Softening of water can be done by two methods. i) External treatment ii) Internal treatment. External Treatment or Conditioning It involves the removal of hardness producing salts from the water before feeding into the boiler. The external treatment can be done by the following methods. Zeolite (or) Permutit process 1.4.1 Zeolite (or) Permutit process Zeolites are naturally occuring hydrated sodium aluminosilicate minerals. The chemical formula is Na O.Al O .XSiO .YH O. The synthetic form of zeolite is called 2 2 3 2 2 permutit and is represented by Na Ze. 2 In this process the hard water is allowed to perlocate through sodium zeolite. The 2+ 2+ sodium ions which are loosely held in this compound are replaced by Ca and Mg ions. When zeolite comes in contact with hard water, it exchanges its sodium ions with calcium and magnesium ions of hard water to form calcium and magnesium zeolites. As sodium ions do not give any hardness to water, the effluent will be soft. The exhausted zeolite is again regenerated by treated with 5 to 10 percent of sodium chloride solution. Na Ze + Ca(HCO) CaZe + 2NaHCO 2 2 3 14 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II Na Ze + Mg(HCO) MgZe + 2NaHCO 2 2 3 Na Ze + MgCl MgZe + 2NaCl 2 2 Na Ze + CaCl CaZe + 2NaCl 2 2 Na Ze + CaSO CaZe + Na SO 2 4 2 4 Na Ze + MgSO MgZe + Na SO 2 4 2 4 Regeration After some time zeolite gets exhausted. The exhausted zeolite is again regerated by treating with 10%solution of NaCl. CaZe + 2 NaCl → Na Ze + CaCl 2 2 MgZe + 2 NaCl → Na Ze +MgCl 2 2 Advantages  No sludge is formed during this process.  Water of nearly zero hardness is obtained.  This method is very cheap because the generated permutit can be used again.  The equipment used is compact and occupies a small space.  Its operation is also easy.  The process can be made automatic and continuous. Disadvantages  This process cannot be used for turbid and acidic water as they will destroy the zeolite bed.  This treatment replaces only the cations, leaving all the anions like 15 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II 2– – (CO ) in the soft water. (HCO ) and 3 3  When such water is boiled in boilers, CO is liberated. Free CO is weakly 2 2 nature and extremely corrosive to boiler metal. acidic in Na CO + H O→2NaOH + CO 2 3 2 2  Due to the formation of sodium hydroxide, the water becomes alkaline and can cause cause caustic embrittlement.  Water containing Fe, Mn cannot be treated, because regeneration is very difficult.  This process cannot be used for softening brackish water. Because + brackish water also contains Na ions. So, the ions exchange reaction will not take place. 1.4.2 Ion exchange or Demineralisation process Ion exchange or demineralisation process removes almost all the ions (both anions and cations) present in the hard water. The soft water, produced by lime-soda and zeolite processes, does not contain 2+ 2+ + + 2– hardness producing Ca and Mg ions, but it will contain other ions like Na , K , SO 4 – , Cl etc., On the other hand demineralised (DM) water does not contain both anions and cations. Thus a soft water is not demineralised water whereas demineralised water is soft water. This process is carried out by using ion exchange resins, which are long chain, cross linked, insoluble organic polymers with a micro process structure. The functional groups attached to the chains are responsible for the ion exchanging properties. (i) Cation exchanger Resins containing acidic functional groups (–COOH, – SO H) are capable of 3 + exchanging their H ions with other cations of hard water. Cation exchange resin is represented as RH . 2 Examples:  Sulphonated coals  Sulphonated polystyrene R–SO H; R–COOH ≡ RH 3 2 (ii) Anion Exchanger 16 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II Resins containing basic functional groups (–NH , –OH) are capable of 2 exchanging their anions with other anions of hard water. Anion exchange resin is represented as R (OH) . 2 Examples:  Cross-linked quaternary ammonium salts. Urea-formaldehyde resin. R–NR OH; R–OH; R–NH ≡ R (OH) 3 2 2 Process The hard water first passed through a cation exchange which absorbs all the 2+ 2+ + + cations like Ca , Mg Na , K , etc. present in the hard water. RH + CaCl → RCa + 2HCl 2 2 RH + MgSO → RMg + H SO 2 4 2 4 RH + NaCl → RNa + HCl The cation free water is then passed through a anion exchange column, which – 2 – absorbs all the anions like Cl , SO , HCO , etc., present in the water. 4 3 R' (OH) + 2HCl → R'Cl + 2 2 2H O R'(OH) + H SO → 2 2 2 4 R'SO + 2H O 4 2 The water coming out of the anion exchanger completely free from cations and 17 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II anions. This water is known as demineralised water or deionised water. Regeneration When the cation exchange resin in exhausted, it can be regenerated by passing a solution of dil.HCl or dil.H SO . 2 4 RCa + 2HCl → RH + CaCl 2 2 RNa + HCl → RH + NaCl Similarly, when the anion exchange resin is exhausted, it can be regenerated by passing a solution of dil.NaOH. R'Cl + 2 NaOH → R'(OH) + 2 NaCl 2 2 Advantages  The water is obtained by this process will have very low hardness.  Highly acidic or alkaline water can be treated by this process. Disadvantages  The equipment is costly.  More explosive chemicals are needed for this process.  Water containing turbidity, Fe and Mn cannot be treated, because turbidity reduces the output and Fe, Mn form stable compound with the resins . 1.5INTERNAL TREATMENT Internal treatment involves adding chemicals directly to the water in the boilers for removing dangerous scale – forming salts which were not completely removed by the external treatment for water softening. This method is used to convert scale to sludge which can be removed by blow-down operation.  Calgon conditioning  Carbonate conditioning  Phosphate conditioning  Colloidal conditioning Calgon conditioning Calgon is sodium hexa meta phosphate with a Composition Na (Na (PO ) ). A highly soluble complex containing Ca is formed by 2 4 3 6 replacing the sodium ions and thus prevents their formation of scale forming salts 18 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II like CaSO . The reaction is as follows: 4 2CaSO + Na Na (PO ) → Na Ca (PO ) + 2Na SO 4 2 4 3 6 2 2 3 6 2 4 Since the complex is highly soluble there is no problem of sludge disposal. Carbonate conditioning Scale formation due to CaSO in low pressure boilers can be avoided by adding 4 Na CO to the boilers. 2 3 CaSO + Na CO → CaCO + Na SO 4 2 3 3 2 4 2- The forward reaction is favored by increasing the concentration of CO .CaCO 3 3 formed can be removed easily. Phosphate conditioning In high pressure boilers, CaSO scale whose solubility decrease with increase of 4 temperature. Such scale can be converted into soft sludge by adding excess of soluble phosphates. 3CaSO + 2Na PO → Ca (PO ) +2Na SO 4 3 4 3 4 2 2 4 There are three types of phosphates employed for this purpose. Tri-sodium phosphate – Na PO (too alkaline): used for too acidic water. 3 4 Di-sodium hydrogen phosphate – Na HPO (weakly alkaline): Used for weakly 2 4 acidic water.Mono sodium di hydrogen phosphate NaH PO (acidic) used for alkaline 2 4 acidic water. Colloidal conditioning The colloidal conditioning agents are kerosene, agar-agar, gelatin, glue, etc. They are Used in low pressure boilers. The colloidal substances convert scale forming substance like CaCO , CaSO into a Non-adherent, loose precipitate called 3 4 sludge, which can be removed by blow-down Operation. 1.6 DESALINATION OF BRACKISH WATER Depending upon the quantity of dissolved solids, water is graded as Fresh water has 1000 ppm of dissolved solids. Brackish water has 1000 but 35,000 ppm of Dissolved solids. Sea water has 35,000 ppm of dissolved solids. Water containing dissolved salts with a peculiar salty or brackish taste is 19 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II called brackish water. It is totally unfit for drinking purpose. Sea water and brackish water can be made available as drinking water through desalination process. The removal of dissolved solids (NaCl) from water is known as desalination process. The need for such a method arises due to the non-availability of fresh water. Desalination is carried out either by electro dialysis or by reverse osmosis. 1.6.1 Reverse Osmosis When two solutions of different concentrations are separated by a semi-permeable membrane, flow of solvent takes place from a region of low concentration to high concentration until the concentration is equal on both the sides. This process is called osmosis. The driving forces in this phenomenon are called osmotic pressure. If a hydrostatic pressure in excess of osmotic pressure is applied on the higher concentration side, the solvent flow reverses, i.e., solvent is forced to move from higher concentration to lower concentration .This is the principle of reverse osmosis. Thus, in reverse osmosis method pure water is separated from its dissolved solids. sing this method pure water is separated from sea water. This process is also known as super-titration. The membranes used are cellulose acetate, cellulose butyrate, etc. Advantages  The life time of the membrance is high.  It can be replaced within few minutes.  It removes ionic as well as non-ionic, colloidal impurities.  Due to simplicity low capital cost, low operating, this process is used for converting sea water into drinking water 20 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II 1.7 Glossary Hardness Harness is the property or characteristics of water, which does not produce lather with soap solution. Soft water Water which produces lather readily with soap solution is called soft water. Soft water is free of calcium & magnesium salts. Temporary hardness Temporary hardness is due to the presence of bicarbonates of calcium and magnesium. Since these salts can be easily removed by simple physical methods such as boiling and filtering. Permanent hardness Permanent hardness is due to the presence of soluble chlorides and sulphates of calcium and magnesium. These salts can be removed by chemical treatments only. Alkalinity - Alkalinity of water is due to the presence of soluble hydroxide ( OH ), carbonate ( 2- CO ) and bicarbonate ( HCO3-) ions. 3 Boiler feed water The water which is free from dissolved salts, dissolved gases, hardness, oils and alkalinity is known as boiler feed water. Sludge The loose and slimy precipitate is called sludge. Scale The hard and adherent precipitate on the inner walls of the boiler is called scale. Priming Some droplets of liquid water are carried along with steam during the production of steam in the boiler is called priming. Foaming The formation of stable bubbles above the surface of water is called foaming. Caustic embrittlement Formation of irregular, intergranular cracks at the welded joints, rivets etc. in high pressure boilers is called caustic embrittlement. Brackish water The water containing high concentration of dissolved salts with salty or brackish taste is called brackish water. Reverse osmosis The solvent flows from higher concentration to lower concentration. 21 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II UNIT II ELECTRO CHEMISTRY AND CORROSION 2.1 INTRODUCTION Electrochemistry is the branch of science which deals with the relationship between chemical reaction and electricity. Electrochemistry is the study of the process involved in theinterconversion of electrical energy to chemical energy and vice – versa Electric current is a flow of electrons generated by a cell or a battery when the circuit is completed. A substance which allows electric current to pass through it ,is called a conductor, e.g. metals, graphite, fused metallic salts while non – conductor is a substance which does not conduct the electric current e.g. plastics, wood, non – metals ect, IMPORTANT TERMS INVOLVED IN ELECTROCHEMISTRY Cell: Cell is an assembly of two electrodes and an electrolyte.Generally, it consists of two half cells. Each half cell contain an electrode material in touch with electrolyte. Current:Current is a flow of electricity through a conductor. It is measured in ampere. Electrode is a material rod/bar/strip which conduct electrons. Anode: Anode is an electrode at which oxidation occurs Cathode: Cathode is an electrode at which reduction occur Electrolyte: Electrolyte is a liquid or solution that conducts electric current. There are three types of electrolytes. (a) Strong electrolytes: these are the substances which ionize completely at any concentration. Example: HCL,aqueous solutions of NAOH, NaCl and KCL (b)Weak electrolytes: Weak electrolytes are the substance which ionize partially in solution. Example: CH COOH, NH OH and aqueous solution of Na CO (b) Non electrolyte: Non electrolyte are the substances which do not ionize at any dilutions. (c) Example: Glucose, Sugar,alcohol, petrol,etc. Galvanic (or) Voltaic (or) Electrochemical cell It is a device that produces electrical energy at the expense of chemical energy produced in a reaction. A cell consists of two half cells or electrodes. A half-cell or electrode contains a metal rod dipped in an electrolytic 22 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II solution. Electrolytic cell: It is a cell in which electrical energy brings about a chemical reaction. Electrochemical cell: It is a device that produces electrical energy at the expense of chemical energy produced in a reaction. Differences between electrolytic cell and electrochemical cell. Electrolytic Cell Electrochemical cell 1. Electrical energy brings about a 1. Electrical energy is produced at the chemical reaction. expense of chemical energy. 2. Anode is positively charged. 2. Anode is negatively charged. 3. Cathode is negatively changed. 3. Cathode is positively charged. 4. Electrons move from anode to cathode through external 4. Electrons move from cathode circuit. to anode through external 5. The extent of chemical reaction at circuit. the electrode is governed by 5. The emf of the cell depends Faraday‟s laws of electrolysis. on concentration of 6. The amount of electricity passed electrolyte and nature of the is measured by a coulometer. metal electrode. 7. One electrolyte and two electrodes 6. The emf of the cell is measured of the same element are generally by a potentiometer. used in these cells. 7. Two different electrolytes and two different electrodes are often used. 2.1.1 TYPES OF CELL Electrolytic cell It is a device in which chemical reaction proceed at the expanse of electrical energy. Ex. Electro plating and electrolysis Electrolysis of NaCl The cell is constituted by dipping two platinum electrode in an appropriate electrolyte ( NaCl in water ) . The electrodes are connected to the two terminals of a battery. The electrode connected to positive terminal acts as anode (attracts anions) and the other electrode connected to the negative terminal acts as cathode (attracts cations). Chlorine is liberated at anode and hydrogen is liberated at cathode Cell reaction: At anode: 2 Cl ▬▬► Cl ↑ + 2e 2 + + At cathode: (i) Na + H O ▬▬► NaOH + H 2 23 SCE Dept.of S & H CY6251 ENGINEERING CHEMISTRY - II + (ii) 2 H + 2e ▬▬► H ↑ 2 Net reaction: 2NaCl + 2H O ▬▬► 2NaOH + H + Cl 2 2 2 ECTROCHEMICAL CELL: ●consists of two half-cells joined by a salt bridge or some other path (porous membrane)that allows ions to pass between the two sides in order to maintain electro neutrality. oxidation occurs at one half cell while reduction takes place at the other half cell. cell with porous membrane cell with salt bridge 24 SCE Dept.of S & H

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