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CY6251 ENGINEERING CHEMISTRY - II

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SCE Dept.of S & H

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


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SCE Dept.of S & H

QUALITY CERTIFICATE

This is to certify that the e-course material Subject Code : CY6251 Subject : Engineering Chemistry II Class : I Year being prepared by us and it meets the knowledge requirement of the university curriculum.

Signature of the Author

Name: K. Mabel Hebsuba, G. Dhinesh

J. Mohammed Ali & S. Vijaya Lakshmi

Designation: Assistant Professor

This is to certify that the course material being prepared by Mrs.K. Mabel Hebsuba, G. Dhinesh, J. Muhamed Ali & S. Vijaya Lakshmi is of adequate quality. He has referred more than five books amount them minimum one is from aborad author.

Signature of HD Name:

SEAL


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SCE Dept.of S & H

TABLE OF CONTENTS

S.NO TITLE PAGE NO

1 UNIT I WATER TECHNOLOGY 8

1.1 Introduction 8

1.2 Hard water and soft water 8

1.3 Boiler Feed water 8

1.3.1 Formation of deposits in steam boilers and Heat exchangers. 9

1.3.2 Caustic embrittlement 10

1.3.3 Boiler corrosion 11

1.3.4 Priming and Foaming 13

1.4 Softening or conditioning Methods 14

1.4.1 Zeolite or Permutit process 14

1.4.2 Ion-Exchange (or) De- ionization (or) Demineralisation Process 14

1.5 Internal Treatment (or) Internal Conditioning (or) Boiler Compounds 14

1.6 Desalination of Brackish water 20

1.6.1 Reverse Osmosis (R.O) 20

1.7 Glossary 21

2 UNIT II ELECTROCHEMISTRY AND CORROSION 22

2.1 Introduction 22

2.1.1 Types of cells 23

2.2 Electrochemical Cells 26

2.2.1 Reversible and Irreversible cells 27

2.3 Electrode Potential 28

2.3.1 Reduction Potential 28

2.3.2 Single electrode Potential 29

2.3.3 Standard electrode Potential 30

2.3.4 Measurement of Standard electrode Potential 30

2.4 EMF of Cell 35

2.4.1 Measurement of EMF of cell 36

2.4.2 Applications of EMF measurement 36

2.5 Electrochemical Series 36

2.5.1 Significance of EMF series 36

2.6 Nernst Equation 38

2.6.1 Nernst equation for reversible cell 39

2.6.2 Problems based on Nernst equation 40

2.7 Corrosion 40

2.7.1 Consequences of corrosion 40

2.8 Types of Theories of Corrosion 40

2.8.1 Dry or Chemical corrosion 41

2.8.2 Wet or electrochemical corrosion 42

2.9 Factors influencing the rate of corrosion 47

2.9.1 Nature of the metal 47

2.9.2 Nature of the environment 47


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2.10 Corrosion Control 48

2.10.1 By modifying metal 48

2.10.2 BY modifying the environment 51

2.11 Paints 52

2.11.1 Characteristics of a good paint 52

2.11.2 Constituents and their functions of paints 52

2.12 Metallic Coatings 53

2.12.1Electroplating 53

2.12.2 Electroless Plating 54

2.13 Glossary 56

3 UNIT – III ENERGY SOURCES 57

3.1 Introduction 57

3.2 Nuclear Energy 57

3.2.1 Nuclear Fission 57

3.2.2 Nuclear fusion 57

3.2.3 Nuclear chain reaction 57

3.3 Nuclear reactor 59

3.3.1 Light water Nuclear Reactor 59

3.3.2 Breeder Reactor 60

3.4 Solar Energy 61

3.4.1 Solar energy conversion 61

3.4.2 Solar cell 61

3.4.3 Application of Solar Cells 61

3.5 Wind Energy 63

3.5.1 Wind Mill 63

3.6 Batteries 64

3.6.1 Types of batteries 64

3.6.2 Alkaline Battery 64

3.6.3 Lead Storage Battery 65

3.6.4 Nickel-Cadmium Battery 65

3.6.5 Lithium Battery 66

3.7 Fuel Cells 68

3.7.1 Hydrogen Oxygen Fuel Cell 68

3.8 Glossary 70

4 UNIT – IV ENGINEERING MATERIALS 71

4.1 Introduction 71

4.2 Abrasives 71

4.2.1 Properties of Abrasives 71

4.2.2 Classification of Abrasives 71

4.2.3 Applications of Abrasives 71

4.3 Refractories 72

4.3.1 Characteristics of Refractories 72

4.3.2 Classification of Refractories 73

4.3.3 Properties of Refractories 73

4.3.4 Manufacturing of Refractories 74

4.4 Portland Cement 74


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4.4.1 Chemical composition of Portland cement 74

4.4.2 Manufacturing methods 75

4.4.3 Setting and hardening 78

4.4.4 Other types of cements 77

4.5 Glass 77

4.5.1 Properties of glass 77

4.5.2 Manufacturing of glass 78

4.5.3 Types and uses of glass 78

4.6 Glossary 80

5 UNIT V FUELS AND COMBUSTION 82

5.1 Definition 82

5.1.1 Characteristics for good fuel 82

5.2.1 Calorific value: 82

5.2.2 Gross or high calorific value (gcv) 82

5.2.3 Net or lower calorific value(ncv) 82

5.2.4 Theoretical calculation of calorific value ( dulong‟s formula) 83

5.3 Solid fuel 83

5.3.1 Advantages of solid fuels: 83

5.3.2 Disadvantages of solid fuels: 83

5.4 Coal 83

5.4.1 Proximate analysis & 5.4.2 significance of proximate analysis 83

5.5 Carbonization of metallurgical coke 84

5.5.1 Metallurgical coke 84

5.5.2 Otto-Hoffman‟s method 84

5.5.3 Recovery of by-products 85

5.5.4 Advantages of otto hoffman‟s process 85

5.6 Liquid fuels 85

5.6.1 Advantages of liquid fuels, disadvantages of liquid fuels 85

5.6.2 Petroleum 86

5.6.3 Refining of petroleum or crude oils 86

5.7. Knocking 87

5.7.1 Causes of knocking in S.I (petrol) engines 87

5.7.2 Improvement of anti knock characteristics 87

5.7.3 Octane number or octane rating 87

5.7.4Anti-knock agent 87 5.7.5Causes of knocking in C.I (diesel) engines 88

5.7.6 Cetane number or cetane rating 88

5.7.8 Diesel index 88

5.8. Hydrogenation of coal 89

5.8.1 Bergius process (indirect method) 89

5.8.2 Fischer-tropics process (indirect method) 90

5.9 Gaseous fuels 90

5.9.1. Compressed natural gas (CNG) 91

5.9.2 Producer gas 91

5.9.3. Water gas 92

5.9.4 LPG- liquefied petroleum gas 93


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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


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SCE Dept.of S & H

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


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SCE Dept.of S & H

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.

2C17 H 35 COONa + Ca ++

→ (C17 H 35 COO) 2 Ca +2Na+

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.


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Requirements of boiler feed water

It should have zero hardness.

It must be free from dissolved gases like O2, CO2, etc.

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

Ca and Mg salts.

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 MgCl2, MgCO3, MgSO4

and CaCl2.

They have greater solubilities in hot water than

cold water.

( i i ) 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 (HCO3)2, CaSO4 and

Mg(OH) 2.


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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 Na2CO3. In high pressure boilers,

Na2CO3 undergoes hydrolysis to produce NaOH.

Na 2 CO 3 + H 2 O → 2NaOH

+CO2

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, Na2FeO2.

Fe + 2NaOH → Na 2 FeO 2

+H2


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SCE Dept.of S & H

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 + 6H2O + 3O2 → 4 Fe

(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(HCO3)2 → CaCO3 + H2O +

CO2

CO2 + H2O → H2CO3

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.

MgCl2 + 2H2O → 2HCl + Mg (OH)2


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Fe + 2 HCl → FeCl2 + H2

FeCl2 + 2H2O → Fe (OH)2 + 2 HCl

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.

2Na2SO3 + O2 → 2Na2SO4

N2H4 + O2 → N2 + 2H2O

Hydrazine is the ideal compound for the removal of dissolved O2 as it forms

only water and inert nitrogen gas during the reaction.

Dissolved CO2 is removed by the addition of ammonium hydroxide.

2NH4OH + CO2 → (NH4)2CO3 + H2O

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.


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High temperature, low pressure and a large exposed surface, reduces the

dissolved gases (O2 and CO 2) in water.

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.


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SCE Dept.of S & H

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 Na2O.Al2O3.XSiO2.YH2O. The synthetic form of zeolite is called

permutit and is represented by Na2Ze.

In this process the hard water is allowed to perlocate through sodium zeolite. The

sodium ions which are loosely held in this compound are replaced by Ca2+

and Mg2+

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.

Na2Ze + Ca(HCO)2 CaZe + 2NaHCO3


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Na2Ze + Mg(HCO)2 MgZe + 2NaHCO3

Na2Ze + MgCl2 MgZe + 2NaCl

Na2Ze + CaCl2 CaZe + 2NaCl

Na2Ze + CaSO4 CaZe + Na 2SO4

Na2Ze + MgSO4 MgZe + Na 2SO4

Regeration

After some time zeolite gets exhausted. The exhausted zeolite is again regerated by

treating with 10%solution of NaCl.

CaZe + 2 NaCl → Na2Ze + CaCl2

MgZe + 2 NaCl → Na2Ze +MgCl2

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


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(HCO3)–

and(CO3)2–

in the soft water.

When such water is boiled in boilers, CO2 is liberated. Free CO2 is weakly

acidic in nature and extremely corrosive to boiler metal.

Na2CO3 + H2O→2NaOH + CO2

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

hardness producing Ca2+

and Mg2+

ions, but it will contain other ions like Na+, K

+, SO4

2–

, 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, – SO3H) are capable of

exchanging their H+

ions with other cations of hard water. Cation exchange resin

is represented as RH2.

Examples:

Sulphonated coals

Sulphonated polystyrene

R–SO3H; R–COOH ≡ RH2

(ii) Anion Exchanger


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Resins containing basic functional groups (–NH2, –OH) are capable of

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–NR3OH; R–OH; R–NH2 ≡ R (OH)2

Process

The hard water first passed through a cation exchange which absorbs all the

cations like Ca2+

, Mg2+

Na+, K

+, etc. present in the hard water.

RH2 + CaCl2 → RCa + 2HCl

RH2 + MgSO4 → RMg + H2SO4

RH + NaCl → RNa + HCl

The cation free water is then passed through a anion exchange column, which

absorbs all the anions like Cl–, SO4

2, HCO3

–, etc., present in the water.

R' (OH) 2 + 2HCl → R'Cl2 +

2H2O R'(OH) 2 + H2SO4 →

R'SO4 + 2H2O

The water coming out of the anion exchanger completely free from cations and


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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.H2SO4.

RCa + 2HCl → RH2 + CaCl2

RNa + HCl → RH + NaCl

Similarly, when the anion exchange resin is exhausted, it can be regenerated by

passing a solution of dil.NaOH.

R'Cl2 + 2 NaOH → R'(OH)2 + 2 NaCl

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 Na2(Na4 (PO3)6). A highly soluble complex containing Ca is formed by

replacing the sodium ions and thus prevents their formation of scale forming salts


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like CaSO4. The reaction is as follows:

2CaSO 4 + Na 2 [Na 4 (PO3) 6] → Na 2 [Ca 2 (PO3) 6] + 2Na 2SO4

Since the complex is highly soluble there is no problem of sludge disposal.

Carbonate conditioning

Scale formation due to CaSO4 in low pressure boilers can be avoided by adding

Na2CO3 to the boilers.

CaSO4 + Na2 CO3 → CaCO3 + Na2SO4

The forward reaction is favored by increasing the concentration of CO32-

.CaCO3

formed can be removed easily.

Phosphate conditioning

In high pressure boilers, CaSO4 scale whose solubility decrease with increase of

temperature. Such scale can be converted into soft sludge by adding excess of

soluble phosphates.

3CaSO4 + 2Na3 PO4 → Ca3 (PO4)2 +2Na2SO4

There are three types of phosphates employed for this purpose.

Tri-sodium phosphate – Na3PO4 (too alkaline): used for too acidic water.

Di-sodium hydrogen phosphate – Na2HPO4 (weakly alkaline): Used for weakly

acidic water.Mono sodium di hydrogen phosphate NaH2PO4 (acidic) used for alkaline

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 CaCO3, CaSO4 into a Non-adherent, loose precipitate called

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


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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


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SCE Dept.of S & H

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 (

CO32-

) and bicarbonate ( HCO3-) ions.

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.


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SCE Dept.of S & H

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


23

SCE Dept.of S & H

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

chemical reaction.

2. Anode is positively charged.

3. Cathode is negatively changed.

4. Electrons move from anode to

cathode through external

circuit.

5. The extent of chemical reaction at

the electrode is governed by

Faraday‟s laws of electrolysis.

6. The amount of electricity passed

is measured by a coulometer.

7. One electrolyte and two electrodes

of the same element are generally

used in these cells.

1. Electrical energy is produced at the

expense of chemical

energy.

2. Anode is negatively charged.

3. Cathode is positively charged.

4. Electrons move from cathode

to anode through external

circuit.

5. The emf of the cell depends

on concentration of

electrolyte and nature of the

metal electrode.

6. The emf of the cell is measured

by a potentiometer.

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

▬▬► Cl2 ↑ + 2e

At cathode: (i) Na +

+ H2O ▬▬► NaOH + H+


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SCE Dept.of S & H

(ii) 2 H+

+ 2e

▬▬► H2↑

Net reaction: 2NaCl + 2H2O ▬▬► 2NaOH + H2 + Cl2

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


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SCE Dept.of S & H

Note:Liquid junction potential is higher when porous membrane is used for

joining two half cells while it is minimum in a cell with a salt bridge.

Liquid junction potential occurs when two solutions of different

concentrations/compositions are in contact with each other through porous

membrane. It is due to formation of an electrical double layer of positive and

negative charges at the junction of the two solutions as the ions diffuse

through membranes.

Salt bridge: Contains a solution of a salt (KCl or KNO 3 or NH4NO3) that

literally serves as a bridge to complete the electric circuit, maintain electro

neutrality of electrolyte

and minimize the liquid junction potentia. For precise measurements of

potential, a salt bridge is used.

Fundamental components of electrochemical cell

1. Anode: oxidation half-reaction takes place ; ex. Zn Zn 2+ +2 e -

Oxidation of metal releases metal ions into the solution of oxidation half-

cell, leaving behind the electron at the surface of the metal electrode.

2. Cathode: reduction half-reaction occurs; ex. Cu2++ 2e - Cu

Movement of metal ions from the solution to the electrode to

gain es in reduction half cell leads to accumulation of positive

charge at the surface of the electrode.

3. Electrolyte:Internal conducting environment that allows ions to migrate

between both half cells so as to preserve electro neutrality

4. External circuit : Two half-cells are joined together by wire through which

electrons flow.

5. Salt bridge/porous membrane: Serves as a bridge to complete the

electric circuit and maintain electro neutrality in the electrolyte.

2.2 Electrochemical Cell or Galvanic cell

It is a device in which a redox reaction is used to derive electrical energy. During

the working of the cell the stored chemical energy decreases and this decrease is

gained as electrical energy.

In the electrochemical cell the electrode at which oxidation occurs is called anode

(− ve) and the electrode at which reduction occurs is called cathode (+ ve).

Example: Zn acts as anode and Cu acts as cathode in Daniel cell


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SCE Dept.of S & H

It consists of zinc electrode dipped in 1M zinc sulphate solution and a

copper electrode dipped in 1M copper sulphate solution. Each electrode acts as a half

cell connected by a salt bridge through a voltmeter. The two

solutions can seep through the salt bridge without mixing.

At anode: Oxidation takes place

Zn (s) ▬▬► Zn 2+

+ 2e _

At cathode: Reduction takes place.

Cu 2+

(aq) + 2e _

▬▬► Cu (s)

Net reaction: Zn (s) + Cu 2+

(aq) ▬▬► Zn 2+

(aq) + Cu (s)

Representation of a galvanic cell

(i) Galvanic cell consists of two electrodes, anode and cathode

(ii) The anode is written on the left hand side while the cathode is written on right side.

(iii) The anode is written with the metal first and then the electrolyte .The two are

separated by a vertical line or semicolon

Ex: Zn / Zn 2+

(or) Zn / ZnSO4 (or) Zn; Zn 2+

(iv) The cathode is written with electrolyte first and then the metal both are separated

by vertical line or semicolon


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SCE Dept.of S & H

Ex: Cu2+

/ Cu (or) CuSO4 / Cu (or) Cu ; Cu2+

(v) The two half cells are connected by a salt bridge which is indicated by two

parallel lines.

▬ +

Zn / ZnSO4 (1M ) ║ CuSO4 ( 1M ) / Cu

Salt bridge: It consists of a U tube filled with a saturated solution of KCl or

(NH4)2NO3 in agar-agar gel. It connects the two half cells and performs the following

functions

• It eliminates the liquid junction potential.

• It provides path for the flow of electrons between two half cells.

• Completes the circuit.

• Maintains electrical neutrality in the two compartments by migration of ions

through the porous material thus ensures the chemical reactions proceed

without hindrance

• Prevents mixing of the electrode solutions

2.2.1 Reversible cells

A cell works reversibly in the thermodynamic conditions. Ex. Daniel

cell, Secondary batteries, Rechargeable batteries.

The cell is reversible if it satisfies all the following conditions:

(i) If applied emf is equal to derived emf then the net reaction is zero

(ii) If applied emf is infinitesimally smaller than the derived emf then the cell should

act as electrochemical cell (forward reaction)

(iii) If applied emf is infinitesimally greater than the derived emf then the cell should

act as electrolytic cell (reverse reaction)

Irreversible cells

Cells which do not obey the (above) conditions of thermodynamic reversibility are

called irreversible cells. If one of the products escapes from the cell then that cell cannot

be made reversible by applying an external current.

Ex. Zinc-Silver cell, Primary cells

Zn –Ag Cell

Zn / H2SO4 (aq) / Ag


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SCE Dept.of S & H

Cell reaction:

Anode: Zn + H2SO4 ▬▬► ZnSO4 + H2 ↑

Cathode: 2Ag +

+ 2e_

▬▬► 2Ag

When two electrodes are connected from outside, zinc dissolves liberating hydrogen

gas. Since one of the product hydrogen escapes, the cell reaction cannot be reversed

when connected to an external EMF. The cell does not obey the conditions of

reversibility and is called irreversible cell.

2.3 Electrode Potential :

2.3.1 Oxidation potential

When a metal M is dipped in its salt solution, one of the following

reactions occurs depending on the metal :

1. Positive metal ions pass into the

solution : M →Mn+ + ne-

(oxidation)

(e.g.) When Zn rod is dipped in ZnSO4 solution, Zn goes into solution as

Zn2+.

The electrons attach to Zn rod, giving it a negative charge. The negative charge

on the rod attracts positive ions from solution. Thus a double layer of ions is

formed close to the rod.


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SCE Dept.of S & H

2.3.2 REDUCTION POTENTIAL

2. Positive ions from the solution deposit over the

metal.

Mn+ + ne- → M (reduction)

When Cu rod is dipped in CuSO4 solution, Cu+2 ions from the

solution deposit on metal rod. They attract negative ions from solution. Thus a

double layer of ions is formed close to the metal rod. This is called Helmholtz

double layer.

As a result a potential difference is set up between the metal and the

solution. The equilibrium value of the potential difference is known as electrode

potential.

Factors affecting electrode potential or emf of Cell :

(i) nature of the metal

(ii) temperature

(iii) concentration of metal ions in the solution

2.3.3 Single electrode potential (or) electrode potential :

It is a measure of the tendency of the metal electrode to lose or gain

electrons, when it is in contact with its own salt solution. It is developed due to

the formation of a double layer around the metal rod.

2.3.4 Standard electrode potential :


30

SCE Dept.of S & H

It is a measure of the tendency of the metal electrode to lose or gain

electrons, when it is in contact with its own salt solution of 1M strength at 25C.

Measurement of single electrode potential : It is impossible to determine the value of a single electrode potential.

But we can always measure the potential difference between two electrodes

using a potentiometer, by combining the two electrodes to form a cell. For

this purpose we reference electrode. Standard hydrogen electrode is called

primary reference electrode. Calomel electrode is called secondary reference

electrode.

Standard hydrogen electrode (SHE) (Primary Reference Electrode)

It has a platinum foil connected to platinum wire and sealed in a glass

tube. The platinum foil is dipped in 1M HCl. Hydrogen gas 1 atm

pressure is passed through the side arm of glass tube as shown in the

figure. The standard electrode potential of SHE is taken as zero. The electrode

is represented,

Pt | H2(g) (1 atm) | H+ (1M)

The electrode reaction

is

2H+ + 2e- H2


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SCE Dept.of S & H

Limitations (or) drawbacks of SHE:

1) H2 gas reduces many ions like Ag+ and affects compounds of Hg, Ag etc.

2) It is difficult to get pure H2.

3) The pressure of H2 is to be kept 1 atm all the time.

4) It is difficult to set up and transport.

5) The electrode potential changes with barometric pressure.

6) A large volume of test solution is required.

7) It cannot be used in solutions of redox systems.

8) The solution may poison platinum surface. So we use a secondary reference

electrode.

Calomel Electrode (Secondary reference electrode):

It consists of a glass tube containing pure mercury at the bottom. A paste

of mercurous chloride covers the mercury. A solution of potassium chloride

is present over the paste. The bottom of the tube is sealed with a platinum

wire. There is a side tube for electrical contact. The electrode is represented as,

Hg | Hg2Cl2(s) | KCl(aq)

The electrode reaction is,

Hg2Cl2 + 2e- 2Hg + 2Cl-

The electrode potential is,

E = E° RT

ln [Cl-

]2

2F

E = E° RT

ln [Cl-

]

F

At 25oC,

E = E – 0.0591 log (Cl-)

For saturated KCl, E = +0.242 volt.


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SCE Dept.of S & H

Measurement of single electrode potential using a reference electrode

(saturated calomel electrode): The given electrode, say zinc electrode, is coupled with saturated

calomel electrode as in the figure. Since the reduction potential of zinc

electrode less than that of calomel electrode, zinc acts as anode and calomel as

cathode. The cell reaction will be

Zn/ ZnSO4 (1 M) // KCl (satd )/ Hg2Cl2/Hg

Zn + Hg2Cl2 Zn2+ + 2Hg + 2Cl-

The emf of the cell is measured using a potentiometer. The value of Ecell

=

1.002 volt.

Now, Ecell = EOright – E0

left

= EOcal – E

OZn

1.002 = 0.242 – E0Zn

EZn = 0.242-1.002

EZn = - 0.76 volt.

Advantages of Reference Electrode (Calomel Electrode):


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SCE Dept.of S & H

1. Easy to set up.

2. Easily transportable

3. Long shelf life

4. Reproducibility of emf

5. Low temperature coefficient

6. Electrode can be used in a variety of solutions.

7. Eo value is accurately known.

Ion sensitive electrode :

Ion sensitive electrodes have the ability to respond only to a specific ion

and develop a potential ignoring other ions in the solution.

The types (classification) of ion-sensitive electrode :

1) Glass membrane electrodes

2) Solid state electrode

3) Pungor or precipitate electrodes

4) Liquid – liquid electrode

Glass Electrode (or) Measurement of pH using glass electrode :

Glass electrode contains a thin-walled glass bulb. The glass has low

melting point and high electrical conductivity. 0.1M HCl is present in the

bulb. A platinum wire is inserted in the acid.

When the glass membrane separates two solutions differing in pH,

exchange of H+ ions takes place between the solutions. As a result a

potential is developed across the membrane. The potential EG is given by,

EG = EG + 0.0591

pH


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SCE Dept.of S & H

Measurement of pH :

The glass electrode is dipped in the given solution. This system is

connected to saturated calomel electrode as in the figure. The emf of the

resulting cell is measured using a potentiometer.

From the emf, the pH of the solution is calculated as

below: Ecell = Eright – Eleft

Ecell= Ecal – Eglass

Ecell = 0.242 – (EG + 0.0591v pH)

Ecell = 0.242 - EG - 0.0591 pH

PH =Ecell – 0.2422+E

0Glass/0.0592

Advantages of Glass Electrode : i)It is easily constructed and used

ii)Results are accurate

iii)Electrode is not easilpoisoned

iv) Equilibrium is quickly attained

v) It can be used in strong oxidizing solutions, coloured solutions and

in presence of metal ions

vi) Using special glass electrode, pH can be measured from 0 to 12.


35

SCE Dept.of S & H

vii) It is used in chemical, industrial, biological and agricultural laboratories.

Disadvantages or Limitations : i) Glass has high resistance. So special electronic potentiometer

must be used.

ii) It cannot be used in highly alkaline solutions, in pure ethanol or in

acetic acid. If the solution pH is more than 12, glass membrane is

affected by cations.

Applications of Ion-sensitive electrode :

i) To determine ions like H+, K+, Li+, etc.

ii) To determine hardness of water (Ca+2 and Mg+2 ions)

iii) To determine concentration of F-, NO3-, CN- etc.

iv) To determine concentration of a gas using gas-sensing

electrodes.

v) To determine pH of a solution using H+ ion sensitive electrode.

2.4 EMF of an electrochemical cell

● Difference of potential which causes the flow of current from the electrode

of higher potential to one of lower potential

● It is the algebraic sum of the single electrode potentials provided the proper sign

are given according to actual reaction taking place on the

Nernst eqn for electrochemical cell

ECell=E right-Eleft, or Ecell =E cathode -Eanode

E° cell+

0.0592Vlog[reactant]/[p

roduc)

2.4.1Instrument:

● emf of the cell can be measured by potentiometer-using

Poggendorff’s compensation principle(wheatstone's bridge).

● Voltmeter cannot be used to determine accurate value of emf because during

measurement a part of cell current is drawn which causes change in the emf.

Factors affecting the emf of a cell

1.Nature of the electrolytes and electrodes.

2.Concentration and composition of the electrolytes.

3.pH and temperature of the solution.

2.4.2 Applications of EMF Measurements

The valency of an ion can be determined.

Solubility of a sparingly soluble salt can be determined.

Potentiometric titrations can be carried out.

Hydrolysis constant can be determined.


36

SCE Dept.of S & H

Determination of standard free energy change and equilibrium constant

Determination of pH by using a standard hydrogen electrode.

2.5 ELECTROCHEMICAL SERIES (e.m.f series)

A series in which elements are arranged in the ascending (increasing ) order

Of their standard reduction potential is called emf series.

2.5.1Significance of EMF series

Half cell reaction Eo

(V)

Li+

+ e-

→ Li - 3.04

Mg2+

+ 2e- -

→ Mg - 2.37

Al3+

+ 3e- -

→Al - 1.66

Zn2+

+ 2e- -

→ Zn - 0.76

Fe2+

+ 2e- -

→ Fe - 0.44

2H+

+ 2e- -

→ H2 (g) 0.00

Hg 22+

+ 2e▬ → Hg (l) 0.2422

Cu2+

+ 2e- -

→ Cu 0.34

Cu+

+ e--

→ Cu 0.52

Pt,Fe3+

+ e▬ → Fe2+

0.77

Ag+

+ e--

→ Ag 0.80

Au+

+ e-

→ Ag 1.69

F2 + 2e- -

→ 2F-

2.8

Application / Significance of electrochemical series

(i) Relative ease of oxidation or reduction • The metals which lie above hydrogen in the series undergo spontaneous

oxidation and the metals which lie below SHE undergo reduction

spontaneously ( ie. Acts as Anodes and Cathodes respectively)

• The metals which lie above hydrogen are good reducing agents and which lies

below hydrogen will act as good oxidizing agents

(ii) Replacement tendency • The metal lying above in emf series displaces the metal lying below it from an

electrolyte of the later.

Example 1: Ni spatula cannot be used to stir copper sulphate solution due to

the following reaction

Ni(s) + Cu2+

(aq) ▬▬► Ni 2+

(aq) + Cu (s)


37

SCE Dept.of S & H

Example 2: when zinc is dipped in copper sulphate solution copper gets deposited

(displaced)

Zn (s) + CuSO4 (aq) → Zn SO4 (aq) + Cu (s)

(iii) Liberation of Hydrogen • The metal with negative reduction potential will displace H2 from an acid

solution

Zn (s) + 2 HCl (aq) → Zn Cl2 (aq) + H2 ↑

Hence acids cannot be stored in galvanized steel containers.

For exactly the same reason galvanized steels are not used to store food stuffs

containing vinegar. (Vinegar is used as food preservative- vinegar is acetic acid)

(iv) Calculation of equilibrium constant (Keq)

∆GO

= n F Eo

−∆GO

= 2.303 RT log K(eq) Therefore

log K(eq) = n F Eo

2 .303 RT

log K(eq) = nEo

0.0591

(v) Calculation of Standard emf of the cell E cell = E cathode − E anode

( if both reduction potentials are considered)

E cell = E cathode + E anode

( if oxidation potential of anode and the reduction potential of cathode

are considered)

(vi) Corrosion • The metals higher in the series are anodic and are more prone to corrosion.

• The metals lower in the series are noble metals (cathodic) and they are less prone

to corrosion.

(vii) Predicting the spontaneity of cell reaction • Spontaneity of the redox reaction can be predicted from the emf value of

complete cell reaction.

If the value of Ecell is positive, the reaction is feasible. as ∆G will be negative

( i.e. it is an electrochemical cell)

If the value of Ecell is negative, the reaction is not feasible. as ∆G will be positive

( i.e. it is an electrolytic cell)


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SCE Dept.of S & H

2.6 NERNST EQUATION

If an emf of a reversibile cell is E volts and the cell reaction that takes place is

accocated with the passage of an faradays or nF coulombs, the electrical work done by the

system is given by the product of the emf and total charge.

Electrical work done by the system = nFE, where E is the reversibile emf of the

cell and n is the no.of electrons involved in the cell reaction.

he work done on the system at constant temperature is the increase in the free

energy ΔG of the system

The free energy change of a reaction depends on the activities of the reactants and

the products. Therfore, in the case of cell reactions of a reversibile, the emf of the cell will

also depend on the activities of the reactant and products.

Applications of Nernst Equation :

I)To calculate electrode potential of unknown metal.

ii)To predict corrosion of metals.

iii) To set up electrochemical series.


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SCE Dept.of S & H

2.6.1Nernst equation for a reversible cell :

Let us consider the reaction in a reversible cell :

A + B C + D ----- (1)

The free energy change G of this equilibrium is related to the

equilibrium

constant K by the Van‟t Hoff isotherm.

ΔG = ΔG° + RTln

[Product]

[Reactant]

-----(2)

We know, G = - nFE

G = -nFE ------ (3)

Substituting equations (1) & (3) in equation (2)

-nFE = -nFE° RTln [C][D]

[A][B]

-----(4)


SCE Dept.of S & H

E = E° 0.0591

log [C][D]

n [A][B]

This is Nernst equation for a reversible cell.

2.6.2 PROBLEM BASED ON NERNST EQUATION

Calculate the reduction potential of lead electrode in contact with a solution of 0.015M Pb2+

ions.( E0

= -0.13 volt)

GIVEN

Std Oxidation potential is given as

Pb2+

+ 2 e- → Pb; E

0 = -0.13v

Concentration of Pb2+

= 0.015M

Solution

The Nernst equation for reduction potential is

E = E0

red + 0.0591/n log(Pb

2+)

E = -0.13 +.o2955(-1.824)

E = -0.1839V

Oxidation potential of Pb = -0.1839V

2.7 Corrosion

It is the gradual deterioration of metals by chemical, electrochemical or

biochemical interaction with the environment.

Causes of Corrosion

Metals occur in nature as their oxides, sulphides carbonates etc. The

chemically combined state is thermodynamically more stable. When we extract a

metal from its ore, the metal is in a higher energy state, which is thermodynamically

unstable. So it tries to go back to the stable state by chemical or electrochemical

interaction with the environment.

2.7.1 Consequences or effects of Corrosion

1. Efficiency of the machine decreases.

2. Plant has to be shut down due to failure.

3. Product is contaminated.

4. The toxic products of corrosion cause health hazards.

5. There is a necessity to over design to allow for corrosion.

2 . 8 T y p es or Theories of Corrosion

I. Dry or Chemical Corrosion

II. Wet or Electrochemical Corrosion

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SCE Dept.of S & H

2.8.I. Dry or Chemical Corrosion

It is due to the attack on metal surface by atmospheric gases like O2, SO2, H2S etc.

(e.g.) Tarnishing of silver by H2S.

There are three types of dry or chemical corrosion.

1) Oxidation Corrosion

2) Corrosion by Hydrogen

3) Liquid Metal Corrosion

(1) Oxidation Corrosion

It is due to the direct attack of oxygen on metal surface in the absence of

moisture. Alkali and Alkaline earth metals are corroded at low temperatures. At high

temperatures, most metals except Au, Pt and Ag are oxidized.

Mechanism

i) Oxidation occurs at the surface of the metal to form M2+ ions.

M M2+ + 2e-

ii) Oxygen takes up the electrons. O2 is reduced to O2-

½ O2 + 2e- O2-

iii) O2- ion reacts with M2+ to form metal oxide.

M2+ + O2- MO

The metal surface is converted to a monolayer of metal oxide. Further

corrosion occurs by diffusion of M2+ ion through the metal oxide barrier. The growth

of oxide film is perpendicular to the metal surface.

Different types of oxide films are formed.

(i) Porous and Non-Porous Oxide Film (or) Pilling-Bedworth Rule

(a) If the volume of the oxide layer formed is less than the volume of the metal

41


SCE Dept.of S & H

consumed, the oxide layer is porous. (e.g.) The volumes of oxides of alkali and alkaline earth

metals are less than the volume of the metal consumed. So the oxide layer is porous and non-

protective

(b) If the volume of the oxide layer formed is greater than the volume of the metal consumed, the

oxide layer is non-porous.(e.g.) The volumes of oxides of heavy metals such as Pb, Sn are greater

than the volumes of the metal consumed. So the oxide layer is non-porous and protective.

(ii) Stable Oxide Layer

A stable oxide layer is firmly adsorbed on the metal surface. The layer is

impervious and prevents further corrosion. So the layer itself acts as a protective

coating. (E.g.) Oxides of Al, Cu etc.

(iii) Unstable oxide Layer

This is mainly produced on the surface of noble metals such Ag, Au etc. The unstable oxide

decomposes to stable metal and oxygen. Metal Oxide Metal + Oxygen

(iv) Volatile Oxide

The oxide film volatilizes as soon as it is formed. It leaves fresh metal surface for further

continuous attack. (e.g.) Molybdenum oxide MoO3.

(2) Corrosion by Hydrogen

(a) Hydrogen embrittlement

Definition

It is formation of cracks and blisters on the metal by hydrogen gas when the metal comes into

contact with H2S. Iron liberates atomic hydrogen by reacting with H2S.

Fe + H2S FeS + 2H

Hydrogen atoms diffuse into the voids of metal matrix. When the pressure of the gas increases,

cracks and blisters develop on the metal.

(b) Decarburisation

It is the process of decrease in the carbon content of steel. At high temperature, molecular

hydrogen decomposes to atomic hydrogen. High Temperature

H2 2H

When steel is exposed to this environment, carbon in the steel reacts with atomic hydrogen.

C + 4H CH4

Hence the carbon content in steel decreases. Collection of methane gas in the voids of steel

develops high pressure and causes cracking.

(3) Liquid Metal Corrosion

It is due to the chemical action of flowing liquid metal at high temperature. It involves :

(i) dissolution of a solid metal by the liquid metal.

(ii) Penetration of liquid metal into the solid metal.

2.8.2. Wet (or) Electrochemical Corrosion :

It occurs under the following conditions.

(i) When two dissimilar metals or alloys are in contact with each other in presence of an aqueous

solution or moisture.

(ii) When the metal is exposed to an electrolyte with varying amounts of oxygen.

Mechanism of Wet Corrosion

(1) Metal dissolution occurs at the anode.

M →Mn++ + ne-

(2) Reduction reaction occurs at the cathode in different environments.

(a) Acidic environment : Here hydrogen gas is evolved at the cathode.

2 H+ + 2e- →H2

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SCE Dept.of S & H

(b) Neutral environment : In neutral or slightly alkaline medium, hydroxide ions are formed at the

cathode.

½ O2 + 2e- + H2O →2OH-

(a) Hydrogen Evolution type corrosion (In Acidic Medium)

All metals above hydrogen in the electrochemical series tend to get dissolved

in acidic solution with simultaneous evolution of H2 gas. e.g.) When iron comes into contact with

non-oxidising acid like HCl, hydrogen evolution occurs.

At anode : Iron is oxidized to Fe2+

Fe →Fe+2 + 2e-

At cathode : H+ ion is reduced to H2.

2 H+ + 2e- →H2

oxygen, OH- ions are formed.

At anode : Iron is oxidized to Fe

(b) Absorption of Oxygen (or) Formation of hydroxide ion type corrosion (In

neutral or weakly alkaline medium)

The surface of iron is normally coated with a thin film of iron oxide. But if some cracks develop

on the film, anodic areas are created on the surface. The rest of the metal part acts as cathode.(e.g.)

When iron is in contact with an electrolyte solution in presence ofoxygen, OH- ions are formed.

At anode : Iron is oxidized to Fe+2

At cathode : Production of OH- ions (more aeration)

½ O2 + 2e- + H2O →2OH

Waterline corrosion

Let us consider metal tank partially filled up with water. The metal area above water line is

exposed to higher concentration of oxygen (cathode) than the metal below water level. The metal

less exposed to O2 acts as anode and corrodes. This is called water line corrosion.

Examples of differential aeration corrosion

i) Pitting or localized corrosion

ii) Crevice corrosion

iii) Pipeline corrosion

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SCE Dept.of S & H

iv) Corrosion on wire fence

(i) Pitting Corrosion

It is the localized attack resulting in the formation of a hole due to corrosion.

Example : Metal area covered by a drop of water, sand, dirt etc.

The area covered by the drop or dirt acts as anode and corrodes. Theuncovered area exposed to air

or O2 acts as cathode.The rate of corrosion is more if the cathodic area is larger and anodic area is

smaller. Thus more material is removed from the same area and a pit is formed.


Fe →Fe2+ + 2e-

At cathode : O2 is reduced to OH-.

½ O2 + H2O + 2e- →2OH

Overallreaction :

Fe2+ + OH- →Fe(OH)2


Fe →Fe2+ + 2e-


½ O2 + 2e- + H2O →2OH

OverallReaction

Fe+2 + 2OH- →Fe(OH)2

If enough oxygen is present, Fe(OH)2 is oxidized to Fe(OH)3.

4Fe(OH)2 + O2 + H2O →4Fe(OH)3

Differences between chemical corrosion and electrochemical corrosion:

Chemical Corrosion Electrochemical Corrosion

1. It occurs in dry condition It occurs in presence of moisture or electrolyte.

2. It occurs by the direct chemical attack on the metal by the environment.It occurs by the

formation of a large number of anodic and cathodic areas.

3. Even a homogenous metal surface iscorroded.Only heterogeneous or bimetallicsurface is

corroded.

4. Corrosion products gather at the placeof corrosion.Corrosion occurs at the anode, whilethe

products form elsewhere.

5. It is a self controlled process It is a continuous process

6. It takes place by adsorption mechanism. It follows electrochemical reaction.(e.g.) Mild scale

formation on iron surface (e.g.) Rusting of iron under moist atmosphere

Types of electrochemical corrosion

There are two types:

(i) Galvanic corrosion

(ii) Differential aeration or Concentration cell corrosion

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SCE Dept.of S & H

(i) Galvanic corrosion

When two different metals are in contact with each other in presence of aqueous solution or

moisture, galvanic corrosion takes place.

The metal with more negative electrode potential acts as anode. Metal with less negative electrode

potential acts as cathode. In the Zn-Fe couple as shown in the figure, zinc with more negative

electrode potential, dissolves in preference to iron. Zn acts as anode and Fe as cathode.

Example :

Steel screw in a brass marine hardware easily undergoes corrosion. Iron has E0 = -0.44V. For Cu

E0 = +0.34 V. Iron corrodes in preference to Cu.

Prevention

Galvanic corrosion is minimized by providing an insulation between the two metals.

(ii) Differential aeration (or) concentration cell corrosion

Let a metal be partially immersed in a conducting solution. The part of the

metal above the solution is more aerated and acts like cathode. The less aerated metal part inside

the solution acts as anode and corrodes.

At anode : Corrosion occurs (less aeration)

M →M2+ + 2e

At cathode : Production of OH- ions (more aeration)

½ O2 + 2e- + H2O →2OH

Wateline corrosion

Let us consider metal tank partially filled up with water. The metal area above water line is

exposed to higher concentration of oxygen (cathode) than the metal below water level. The metal

less exposed to O2 acts as anode and corrodes. This is called water line corrosion.

Examples of differential aeration corrosion

i) Pitting or localized corrosion

ii) Crevice corrosion

iii) Pipeline corrosion

iv) Corrosion on wire fence

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SCE Dept.of S & H

(i) Pitting Corrosion

It is the localized attack resulting in the formation of a hole due to corrosion.

Example : Metal area covered by a drop of water, sand, dirt etc.

The area covered by the drop or dirt acts as anode and corrodes. The uncovered area exposed to air

or O2 acts as cathode. The rate of corrosion is more if the cathodic area is larger and anodic area is

smaller. Thus more material is removed from the same area and a pit is formed.


Fe →Fe2+ + 2e-


½ O2 + H2O + 2e- →2OH

Overall reaction :

Fe2+ + OH-→Fe(OH)2

(ii) Crevice Corrosion

Let a crevice or crack between two different metallic objects be in contact

with a liquid. The crevice acts like anode due to less oxygen availability and corrodes.

The exposed area acts as cathode.

(e.g.) rivets, joints.

(iii) Pipeline Corrosion

Buried pipelines or cables passing from one type of soil (clay, less aerated) to another type (sand,

more aerated) get corroded due to differential aeration.

(iv) Corrosion on wire fence

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In a wire fence, the wires at the crossings are less aerated than the rest of the

fence. So corrosion takes place at the wire crossings, which become anodic.

2.9 Factors influencing corrosion

2.9.1. Nature of the metal

(i) Position in emf series

Metals above hydrogen in the electrochemical series corrode easily because they have negative

reduction potential. When two metals are in contact, the more active metal with a higher negative

potential corrodes.

(ii) Areas of anode and cathode

Corrosion will be severe if the anodic area is smaller and cathodic area is larger. The larger

cathodic area demands more electrons. So the anodic area corrodes faster.

(iii) Purity

100% pure metal will not corrode. (e.g.) Pure Zn does not corrode. If the metal has trace amount of

impurity, it corrodes. (e.g.) Zinc metal with iron or copper impurity forms an electrochemical cell.

The base metal Zn acts as anode and corrodes.

(iv) Over Voltage

Corrosion rate is inversely proportional to the over voltage of the metal in a corrosive

surroundings. (e.g.) The hydrogen over voltage of Zn in 1M H2SO4 is 0.7V. So the rate of

corrosion is low. But when some Cu impurity is present, the over voltage is reduced and corrosion

rate increases.

(v) Nature of the Film

Nature of film formed on the metal surface determines extent of corrosion.(e.g.) In the case of

alkali and alkaline earth metals, the oxide film formed is porous .The corrosion continues. In the

case of heavy metals, the oxide film is non-porous. The film acts as a protective layer.

(vi) Nature of corrosion product

If the corrosion product is soluble in the corroding medium, corrosion rate is faster. Similarly if the

corrosion product is volatile (e.g. MoO3), corrosion will be more.

2.9.2. Nature of Environment

(i) Temperature

Increase of temperature increases corrosion rate because the rate of diffusion of ions increases.

(ii) Humidity

Rate of corrosion is more, if humidity of environment is high. Moisture acts as

solvent for O2, CO2 etc, to produce electrolyte necessary for formation of corrosion

cell.

(iii) Corrosive gases

Acidic gases like CO2, SO2, H2S etc, produce electrolytes and increase corrosion.

(iv) Presence of suspended particles

Particles like NaCl, (NH4)2SO4 along with moisture are powerful electrolytes and increase rate of

corrosion.

(v) Effect of pH

Generally in alkaline medium, the rate of corrosion is less compared to acidic medium.

The effect of pH on the corrosion of iron in water is shown in the Pourbaix diagram as indicated in

the figure.

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SCE Dept.of S & H

The figure shows zones of corrosion, immunity and passivity. Z is the point at which pH=7 and

corresponding electrode potential is E= -0.2V. This is in the corrosion zone. So iron rusts under

these conditions.

The rate of corrosion can be altered by shifting the point Z to different regions.

1) If the potential is changed to -0.8V by applying external current, iron becomes immune to

corrosion.

2) If the potential applied is positive, iron becomes passive.

3) If the pH is increased to more than 7, corrosion rate decreases.

4) If the pH is reduced to less than 7, rate of corrosion increases

2.10 CORROSION CONTROL

The rate of corrosion can be controlled by modifying the metal or environment. Some control

methods are

1) proper selection of metals

2) Use of pure metals

3) Use of metal alloys

4) Cathodic protection

a. Sacrificial anode protection

b. Impressed current cathodic protection

5) Changing the environment

6) Use of inhibitors

a. Anodic inhibitors

b. Cathodic inhibitors

7) Applying protective coatings

1) Proper selection of metals

Noble metals are used in ornaments and in surgical instruments, because they do not corrode.

Contact of dissimilar metals far away from each other in electrochemical series should be avoided.

2) By using pure metals

Pure metals have high corrosion resistance. Even a trace of impurity will lead to corrosion, the

base metal becoming anode.

3) Use of alloys

Use of metal alloys is a good method of protection against corrosion. (e.g.) Stainless steel

containing chromium forms a coherent oxide film which protects steel against further attack

2.10.1 Proper designing i. Complicated designs with more angles, sharp edges and corners should be avoided.

ii. Direct contact of dissimilar metals lead to galvanic corrosion. So insulating material between

the two metals should be inserted.

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SCE Dept.of S & H

iii. Smaller area for cathode and larger area for anode must be provided.

iv. Tanks and containers should be designed such that the liquid should be drained off completely.

v. Crevices should be avoided or they should be filled using fillers.

vi. Bendings should be smooth.

vii. Annealing minimizes corrosion.

5) Cathodic Protection

The metal to be protected is made to act like a cathode. This is achieved in two ways.

a) Sacrificial anodic protection

Here the metal to be protected is made cathode by connecting it to a more active metal (anodic

metal) called sacrificial anode. Only the more active metal will

a) Sacrificial anodic protection

Here the metal to be protected is made cathode by connecting it to a more active metal (anodic

metal) called sacrificial anode. Only the more active metal will be corroded, protecting the parent

metal. Since the anodic metal is sacrificed, the method is called sacrificial anodic protection. Mg,

Zn are used as sacrificial anodes.Metal to be protected

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Applications

i) Protection of buried pipelines, cables

ii) Protection of ships and boats

iii) Calcium metal is used to minimize engine corrosion

iv) Magnesium sheets are inserted into domestic water boilers to prevent rust

formation.

b) Impressed current cathodic protection method

Here an impressed current is applied in an opposite direction to annul the corrosion current. Thus

the corroding metal is converted to cathode from anode.The negative terminal of battery is

connected to the metal to be protected. The positive terminal is connected to an inert electrode like

graphite. The anode is buried in a „back-fill‟ (containing a mixture of gypsum, coke breeze and

sodium sulphate) to increase electrical contact.

Mg Metal to be protected

APPLICATION

i) Protection of tanks, transmission line towers, underground water pipes, oil pipe line, ships etc.

Limitations

i) It is costly

ii) It fails when current is switched off.

Corrosion inhibitors

A corrosion inhibitor is a substance that reduces corrosion, when added to the corrosive

environment. There are three types of inhibitors.

i) Anodic inhibitors - chromate, nitrate

ii) Cathodic inhibitors - amines

iii) Vapour phase inhibitors - benzonitrile.

i) Anodic inhibitors

(e.g.) chromate, nitrate, phosphates, tungstate.

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The inhibitors form insoluble compound with the newly produced metal ions and prevent

corrosion. This compound is adsorbed on the metal surface to form a passive film. Anodic

inhibitors are used to repair

i) the crack of oxide film on metal surface

ii) pitting corrosion

iii) porous oxide film on metal surface

ii) Cathodic inhibitors

There are two types depending on the nature of cathodic reaction in an electrochemical reaction.

a) In acidic solution

Example : amines, thiourea, mercaptans act as inhibitors.Here evolution of H2 is the cathodic

reaction.

2 H+ + 2e- →H2

The corrosion is controlled by slowing down the diffusion of H+ ions to cathode by addition of the

inhibitor which is adsorbed on the metal surface.

b) In neutral solution

Example : hydrazine, sodium sulphite act as inhibitors.Here OH- ions are formed at cathode.

H2O + ½ O2 + 2e- →2 OH

Corrosion is controlled by eliminating O2 from the corroding medium by

adding Na2SO3. The OH- ions can be eliminated by salts of Mg, Zn etc.

iii) Vapour phase inhibitors

(e.g.) benzotriazole, dicyclohexyl ammonium chromate act as inhibitors.These organic inhibitors

readily vapourise and form a protective layer on the metal surface.

2.10.2 Control of corrosion by modifying the environment:

There are five methods

1. Deaeration :

Presence of oxygen increases corrosion rate. Deaeration involves removal of dissolved oxygen by

increasing the temperature together with the mechanicalagitation. This also removes dissolved

oxygen.

2. Deactivation:

It is the removal of dissolved oxygen by adding chemicals in aqueous solution.

(E.g.) 2Na2SO3 + O2 →2Na2SO4

3. Dehumidification:

It is the removal of moisture from the air by reducing the relative humidity of the surrounding air.

It can be achieved by adding silica gel or alumina which absorbs

moisture.

4. Alkaline neutralization:

The acidic nature of the corrosive environment is due to the presence of HCl,

SO2, CO2 etc. They are neutralized with alkali spray. E.g. NaOH lime etc.

5. Using corrosion inhibitors :

A corrosion inhibitor is a substance that reduces the corrosion of a metal when added to corrosive

environment

Applications

To prevent corrosion in closed space, storage containers, sophisticated equipment etc.

PROTECTIVE COATINGS

Metal surface is covered by a protective coating to prevent corrosion. The coating acts as a

physical barrier between the metal surface and the environment. The coating gives a decorative

appeal and also imparts hardness, oxidation resistance and thermal insulation to the surface. The

main types of coating are:

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SCE Dept.of S & H

1) Metallic coating

2) Chemical conversion coating

3) Organic coating

4) Non-metallic coating

2.11 PAINT

Paint is a mechanical dispersion of one or more fine pigments in a medium (thinner + vehicle).

When a paint is applied to metal surface, the thinner evaporates. The vehicle undergoes slow

oxidation to form a pigmented film.

2.11.1 Requirements or requisites of a good paint

A good paint should,

i) have good covering power

ii) spread easily on the surface

iii) not crack on drying

iv) adhere well to the surface

v) give a glossy film

vi) be corrosion and water resistant

vii) have stable colour

2.11.2Constituents of Paint

Pigment

Vehicle

Thinner

Drier

Filler

Plasticizer

Anti skinning agent

1. Pigment

It is a solid that gives colour to the paint.

Functions:

1.To give colour and opacity to the film.

2.To provide strength to the film.

3.To protect film by reflecting U.V. rays.

4.To provide resistance to abrasion and weather.

Example:

White pigment - White lead, TiO2

Blue pigment - Prussion blue

Green pigment - Chromium oxide

Red pigment - Red lead, Fe3O4

2. Vehicle (or) Drying Oil

It is the film-forming liquid. It holds the ingredients of the paint. It is a nonvolatile high molecular

weight fatty acid of vegetable or animal.

Function

1.To hold the pigment on the surface.

2.To form a protective layer by oxidation and polymerization.

3.To impart water repellency, toughness and durability of film.

4.To improve adhesion of film.

Example

Lin seed oil, Castor oil.

3. Thinner

It is the volatile portion of paint. It is added to reduce the viscosity of the paintfor easy application

on the surface. It easily evaporates after paint is applied.

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Functions

1.To reduce viscosity of paint.

2.To dissolve vehicle and other additives.

3.To suspend the pigments.

4.To increase elasticity of film.

5.To increase penetration of vehicle.

6.To improve drying of film.

Example

Turpentine, Dipentine, Xylol.

4. Drier

It is a substance used to speed up drying of the paint.

Functions

1.To act as oxygen carrier or catalyst.

2.To provide oxygen essential for oxidation and polymerization of drying oil.

Example

Metallic soap, linoleate and resinate of Co, Mn etc.

5. Extender or Filler

These are white pigments that form bulk of the paint.

Functions

1.To reduce cost of paint

2.To prevent shrinkage and cracking of film

3.To modify shades of pigment

4.To retard settling of pigments in paint.

Example

Talc gypsum, china-day.

6. Plasticizer

It is added to the paint to provide elasticity to the film and prevent its cracking.

Example

Triphenyl phosphate, Tricresyl phosphate

7. Antiskinning agent

It is a chemical added to the paint to prevent gelling and peeling of the paint.

Example

Polyhydroxy phenols.

Pigment Volume Concentration (P.V.C.)

The P.V.C. of a paint is calculated using the equation.

P.V.C. =

Volume of pigment in the paint =Volume of pigment in the paint + Volume of non-volatile vehicle

in the paint

If P.V.C. is high, durability, adhesion and consistency of the paint will be low.

Failure of Paints

A paint may fail due to any one of the following reasons:

i) Chalking : It is the gradual powdering of the paint film on the painted surface. This happens due

to improper dispersion of pigment in vehicle.

ii) Cracking : A paint film cracks due to unequal expansion or contraction of paint coats.

Evasion : This is very quick chalking.

iv) Blistering : It is due to improper surface exposure of paint to strong sunshine.

2.12 Metallic Coating

2.12.1 Electroplating or Electro-deposition

It is the deposition of coat metal on the base metal by passing direct current through an electrolytic

solution of a soluble salt of the coat metal. The base metal to be electroplated acts as cathode. The

coat metal or an inert electrode forms anode. The electrolyte is a soluble salt of coat metal.

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Objectives or uses or applications of Electroplating:

i) To enhance resistance to corrosion of base metals.

ii) To give a decorative appearance.

iii) To enhance resistance to chemical attack.

iv) To improve hardness and wearing resistance.

v) To obtain polished surface.

Theory If the coating metal itself forms the anode, the concentration of electrolyte bath does not change

during electrolysis. The metal ions deposited on the cathode are replenished continuously by

dissolution of the anode.

Example

Electroplating of Gold

The object to be gold plated is treated with organic solvent like acetone, CCl4

to remove grease, oil etc. It is then washed with dil H2SO4 to remove scales, oxides

etc. The cleaned object is made cathode of electrolytic cell. Anode is a gold plate.

AuCl3 solution is the electrolyte. When current is passed into the solution, gold ions

migrate to the cathode, get reduced and deposit on the object.

Ionisation : AuCl3→Au+3 + 3Cl-

At Cathode :

Au+3 + 3e- →Au

At anode :

Au →Au+3 + 3e-

Au+3 + 3Cl- →AuCl3

To achieve a strong adherent and smooth deposit, glue or gelatin is added to the electrolyte bath.

To enhance the brightness of the deposit, brightening agents are added to the bath.

Conditions

i) Temperature : 600C

ii) Current density : 1 to 10 mA/cm2

iii) Low metal ion concentration

iv) Buffer solution to maintain pH.

Applications or Uses or Objectives

i) To give a decorative appearance.

ii) Electrical and electronic applications

iii) To get a thin coating of gold on cheap jewellery

iv) To achieve oxidation resistance, corrosion resistance etc.

2.12.2 Electroless Plating

It is the deposition of a noble metal (from its salt solution) on a catalytically active metal surface

using a reducing agent without use of electric current.The reducing agent reduces the metal ions.

The metal atoms get deposited over the surface to give a thin uniform coating.

Metal ions + reducing agent metal (deposited) + oxidation product

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Example

Electroless nickel plating

The various steps are:

Step I : Pretreatment and activation of the surface:

The surface to be plated is degreased by using organic solvents or alkali and then accompanied by

acid treatment.

i) The surface of stainless steel is activated by dipping in hot solution of 50% H2SO4.

ii) Mg alloy surface is activated by giving a thin coating of zinc and copper overit.

iii) Al, Cu, Fe, brass etc, do not require activation.

iv) Plastic, glass etc, are activated by dipping in a solution of SnCl2/HCl and then

in PdCl2 solution. On drying a thin layer of palladium is formed on the surface.

Step II : Preparation of plating bath:

The plating bath consists of:

i) Coating Metal : A solution of NiCl2 20g/lit.

ii) Reducing agent : Sodium hypophosphite 20g/lit.

iii) Exaltant to accelerate coating rate and complexing agent : Sodium succinate

15g/lit.

iv) Buffer to maintain pH at 4.5 : Sodium acetate 10g/lit.

v) Temperature 93oC

The pretreated object is immersed in the plating bath for required time. The following reactions

occur and nickel is coated on the object.

Cathode : Ni2+ + 2e- →Ni

Anode : H2PO2- + H2O →H2PO3- + 2H+ + 2e

Overall Reaction : Ni2+ + H2PO2- + H2O →Ni + H2PO3- + 2H+

Uses of Nickel Plating

i) For decorative coating of jewellery, decorative items and automobile spares

ii) For coating of polymers for decorative purpose.

iii) For electronic appliances.

Advantages of electroless plating over electro plating

i) Electricity is not necessary

ii) Complicated parts are uniformly coated

iii) Plastics, glass etc, are easily coated

iv) Good mechanical, chemical and magnetic properties are obtained.

Differences between Electroplating and Electroless plating:

Electroplating Electroless Plating

1. It is done by passing current. It is done by auto catalytic redox reaction.

2. Separate anode is required. Catalytic surface of the object acts as anode.

3. Anode reaction: M →Mn+ + ne- Anode reaction : R →O + ne-

4. Cathode reaction: Mn+ + ne-→M Cathode reaction: Mn+ + ne- →M

5. Irregular objects are not satisfactorily plated All objects are satisfactorily plated.

6. Object to be coated forms the cathode .Object to be coated forms catalytically active surface.

7. It is carried out on conducting materials.It is carried out even on insulators.

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GLOSSARY

CELL: Cell is an assembly of two electrodes and an electrolyte. It consists of two half cells. Each

half cell contains an electrode material in touch with electrolyte.

CURRENT: Current is flow of electricity through a conductor. It is measured in ampere.

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ELECTRODE: Electrode is a material or a metallic rod/bar/strip which conducts electrons.

ANODE: Anode is an electrode at which oxidation occurs.

CATHODE: Cathode is an electrode at which reduction occurs.

ELECTROLYTIC CELLS: Electrolytic cells are cells in which electrical energy is converted

into chemical energy.

ELECTOCHEMICAL CELLS: Electrochemical cells are cells in which chemical energy is

converted into electrical energy.

SALT BRIDGE: Salt bridge consists of a U – tube containing a gel saturated with KCL or NH4

NO3 in agar – agar. It consists the two half cells of the galvanic cells by the electron movement

through it.

OXIDATION POTENTIAL: Electrode lose electrons

REDUCTION POTENTIAL: Electrode gain electrons.

SINGLE ELECTRODE POTENTIAL: Measure of the tendency of a metallic electrode to lose or

gain electrons, when it is in contact with a solution of its own salt.

STANDARD ELECTODE POTENTIAL: Measure of the tendency of a metallic electrode to

lose or gain electrons, when it is in contact with a solution of its own salt of a 1 molar

concentration at 250C.

ION – SELECTIVE ELECTRODES: These are electrodes which have the ability to respond

only to a particular ion and develop potential, ignoring the other ions in a mixture. The potential

developed by an ion – selective electrodes depends only on the concentration of particular ions.

ELECTROCHEMICAL SERIES: An increasing order of the standard reduction potentials

CORROSION: Corrosion is defined as the gradual destruction of metals by the chemical or

electrochemical reaction with its environment.

PILLING BEDWORTH RATIO; The ratio of the volume of the oxide formed to the volume of

the metal consumed is called ”Pilling – Bed worth ratio” or “Pilling – Bed worth rule”.

PAINT: Paint is a uniform dispersion of finely dived pigments, filters and driers in a liquid

called medium (thinner + vehicle ).When a paint applied to a metal surface , the thinner

evaporates, while the vehicle undergoes slow oxidation forming a pigmented film.

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UNIT-III

ENERGY SOURCES

3.1 INTRODUCTION

Sufficient sources of energy are necessary for industrialized nations. Energy is used for

heating, cooking, transportation and manufacturing. Energy sources can be generally classified as

conventional and non-conventional. Over 85% of the energy used in the world is from

conventional sources such as fossil fuels (coal and oil) and nuclear power.

The convenrtional energy sources depend on coal and oil. The burnt fuels result in the

release of CO2 and other gases into the atmosphere causing environmental damage. There are

abundant renewable sources of energy such as wind, sun, water, and biomass. These sources are

pollution ofree and known as “ green energy “.

3.2 Nuclear Energy :

The enormous energy thus released during the Nuclear Fission & Fusion reaction is

known as Nuclear Energy.

3.2.1 N uclear fission: It is the nuclear reaction in which heavy isotopes are split into

lighter nuclei on bombardment by neutrons. Fission reaction of U235

is given below

3.2.2 Nuclear fusion:

Process of combination of lighter nuclei into heavier nucleus with simultaneous

liberation of largeamount of energy. (e.g) solar system

2 2

1H + 1H→ 2 He + Energy

Nuclear fusion reaction occurs in sun.

3.2.3 Nuclear chain reaction:

235

92U

+ 0n1 → 36Kr

92

+ 56Ba

141 1

+ 3 0n

+ energy

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Differences between fission and fusion reaction

S.No

Nuclear fission

Nuclear fusion

1

It is a process of breaking at

heavier nucleous.

It is a process of

combination of lighter

nuclei.

2

It emits radioactive rays

It does not emit any kind

of radioactive rays

3

The mass number and

atomic number of new

elements are lower than

The mass number and

atomic number of

product is higher than

that of starting elements

4

It occurs at ordinary

temperature

It occurs at high

Temperature

5

It gives rise to chain

reaction

It does not give rise to

chain reaction

6

It emits neutrons

It emits positrons

7

It can be controlled

It canot be controlled

3.3 Nuclear Reactor

3.3.1 Light water nuclear power plant

Definition

Light water nuclear power plant is one in which U235

feed rods are submerged in

water.

Here the water acts as coolant and moderator.

The fission reaction is controlled by inserting or removing the control rods of

B10

automatically.

from the spaces I between the fuel rods.

The heat emitted by

U235

absorbed by the coolant in the fuel core is

Heat is transferred to sea water and then converted into steam.

The steam then drives the turbines, generating electricity.

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SCE Dept.of S & H

(Structure of light water nuclear power plant)

3.3.2 Breeder reactor A nuclear reactor with conversion or multiplication factor greater than one is a breeder

reactor. A

breeder reactor generates fissionable nuclei from fertile

nuclei. E.g., the fertile material like uranium-238 is

converted into fissile

94 Pu239

by using slow neutrons. 94 Pu239

undergoes fission and produces energy.

Working :

`In breeder reactor, 92U235

is used as trigger to produce sufficient neutrons. These

are used to convert 92U235

to Plutonium undergoes fission with the production of three

neutrons. One neutron is used to propagate fission chain. The other two neutrons react with

92U238

to fissionable 94 Pu239

. Thus breeder reactor produces two 239

Pu atoms for

each 238

U consumed. Thus more fissionable material is produced than consumed. Hence

the reactor is called breeder reactor.

Critical Mass:

The minimum amount of fissile material (U235

) required to continue the nuclear chain

reaction is called critical mass.

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w

3.4 Solar Energy

Solar energy

The energy that derive directly from sunlight and can be converted into more

useful forms is known as Solar energy.

3.4.1 Solar Energy Conversion

The energy conversion my occur iterms of heat & current.

3.4.2 Photo galvanic cell or Solar cell

PRINCIPLE: The principle of Solar cell is based on photovoltaic effect. When light radiation falls on

the p-n junction semi conductor device, charge separation takes place and a potential

difference is setup. This causes flow of electrons and produces electricity.

Working: When sun rays all on the top layer of p- type semiconductor, electrons from valence

band are promoted to conductance band and cross the p-n junction into the n-type

semiconductor. A potential difference is set up between the two layers. This causes flow of

electrons and produces electricity. When the „p‟ and „n‟ layers are connected to an external

circuit, electrons flow from „n‟ layer to „p‟ layer and current is generated.

3.4.3 Application of Solar Cell 1. Lighting purpose

Now a days electrical street lights are substituted by solar street lights.

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2. Solar pumps are run by solar battery A large number of solar cells are connected in series to form a solar battery. Solar battery

produces enough electricity to run water pump, etc., They are also used in remote areas

where conventional electricity is not available.

SOLAR BATTERY

3. Solar cells are used in calculators, electronic watches etc.

4. Solar cells are superior to other type of cells, because they are non-polluting and eco-

friendly.

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5. Solar cells are used to drive vehicles.

6. Silicon solar cells are used as a source of electricity in space crafts and satellites.

Advantages of Solar cells

1. Solar cells are used in remote areas, forests and hilly regions.

2. Maintenance cost is minimum.

3. Solar cells are pollution free.

4. They have long life.

Disadvantages

1. Solar cells are costly.

2. Storage of energy is not possible with solar cells.

3.5 WIND ENERGY

Moving air is called wind. Energy recovered from the forces of wind is called wind

energy.

3.5.1 Generation of electricity from wind mill Wind energy is used to generate electricity with the help of wind mills. The crank of

the wind mill is connected to a dynamo. When the blades of wind mill rotate, they turn the

coil of the dynamo and produce electricity. Usually a number of wind mills are erected side-

by-side. The outputs from the wind mills are coupled to generate electricity for commercial

purpose. This type of system is wind energy farms.

Condition: Wind speed should be more than 15km/hr.

Advantages of wind energy

(i) It is cheap and

economical. (ii) It is

renewable

(iii) It does not cause pollution.

Disadvantages

(i) They produce noise.

(ii) Wind farms erected on the migratory routes of birds create

problems. (iii) Wind turbines interfere with electromagnetic

signals.

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3.6 BATTERIES (or) ENERGY STORAGE DEVICES

Battery:

It is an arrangement of several electrochemical cells connected in series that

can be used as a source of direct electric current.

3.6.1 TYPES OF BATTERIES

Secondary battery or secondary cells

In these cells, the electrode reactions can be reversed by passing an external

energy.

They can be recharged by passing electric current.

They are called storage cells or accumulators.

Ex: Lead acid storage cell, Nickel- cadmium cell.

3.6.2 Alkaline Battery

Here the powdered zinc is mixed with KOH and MnO2 to get a gel. A Carbon rod

acts as cathode. IT is immersed in KOH The outside cylindrical body is made up of zinc.

Cell reactions

At anode : Zn (s) + 2OH- → Zn(OH)2 + 2e-

At cathode: 2MnO2 + H2O(l) + 2e- → 2OH- +Mn2O3

Overall : Zn (s) + 2MnO2 + H2O(l) → Zn(OH)2 + Mn2O3

Uses: It is used in calculators, watches etc.,

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3.6.3 Lead storage cell

Description:

It consists of number of voltaic cells connected in series Pb is anode and

PbO2 is cathode

Number of Pb plates and PbO2 plates are connected in parallel.

Plates are separated from adjacent ones by insulators like rubber or glass

fiber.

This arrangement is immersed in dil. H2SO4

Cell reactions

At anode : Pb (s) + SO42-

→ PbSO4 (s) + 2e- +

At cathode: PbO2(s) + SO42-

+ H + 2e- →PbSO4 + 2 H2O

Overall reaction:

Pb (s) + PbO2(s) + 2H2SO4 → PbSO4 + H2O + energy

Uses:

* It is used to supply current mainly in automobiles such as cars. Buses,

trucks, etc.,

* It is also used in gas engine ignition, telephone exchanges, hospitals,

power stations.

3.6.4 Nickel – Cadmium Battery Description

It consists of a cadmium anode. A metal grid containing a paste of NiO2 acting as a cathode.

KOH is electrolyte

Ni-Cd battery

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Cell reactions

At anode: Cd(s) + 2OH- → Cd(OH)2(s) + 2e-

At cathode: NiO2 + 2H2O(l) + 2e- → 2OH- +Ni(OH)2 (s) + energy

Overall reaction:

Cd(s) + NiO2 + 2H2O(l) → Cd(OH)2(s) + Ni(OH)2 (s) + energy

Uses: It is used in calculators. Electronic flash units, transistors and cordless appliances.

3.6.5 Lithium Battery

Description

It consists of a lithium anode and a TiS2 cathode.

A solid electrolyte generally a polymer is packed in

between the electrodes.

The electrolyte permits the passage of ions but not

electrons.

Cell reactions

+

At anode: Li(s) → Li

+ e-

At cathode: TiS2 + e- → TiS2-

Overall reaction:

+

Li(s) + TiS2 → Li

+ TiS2-

Other types of secondary lithium batteries

(i) Li/ MnO2

(ii) Li/V2O5

(iii) Li/MoO2

(iv) Li/Cr3O8

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SCE Dept.of S&H

.

Advantages of Li battery

It is the cell future. Why?

Its cell voltage is high, 3.0VSince Li is a light weight metal, only 7kg material required to

produce 1mole of electrons.Since all the constituents of the battery are solids, there is no risk of

leakage from the battery. This battery can be made in a variety of shapes and sizes.

Disadvantages of Li

battery

Li battery is more expensive than other batteries

Uses

Button sized batteries are used in calculators, watches, cameras, mobile phones,

laptop computers.

Lithium Battery

It is a solid state battery. Solid electrolyte is

used.

Construction

It has a lithium anode and a TiS2 cathode. A solid electrolyte, a polymer, is

packed in between the electrodes. The polymer electrolyte permits the passage of ions but

not that of electrons.

Working (Discharging)

The anode is connected to cathode through the polymer electrolyte.

Lithium ions and electrons are produced at the anode . The cathode receives the lithium ions

and electrons.

At anode: Li(s) Li+ + e-

At Cathode: TiS2(s) + e TiS2-


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Overall reaction: Li(s) + TiS2(s) Li+ + TiS2- LiTiS2

Recharging

The battery is recharged by passing an external current, which drives the lithium

ions back to the anode. The overall reaction is

LiTiS2 Li+

+ TiS2

This cell has a voltage of 3.oV.

Uses:

It possesses very small size and high energy density. So it is used in calcutors,

electronic flash units, computers, transistors head phones etc.

Advantages

(i) The cell has a voltage of 3.0V.

(ii) Li is a light-weight metal. Just 7g (1 mole) of Li is required to produce 1 mole of

electrons.

(iii) Li has the most negative Eo value. So it gives a higher voltage than other cells. (iv)

It is a total solid state battery. There is no risk of current leakage from the battery. (v)

It is manufactured in a variety of sizes and shapes.

Disadvantages

It is more expensive than other batteries.

3.7 FUEL CELLS:

Definition

Fuel cell is a voltaic cell. It converts chemical energy of the fuels directly into

electricity without combustion. In these cells, the reactants and electrolytes are continuously

supplied to the cell.

Fuel + Oxygen Oxidation products + Electricity.

Examples : Hydrogen - oxygen fuel cell.

3.7.1 Hydrogen - oxygen fuel cell

It is the simplest and most successful fuel cell. The fuel-hydrogen and the oxidiser-

oxygen and the liquid electrolyte are continuously supplied to the cell.

Description

The cell has two porous electrodes, anode and cathode. The electrodes are made of


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SCE Dept.of S&H

compressed carbon containing a small amount of catalyst (Pt, Pd, Ag). Between the two

electrodes an electrolytic solution, 25% KOH is filled

Working

Hydrogen passes through the anode compartment, where it is oxidised. Oxygen

passes through the cathode compar tment, where it is reduced.

(Hydrogen – Oxygen fuel cell)

Cell reactions At anode: 2H2+4OH-

4H2O +4e- At cathode: O2 +2H2O + 4e-

4OH Overall cell

reaction: 2H2+O 2 2H2O

emf of the cell = 0.8 to 1.0V

Advantages of Fuel Cells 1. They are efficient and instant in operation.

2. They are pollution free.

3. They produce electric current directly from the reaction of a fuel and an oxidiser.

4. They are light in weight

Disadvantages 1. Fuel cells cannot store electric energy.

2. Electrodes are expensive and short lived.

3. H2 should be pure.

Applications 1. H2 - O2 fuel cells are used in space crafts, submarines to get electricity

2. In H2 - O2 fuel cell, the produt water is a valuable source of fresh water for astronauts


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Glossary

Nuclear fission

The process of splitting of heavier nucleus into two or more smaller nuclei with

simultaneous liberation of large amount of energy is called Nuclear Fission.

Nuclear Fusion

The process of combination of lighter nuclei into heavier nuclei, with simultaneous

liberation of large amount of energy is called Nuclear Fusion.

Nuclear Chain Reaction

A fission reaction, where the neutrons from the previous step continues to propagate

and repeat the reaction is called Nuclear Chain Reaction.

Breeder reactor

Breeder reactor is the one which converts non-fissionable material ( U238

, Th232

) into

fissionable material ( U235

, Pu239

). Thus the reactor produces or breeds more fissionable

material than it consumes.

Fissionable nucleides (or ) Fissile nucleides

The nucleides like U235

, Pu239

which undergo fission reaction is known as Fissle

nucleides.

Non- fissionable nucleides (or) Fertile nucleides

The nucleides like U238

, Th232

which do not undergo fission reaction is known as

Fertile nucleides.

Critical Mass:

The minimum amount of fissile material (U235

) required to continue the nuclear

chain reaction is called critical mass.

Solar energy

The energy that derive directly from sunlight and can be converted into more

useful forms is known as Solar energy.

Photogalvanic cell

Photogalvanic cell is one, which converts the solar energy (energy obtained from the

sun ) directly into electrical energy.

Wind energy

Moving air is called wind. The energy possessed by wind is because of its high speed.

The wind energy is harnessed by making use of wind mills, sky sail, ladder mill, kite ship etc.

Fuel cell

Fuel cell is a voltaic cell which converts the chemical energy directly into electricity

without combustion. In these cells, the reactants, products and the electrolytes pass through the

cell.

Battery

Battery is an arrangement of several electrochemical cells connected in series that can

be used as a sources of direct electric current.

Electrolytic Cell

Electrolytic Cell converts electrical energy into chemical energy. Eg. Decomposition

of water into hydrogen and hydroxide ion.

Electrochemical Cell

Electrochemical Cell converts chemical energy into electrical energy. Eg. Galvanic (or)

Voltaic cell.


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UNIT – IV

ENGINEERING MATERIALS

4.1 INTRODUCTION

Materials which are used in manufacturing of wools and equipments and in construction

of buildings, where very specific requirements are needed are called engineering materials.

These include cement, refractories, abrasives, lubricants, etc,

4.2 ABRASIVES Abrasives are very hard substances used for grinding, shaping and polishing

other materials

4.2.1 PROPERTIES

Have very high melting point

Chemically inert

High abrasive power (ability to scratch away pr sharp other materials)

Sometimes hard and brittle or soft and flexible

4.2.2 CLASSIFICATION OF ABRASIVES – TYPES &

4.2.3 APPLICATIONS OF ABRSASIVES

natural abrasives – Eg. Diamond, corundum

synthetic abrasives – Eg. carborundum, norbide

Hardness is measured in terms of moh‟s scale.

Diamond is taken as the reference and hardness of other materials are determined

abrasives with Mohr‟s scale 1-4 are called soft abrasives

NATURAL ABRASIVES

Diamond:

Purest crystalline carbon - Hardest natural substance

Mohr‟s scale value is 10 -Superior chemical inertness

Used in grinding wheels, drilling tools, cutting glasses, etc

Corundum

Pure crystalline form of alumina - Mohr‟s scale value is 9 - Used in grinding glass, gems etc.

Emery

55-75% alumina, 20-40% magnetite, 12% others - Black and opaque

-Mho‟s scale value is 8 - Used for making abrasive paper, abrasive cloth, etc.

Quartz

Pure silicone - Mohr‟s scale value is 7 - Used in painting industries

Garnet

Trisilicates of alumina, magnetite and Fe oxide used for the manufacture of abrasive paper

and cloth.

ARTIFICIAL ABRASIVES

Silicon Carbide (SiC)

Manufacture Silicon Carbide is manufactured by heating sand (60%)and coke (40%) with some

saw dust

and a little salt in an electric furnace to about 1500°C


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SiO2 + 3C gives SiC + 2CO

The silicon carbide removed from the furnaces, is then mixed with bonding

agent(clay, silicon nitride) and than shaped, dried and fired.

Properties 1. Silicon carbide possesses a high thermal conductivity, low expansion and high

resistance

to abrasion and spalling.

2. They are mechanically strong. Mohr‟s scale value is 9.

3. Bear very high temp. 1650°C

4. Has thermal conductivity between metals and ceramics –

They are electrically intermediate between conductors and insulators.

Uses 1. Silicon carbide are used as heating elements in furnaces in the form of rods or bars.

2. They are also used for partition wall of chamber kilns, coke ovens, muffle furnaces

and floors of heat treatment furnaces.

3. Sic bonded with tar are excellent for making high conductivity crucible.

Norbide or Boran Carbide (B4C)

Manufacture It is prepared by heating a mixture of boran oxide (B2O3) and coke in an electric furnace to

about 2700°C

B2O3 +7C give B4C + 6CO

Properties

1. Its hardness is 9 on Mohr‟s scale.

2. It is light weight and black colored compound.

3. It is highly resistant to chemical attack and erosion.

4. It resists oxidation much better than diamond.

Uses

It is used as hard materials for making grinding dies, and for cutting and

sharpening hard high speed tools.

It is used to prepare scratch and wear resistant coating.

4.3 REFRACTORIES Materials that can withstand high temp without softening and deformation in their

shape.Used for the construction of furnaces, converters, kilns, crucibles, ladles etc.

4.3.1 CHARACTERISTICS

Infusible at operating temp.

Chemically inert towards corrosive gases, liquids etc. Should not suffer change

in size at operating temp. Should have high refractoriness

Should have high load bearing capacity at operating temp.

4.3.2 CLASSIFICATION Based on chemical nature

Acidic refractories – Eg. Silica and Alumina


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Basic refractories – Eg. Magnesite and Dolomite

Neutral refractories – Eg. Graphite and Carborundum

Based on refractoriness

Low heat duty refractories

Intermediate heat duty

refractories

High heat duty refractories

Super heat duty refractories

4.3.3 PROPERTIES Refractoriness It is the ability to withstand very high temp. without softening or deformation

under particular service condition. Since most of the refractories are mixtures of

several metallic oxides, they do not have a sharp melting point. So the refractoriness

of a refractory is generally measured as the softening temperature and is expressed in

terms of pyrometric cone equivalent.(PCE). Pyrometric cone equivalent is the

number which represents the softening temperature of a refractory specimen of

standard dimension (38mm height and 19mm triangular base) and composition.

Objectives of PCE test

To determine the softening temperature of a test refractory material.

To classify the refractories

To determine the purity of the refractoreies

To check whether the refractory can be used at particular servicing temperature.

Refractoriness is determined by comparing the softening temperature of a test cone

with that of a series of segar cones. Segar cones are pyramid shaped standard refractory of

definite composition and dimensions and hence it has a definite softening temperature.

A test cone is prepared from a refractory for which the softening temperature

to be determined, as the same dimensions of segar cones. Then the test cone is placed in

electric furnace. The furnace is heated at a standard rate of100C per minute, during which

softening of segar cones occur along with test cone. The temperature at which the apex of the

cone touches the base is taken as its softening temperature.

RUL – Refractoriness Under Load The temp. at which a std dimensioned specimen of a refractory undergoes 10%

deformation

with a constant load of 3.5 or 1.75 Kg/cm2

The load bearing capacity of a refractory can

be measured by RUL test. A good refractory should have high RUL value

Porosity – ratio of pore volume to the bulk

volume

P = (W- D/W- A) X

100

W – weight of saturated specimen in

air

D – weight of dry specimen


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SCE Dept.of S&H

A – weight of saturated specimen in water

Porosity reduces strength, corrosion resistance thermal conductivity, thermal spalling

and abrasion resistance

Thermal spalling – property of breaking, cracking or peeling of refractory material

under high temp. Thermal spalling may be due to rapid change in temp. or slag penetration.

A good refractory should show good resistance to thermal spalling

Dimensional stability Resistance of refractory to any volume change when exposed to high temp. over a

prolonged time. Refractories may undergo reversible or irreversible dimensional changes

A good refractory should show minimum level of reversible dimensional changes with temp.

4.3.4 MANUFACTURING OF REFRACTORIES

ALUMINA BRICKS Contain 50% of aluminium oxide

Manufacture

Calcined bauxite, silica and grog (calcined fire clay) are ground well and mixed

with water. The pasty mass is converted into bricks by mechanical pressing or slip

casting.The bricks are dried and fired at about 1200 to 14000 C for 6-8 days

MAGNESITE BRICKS Contain maximum Magnesium oxide

Manufacture Calcined magnesite, magnesia or iron oxide are ground well and mixed with water. The pasty

mass is converted into bricks by mechanical pressing or slip casting. The bricks are dried and

fired at about 15000 C for 8 hours then cooled slowly

ZIRCONIA BRICKS

Contain zirconite

Manufacture

Zirconite mineral, colloidal zirconia or alumina are ground well and mixed with water and

made into bricks. Small amount of MgO or CaO is added as stabilizer. The bricks are dried

and fired at about 17000 C

4.4 PORTLAND CEMENT

It is defined as an extremely finely ground product.

It is obtained by heating a mixture of argillaceous (clay containing ) and calcareous (lime

containing ) raw materials to about 1500 c. It is then mixed with gypsum to increase the quick setting

and hardening property.

4.4.1 CHEMICAL COMPOSITION OF PORTLAND CEMENT 3CaO.SiO2 - Tri calcium Silicate

3CaO.Al2O3 - Tri calcium Aluminate

4CaO.Al2O3.Fe2O3 - Tetra calcium alumino Ferrate


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SCE Dept.of S&H

4.4.2 MANUFACTURE OF PORTLAND

CEMENT

Raw materials :

(i) Calcareous materials , CaO Ex: Limestone, chalk.

(ii) Argillaceous materials, Al2O3 and SiO2 Ex: clay, slate etc

(iii) Powdered coal (or) fuel oil.

(iv) Gypsum (CaSo4.2H2O)

Manufacture of Portland cement involves the following

steps:

(i) Mixing of raw materials

(ii) Burning

(iii) Grinding

(iv) Storage and Packing

(i) Mixing of raw materials:

(a) Dry Process (b) Wet Process

(a) Dry Process: In dry process, the raw materials like limestone and clay(3:1) are dried,

and mixed in definite proportions

(b) Wet process : In wet process, the raw materials in definite proportions are finely ground

with water and the slurry ( past like) is fed at the top of the rotary kiln.

(II) Burning

The burning process is usually done in rotary kiln which is a long horizontal steel cylinder

coated with refractory bricks and capable of rotating at 1 rpm 9 Revolution per minute) . The

rotary kiln is set at a slight inclination of about 5-60

in order to allow the raw materials fed at one

end to travel slowly to the firing and discharge exit end.

The slurry of raw materials is allowed to enter from the top end of the rotary kiln.

Simultaneously the burning fuel ( like powdered coal or oil) and air are introduced from the

lower end of kiln . The slurry gradually comes down in the kiln into the different zones ( Drying

Zone at


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SCE Dept.of S&H

400o

:Calcination zone at 700 -1000 o

C and clinkering zone at 1250-1500 o

C of

increasing temperatures.

(a) Drying Zone: The upper part of the rotary kiln is known as drying zone

,where the temperature is about 400 o

C . Due to the presence of hot gases in

this zone, water is evaporated from the slurry.

(b) Calcinations zone: The middle part of the rotary kiln is known as calcining zone

where the temperature ranges from 700 -1000 o

C. In this zone lime stone is

decomposed into CaO and CO2

CaCO3 700 -1000 o

C CaO +CO2

Lime Stone Quick lime

(c) Clinkering Zone : The lowest part of the zone is called as clinkering zone, where

the temperature is maintained about 1250-1500 o

C. In this zone lime reacts with clay (

Containing Al2O3, Fe2O3 and SiO2) and forms aluminates and silicates

2CaO+ SiO2 -------- 2CaO.SiO2

Di calcium Silicate

3CaO+ SiO2 -------- 3CaO.SiO2

Tri calcium Silicate

The mixture is then finely powdered and fed into the top of the rotary kiln.

3CaO+ Al2O3-------- 3CaO.Al2O3

Tri calcium Aluminate

4CaO + Al2O3 + Fe2O3--------- 4CaO.Al2O3.Fe2O3

Tetra calcium alumino Ferrate

(ii) Cooling : the hot clinker is cooled with atmospheric air and the hot air thus

produced is used for drying the coal before grinding.

(iii) Grinding : The cooled clinker is then finely pulverized with 2-6% gypsum acts as

a retarding agent for quick setting of cement.

(iv) Storage and Packing: The cement coming out from the grinding mills is stored in

a concrete storage silos. Then the cement is packed in jute bags by automatic

machines. Each bag contains 50kgs of cement.

PROPERTIES

4.4.3 SETTING AND HARDENING OF CEMENT:

When the cement is mixed with water, hydration and hydrolysis of cement begin, resulting

in the formation of gel and crystalline products.

Setting: It is defined as the stiffening of the original plastic mass, due to initial gel

formation. Hardening: It is defined as the development of strength, due to crystallization.

Chemical reactions involved in setting and hardening of cement:

When water is mixed with cement , hydration of tricalcium aluminate occurs rapidly and

the paste becomes quite hard within a short time. This process is known as initial setting of cement.

3CaO.Al2O3 +6H2O-------3CaO.Al2O3.6H2O

Role of gypsum in cement:

(i) In initial setting process gypsum is added during grinding of cement clinkers to retard the rapid

hydration


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SCE Dept.of S&H

of C3A. Gypsum reacts with C3A to form insoluble calcium sulphoaluminate complex.

C3A + 3CaSO4.2H2O--------C3A.3CaSO4.2H2O

(ii) After the hydration of C3A,C3S begins to hydrate to give tobermonite gel and crystalline

Ca(OH)2. The hydration of C3S takes place within 7days.

2(3CaO.SiO2) + 6H2O--------- 3CaO.2SiO2.3H2O + 3Ca(OH)2 + 500kj/kg

(iii) Dicalcium silicate reacts with water slowly and gets finished 7-28days.

2(2CaO.SiO2) + 4 H2O -------- 3CaO.2SiO2.3H2O +Ca(OH) 2 + 250kj/kg

(iv) Hydration of tetra calcium aluminoferritetakesplace initially, the hardening takes

place finally through crystallization along with C2 S.

4CaO.Al2O3.Fe2O3 + 7H2O----------3CaO.Al2O3.6H2O + CaO.Fe2O3.H2O + 420KJ

Crystalline gel

Thus the final setting and hardening of cement is due to the formation of tobermonite gel

plus crystallization of Ca(OH)2 and hydrated tricalcium aluminate.

4.4.4 OTHER TYPES OF CEMENT - SPECIAL CEMENT

Water Proof Cement :

It is obtained by adding water proofing agents like calcium stearate and gypsum

with tannic acid to ordinary Portland cement during grinding.

Functions of water- Proof cement:

Functions of waterproof cement

(i) To make concrete impervious to water under

pressure.

(ii) To resist the adsorption of water.

White cement or White Portland cement

It is obtained by heating the raw materials free from iron oxides. It is white in

color due to the absence of ferric oxide.

It issued for making tiles, mosaic works with some coloring agents like

yellow ochre, Venetian red etc. It is used for repairing and joining marble pillars and

blocks.

4.5 GLASS

Glass is an amorphous, hard brittle, transparent, super cooled liquid of infinite

viscosity.

Glass may be represented as xR2O.yMO.6SiO2

4.5.1 GENERAL PROPERTIES OF GLASS:

1. It is amorphous.

2. It is very brittle.

3. It softens on heating.

4. It has no definite melting point.

5. It is affected by alkalis.


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SCE Dept.of S&H

6. It is a good electrical insulator.

7. It can absorb, reflect or transmit light.

8. It is not affected by air water, acids and chemical agents.

4.5.2 MANUFACTURE OF GLASS

1. Melting :

The raw materials in proper proportions are mixed and finely powdered.This

homogeneous mixture is known as BATCH is fused with some broken glass called

CULLET in the pot of the furnace.The furnace is heated by burning producer gas and air

mixture over the charge. The cullet melts at a low temp and assists in melting the rest of the

charge.

CaC O3 + Si O2------ CaSiO3 + CO2

Na2C O3 + Si O2----- Na2Si O3 + CO2

Forming and Shaping

The molten glass is then worked into articles of desired shapes by either blowing or

moulding or pressing between rollers.

Annealing:

Glass articles are then allowed to cool gradually to room temperature. Suddencooling must be

avoided, because cracking occurs.Longer the annealing period, the better is the quality of the glass.

Finishing:

All glass articles after annealing, are subjected to finishing processes such as

(a) Cleaning (b) grinding (c) polishing (d) cutting (e) sand blasting

4.5.3 TYPES AND USES OF GLASSES

1.Soda-lime or soda glass

(i) Raw materials: Silica , calcium carbonate and soda ash

(ii) Composition: Na2O. CaO. 6SiO2

Properties (a) They are low in cost.

(b) They are resistant to water

(c) They are attacked by common reagents like acids.

(d) They melt easily‟

2. Potash lime or Hard glass

(i) Raw materials : Silica, CaCO3, K2CO3

(ii) Composition: K2O.

CaO.6SiO2

Properties:

(a) They have high melting

point.

(b) They do not fuse easily.


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SCE Dept.of S&H

(c) They are less acted upon acids alkalis, solvents.

Uses: Used for manufacturing combustion tubes, chemical apparatus

3. Lead glass or Flint glass

(i) Raw materials: Lead

oxide, silicva, K2O

(ii) Composition:

K2O.PbO.6SiO2

Properties:

(a) It is bright and lustrous

(b) It has high specific gravity. (3 to 3.3)

(c) It is more expensive to manufacture.

(d) It has a lower softening temperature than soda glass.

(e) It has higher refractive index.

Uses: (a) These are used for high quality tablewares.

(b) They are used in neon sign tubings, optical lenses, electrical insulators, cathode ray tube.

2. Potash lime or Hard glass

(i) Raw materials : Silica, CaCO3, K2CO3

(ii) Composition: K2O.

CaO.6SiO2 (iii)

Properties:

(a) They have high melting

point.

(b) They do not fuse easily.

(c) They are less acted upon acids alkalis, solvents.

Uses: Used for manufacturing combustion tubes, chemical apparatus

3. Lead glass or Flint glass

(i) Raw materials: Lead oxide, silicva, K2O

(ii) Composition: K2O.PbO.6SiO2

Properties:

(a) It is bright and lustrous

(b) It has high specific gravity. (3 to 3.3)

(c) It is more expensive to manufacture.

(d) It has a lower softening temperature than soda glass.

(e) It has higher refractive index.

Uses: (a) These are used for high quality tablewares.

(b) They are used in neon sign tubings, optical lenses, electrical insulators, cathode ray tube.


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4. Borosilicate glass or Pyrex glass or Jena glass

(i)Raw materials: Silica, borax with small amount of alumina and some oxides.

(ii) Composition : SiO2 (80.5%); B2O3 (13%)

Al2O3 (3%) K2O (3%) Na2O (0.5%)

(iii) Properties:

(1) It possess low thermal coefficient of expansion and high chemical resistance.

(2)It possesses very high softening points and excellent resistivity.

Uses: It is used in industry for pipe lines for corrosive liquids, gauge glasses,

5. Alumina silicate glass

Raw materials: It has 5% or more alumina

(i)Composition: SiO2 Al2O3, B2O3, MgO, CaO, Na2O K2O

Properties:

They possess high softening temperature.

Uses: (a)Used in high pressure mercury discharge tubes

(b)Chemical combustion tubes.

6. Optical or Crookes glass

Raw materials: It contains phosphorus, lead silicate with small amount of cerium oxide.

Properties:

(a) Cerium oxide present in the glass absorbs uv light,

(b) They have low melting point.

Uses: optical glasses are used for making lenses.

7. Glass wool Glass wool is fibrous wool like material It is composed of intermingled fine threads

or filaments of glass.

Properties: It is a very good heat and fire proof materials

Its electrical conductivity is low.

Uses; It is used for heat insulation purposes

It is used for electrical and sound insulation.

Glossary

Abrasives

Abrasives are hard substances, used for polishing, shaping, grinding operations. They

are characterized by high melting point, high hardness and chemically inactive.

Refractories

Refractories are materials that can withstand high temperatures without softening or

deformation in shape.

Refractoriness

Refractoriness is the ability of a material to withstand very high temperature without

softening or deformation under particular service condition.

Pyrometric cone equivalent

Pyrometric cone equivalent is a number which represents the softening temperature of a

refractory specimen of standard dimension ( 38 mm height and 19 mm triangular base ) and

composition.

RUL ( Refractoriness Under Load )


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The temperature at which the refractory deforms by 10% under a load of 3.5kg/cm2 is

called RUL (Refractoriness Under Load)

Porosity

Porosity is defined as the ratio of its pore volume to the bulk volume

Thermal spalling

Thermal spalling is the property of breaking, cracking or peeling off a refractory

material under high temperature. A good refractory must show a very good resistance to

thermal spalling.

Calcination

Heating the ore in absence of air is called Calcination

Cement

Cement is a material with adhesive and cohesive properties which make it capable of

bonding minerals fragments into a compact whole. The name “Portland cement” given

originally due to the resemblance of the colour and quality of the hardened cement to Portland

sonte ( Portland island is England).


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UNIT V

FUELS AND COMBUSTION

5.1 DEFINITION Fuel is a combustible substance ,during combustion of it the atoms of C,H,S and

N etc are combine with oxygen with simultaneous liberation of heat and light.

Ex: C + O2 CO2 +94 K cals

2H2+O2 2H2O +68.5 K cals

5.1.1 CHARACTERISTICS FOR GOOD FUEL

High calorific value.

Moderate ignition temperature.

Low moisture content.

Low contents of non-combustible matters.

Combustion should be controllable.

Easy to transport and readily available at low cost.

Solid Liquid Gas Solid Liquid Gas

5.2.1 Calorific value:

. It is defined as the amount of heat liberated by the complete combustion of a unit mass

of the fuel.

5.2.2 Gross or high calorific value (GCV)

It is defined as the total heat generated when a unit quantity of fuel is

completely burnt and the products of combustion are cooled to room temperature.

5.2.3 Net or Lower Calorific Value(NCV)

It is defined as the “net heat produced when a unit quantity o f fuel is

completely burnt and the products of combustion are allowed to escape.


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NCV = GCV- latent heat of condensation of steam produced.

5.2.4 Theoretical calculation of calorific value ( Dulong‟s formula)

According to Dulong, the calorific value of a fuel is the sum of the calorific

values of its constituent elements.

The calorific values of C, H&S are found to be 8080, 34500 and 2240kcals

when 1kg of the fuel is burnt completely.

Thus,Dulong‟s formulae for GCV is written as

GCV=1/100 [8080(C)+34500(H-O/8)+2240(S)]Cals/kg.

NCV =[ GCV-9/100( H)x 587] Kcals/kg.

The latent heat of steam is 587Kcals/kg.

In Dulong‟s formula C, H, O&S represent the percentage of the corresponding

elements.

5.3 SOLID FUEL

5.3.1 ADVANTAGES OF SOLID FUELS:

Solid fuels are easily available and they are cheap.

Handling and transportation are easy.

They can be stored conveniently without any risk.

They have a moderate ignition temperature.

5.3.2 DISADVANTAGES OF SOLID FUELS:

Combustion process cannot be easily controlled.

The calorific value is comparatively lower.

They form large amount of ash and its disposal is a big problem.

A large space is required for storage.

5.4 COAL

Analysis of coal

5.4.1 Proximate Analysis:

It involves the determination of percentage of following in coal

a. Moisture content

b. Volatile matter

c. Ash content

d. Fixed carbon

(a) Moisture content

1. 1gm of powdered and air-dried coal sample in crucible is heated at 100-

150⁰C in an electric air oven for 1 hour.

2. The loss in weight of the sample is found out and percentage of moisture is

calculated as

% of moisture = Loss in weight of coal ×100

Weight of air-dried coal

(b) Volatile matter:


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In this the crucible with residual coal sample is covered with a lid and heated at

950± 20⁰C for 7mins in a muffle furnace. The loss in weight is found out.

% of volatile matter = Loss in weight of coal ×100

Weight of moisture free coal

(c) Ash Content:

The crucible with residual coal sample is heated without lid at

700 ± 50 ⁰C for ½hour in a muffle furnace. The loss in weight is found out.

% of ash content = weight of ash formed x100

Weight of dried coal

(d) Fixed Carbon:

% of fixed carbon = percentage of (a+b+c)

5.4.2 Significance of proximate analysis:

1. High percentage of moisture content is undesirable because

2. High percentage of volatile matter is undesirable because

3. High percentage of ash content is undesirable because

4. High percentage of fixed carbon content is desirable because

5.5 CARBONISATION OFMETALLURGICAL COKE

When coal is heated strongly in the absence of air , it is converted into lustrous,

dense, porous and coherent mass known as coke. This process of converting coal is known as

carbonization.

5.5.1 METALLURGICAL COKE

When bituminous coal is heated strongly in the absence of air, the volatile matter

escapes out and the mass becomes hard, porous and coherent which is called Metallurgical

coke.

PREPARATION

5.5.2 Otto-Hoffman’s method

Significance of Otto-Hoffman‟s method

(i) To increases the thermal efficiency of the carbonization process and,

(ii) To recover the valuable by products (like coal gas, ammonia, benzyl oil,

etc).


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. The oven consists of a number of silica chambers , each chamber is provided with a

charging hole at the top, it is also provided with a gas off take valve and iron door at

each end for discharging coke.

Coal is introduced into the silica chamber and the chambers are closed.

The chambers are heated to 1200 o C by burning the preheated air and the producer gas

mixture in the interspaces between the chambers.

The air and gas are preheated by sending them through 2nd

and 3rd

hot regenerator.

Hot flue gases produced during carbonization are allowed to pass through 1st and 4

th

regenerators until the temperature has been raised to 1000ċ.

While 1st and 4

th regenerated are heated by hot flue gases, the 2

nd and 3

rd regenerators

are used for heating the incoming air and gas mixture.

When the process is complete, the coke is removed and quenched with water.

The yield of coke is about 70%.

The valuable by products like coal gas, tar, ammonia, H2S and benzyl, etc. can be

recovered from flue gas.

5.5.3 Recovery of by-products

(i) Tar

The flue gases are first passed through a tower in which liquor ammonia is sprayed.

Tar and dust get dissolved and collected in a tank below

(ii) Ammonia

The gases are then passed through another tower in which water is sprayed. Here

ammonia gets converted to NH4OH.

(iii) Naphthalene

The gases are again passed through a tower, in which cooled water is sprayed,

naphthalene gets condensed.

(iv) Benzene

The gases are passed through another tower, where petroleum is sprayed, benzene

gets condensed to liquid.

(v) Hydrogen Sulphide

The remaining gases are then passed through a purifier packed with moist Fe2O3.

Here H2S is retained.

The final gas left out is called coal gas which is used as a gaseous fuel.

5.5.4 Advantages of Otto Hoffman’s process

1. Valuable by products like ammonia, coal gas, Naphthalene etc. are recovered.

2. The carbonization time is less.

3. Heating is done externally by producer gas.

5.6 IQUID FUELS

5.6.1 ADVANTAGES OF LIQUID FUELS:

They have higher calorific value than solid fuel.

They occupy less storage space than solid fuels.

Their combustion is uniform and easily controllable.

Liquid fuels do not yield any ash after burning.

DISADVANTAGES OF LIQUID FUELS:

Liquid fuels are more costly than the solid fuels.

Liquid fuels give unpleasant odor during incomplete combustion.

Special type of burners is required for effective combustion.

Some amount of liquid fuels will escape due to evaporation during storage.


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5.6.2 PETROLEUM

It is naturally occurring liquid fuel.

It is dark brown or black coloured viscous oil

Crude oil is a mixture of paraffinic, olefinic and aromatic hydrocarbons with small

amounts of organic compounds like N, O and S.

The average composition of crude oil is as follows

C = 80-87%

H = 11-15%

S = 0.1-3.5%

N+O = 0.1-0.5%

It is classified into three types

1. Paraffinic-Base type crude oil

It contains saturated hydrocarbons from CH4 to C35H72 with a smaller amount of

naphthenes and aromatics.

2. Naphthenic or Asphaltic Base type crude oil

It contains Cycloparaffins or naphthenes with a smaller amount of paraffin and

aromatics

3. Mixed base type crude oil

It contains both paraffinic and asphaltic hydrocarbons.

5.6.3 REFINING OF PETROLEUM OR CRUDE OIL

The crude oil obtained from the earth is a mixture of oil, water, unwanted impurities

and its subjected to fractional distillation.

During fractional distillation, the crude oil is Thus, the process of removing impurities

and separating the crude oil into various fractions having different boiling points is

called Refining of Petroleum.

The process of refining involves the following steps.

Step 1: Separation of water (Cottrell‟s process)

The crude oil well is an extremely stable emulsion. The crude oil is allowed to flow

between two highly charged electrodes, where colloidal water droplets combine to form large

drops, which is then separated out form the oil.

Step 2: Removal of harmful sulphur compounds

Sulphur compounds are removed by treating the crude oil with copper oxide. The

copper sulphide formed is separated out by filtration.

Step: 3 Fractional distillation

The purified crude oil is then heated to about 400ċ in an iron retort, where the oil gets

vaporized.

The hot vapors are then passed into the bottom of a “fractionating column”.

The fractionating column is a tall cylindrical tower containing a number of horizontal

stainless steel trays at short distances.

Each tray is provided with small chimney covered with a loose cap.

When the vapors of the oil go up in the fractionating column, they become and get

condensed at different trays.

The fractions having higher boiling points condense at lower trays whereas the

fractions having lower boiling points condense at higher trays.

The gasoline obtained by this fractional distillation is called straight-run gasoline.

Various fractions obtained at different trays are given in table.


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5.7. KNOCKING

The rate of ignition of the fuel gradually increases and the final portion of the fuel-air

mixture gets ignited instantaneously producing an explosive sound known as “Knocking”.

5.7.1 Causes of knocking in S.I (Petrol) engines

In a petrol engine fuel used as a mixture of gasoline vapor and air at 1:17 ratio

The mixture is compressed and ignited by an electric spark.

The products of combustion increase the pressure and push the piston down the

cylinder.

If the combustion proceeds in a regular way, there is no problem in knocking.

But in some cases, the rate of combustion (oxidation) will not be uniform due to

unwanted chemical constituents of gasoline.

Knocking property of the fuel reduces the efficiency of engine. So a good gasoline

should resist knocking.

5.7.2 Improvement of anti knock characteristics

(i) Blending petrol of high octane number with petrol of low octane number, so that the

octane number of the latter can be improved.

(ii) The addition of anti-knock agents like Tetra-Ethyl Lead (TEL).

5.7.3 OCTANE NUMBER OR OCTANE RATING

Thus octane number is defined as „the percentage of iso-octane present in a

mixture of iso-octane and n-heptanes.‟Octane number is introduced to express the

knocking characteristics of petrol. On the other hand, iso-octane gives very little

knocking and so, its anti-knock value has been given 100.

CH3 CH3 -CH-CH2-C-CH3 CH3-CH2-CH2-CH2-CH2-CH2-CH3 CH3 N-heptanes (Octane no = 0) CH3 Iso-octane (Octane no = 100) 5.7.4ANTI-KNOCK AGENT Tetraethyl lead (TEL) (C2H5)4 Pb is an important additive added to petrol. Thus the

petrol containing tetra ethyl lead is called leaded petrol. TEL reduces the knocking tendency

of hydrocarbon.

Mechanism of Knocking

. Knocking follows a free radical mechanism, leading to a chain growth which results in

an explosion.

If the chains are terminated before their growth, knocking will cease.

TEL decomposes thermally to form ethyl free radicals which combine with the growing

free radicals of knocking process and thus the chain growth is stopped.

Disadvantages of using TEL

When the leaded petrol is used as a fuel, the TEL is converted to lead oxide and

metallic lead.

To avoid this, small amount of ethylene dibromide is added along with TEL.

This ethylene dibromide reacts with Pb and PbO to give volatile lead bromide, which

goes out along with exhaust gases and creates atmospheric pollution.

But now a day‟s aromatic phosphates are used instead of TEL.


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5.7.5Causes of knocking in CI (Diesel) engines

In a diesel engine, first air is compressed and raises the temperature of the cylinder to

about 500ċ then the oil is sprayed.

The expanding gases push the piston and power stroke begins.

The combustion of a fuel in a diesel engine is not instantaneous and the time

between injection of the fuel and its ignition is called Ignition lag or Ignition delay and

raising the temperature of vapour to its ignition temperature.

.

Which undergo explosion during ignition ,this is responsible for diesel knock.

5.7.6 CETANE NUMBER OR CETANE RATING

Thus the cetane number is defined as “the percentage of cetane present in a

mixture of cetane and 2-methyl naphthalene which has the same ignition lag as the

fuel under test”.

Cetane number is introduced to express the knocking characteristics of diesel.

Cetane (C16H34) has a very short ignition lag and hence its cetane number is taken as

100.

On the other hand 2-methyl naphthalene has a long ignition lag and hence

its cetane number is taken zero.

CH3

CH3-(CH2)14-CH3

n-cetane (cetane no = 100)

2-methyl naphthalene (cetane no = 0)

The cetane number decreases in the following order.

Straight chain paraffin‟s >Cycloparaffins >Olefins >Branched paraffin‟s.

The cetane number of diesel oil can be increased by adding additives called dopes.

Ex: Ethyl nitrate, Iso-amyl nitrate.

5.7.8 DIESEL INDEX

The quality of diesel oil is indicated by diesel index number using the following.

Diesel index number = Specific gravity (API) x Aniline in . F

100

Aniline point and specific gravity is noted from API (American Petroleum Institute)

Comparison of gasoline oil and diesel oil

S.No Gasoline oil Diesel oil

1. Low boiling fraction of petroleum

contains C5-C9 hydrocarbons.

High boiling fraction of petroleum

contains C15-C18 hydrocarbons.

2. Fuel for SI engine Fuel for Cl engine.

3. Knocking tendency is measured in

octane rating

Knocking tendency is measured in cetin

rating.

4. Knocking is due to premature

ignition

Knocking is due to ignition lag.

5. Antiknocking is improved by the Anti knocking is improved by doping


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SCE Dept.of S&H

addition of TEL with ethyl nitrate.

6. Its exhaust gases contain higher

amount of pollutants

Its exhaust gases contain lesser amount

of pollutants.

7. More consumption, lower thermal

efficiency

Less consumption, higher thermal

efficiency.

5.8. Hydrogenation of coal

The preparation of liquid fuels from solid coal is called Hydrogenation of coal.

Coal contains about 4.5% of hydrogen compared to about 18% of in petroleum. So,

coal is a hydrogen deficient compound.

If coal is heated with hydrogen to high temperature under high pressure, it is converted

to gasoline.

There are two methods available for the hydrogenation of coal.

(a) Bergius process (or direct method)

(b) Fischer- Tropsch Process (or indirect method)

5.8.1(a)Bergiusprocess(indirectmethod)

Finely powdered coal + heavy oil+ catalyst powder (tin or nickel) is made into a paste

The paste is pumped along with hydrogen gas into the converter, where the paste is

heated to 400-450ċ under a pressure of 200-250atm.

During this process hydrogen combine with coal to form saturated higher

hydrocarbons, which undergo further decomposition at higher temperature to yield

mixture of lower hydrocarbons.

The mixture is led to a condenser, where the crude oil is obtained.

The crude oil is then fractionated to yield.

(i) Gasoline (ii) Middle oil (iii) heavy oil

The middle oil is further hydrogenated in vapour phase to yield more gasoline.

The heavy oil is recycled for making paste with fresh coal dust.


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The yield of gasoline is about 60% of the coal.

5.8.2 (b) Fischer-tropics process (indirect method)

In this process coal is first converted into coke. Then water gas is produced by passing steam

over red hot coke

C + H2O 1200ċ CO + H2

(Water gas)

The water gas is mixed with hydrogen and the mixture is purified by passing through

Fe 2O3 +Na2CO 3 (to remove sulphur compounds).

The purified gas is compressed to 5 to 25 atm and then led through a converter, which

is maintained at a temperature of 200-300⁰C.

The converter is provided with a catalyst bed consisting of a mixture of 100 parts

cobalt, 5 parts thoria, 8 parts magnesia and 200 parts kieselgurh earth.

A mixture of saturated and unsaturated hydrocarbon is produced as a result

polymerization.

n CO+2nH2 CnH2n + nH2O

n CO+(2n+1)H2 CnH2n +2+ nH2O

The out coming gaseous mixture is led to condenser, where the liquid crude oil is

obtained.

The crude oil is fractionated to yield (i) Gasoline and (ii) Heavy oil.

The heavy oil is used for cracking to get more gasoline.

5.9 GASEOUS FUELS

ADVANTAGES OF GASEOUS FUELS:

Gaseous fuels have high calorific value than solid fuels.

During burning they do not produce any ash or smoke.

Compared to solid and liquid fuels, they have high thermal efficiency.


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They can be easily transported through the pipes.

DISADVANTAGES OF GASEOUS FUELS:

They are highly inflammable and hence the chances for fire hazards are high.

Since gases occupy a large volume, they require large storage tanks.

5.9.1. COMPRESSED NATURAL GAS (CNG)

When the natural gas is compressed, it is called compresses natural gas (CNG).

The primary component present in CNG is methane. It is mainly derived from natural gas.

Properties

1. CNG is the cheapest, cleanest and least environmentally vehicle impacting

alternative fuel.

2. Vehicles powered by CNG produce less carbon monoxide and hydrocarbon (HC)

emission.

3. It is less expensive than and diesel.

4. The ignition temperature of CNG is about 55ċ.

5. CNG requires more air for ignition.

Uses:

CNG is used to run an automotive vehicle just like LPG.

Comparison of emission levels between CNG- driven vehicles and petrol driven vehicles

Pollutants

Emission Levels

Petrol Driven Vehicle CNG Driven Vehicle

CO (gm/km) 0.92 0.05

HC (gm/km) 0.36 0.24

5.9.2 PRODUCER GAS

It is a mixture of CO&N2 with small amount of H2. Its average composition is as follows.

Constituents Percentage (%)

CO 30

N2 51-56

H2 10-15

CO2+CH4 rest

It is calorific value is about 1300 kcal/m3.

Manufacture

The reactor used for the manufacture of producer gas is known as gas producer.

It consists of a tall steel vessel inside of which is lined with refractory bricks.

It is provided with cup and cone feeder at the top and a side opening for producer

gas exists.

At the bottom, it is provided with an inlet pipe for passing air and steam.

When a mixture of air and steam is passes over a red hot coke maintained at about

1100ċ in a reactor, the producer gas is produced.


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1. Ash Zone

This is the lowest zone consists mainly of ash. The incoming air and steam mixture is

preheated in this zone.

2. Combustion or oxidation zone

This is the zone next to ash zone. Both the reactions are exothermic. Hence, the temperature

of the bed reaches around 1,100ċ.

C+1/2O2 CO exothermic

C+O2 CO2 exothermic

3. Reduction Zone

This is the middle zone. Here both CO2 and steam are reduced.

C+CO2 2CO endothermic

C+H2O CO+H2O endothermic

The above reactions are endothermic. Hence the temperature of the coke bed falls to 1000ċ.

4. Distillation or Drying Zone

This is the upper most of the coke bed. In this zone (400-800ċ) the incoming coke is heated

by the outgoing gases.

Uses

1. It is used as a reducing agent in metallurgical operations.

2. It is also used for heating muffle furnaces, open-hearth furnaces etc.

5.9.3. WATER GAS

It is mixture of CO and H2 with small amount of N2. The average composition of water gas

is as follows.

Constituents Percentage

CO 41

H2 51

N2 4

CO2+CH4 rest

Its calorific value is about 2800kcal/m3


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Manufacture

The water gas producer consists of a tall steel vessel, lined inside with refractory

bricks.

It is provide with cup and cone feeder at the top and a side opening of water gas

exist.

At the bottom on it is provide with two inlet pipes for passing air and steam.

When steam and little air is passed alternatively over a red hot coke maintained at

about 900-1000ċ in a reactor, water gas is produced.

Step-I

In the first stage, steam is passed through the red hot coke, where CO &H2 are produced.

The reaction is endothermic. Hence, the temperature of the coke bed falls.

C+ H2O CO+H2 endothermic

Step-II

In the second stage, in order to raise the temperature of the coke bed to 1000ċ, the steam

supply is temporarily cut off and air blown in; the reaction is exothermic.

C+O2 CO2 exothermic

Thus the steam-run and air blow are repeated alternatively to maintain proper temperature.

Uses

1. It is used for the production of H2 and in the synthesis of ammonia.

2. It is used to synthesis gasoline in Fischer-Tropics process.

3. It is also used in the manufacture of power alcohol and carbureted water gas (water gas

+ oil gas).

5.9.4 LPG- Liquefied Petroleum gas

It is also known as bottled gas or refinery gas.

It is obtained by a by-product during the fractional distillation of heavy oil or

cracking of higher hydrocarbons.

It can easily be liquefied under pressure, but exist as gas at atmospheric

pressure.


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LPG consists of the following hydrocarbons containing carbons atoms up to

4(C4).

The average composition of LPG is

Propane - 24.7%

Butane - 38.5%

Isobutene - 37.7%

LPG has the calorific value of about 2500 Kcals/m3.

Uses

1. LPG is supplied with the trade name like indene bharath gas etc. It is mainly used as

domestic and industrial fuel.

2. It is also used as motor fuel, because it easily mixes with air and burns without any

pollution creating residue.

Advantages

1. It possesses high efficiency and heating rate.

2. Burns completely without smoke.

3. Needs only little care in maintenance.

4. Easily transported using steel cylinder to any places.

5. It is very cheaper than gasoline.

Disadvantages

1. Handling should be only under pressure.

2. User of LPG in engines is possible only if it works under high compression ratio.

3. Its response to blending is poor and so its uses are selective.

ii) Nitrogen Content:

Kjeldahl’s method: Con.H2SO4,

Powdered Coal Ammonium sulphate (clear solution)

K2SO4 Catalyst

Std N/10 HCl NaOH

Reactions :

Neutralisation takes place NH3

2N + 3H2+H2SO4 (NH4)2SO4

(NH4)2SO4+2NaOH 2 NH3 + Na2SO4 +2 H2O NH3 + HCl

From the volume of HCl consumed % of Nitrogen is calculated

% of N2 in coal = 1.4 x volume of acid consumed x Normality

Weight of coal sample

i) Sulphur Content :

Burnt in


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A known amount of coal Sulphate

Bomb calorimeter Extracted with water

Extract

BaCl2

Filtered, dried & weighed BaSO4

From the weight of BaSO4, % of Sulphur is calculated

% of Sulphur = 32 x weight of BaSO4 obtained / 233 x weight of coal sample x 100

iv) Ash content :

Moisture & volatile ½ hr

matter removed coal Loss in weight of coal is noted

without lid 700 + 500

c

Loss in weight of the coal

% of volatile matter in coal = x 100

Weight of air-dried coal v) Oxygen

content :

% of Oxygen in coal = 100 - % of ( Carbon + Hydrogen + Sulphur

+ Ash content

S.NO CONTENTS SIGNIFICANCE

1

High Carbon

& Hydrogen

i) Increases calorific value of coal ii) Helps in the

2 High Nitrogen No calorific value

High

Sulphur

i) Increase calorific value ii) It produces SO2 , SO3 and corrosion takes place

4

High

Oxygen

i) Low calorific value ii)

Increases moisture

holding capacity of coal

Draw backs of presence of S in Coal:

1. The combustion products of sulphur, SO2 and SO3 are harmful and have

corrosion effects on equipments

2. The coal containing sulphur is not suitable for the preparation of etallurgical

coke as it affects the properties of the metal.


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Description of Orsat's apparatus

It consists of a horizontal tube having three way stop cock at one end. The end of three way stop

cock is connected to a U tube containing fused CaCl2 to remove moisture in the gas. The another end of

the tube is connected with a graduated burette. The burette is surrounded by a water jacket in order to keep

the temperature of gas constantly. The lower end of the burette is connected by a water reservoir by means

of Rubber tube. The level of the water in burette can be raised or loweredby raising or lowering the

reservoir. The middle of the horizontal tube is connected with 3 bulbs(A, B and C) for absorbing flue gases

as follows:

i. Bulb 'A' containing KOH solution and it absorbs only CO2

ii. Bulb 'B' containing alkaline pyrogallol solution and it absorbs only O2

iii. Bulb 'C' containing ammoniacal cuprous chloride solution and it absorbs CO.

Working of Orsat apparatus

The bulbs A, B and C are cleaned and filled by respective solutions. Now, the three way stop cock

is opened and the burette is filled with water, by raising the water reservoir to remove air from the burette.

Then, the flue gas is taken in the burette up to 100 cc by raising and lowering the reservoir. The 3 way stop

cock is now closed.

Absorption of gases in bulbs

i) Absorption of CO2

The stopper of the bulb 'A' is opened and the flue gas is allowed to pass by raising the water

reservoir. CO2 present in flue gas is absorbed by KOH. This process is repeated several times by raising

and lowering the water reservoir until the volume of burette becomes constant. The decrease in volume of

burette indicates the volume of CO2 in 100 cc of the flue gas. Now the stopper of the bulb „A' is closed.

ii) Absorption of O2

The stopper of the bulb 'B' is opened and the flue gas is allowed to pass. O2 present in the flue gas

is absorbed by alkaline pyrogallol. This process is repeated several times until the volume of burette

becomes constant. The decrease in volume of flue gas in burette indicates the volume of O2 . Now the

stopper is closed.

c) Absorption of CO

The stopper of the bulb 'C' is opened and the flue gas is allowed to pass. 'CO' present in the flue gas

is absorbed by ammoniacal cuprous chloride solution. This process is repeated several times until the

volume of burette becomes constant. The decrease in volume of flue gas in burette indicates the volume of

CO. The remaining gas in the burette after the absorption of CO2 , O2 and CO is taken as nitrogen. The %

of N2 = [100 (% of CO2 + % of O2 + % of CO)]


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Glossary

Fuel

Fuel is a combustible substance, containing carbon as the main constituent, which on

burning gives large amount of heat that can be used for domestic and industrial purposes.

Calorific value

Is the total quantity of heat liberated when a unit mass of the furel is burnt completely

Calorie

Is the amount of heat required to raise the temperature of one gram of water through one

degree centigrade.

Cracking

Is defined as “ the decomposition of higher boiling hydrocarbons of high molecular weight

into simpler, lower boiling hydrocarbons of lower molecular weight”.

Synthetic petrol

The preparation of liquid fuels from solid coal is called hydrogenation of coal (or) synthetic

petrol.

Knocking

Is a kind of explosion due to rapid pressure rise occurring in an IC engine.

Octane number

Is defined as “ the percentage of iso-octane present in a mixture of iso-octane and n-

heptanes, which matches the fuel under test in knocking characteristics”.

Cetane number

Is defined as “ the percentage of hexadecane present in a mixture of hexadecane and alpha

methyl naphthalene, which has the same ignition characteristics as the diesel under test”.

Water gas

Is a mixture of combustible gases (CO and H2) with small amount of non combustible gases

(CO2, N2).

Producer gas

Is a mixture of combustible gases ( CO & N2 ) with small amount of non-combustible gases

( N2, CO2 etc).

Power alcohol

Is known as power alcohol. It is used as additive to motors fuels, when blended with petrol

at concentration of 5-10%, it is called power alcohol.


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Unit – I, WATER TECHNOLOGY

01. Define boiler feed water. The water fed into the boiler for the production of steam is known as boiler feed water.

02. What are the requirements of boiler feed water?

It should free from

(i). Suspended solids. (ii). Dissolved salts like MgCl2

(iii). Hardness. (iv). Alkalinity. (v). Dissolved gases like O2 and CO2

03. What are the disadvantages using hard water in boiler?

1. Scale and sludge formation. 2. Caustic Embrittlement.

3. Priming and Foaming. 4. Boiler corrosion.

04. What are the disadvantages formation of deposits in steam boilers and heat exchangers?

1. Wastage of fuels. 2. Decrease in efficiency. 3. Boiler explosion.

05. Define 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.

06. What are the methods used to be prevention of scale formation?

Prevention of scales formation: (i). External Treatment – zeolite process

(ii). Internal Treatment.- carbonate conditioning. 07. What are the methods used to be softening hard water?

(i). External Treatment – zeolite process, Demineralisation process

(ii). Internal Treatment.- carbonate conditioning, calgon conditioning.

08. What is meant by internal conditioning of water? Give one example.

Treating the boiler water in the boiler itself by adding chemicals to remove scale forming

substance is called internal conditioning.

Ex: carbonate conditioning, phosphate conditioning, calgon conditioning.

09. What is meant by external conditioning of water? Give one example.

Treating the boiler water before feeding it into boiler is called external conditioning.

Ex: zeolite process, Demineralisation process

10. Distinguish between internal contioning and external conditioning methods.

Internal contioning External conditioning


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To remove hardness producing salts, chemicals were added to the boiler water in

the boiler itself and that treatment is

known as internal conditioning.

Treating the boiler water before feeding it into boiler is called external

contioning.

11. What is meant by caustic embrittlement? How is it prevented?

Caustic embrittlement means intercrystalline cracking of boiler metal. It is prevented by

using softening agent like sodium phosphate and by adding tannin and lignin.

12. What is desalination?

The process of removing common salt (NaCl) from the water is known as desalination. The

water contains dissolved salts with brackish taste is called brackish water.

Desalination = Removal of common salt (NaCl ) from water

13. What are boiler compounds? Mention two different boiler compounds and their

actions.

The chemicals directly added into the boiler for removing scale forming

substances is known as boiler compounds. Ex: Sodium carbonate and Sodium

phosphate

14. What is Calgon? How does it function in water treatment?

Calgon – Sodium Hexa Meta Phosphate

It interacts with calcium ions forming a highly soluble complex and thus prevents

the precipitation of scale forming salt.

15. What is Reverse osmosis?

When two solutions of different concentrations are separated by a semipermeable

membrane, when a pressure is applied on the concentrated side, the solvent flow from higher

concentration to lower concentration.

16. What are the advantages of reverse osmosis method?

i. Low capital cost, easy operating.

ii. RO method is used for converting sea water into drinking

water.

iii. It removes all types of impurities like non-ionic and

colloidal.

iv. The life time of membrane is high and it can be replaced within few minutes.


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Unit – I Water Technology.

16 Marks Questions.

01. Define Boiler Feed Water. What are the requirements of Boiler Feed Water?

Boiler Feed Water: The water fed into boiler for the production of steam is called boiler

feed water.

It should be free from turbidity, oil, dissolved gases, alkali and hardness

producing substances.

Requirements of Boiler Feed Water:

i). It should has zero hardness.

If hardness present in boiler feed water, it produces scales and sludges, which prevents

efficient heat transfer.

ii). It must free from dissolved gases like O2 and CO2.

If dissolved gases present in boiler feed water, it leads to boiler corrosion.

iii). It should be free from suspended impurities.

If it is present in boiler feed water, it produces wet steam.

iv). It should be free from dissolved salts and alkalinity.

If it is present in boiler feed water, it produces caustic embrittlement, which causes

brittlement of boiler parts.

02. What is Zeolite? How is water softened by zeolite? Give equations.

Zeolite:Sodium Aluminum Orthosilicate.

Na2O.Al2O3.xSiO2YH2O.

The synthetic form of zeolite is known as

PERMUTIT, which is porous and possess gel

structure and Ze stands for insoluble zeolite.

Principle: The sodium ions which are loosely

held in Na2Ze are replaced by Ca2+

and Mg2+

ions present in water.

Process: When hard water is passed through a bed of

zeolite placed in a closed cylinder, the hardness


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causing ions like Ca2+

and Mg2+

ions are taken

up by zeolite. Sodium salts are released during the

reaction as byproduct.

For softening of water by zeolite process, hard water is percolated at a specified rate

through a bed of zeolite, kept in a cylinder.

The hardness causing ions like Ca2+

and Mg2+

are retained by the zeolite as CaZe and

MgZe. While the outgoing water contains sodium salts.

The various reactions taking place may be

Na2Ze + Ca(HCO)2 CaZe + 2NaHCO3

Na2Ze + Mg(HCO)2 MgZe + 2NaHCO3

Na2Ze + MgCl2 MgZe + 2NaCl

Na2Ze + CaCl2 CaZe + 2NaCl

Na2Ze + CaSO4 CaZe + Na 2SO4

Na2Ze + MgSO4 MgZe + Na 2SO4

Regeration

After some time zeolite gets exhausted. The

exhausted zeolite is again regerated by treating

with 10%solution of NaCl.

CaZe + 2 NaCl → Na2Ze + CaCl2

MgZe + 2 NaCl → Na2Ze +MgCl2

Advantages: (i). Output water has only 1-2 ppm. (ii).

Operation is easy.

(iii). No sludge is formed during the process.

03. Explain formation of deposits in steam boilers and heat exchangers. Steamboilers: Sealed vessel where water is converted to steam.

A steam boiler is a type of generator that is used to create steam. Heat exchangers: A device for transferring heat from one medium to another.

Heat exchangers are designed to remove excess heat from aircraft engines,

optics, x-ray tubes, lasers, power supplies, military equipment, and many

other types of equipment that require cooling beyond what air-cooled heat

sinks can provide.

http://www.lytron.com/Heat-Exchangers/Standard

http://www.lytron.com/Industries/Medical-Equipment

http://www.lytron.com/Industries/Laser-Cooling

http://www.lytron.com/Industries/Power-Generation

http://www.lytron.com/Industries/Defense-And-Aerospace-Thermal-Solutions

http://www.lytron.com/Industries/Defense-And-Aerospace-Thermal-Solutions


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Formation of deposits: Scale and sludge formation. Scale:If the precipitate forms hard and adherent coating on the inner walls of the boiler is known

as scale. It is formed by the substances like CaSO4, Mg(OH)2 and Ca(HCO3)2.

Scales are difficult to remove even with the help of hammer and chisel. Scales are the main

sources of boiler troubles. Formation

of scales may be due to

(i). Decomposition of calcium bicarbonate.

(ii). Deposition of calcium sulphate.

(iii). Hydrolysis of magnesium salts.

(iv). Presence of silica. Sludge: In boiler, water contains the precipitate loose and slimy is known as sludge. It is

formed by the substances like CaCl2, MgCl2, MgSO4 and MgCO3. Disadvantages:

(i). Poor conductor of heat.

(ii). Excessive sludge formation disturbs the working of the boiler.

(iii). It forms along with scales, then former gets entrapped in the latter and both get

deposited scales.

Prevention of sludge formation: (i).

By using well softened water.

(ii). By frequently below down opearation.

04. What are the disadvantages in scale formation? Explain in detail.(Or)

What are the disadvantages formation of deposits in steam boilers and heat

exchangers?(or)

Write short notes on (i). Wastage of fuels. (ii). Decrease in efficiency. (iii). Boiler

Explosion.


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Disadvantages: (i). Wastage of fuels.

(ii). Decrease in

efficiency. (iii). Boiler

Explosion.

(i). Wastage of fuels:

(ii). Decrease in efficiency:

Scales may sometimes deposit in the valves and condensers of the boiler and choke then

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. 05. What are the disadvantages using hard water in boilers?

Disadvantages: 1. Scale and sludge formation. [see Q.No: 3]

2. Priming and Foaming. [see Q.No: 10]

3. Caustic Embrittlement. [see Q.No: 7]

4. Boiler Corrosion. [see Q.No: 9]

6. Explain prevention of scales formation. Prevention of scales formation: (i). External Treatment.

(ii). Internal Treatment. (i). External Treatment:Softening of water (i.e) removing hardness- producing constituents of

water. Ex: Zeolite process and Demineralization process.

(ii). Internal Treatment:Sequestration process.

An ion is prohibited to exhibit its original character by complexing or converting it into other

more soluble salt by adding suitable reagent.


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An internal treatment – by adding proper chemical to the boiler water either

(a). To precipitate the scale forming impurities in the form of sludge, which can be remove by blow –

down operation (or)

(b). To convert them into compounds, which will stay in dissolved form in water and thus do not cause

any harm Blow down operation: It is partial removal of hard water through top at the bottom of

boiler,

when extent of hardness in the boiler becomes alarmingly high. Internal treatments methods are, generally, followed by blow-down operation , so that

accumulated sludge is removed.

Internal treatment methods are, (i). Colloidal Conditioning (ii).

Phosphate Conditioning (iii).

Carbonate Conditioning (iv).

Calgon Conditioning. 07. Write short notes on Caustic Embrittlement.

Caustic Embrittlement. = Intercrystalline cracking of boiler metal

It is a type of boiler corrosion, caused by using highly alkaline water in the boiler.

In boiler water, it contains a small amount of sodium Carbonate. In high pressure, it

decomposes to give sodium hydroxide.

Na2CO3 + H2O --- 2NaOH + CO2

Then this NaOH reacts with boiler material and it forms sodium Ferrate and this leads to

brittlement of boiler parts like joints, bends etc.,

Fe +2NaOH --- Na2FeO2 + H2

It is prevented by or Avoided by

(i). By using sodium phosphate as softening reagent instead of sodium carbonate. (ii). By

adding Tannin or Lignin to boiler water for blocks hair-cracks.

(iii). By adding sodium sulphate to boiler water also blocks hair-cracks.

08. Explain Internal conditioning methods of softening hard water. To remove hardness producing salts, chemicals were added to the boiler water in the boiler itself and

that treatment is called internal conditioning.

(i). Colloidal Conditioning (ii). Phosphate Conditioning

(iii). Carbonate Conditioning (iv). Calgon Conditioning. (i) Colloidal Conditioning:

In low-pressure boilers, scale formation can be avoided by adding organic substances like

kerosene ,tannin, agar-agar (a gel), etc., which get coated over the scale forming precipitates, thereby

yielding non-sticky and loose deposits, which can easily be removed by pre-determined blow-down

operations.


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(ii). Phosphate conditioning:

Three types of phosphates- mono, di and trisodium phosphates are employed in phosphate

conditioning. The advantages of phosphate conditioning over carborate

conditioning are (i). It can be applied to high pressure boilers and

(ii) It can be used for softening/ conditioning acidic, neutral or alkaline water sample. If acidic water is to be conditioned, trisodium phosphate can be used. For neutral and alkaline

water samples disodium phosphate and monosodium phosphate can be used respectively.

(iii) Carbonate conditioning:

In low pressure boilers, calcium ions are converted into soft and loose sludge by adding sodium

carbonate solution. It forms soft CaCO3 which can be removed by blow-down operation.

Calgon conditioning:

Calgon interacts with calcium ions forming a highly soluble complex and it prevents the Calgon=Sodium HexaMeta phospha)6precipitation of scale forming salt. The complex Na2[Na4(PO3)6] is soluble in water and no problem for its sludge disposal.

09. Write short notes on Boiler corrosion / Explain boiler corrosion in detail / Write

short notes on Boiler troubles – Boiler corrosion. Boiler corrosion taken place in boiler in the presence of gases like dissolved oxygen, dissolved

CO2 and dissolved salts.

Dissolved oxygen: It attacks the boiler material at high temperature and causes the

Corrosion.

4Fe +6H2O + 3O2 --- 4Fe(OH)3

It is removed by chemical and mechanical method.

Chemical Method: Sodium sulphite, Hydrazine used to remove dissolved oxygen.

Mechanical Method: To remove by De-aeration method.

Water spraying in a perforated plate-fitted tower, heated from sides and

connected to vaccum pump. High temperature, low pressure and large exposed

surface reduces the dissolved oxygen in water.


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Dissolved CO2: It produces carbonic acid, which is acidic and corrosive in nature

CO2 +H2O --- H2CO3 (Carbonic acid )

It can be removed by adding calculated amount of NH4OH into water and

also removed by de-aeration method.

Dissolved salts(MgCl2): Salts like CaCl2 and MgCl2 undergoes hydrolysis at higher

temperature , to give HCl , which corrodes the boiler

MgCl2 + 2H2O --- Mg(OH)2 +2HCl

Fe +2HCl --- FeCl2 + H2

FeCl2 + 2H2O ---- Fe(OH)2 +2HCl

It can be avoided by adding alkali into the boiler water.

10. Explain priming and foaming (carry over). 1.Priming: It is the process of production of wet steam.

Wet steam – Steam contains droplets of liquid water

It is caused by

i) High steam velocity

ii) Very high level water in the boiler

iii) Sudden boiling of water

iv).Very poor boiler design.

Prevention: It is controlled by

1) Controlling the velocity of steam

2) Keeping the water level lower

3) Good boiler design


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4) Using treated water 2) 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.

It is caused by

i) Presence of oil and grease

ii) Presence of finely divided particles.

It can be prevented by

i). Adding coagulants like sodium aluminate

ii).Adding anti-foaming agents like synthetic polyamides.

11. What is Desalination? Describe desalination of by Reverse Osmosis method with neat

diagram.(or)Explain the reverse osmosis process for desalination of brackish water in detail.

Desalination = Removal of common salt-NaCl from water.

Brackish water = Water containing dissolved salts with a peculiar salty taste. Ex: Sea water

Reverse

Osmosis(R

O):

When a pressure greater than osmotic pressure applied on the concentrated side, the

solvent flow takes place from higher concentration to lower concentration is known

as reverse osmosis.

1) In this RO process, Pure water is separated from salt water.

2) This RO process is also known as Super filtration or Hyper filtration

3) When the pressure is applied from the higher concentration side, the solvent flow

takes place to lower side and these two concentrations are separated by

semipermeable membrane, the salt water is converted into pure water.

4) The membranes used as cellulose acetate, polyamide and some polymers.

Advantages &

RO method:

1. Low capital cost, easy operating.

2. It is used for converting sea water into drinking water

3. It removes all types of impurities like non-ionic and colloidal

. 4. The life time of membrane is high and it can be replaced within few

minutes.

12 . Describe Demineralisation process.


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Unit – II electrochemistry & corrosion.

01. Define an Electrochemical cell. Give one example. A cell which converts chemical energy into electrical energy is known as electrochemical cell. Example:

Daniel cell , Batteries.

02. What do you mean by redox reaction? Both reduction and oxidation takes place simultaneously in a cell reaction then it is known as redox

reaction of an electrochemical cell.

03. What is electrode potential? It is the measure of tendency of a metallic electrode to lose or gain electrons, when it is in contact with its

own salt solution. It is denoted as “E”

04. Define an origin of electrode potential. When a metallic electrode is placed in its own salt solution, two types of reaction takes place.

1. +ve ions may pass into the solution. M → Mn+ + ne- (oxidation)

2. +ve ions from the solution may deposit over the metal. Mn+ + ne-→M (reduction)

The above reaction takes place in an electrode then it is known as an origin of electrode potential.

05. Define oxidation potential and reduction potential. Oxidation potential: The tendency of a metallic electrode to lose electrons, Reduction potential: The

tendency of a metallic electrode to gain electrons

06. How an electrochemical is measured? Define EMF of an electrochemical cell. It is measured by EMF.: “The difference of potential which causes flow of electrons from one

electrode of higher potential to the other electrode of lower electrode potential”.EMF= ER-EL

07. What are the applications of electrochemical cell? Determination of sparingly soluble salt.,Determination of the valency ion.

Determination of standard free energy change and K.

Potentiometric titrations can be carried out.,Hydrolysis constant can be determined.

08. Define electrochemical series. When various types of metallic electrodes are arranged in their increasing order of standard reduction

potential on the basis of hydrogen scale is known as emf series.

09. Write the significance of electrochemical series. To calculate the standard emf of the cell.,Relative ease of oxidation or reduction.

Displacement of one element by the other., Hydrogen displacement behavior.

Determination of equilibrium constant (K) for the reaction.

10. Write the mathematical form of Nernst equation and give one application. E = Eo + 0.0591/n log [Mn+]

Application: 1.It is used to calculate electrode potential of unknown metal.

2. Corrosion tendency of metals can be predicted.3. Applications of emf series.

11. Define corrosion. The gradual destruction of the metal or an alloy surface by the chemical or electrochemical reaction with

its environment. (i.e) metal into metal oxide.

12. What are the types of corrosion?

(i). Chemical corrosion. (ii). Electrochemical corrosion 13. What are the factors which affect

corrosion?

(i).Air and moisture. (ii). Electrolytes in water.(iii). Presence of impurities in metal.

(iv). Presence of gases like CO2 and SO2 (v). Differential aeration.

14. Define chemical corrosion. What are the types of chemical corrosion?

It is known as dry corrosion. It is due to the local attack of metal surfaces by the atmospheric

gases like oxygen,H2S,SO2 , N2, etc.


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3 types. (i). Oxidation Corrosion. (ii). Corrosion by other gases. (iii). Liquid-metal corrosion.

15. Define an Electrochemical corrosion. What are the types of Electrochemical corrosion?

It occurs due to the existence of separate “anodic” and “cathodic” areas between which current

flows through the conducting solution. At anodic area oxidation occurs and anodic part of metal is

destroyed. (i). Galvanic corrosion. (ii). Concentration cell / Differential aeration corrosion.

16. What are the differences between chemical and electrochemical corrosion?

Chemical corrosion Electrochemical corrosion

1) It occurs dry state Wet state (i.e) presence in moisture

2) Local attack to metal by environment Large number of cathodic and anodic areas

3) Chemical corrosion is self-controlled It is continuous process

4) It follows adsorption mechanism It follows electrochemical reaction

5) Ex: formation of mild scale on iron surface Ex: rusting of iron in moisture

17.What isBedworth rule?

The ratio of the volume of the oxide formed to the volume of the metal consumed is pilling

bedworth rule

18.Bolt and Nut made of the same metal is preferred in practice . Why?

Because such a combination will not permit galvanic corrosion to take place.

19.What are the methods used to control the corrosion?

1. Sacrificial anodic method 2.Impressed current cathodic metho

20 What is paint?

Paint is a mechanical dispersion of one or more finely divided pigments in a medium (thinner +

vehicle) .When paint is applied to a metal surface, the thinner evaporates, while the vehicle undergoes

slow oxidation forming a pigmented film.

21.Differenciate Electroplating and Electro less plating

Electroplating Electroless plating

It is carried out by passing current Used reducing agent

Separate anode is employed Catalytic surface of the substrate acts as an

anode

Anodic reaction is

M → Mn+

+ ne-

Anodic reaction is

R → 0 + ne-

Thickness of the plating is 1-100μm It has a thickness of 1-100μm

22. Define Electro plating and Electroless plating

Electro plating:

It‟s a process in which the coating metal is deposited on the base metal by passing a direct

current through an electrolytic solution containing the soluble salt of the coating metal.

Electro less plating:

It is a technique of depositing of a noble metal from its salt solution on a catalytically active

surface of a base metal by using a suitable reducing agent.


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UNIT II

ELECTROCHEMISTRY AND CORROSION

16 MARKS

01. What is electrochemical cell? Explain with example of Daniel cell.

Electrochemical cell: Cells in which the e-s transferred due to redox reaction and

converted into electrical energy.

Example : Daniel cell.

Daniel cell consists of two electrodes like Zn and Cu.

Zinc electrode is dipped in 1M ZnSO4 solution and Cu electrode dipped in 1M

CuSO4 solution.

The two solutions are inter connected by a salt bridge and two electrodes are

connected by a wire through the volt meter.

Each electrode is known as half-cell.

The e-s liberates from Zn electrode flow through the external wire and is consumed by

copper ions at the cathode.

Salt Bridge: U- tube containing saturated solution of KCl in agar-agar gel. It

connects two half cells.

Function of salt bridge: It eliminates liquid junction potential and electrical

continuity between two half cells.

02. How electrochemical cell is measured by potentiometrically? Or

What is emf? How emf is measured by pogendroff’s method?


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03. What are electrochemical series? Give its applications.

Electrochemical series: An arrangement in which the standard electrode reduction potential

of different metals are arranged in increasing order on the basis of

hydrogen scale is known as emf series.

Applications of EMF series: (i). To calculate the std. emf of a cell.

(ii). Relative ease of oxidation or reduction.

(iii). Displacement of one element by other.

(iv). To determine K of a reaction.

(v). Hydrogen displacement from acid solutions.

(vi). Predicting spontaneity of redox reations.

(i). To calculate the std. emf of a cell.

The emf of a cell is calculated as follows, Ecell = ER-EL

(ii). Relative ease of oxidation or reduction.

+ve value – higher reduction potential – high reduction. F-

=

2.87, higher reduction – get oxidized

Li+

= -3.07, lower reduction – high oxidation.

(iii). Displacement of one element by other.

Metals which lie higher in the series can displace those elements which lie below them in

the series. Cu can easily displaced by Zn

(iv). Determination of Equilibrium Constant K of a reaction.

From the value of Eo, we can be determined the equilibrium constant K (v)

Hydrogen displacement Behavior

Metals with above H2 scale will displace the hydrogen from an acid solution (ie)-ve

reduction potential. Zinc reacts with hydrogen but not in Ag. Why?

Because it has positive reduction potential

(vi) Predicting Spontaneity of Redox Reactions:

Eo

value is positive=Reaction is Spontaneous

Eo

value is –ve =Non-Spontaneous Process.

04. Derive Nernst equation.


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Applications:

(i) To calculate electrode potential of unknown metal

(ii) Application of emf series

(iii) Corrosion tendency of methods can be predicted


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05.


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What is Chemical corrosion? Explain with its types.

It is classified into 3 types. i). Oxidation corrosion.

ii).Corrosion by the other gases – Hydrogen

iii).Liquid – Metal corrosion

01. Oxidation corrosion

Oxygen present in atmosphere attacks metal surface resulting in the formation of metallic

oxide which is a corrosion product and known as oxidation corrosion.

Oxidation occurs first at the surface of the metal and the resulting metal oxide scale forms a

barrier that tends to restrict either further oxidation. For oxidation to continue either the metal must

diffuse outwards through the scale to the surface or the oxygen must diffuse inwards through the scale,

to the underlying metal.

Both transfer occurs, but the outward metal diffusion is generally, much more rapid than

the inward diffusion of oxygen. Since the metal ion is appreciably smaller than the oxygen ion

and consequently of much higher mobility.

02. PILLING – BEDWORTH RULE:

The ratio of the volume of the oxide formed to the volume of the metal consumed is pilling bed

worth rule.

03. Corrosion by other gases – Hydrogen.

i) At ordinary Temperature : Hydrogen Embrittlement.

The process of formation of cracks and blisters on the metal surface, due to high

pressure of hydrogen gas is called hydrogen embrittlement

(i) Metal direct contact with H2s and forms atomic hydrogen

(ii) Atomic hydrogen diffuses readily into metal to form molecular hydrogen

2) At high temperature – Decarburization

The process of decrease in carbon content in steel is termed as decarburization of

steel.

04) Liquid – metal corrosion

This is due to the chemical action of following liquid metal at high temperature. The

corrosion reaction involves

(i) Either dissolution of a solid metal by a liquid metal.


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(ii) Liquid metal may penetrate into the solid metal.

06. Explain Electrochemical corrosion with its types.

01. Galvanic corrosion:

The corrosion occurs when two different metals are in contact with each other in

presence of an aqueous electrolyte solution or moisture is known as Galvanic corrosion.

The more active metal acts an anode the less active and metal acts as a cathode

In Zn-Fe bimetallic couple, Zn undergoes to

corrosion, because compare with Fe, it has

higher –Ve emf Value. So it acts as

an anode. Iron acts as cathode and it is

protected.

Prevention of Galvanic corrosion:

But in the case of Fe – copper bimetallic

couple Fe dissolves and copper is protected.

(ie) Fe acts an anode and copper as a

cathode.

To prevent Galvanic corrosion, the bolt and but are made by same metal. If they are n‟t

corrosion will take place easily.

It may be minimized by putting an insulating material between the two metals.

02. Differential aeration corrosion:- Otherwise it is known as concentration cell corrosion.

It is due to the formation of concentration cell formed by the variation of concentration mainly of

oxygen or any electrolyte on the surface of the base metal.

i).When a metal is partially immersed in a solution,

the metal inside the solution has very poor aeration

compared with the metal that is outside the solution.

ii).The difference in the air concentration of the base

metal can produce the anode with less aerated area and

cathode.

iii).Corrosion will take place at the anode area where the

metal is converted into metal ion. Example: A zinc metal

partially dipped in a brackish solution.

Example: Pitting Corrosion - The holes are formed at corrosion in concentrated places.

Low oxygen part= anode , More oxygen part=cathode.

It is a localized attack, resulting in the formation of a hole around which the metal is relatively

unattached. Ex: metal area covered by a drop of water, sand, dust, etc.


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07. Distinguish between Chemical corrosion and Electrochemical corrosion.

Chemical corrosion Electrochemical corrosion

1) It occurs dry state Wet state (i.e) presence in moisture

2) Local attack to metal by environment Large number of cathodic and anodic areas

3) Homogeneous metal surface gets

corroded

Heterogeneous surface

(i.e) Bimetallic contact.

4) Corrosion products accumulate in the

same place, where corrosion occurs

Corrosion occurs at the anode, while the

products formed else where

5) Chemical corrosion is self-controlled It is continuous process

6) It follows adsorption mechanism It follows electrochemical reaction

7) Ex: formation of mild scale on iron surface Ex: rusting of iron in moisture

8) 3 types. i). Oxidation corrosion

ii). Corrosion by other gases- H2

iii). Liquid – Metal corrosion.

2 types.

i). Galvanic corrosion.

ii). Differential aeration corrosion.

08. What are the factors influencing corrosion?

The rate of corrosion is mainly depends on i)

Nature of the metal

ii) Nature of the environment

i). Nature of the metal

a). Position in emf series: Metals above hydrogen in emf series, corroded easily.

Metals have high –ve reduction potential undergoes corrosion.

b) Relative areas of the anode and cathode: Rate of the corrosion has higher % in anodic area, the

rate of corrosion will be more, when the cathodic area is larger.

c) Purity of the metal: The 100% pure will not undergo any type of corrosion. If

impurity present in higher percentage, corrosion takes places at anode.

d) Over voltage: Corrosive environment is inversely proportional to corrosion rate. e)

Nature of the surface film: It is known as pilling- bed worth rule.

f) Nature of the corrosion product: Corrosion is faster when the corrosion product is soluble in

corroding medium. If the corrosion product is volatile, the corrosion rate will be faster.

ii). Nature of the environment:

i). Temperature: Rate of corrosion α temperature.

The rate of corrosion increase with temperature

ii). Humidity: If the humidity is high in the environment, corrosion will be more.

iii). Presence of corrosive gases: The acidic gases like CO2,SO2,H2S are produce electrolytes, which

are acidic and increases the electrochemical corrosion.

iv). Presence of suspended particles: Particles like NaCl present in the moisture and acts a

powerful electrolyte and then enhance the electrochemical corrosion.

v). Effect of pH; the rate of corrosion will be maximum when the corrosive environment is acidic

(ie) PH<7.


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09. Explain the Sacrificial anode and impressed current techniques for the preventions of

corrosion.

i) Sacrificial anode method:

i). The application of sacrificial anodes in cathodic protection is based on the differences in

electrochemical reactivity of metals.

ii). In this method, the metal to be protected from corrosion is connected to more active metal which

acts as an anode.

iii). In a redox reaction involving iron and zinc, the zinc will serve as the anode, and iron the

cathode.

iv). The zinc anode will oxidize and provide electrons for the reduction of Fe2+

to elemental iron.

v). This is called cathodic protection. The zinc anode is termed a sacrificial anode.

vi). Iron pipes buried in the ground, and designed to carry water, would normally be expected to rust

pretty quickly.

vii).If they are buried along with a piece of zinc, and connected by a wire the zinc will provide

cathodic protection.

2. Corrosion control through Impressed current method:

An alternative method of providing the current to protect a system is to use some sort of

external power supply. As with the sacrificial system , the structure to be protected is made the

cathode, the difference being that the driving force. Behind the current is not the difference in

potential between the anode and cathode of the system but from the power supply.

Both these types of cathodic protection may be applied to buried pipelines and to steel hulled

ships. For oil drilling platforms, however the method employed is sacrificial in the vast majority of

cases in the north sea.

This technique is useful for protecting large structures like water tanks, underground oil collers,

laid up ships,etc.


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10. What is paint? What are the constituents and their functions in paint?

Paint: paint is a mechanical dispersion of one or more finely divided pigments in a medium (thinner +

vehicle) . When paint is applied to a metal surface, the thinner evaporates, while the vehicle undergoes

slow oxidation forming a pigmented film.

Constituents:

1. Pigments. 2. Vehicle or drying oil. 3. Thinner. 4. Drier. 5. Filler or extender. 6. Plasticizer

7. Antiskinnig agent.

i) Pigments: They are colour producing substances in paint.

Ex: white colour- white lead, black=- carben black red-indian red, green –

chromium oxide.

Function: i). It gives colour and opacity to the film.

ii). It also provides strength to the film.

iii). It provides weather resistance of the film.

ii) Vehicle (or) drying oil: This is a non-volatile portion of a medium and film forming

constituent of the paint. These are high molecular weight fatty acids present in vegetable and animal

oils. Ex: Linseed oil, castor oil.

Function:

i) They form a protective film by the oxidation and polymerization of the oil ii)

They hold the pigment particles together on the metal surface

iii) They impart water repellency , toughness and durability to the film.

iii) Thinners or solvents: Thinner is a volatile substance present in the medium.

Ex: turpentine, toluol, xylol etc.

Function:

I) It increases the elasticity of the film

II) It helps easy drying of the paint film

III) It increases the penetrating power of the vehicle.

iv) Extenders or fillers: These are colourless (white) pigments which improve the quality of the paint.

Ex: Gypsum, Jal ,china clay, silica, etc.

Functions:

i) It reduces the cost of paint

ii) It prevents the cracking and shrinkage of the film iii)

It modifies the shades of the pigments

v).Driers: these are the substances, used to accelerate the process of crying.

Ex: metallic soaps.

Functions: i).They are oxygen –carriers (or) catalists.

ii). They provide oxygen , which is essential for oxidation, polymerization of drying oil.

vi).Plasticisers: these are chemicals added to the paint to provide elasticity to the film and to prevent

cracking of the film. Ex: triphenyl phosphate, tricresyl phosphate, etc.

vii). Antiskinning agents: These are chemicals added to the paint to prevent gelling and

skinning of the paint. Ex: polyhydhroxy phenol.

11. Explain the electroplating of Gold.

Electroplating: It‟s a process in which the coating metal is deposited on the base metal by passing a


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direct current through an electrolytic solution containing the soluble salt of the coating metal.

Ex: Electroplating of Au (gold) over copper object.

The copper object to be plated is first treated with dilute acid.

Then this copper object and gold rod are immersed in an electrolyte.

Copper object acts cathode and gold rod is an anode.

These electrodes are connected with direct power supply.

When current is passed from battery through the solution, gold dissolves in electrolyte and

deposits uniformly in copper object.

12. Explain the Electroless plating of Nickel.

Metal ions + Reducing Agent Metal (Deposited) + Oxidised Product

Definition: it is a technique of depositing of a noble metal from its salt solution on a

catalytically active surface of a base metal by using a suitable reducing agent.

Electroless Nickel plating:

Step 1: Pretreatment and activation of the surface:

The surface to be plated is first degreased by using organic solvents or alkali followed by acid

treatment.


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Step 2: preparation of plating bath composition:

Coating solution NiCl2.6H2O 30 g/l

Reducing agent NaH2PO2H2O

(sodium hypophosphite)

10 g/l

Complexing agent Sodium succinate 10 g/l

Buffer Sodium acetate 10 g/l

Temp 85-95

oc

pH 4-6

The pretreated object is immersed in the plating bath for the required time during which the

following reduction reaction will occur and the Ni gets coated over the object.

UNIT – III ENERGY SOURCES

2 MARKS QUESTION BANK

1. What is nuclear energy?

The energy released by the nuclear fission reaction is called nuclear energy. 2. What is nuclear fission reaction?

The process of splitting of heavy nucleus into two or smaller nuclei with

Simultaneous liberation of large amount of energy in the form of heat is known as nuclear fission

reaction.

92U235

+ 0n1→ 56Ba

140+ 36Kr

93+30n

1

3. What is nuclear fusion reaction? Give one example.

The process of combination of two or more lighter nuclei into heavier nuclei,

with simultaneous liberation of large amount of energy is known as nuclear fusion reaction.

Ex: 1H1+ 1H

12H

4+energy

4. Differentiate nuclear fission and fusion reaction.


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Nuclear fission Nuclear fusion

It is the process of breaking the heavier nucleus.

It is the process of combination of lighter nuclei.

It emits radioactive rays Does not emit radioactive rays.

Occurs at ordinary temperature At high temperature(>106

K)

Chain reaction takes place No chain reaction.

Emits neutrons Emits positrons

It can be controlled Cannot be controlled

5. Define nuclear chain reaction. It is a nuclear reaction, in which neutrons from a previous step propagate and repeat the reaction.

It takes place only nuclear fission reaction.

6. What are fissile and fertile nucleides?

Fissile – Fissionable materials like U235

Fertile – Non- Fissionable materials like U238

7. What are solar cells? Give one example./What is a photo galvanic cells?

It is a device, Converting solar energy directly into electrical energy.

It provides power supply for space satellites. Ex: solar water heater , photovoltaic cell 8. What is solar energy conversion?

It is the process of conversion of direct sunlight into more useful forms.

It undergoes mainly two types of mechanisms. 1. Thermal conversion. 2. Photo conversion 9. What

are the merits of wind energy?

a) Nonpolluting and sustainable energy source.b).The scope of wind energy is enormous.

c). It is a renewable and sustainable energy source.

d). It is available in many offshore, onshore and remote areas.

10. What are batteries?

It is a device, converted chemical energy into electrical energy. It is an arrangement of

electrochemical cells connected in series,and it can be used as a source of direct electric

current.

11. Define breeder reactor.

Breeder reactor is one which converts non-fissionable material into fissionable material.

Non-fissionable material ------------> fissionable material.

Ex: 92U238

+ 0n1→94Pu

239 + 2 e

-

Non-fissionable fissionable

94Pu239

+ + 0n1

→ fission products + 3 0n1

12. What are non-conventional energy sources? Give two examples.

Non-conventional energy sources are those energy sources which are exposed to use from

modern technological advancements; rather than the normal use of conventional fuels as

energy sources like gas or oil.

Ex: 1. Wind energy 2. Solar energy 3.Tidal energy 4. Hydropower energy

13. What is a fuel cell?


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It is a device in which the chemical energy of the fuel hydrogen is directly converted into

electrical energy without combustion.

Fuel + Oxygen → Oxidation products + Electricity.

14. What are the advantages of fuel cell? Or What are the advantages of H2-O2 fuel cell?

It is used as an auxiliary energy source in space vehicles, submarines.

Used in military vehicles. It is a pollution free one.The product is water, so it is a valuable source

of fresh water by the astronauts.

15. What are the applications of lithium batteries?

Used in cell phones, Digital cameras, Watches, Remote cars, Calculators, Toys, Backup

batteries in computers, etc.

16. List any two advantages of lithium batteries.

Its cell voltage is high, 3V. Li is a light-weight metal, only 7g (1 mole) material is required to

produce 1 mole of electrons. It contains solid material so there is no leakage from battery. This

battery can be made in a variety of sizes and shapes.

17. Define alkaline battery.

It is called as a dry cell.>It has the electrolyte of KOH

A zinc cylinder is filled with an electrolyte of Zn,KOH and MnO2

A carbon rod acts as a cathode and zinc body acts as a anode.

18. What are the general components in nuclear reactor?

The components are fuel rods, control rods, moderators, reflectors, coolants and pressure

vessel, Turbine

UNIT – III ENERGY SOURCES


01. Distinguish between Nuclear fission and Nuclear fusion reaction.

Nuclear fission

Nuclear fusion

1. Breaking a heavy nucleus into smaller

Combination of two smaller nuclei.

2. Emits radioactive rays

Does not emit radioactive rays

3. Reaction takes place at ordinary

temperature

At high temperature. > 106K.

4. It gives chain reaction.

No chain reaction

5. Emits neutron

Emits positron

6. It can be controlled

It cannot be controlled.

7. The mass number and atomic number of new

elements are lower than the parent nucleus.

Higher than that of starting elements.

8. Ex: Fission reaction of 92U235

, power

generation in Nuclear Power Plant,

Kalpakkam&koodankulam, Tamilnadu.

Ex: Fission reaction takes place in sun.


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2. Define Nuclear fission reaction. Explain with one example in detail.


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C

03. Explain Nuclear Reactor-Power Generator with neat diagram./ Light Water Reactor Nuclear Reactor: A device used for power generation, in which a nuclear chain reaction is

initiated, maintained and controlled to produce the heat energy is

known as nuclear reactor

.

Nuclear Energy Converted Thermal Energy

(or)

Heat Energy

onverted Electrical Energy


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Components:

1) Fuel Rods: It produces heat energy and neutrons.

Ex: Natural Uranium (99.28% U238

and 0.714 % U235

) and Pu239

2) Control Rods: To keep power production at a steady state.

Ex: Boron and Cadmium rods.

3) Moderators: Function to reduce the kinetic energy of fast fission neutrons to slow

neutron and this is done in a small fraction of a second.

Ex: Graphite, Be, Ordinary water and Heavy water.

4) Coolants: To remove the intense heat produced in the reactor and to bring it out for

utilization. Ex: Ordinary water, Heavy water, liquid metals and gases.

5) Reflector: It placed around the core to reflect back some of the neutrons that leak out

from the surface of the core.

6) Pressure vessel: It enclosed the core and reflector. It also provides the entrance and exit passages

for coolant. (Pressure 200 kg/cm2)

7) Shielding: To attentiate the Gama rays and other radiations coming out from the reactor.

2 Types. (i). Thermal shield (ii). Biological shield.

8) Turbine: The steam at high pressure, generated in the heat exchanger is used to operate a

steam turbine, which derives a generator to produce electricity.

04. Explain Breeder reactor with reactions.


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05.

Define solar cell. Explain solar energy conversion in detail. Solar cell = Photogalvanic cell : It is a device used for converting solar energy into

electricity. It is made by interconnecting a large number of photovoltaic cells.


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Solar Energy Conversion: It is the process of conversion of direct sunlight into more useful forms.

Conversion may be in two forms. 1. Thermal Conversion. 2. Photo Conversion.

01. Thermal Conversion:

It involves absorption of thermal energy in the form of IR radiation. Temperature below

100oC, is useful for heating purpose of water and refrigeration.

Methods: (i).Solar heat collectors. (ii).Solar water heater.

(i). Solar heat collectors: It consists of natural materials like stones, bricks which can absorb heat

during the day time and release it slowly at night.

Uses: It is used for houses in cold condition. (ii). Solar Water Heater:

It consists of an insulated box inside of which is painted

with black paint. There is a provision for sun light absorption

using a glass lid and store solar heat. Inside the black

painted, copper coil and cold water is flow in and gets

heated and storage in a tank. 02. Photo Conversion: It involves conversion of light energy directly into electrical energy.

Methods: Solar Cell.

Solar Cell: Ex: Solar light, solar pump, solar battery. It is a device, converting solar energy directly into

electrical energy. Principle: When solar rays fall on a two layer of

semi-conductor devices, a potential difference between the two layers is produced. This potential

difference causes flow of electrons and produces electricity. Working: When the solar ray falls on the top layer and the e-s promoted to the conduction into n-type

semiconductor. The potential difference occurs; it should lead current increasing (i.e) flow e-s. They

are connected with an external circuit, and current is generated. Applications of Solar Energy :(i). Used in calculators,Watches, etc. (ii).

Used to drive Vehicles.

(iii). Used in boilers to produce hot water for domestic and Industrial uses.

(iv). Used for lighting purposes. (v). Used as a source power in space crafts and satellites. (vi).

Used for producing hydrogen by hydrolysis of H2O. Demerits of Solar Energy: (i). Huge capital cost.

(ii). Not available during night and cloudy days. (iii). Storage of energy is not possible


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06. Write a short note on Wind Energy.

Moving energy is called Wind.Energy recovered from the force of the wind is called wind energy.

The wind energy is harnessed by making use of wind mills.

01.Wind Mills:

The strikes of blowing wind on blades of the

wind mill make it rotating continuously.

The rotational motion of the blade drives a

number of machines like water pump, flour mills and

electric generators. Wind mills are capable of generating about 100 kw

electricity.

02. Wind Farms. It is known as the large number of joining wind mills called wind farm. It produces a large

amount of electricity.The minimum speed required for working of a wind generator is 15 km/hr

03. Other methods: (i). Sky Sail (ii). Ladder

mill. Advantages/ Merits of Wind energy:

i. It does not cause any Air pollution.

ii. It is very cheap and economic.

iii. It is renewable.

Disadvantages of Wind energy:

i. It produces noise.

ii. It produces unwanted sound.

iii. Affects bird‟s life.

iv. Affected to the radio signals.

07. Define Battery. Explain with its types.

Battery:

It is a device, converted chemical energy into electrical energy. It contains

several anodes and cathodes.

It is an arrangement of several electrochemical cells connected in series and it can be


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used as a source of direct electric current. (D.C).

TYPES OF BATTERIES:

01. PRIMARY BATTERY

i. It is known as Non-Reversible battery.

ii. Electrode and electrode reactions cannot be reversed by passing an external current. iii.

Reactions take place only once and after use they become dead.

iv. They are not chargeable.

v. Ex: Dry Cell, Mercury Cell, Leclanche‟s cell.

02. SECONDARY BATTERY

i. It is known as Reversible battery.

ii. Electrode and electrode reactions are reversiblewhen an external current is passing iii. It

can be recharged again and again.

iv. Also called as Storage cells or Accumulators. v.

Ex: Lead-Acid Battery, Ni-Cd.

03. FLOW BATTERY

In these cells, the reactants, products and electrolytes and continuously passing through

the cell. Here chemical energy is converted to electrical energy.

Ex: H2-O2 fuel cell.

08. Explain Alkaline Battery with neat diagram and cell reactions.

Anode Zinc body

Cathode Carbon rod / Graphite rod

Electrolyte Powdered KOH, MnO2 in the form of paste.


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Alkaline battery consists of electrolyte KOH and a Zinc cylinder filled with powdered Zn, KOH

and MnO2 in the form of paste using starch and water. A carbon rod [Graphite] acts as a cathode

and it is immersed in the electrolyte in the centre of the cell. The outside cylindrical

zinc body acts as an anode. Advantages:

i. Zinc doesn‟t dissolve in a basic medium.

ii. Its life is longer than dry battery because there is no corrosion on Zn. iii. It

maintains its voltage, when the current is drawn from it.

Uses: It is used in calculators and watches.

09. Explain Lead acid storage battery.

Cell Representation: Pb/Pb(SO)4//H2(SO)4(aq)/PbO2/Pb

Anode Lead - Pb

Cathode Lead Oxide PbO2

Electrolyte Sulphuric acid - H2(SO)4


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This is also a rechargeable battery.It consists of number of voltaic cells. In each cell, anode is Pb

plate and cathode is PbO2

A known number of lead plates are connected with parallel and a number of PbO2 plates

and also connected in parallel. Various plates are separated by insulators like glass fibre. The total

anodes and cathodes are immersed in dil.H2(SO)4 Solution.

Cell reactions: Discharging.-----------------------------------------------------------------------------------------------

Advantages:

(i). It is made easily. (ii). It produces very high current.

(iii). Effective one at low temperature. (iv). Self- discharging rate is low.

Uses:

(i). Used in automobiles like Car, Bus, Van, Lorry, Bike etc.

(ii). Used in Hospitals, Power stations, Telephone exchanges etc.

10. Explain Nickel – Cadmium battery with cell reactions.

Cell Representation: Cd/Cd(OH)2//KOH(aq)/NiO2/Ni

Anode Cadmium (Cd)

Cathode A metal grid containing a paste of NiO2

Electrolyte KOH

This is also a rechargeable battery. It consists of Cd anode and a metal grid containing a paste of NiO2 acts as a cathode. The


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Charging

Discharging Cd(S

)

electrolyte is KOH.

Discharging: When Ni-Cd battery operates, Cd gets oxidation and forms Cd2+

and insoluble

Cd(OH)2 is formed. Its emf value is 1.4 V.

Cell Reaction:

Recharging: When an current is passed opposite direction, the cell reaction is reversed. As a

result, Cd gets deposited on anode and NiO2 on cathode.

Cd(OH)2 Ni(OH)2

Energy

NiO2(S) 2H 2O


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Advantages:

(i). It is smaller and lighter.

(ii). It has longer life than lead storage cell.

Uses:

(i). It is used in calculators, Electronic devices.

(ii). Used in transistors, cordless appliances.

11. Explain Lithium batteries in detail.

It is known as solid state battery because the electrolyte is used here at solid state.

Anode Lithium (Li)

Cathode TiS2

Electrolyte Polymer


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2

2

2

Anode : Li(S) Li+

+ e-

Cathode: TiS2 (S) + e-

TiS-

Over all Li(S) + TiS2 (S) Li+

+

TiS-

LiTiS2

Discharging:

When the anode is connected to cathode, Li+ ions move from anode to cathode. Anode

is Li and cathode is TiS2 and the electrolyte is solid polymer. The cathode is a material capable of

receiving the Lithium ions and electrons.

Recharging:

LiTiS2 Li+

+ TiS-

It is recharged by applying an external current. Emf of Li cells = 3.0V. Advantages: (i). Itsemf is high 3.0V.

(ii). It is a light weight material only 7g required for produce 1 mole of e-

s.

12. Explain Hydrogen- Oxygen Fuel cell / [H2 – O2] Fuel cell. Fuel Cell: It is a device in which the chemical energy of the fuel hydrogen is

directly converted into electricity without combustion.

Anode Hydrogen

Cathode Oxygen ( oxidizer)

Electrolyte 25% KOH or NaOH


SCE Dept. of S & H

Two porous electrodes – Made of compressed carbon

containing a catalyst like pt / pd.

It consists of two porous electrodes anode and cathode. In

between two electrodes an electrolytic solution 25% KOH or

NaOH filled.

When H2 is bubbled through the anode compartment,

where it is oxidized. The O2 is bubbled at the cathode

compartment where it is reduced. The emf of the cell = 0.8 to 1.0 V. Merits:

i. High efficiency.

ii. No unwanted noise and less maintenance. iii. No

pollution

iv. No need to change electrode often. Uses:

i. Used in military vehicles and space vehicles. ii. H2 – O2 fuel cell, the product is water, so no need of fuel because fuel is water UNIT – IV ENGINEERING MATERIALS 2 MARKS QUESTION BANK 1. What are abrasives? Abrasives are hard substances used for Grinding, cutting, Shaping, drilling, poslishing andsharpening operations. Ex: Diamond,Talc Property of an Abrasive = Hardness 2. How abrasives are classified? Give example for each. Abrasives are classified into two types. 1. Natural abrasive Ex: Talc,Diamond. 2. Synthetic abrasive Ex: SiC and B4C. 3. What are natural abrasives? Abrasives are obtained from nature is called natural abrasives. Ex: Talc,Diamond,Topaz. 4. What are synthetic abrasives? Abrasives are prepared from man - made and obtained some chemical reactions. Ex: silicon carbide (SiC) 5. What is moh‟s scale of hardness? It is a scale in which common abrasives are arranged in their increasing order of hardness. In this scale, lowest: talc (hardness number is 1) highest: diamond(hardness number is 10) 136


SCE Dept. of S & H

6. Define Refractories. Refractories are materials that can withstand very high temperature without softening or deformation in shape.Ex: Alumina bricks, Magnesite bricks, Zirconia bricks. 7. How refractories are classified? Give example for each. Refractories are classified into three types. 1. Acidic refractories Ex:Alumina bricks. 2. Basic refractories Ex: Magnesite bricks. 3. Neutral refractories Ex: Zirconia bricks. 8. What are the properties of Refractories? (i). Refractoriness (ii). RUL- Refractoriness Under Load (iii). Porosity (iv). Thermal spalling (v).Dimensional stability. 9. What is RUL? The temperature at which the refractory deforms by 10% is called RUL. i.e The load bearing capacity of a refractory can be measured by RUL Test. 10. Define refractoriness of a refractory. [Jan‟10] It is the ability of a refractory material to withstand very high temperature without softening or deformation under the working conditions. 11. What is meant by thermal spalling with respect to a refractory? [Jun‟10] It is the property of a refractory breaking, cracking or peeling of refractory material under high temperature. It is mainly due to (i).rapid change in temperature. (ii).slag penetration. 12. What is Porosity? It is defined as the ratio of its pore volume to the bulk volume. Porosity = W-D ------- X 100 W-A 13. How SiC is prepared? It is prepared by heating a mixture of 60% sand (i.e) SiO2 and 40% coke with a small amount of saw dust and a little salt in an electric furnace to about 1650oC for 36 hours. SiO2+ 3C SiC + 2CO 14. What do you mean by Portland cement? A paste of cement with water on setting and hardening resembles in colour and hardness to Portland stone, a lime stone quarried in Dorset. It is obtained by heating clay containing material and lime containing material to about 1500oC and mixed with gypsum. 15. What are the properties of Portland cement? Setting and hardening and Heat of Hydration. 16. What do you mean by hardening and setting of cement? Setting: Stiffening of the original plastic mass, due to initial gel formation. Hardening: Development of strength, due to crystallization. 17. Give two examples of special cement. White cement , water proof cement. 18. Name 2 water proofing agents added to make water proof cements? Calcium stearate and Aluminium stearate 19. Give any 2 uses of waterproof cement. Used in the construction bridges and structures under water. 20. Give any 2 uses of white cement. (i). Repairing and joining marble pillars and blocks, manufacture of tiles and mosaic works. (ii). Used in the construction bridges and structures under water. 137


SCE Dept. of S & H

21. What is glass? It is an amorphous, hard, brittle, transparent or translucent, super cooled liquid obtained by fusing a mixture of a number of silicates. Silicates of Na, Ca, Pb are mostly used. 22. Mention some types of glass. Soft glass , hard glass, laminated glass, glass wool, flint glass , silica glass. 23. How is bullet resistant glass prepared? By pressing together several thin layers of glass with vinyl resin in alternate layers. 24. What is the chemical difference between soft glass and hard glass? Na2O.CaO.6SiO2 and K2O.CaO.6SiO2

UNIT – IV ENGINEERING MATERIALS


01. What are Abrasives? Explain with classification.

Abrasives are hard substances used for Grinding, cutting, Shaping, drilling, poslishing

andsharpening operations.

Ex: Diamond,Talc

04. Explain the properties of refractories.

Properties : Refractoriness, RUL, Dimensional stability, Thermal spalling,

Thermal expansion , porosity.

01. Refractoriness:

1. It is ability of a refractory material to withstand very high temperature without softening or

deformation under the working conditions

2. It is measured by PCE test.

3. PCE= Pyrometric cone equivalent

4. P.C.E number and softening temperature of some refractories are as follows a. Silica

bricks

b. Alumina bricks

c. Magnesite bricks

5. PCE is the number which refer to the softening temperature of a refractory specimen of

standard dimension 38mm height and 19mm triangular base.

Measurement:

1) This measurement is called segarcone test.

2) A test cone is determined by comparing its softening

temperature with softening temps.of a series of standard

pyrometric cones.

3) A test cone is prepared from the sample refactory

then it is placed along with the standard segar cone in an electric furnace and heated at

a rate of 10oC per minute.

4) The temperature at which apex (top) of the cone touches the base is taken as the softening

temperature .the PCE number of the standard cone which behaves identically is taken as

the PCE of the test sample.

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2)RUL: Refractoriness Under Load:

The temperature at which the refractory deforms by 10 % is called refractoriness under load.

(i.e).The load bearing capacity of a refractory can be measured by RUL test.

A good refractory should have high RUL value.

RUL TEST:

RUL test is conducted by applying a constant load of 3.5 or 1.75 kg/cm2

to the test refractory

specimen of size base 5 cm2

and height 75 cm and heating in a furnace at a standard rate of 10oC per minute.

03.Porosity: It is defined as the ratio of its por volume to the bulk volume

If porosity of a refractory is lower

i) posseses lower thermal conductivity ii) it

reduces thermal spalling

If porosity of a refactory is high

i). it reduces the strength

ii) .It reduces resistance to abrasion

iii). It reduces the resistance to corrosion

Porosity is used to determine like strength, corrosion and thermal conductivity.

A good refractory should have low porosity.

05. How Alumina, Magnesite and Silicon carbide are manufactured?

Manufacture of Alumina bricks:

Alumina bricks is an example for acidic refractories.

Alumina bricks are prepared from minerals silimanite and kyanite

They are anhydrous aluminosilicate materials Al2O3.SiO2

This mineral is fixed with coal in the ratio 1:1 along with plastic clay as binders.

The raw materials are mixed and moulded into bricks

The bricks are then dried and fired at 1600oc

The final product material contains 63 % alumina and 34% silica approximately.

These are used in steel industries.

Manufacture of Magnesite bricks:

Magnesite bricks are an example for basic refractories

Powdered caleinedmagnoite (Mgo) is mixed with caustic magnesia or iron oxide, as a birder with

water mixed and moulded into bricks.

The bricks are then dried and fired at 1500oC

The final product material containsMgO=85%,CaO=2.5% & SiO2=5.5%

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There are used in open hearth furnaces, libing converters and reverberatory furnaces.

Manufacture of Silicon carbide (SiC): CARBORUNDUM

SiC is an example for very hard synthetic abrasive.

It is a mixture of SiO2 and coke.

Its hardness is 9.8 on Moh‟s scale.

It is chemically inert and can withstand high temperature.

Preparation Or Manufacture:

It is prepared by heating a mixture of 60% sand (i.e) SiO2 and 40% coke with a small amount of saw

dust and a little salt in an electric furnace to about 1650oC for 36 hours.

SiO2

+ 3C SiC + 2CO

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2. Burning: Dry raw mix / slurry is carried out in rotary kiln.

The Dry raw mix / slurry is fed into the kiln from upper end and the flame is forced into the lower end.

Due to slope and slow rotation, the material gradually descends in the klin into different zones of

increasing temperatures.

(i). Drying Zone: Upper part of the kiln - About 400oc, water in slurry gets evaporated.

(ii). Calcination zone: Center part of the kiln - About 1000oc, limestone gets decomposed

intoCaO and CO2. CaCO3 ----- CaO + CO2

(iii). Clinkering Zone: Lowest part of the kiln - About 1350-1500oc, limestone reacts with

clay to form Bogue compounds. C2S,C3S,C3A,C4AF.

The Bogue compounds fuse together to form small, hard, grayish

coloured stone like mass called cement clinkers. 3. Grinding : The hot clinkers are cooled with atmospheric air and then pulverized together with 2-

3% gypsum in ball mills. Gypsum act as retarding agent for quick setting

cement.

4. Storage and Packing: The cement coming out of the grinding mill is stored in a concrete storage

silos. Then the cement is packed in jute bags by automatic

machine.

07. Explain the properties of Portland cement.

Properties: (i). Setting and Hardening of cement. (ii). Heat of Hydration. (i).

Setting and Hardening of cement.

Setting: It is defined as the stiffening of the original plastic mass, due to the formation of

tobermonite gel.

Hardening: It is defined as the development of strength due to formation of crystals. When

cement is mixed with water, results formation of gel and crystalline products.

Chemical reactions:

(i). Flash set – When cement is mixed with water, hydration of C3A takes place and the paste becomes

quite rigid within a short time.(1 Day)

C3A + 6H2O C3A.6H2O

(II). Formation of Tobermonite gel: After the hydration of C3A, C3S begins to hydrate to give Tobermonite

gel and crystalline Ca(OH)2. This is responsible for the development of initial strength of cement. The

hydration of C3S gets completed within 7 days. It does not contribute much to the

strength of cement.

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SCE Dept. of S & H

C3S + 6H2O C3S2 .3H2O + 3Ca(OH)2

(III). Dicalcium silicate (C2S) reacts with water very slowly and gets completed in 7 to 28 days.

2C2S + 4H2O C3S2 .3H2O +Ca(OH)2

The increase in strength between 7 to 28 days is due to the formation of tobermonite gel and

crystalline Ca(OH)2.

(iv). The hydration of C4AF takes place initially, the hardening takes place finally through

crystallization along with C2S.

C4AF + 7H2O C3A.6H2O

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Thus the final setting and hardening of cement is due to the formation of tobermonite gel plus

crystallization of Ca(OH)2 and hydrated tricalcium aluminate.

ii). Heat of Hydration:

When water is mixed with Portland cement some

amount of heat is liberated due to hydration and

hydrolysis reactions of Bogue compounds. The average

quantity of heat is liberated is 500 kJ/kg. 143


SCE Dept. of S & H

08. Write short notes on special cements like waterproof and white cement.

(i). Waterproof cement:

It is obtained by adding waterproof materials like calcium stearate, aluminium stearate and gypsum

with tannic acid to ordinary Portland cement during grinding.

Functions: (i). To make concrete impervious to water under pressure. (ii).

To resist the absorption of water.

Properties:

(i). It is more expensive than ordinary Portland cement. (ii).

It act as pore-blocking and water- repelling agent.

Uses: (i). Used in the construction bridges and structures under water.

(ii). White Cement:

It is white in color due to absence of iron compounds.

Such cements are made from raw materials which are free from iron oxide.

Properties:

(i). It is more expensive than ordinary Portland cement.

(ii). It act as pore-blocking and water- repelling agent.

Uses: works.

(i). Repairing and joining marble pillars and blocks, manufacture of tiles and mosaic (ii). Used in the construction bridges and structures under water.

09. Explain the manufacture the glass.

Raw materials:

a) Sodium is soda , Na2CO3 – Soft glass.

b) Potassium is potash, K2CO3 – Hard

glass. c) Calcium are limestone, chalk

and lime

d) Lead are litharge, and red lead – Flint

glass e) Silica are quartz, white sand.

f) Zinc is zinc oxide – Heat and shock proof glass

g) Borate are borax , and boric acid - Heat and shock proof

glass h) Cutlets or pieces of broken glass to increase the

fusibility. 4 steps involved for the manufacturing of glass.

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1. Melting: The raw materials in proper proportions are mixed and finely powdered. This mixture called

batch is fused with some broken glass, called cullet in the pot of the tank furnace, in which heating is done

by burning produces gas and air mixture over the charge.

Heating is continued, till the molten mass is free from bubbles and glass-balls, and then cooled to about

800oC.

2. Foaming and shaping:

Molten glass is then worked into articles of desires shapes by either blowing or moulding or pressing

between rollers.

3. Annealing:

Glass articles are then allowed to cool gradually to room temperature( sudden cooling must be

avoided, because cracking occurs ). The longer the annealing period, the better quality of glass. 4. Finishing:

All glass articles after annealing are subjected to finsish processes such as cleaning, grinding,

polishing, cutting etc.

10. Explain the types and properties and uses of glass.

1) . Soda lime or soft glass:

Raw materials: silica, calcium carbonate and soda ash.

Approximate composition: Na2O.CaO.6SiO2

Properties:

a) They are low cost.

b) It is resistant to water.

c) It is attacked by common reagents like acids.

Uses: Window glasses, electric bulbs, bottles, jars, cheaper table wares, where high temperature –

resistance and chemical stability required. 2). Potash lime or Hard glass:

Raw materials: silica, calcium carbonate and Potassium carbonate.

Approximate composition: K2O.CaO.6SiO2

Properties:

a) Possess high melting point so it will not fuse easily.

b) Less acted upon by acids, alkali and other solvents than ordinary glasses. Uses:

Chemical apparatus, combustion tubes, which are to be used for heating operations. 3). Lead glass or Flint glass:

Raw materials: Lead oxide and silica are fused.

Approximate composition: K2O.PbO.6SiO2

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SCE Dept. of S & H

Properties:

a) Lower softening temperature than soda- glass. b) Higher

refractive – index.

c) Has excellent electrical properties. d) High

specific gravity ( 3 to 3.3) Uses:

High quality table wares, optical purposes(like lenses), neon sign tubing, cathode ray tubes,

electrical insulators and in art objects.

High lead content glasses are used for extra-dense optical glasses for windows and

shields to protect personnel from x-rays and gamma rays in medical and atomic energy

fields respectively.

4). Boro silicate glass / Pyrex glass / Jena glass.

Raw materials: silica, boron with a small amount of alumina and some oxides. Approximate

composition: SiO2(80.5%),B2O3(13%), Al2O3(3%),K2O(3%),Na2O(0.5%)

Properties:

a) Low thermal efficient of expansion

b) High chemical resistance.

c) Very high softening points.

d) Excellent shock-proof.

Uses:

Used in industry for pipelines for corrosive liquids, gauge glasses, superior laboratory

apparatus, kitchenware, chemical plants, television tubes, electrical insulators.

5). Aluminosillicate glass

Raw materials: alumina, silica, boron with a small amount of and some oxides.

Approximate composition: SiO2(55%),Al2O3(23%),B2O3(7%), MgO(9%),CaO(5%),

Na2O+K2O(1%)

Properties:High softening temperature

Uses:

High-pressure mercury discharge tubes, chemical combustion tubes, certain domestic

equipments, etc, 6). Glass wool:

It is a fibrous wool-like material, composed of intermingled fine threads of filaments of glass. They are completely alkali free. The glass filaments are obtained by forcing molten glass through small offices. The average diameter of the office is 0.0005 to 0.007mm. Then the filaments of glass so obtained are thrown over a rapidly rotating drum to get wool-like materials

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. Properties:

a). fire- proof and heat proof material.

b). low electrical and thermal conductivity.

c). resistant to water and most of chemicals. d). high

tensile strength – 8 times that of steel.

Uses:

1. Heat insulation purpose- domestic and industrial appliances.

2. Air filters and dust filtering materials.

3. Insulation of metal pipelines and walls and roofs of houses.

4. used in filtration of corrosive liquids like acids.

5. Manufacture of fibre-glass, by blending with plastic resins.

UNIT V

1. How fuels are classified. Give one example for each.

Based on Occurrence - 2 Types.

1. Primary Fuels – It occurs in nature such as. Ex: Coal, Crude oil, Natural Gas.

2. Secondary Fuels – It is derived from primary fuels. Ex: Coke, Petrol, Coal gas. Based

on physical state – 3 types

1. Solid fuels, eg., coal, coke,

2. Liquid fuels eg., gasoline, diesel.

3. Gaseous fuels eg., coal gas, natural gas.

2. Define calorific value. Explain higher & lower calorific value.

It is defined as the amount of heat obtainable by the complete combustion of a unit mass of the fuel.

Units of Calorific value ; Calorie, kilocalorie, British thermal unit, centigrade heat unit

Higher calorific value / Gross calorific value (GCV)

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GCV = The total amount of heat is produced, when a unit quantity of the fuel is completely burnt and the

products of combustion are cooled to room temperature is known as GCV.

Lower calorific value / Net calorific value (GCV)

NCV = The total amount of heat is produced, when a unit quantity of the fuel is completely burnt and

products of combustion are allowed to escape is known as NCV

NCV = GCV -0.09H x 587.

3. Explain proximate analysis. Give its significance.

It involves the determination of % of moisture content, volatile matter, ash content &

fixed carbon in coal.

(i) Moisture content: About 1 gram of air-dried powdered coal sample is taken in a crucible & it is heated

at 100-105°C for 1 hour.

%of moisture in coal= loss in weight of the coal

X100

weight of Air- dried coal

(ii) Volatile matter: After the analysis of moisture content, the crucible with residual coal sample is

converted with lid & it is heated upto 950°C for 7 minutes.

% of Volatile mater in coal=

148

loss in weight of the coal

weight of moisture- free coal

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SCE Dept. of S & H

(iv) Fixed carbon: It is determined by the subtracting the sum total of moisture, volatile & ash content

from 100.

% of fixed carbon in coal = 100 - % of (moisture content + volatile matter + ash content)

Significance or importance of Proximate Analysis:

(i) High % of moisture is undesirable because it reduces the calorific value of a fuel, & increases the transport

cost.

(ii) High % of matter is undesirable because it reduces the calorific value of a fuel & coal burns with a long

flame & high smoke.

(iii) High % of ash content is undesirable because it reduces the calorific value of a fuel & makes the

additional cost of disposal of ash.

(iv) High % of fixed carbon in a coal, is greater its calorific value.

4. Explain ultimate analysis. Give its significance.

It involves the determination of % of Carbon, Hydrogen, Nitrogen, Sulphur & Oxygen in coal.

1. Carbon & Hydrogen: A known aount of coal sample is burnt with o2 in a combustion apparatus. In coal

carbon & hydrogen are converted into CO2 & H2O & those are absorbed by KOH tubes &

CaCl2 tubes.

C + O2 CO2

12 44

% of Carbon in coal =

Increase weight in KOHtubes X

12 X 100

Weight of coal sample taken 44

H2 + ½ O2 H2O

2 18

% of Hydrogen in coal = Increase weight inCaCl2 tubes

X 2

X 100

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Weight of coal sample taken 18


SCE Dept. of S & H

1. Nitrogen:

It is carried out by Kjeldahl‟s method. A known amount of coal is heated

with Con. H2SO4 in presence of K2 SO4 catalyst in a long necked flask

called Kjeldahl‟s flask. Nitrogen is converted into ammonium sulphate.

Then it is heated with NaOH & absorbed by HCl.

% of Nitrogen in coal = 1.4 X volume of acid consumed

Weight of coal sample

3. Sulphur: A known amount of coal is completely burnt in a bomb calorimeter. Here

sulphur is converted into sulphate & treated with BaCl2, BaSO4 is obtained.

4. Oxygen: The % of oxygen is calculated as follows,

% of Oxygen in Coal = 100 - % of ( C + H+N+S+ash)

Importance or significance of Ultimate analysis:

1. Higher % of carbon & hydrogen, better is the quality of coal & higher its calorific value.

2. Should have very little nitrogen content.

3. Presence of sulphur is undesirable because it forms harmful gases during the combustion.

4. Lower of the Oxygen increases the higher its calorific value.

5. How Metallurgical coke is manufactured by Otto-Hoffman’s method?

When bituminous coal is heated strongly in the absence of air, the volatile matter

escapes out & the mass becomes hard, strong, porous, which is called metallurgical coke.

Manufacture of Metallurgical coke by “Otto- Hoffman’s by products”

method:

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In order to (i) Increase the thermal efficiency of the carbonization

process & (ii) Recover the valuable by products by this

method.

This oven consists of a number of silica chambers.

Each chamber is about 10-12 m long, 3-4 m height & 0.4 – 0.45m wide.

Coal is introduced into the silica chambers & heated 1200°C by air & producer gas.

I & IV regenerators are heated by hot flue gases & II & III regenerators are

heated by incoming air & producergas.

When the process is complete, the coke is removed & cool by with water.

Time taken for this process is 12-20 hours.

The yield of the coke is about 70%

From out coming flue gas, it gives valuable products like Tar, ammonia, benzene.

H2S are obtained.

Recovery of by products:

(i) Tar: the flue gases are first passed through a tower, in which liq. NH3 is sprayed,

tar is collected at the bottom of the tank.

(ii) Ammonia: The gases are then passed through 2nd

tower, in which water is sprayed &

NH3

gets in the form of NH4OH.

(iii) Naphthalene: The gases are again passed through next tower, cooled water is

sprayed, Naphthalene gets & condensed.

(iv) Benzene: The gases are passed through next tower, petroleum oil is sprayed, benzene

gets condensed.

(v) H2S gas: The remaining gases are then passed through a purifier, H2S gas is obtained.

The final gas left out is called coal gas.

Advantages: (i) Valuable by product like NH3, benzene, etc are obtained.

(ii) The carbonization time is less.(iii) Heating is done by extremely by producer gas.

6. What do you mean by hydrogenation of coal? How Synthetic petrol is manufactured by

Bergius Process? Or How solid fuel is converted into liquid fuel? Explain in detail.

The preparation of liquid fuels from solid coal is known as hydrogenation of coal &

this gasoline is known as synthetic petrol.

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SCE Dept. of S & H

Bergius Process – Direct method:

Input Powdered coal + Ni Oleate catalyst + Heavy oil are made into paste + H2

Heating in

400 - 450°C.

Pressure

200-250atm.

Process: Powdered coal is converted into gasoline.

In this process powdered coal is mixed with Ni oleate & heavy oil & made into paste.

It is pumped along with H2 gas into the converter, the paste is heated to 400 - 450°C

under pressure of 200-250atm.

Crude oil comes & it is fractionated into 3 parts.

1. Gasoline. 2. Middle oil. 3. Heavy oil.

The middle oil is further hydrogenated in vapour phase to get

gasoline. The heavy oil is recycled for making paste again coal

powder.

60% of yield is obtained from this process.

7. Explain the following (i) Compressed natural

gas (CNG) (ii) Liquid

petroleum gas. (LPG)

(i) Compressed Natural Gas: (CNG) Natural gas (CH4) compressed to a pressure of about 1000 atm is known as CNG.

Its calorific value is 12000-14000kcal/m3.

It is fully of methane only & derived from natural gas.

Its composition is as follows.

CH4 = 70-90% C2 H2 = 5- 10 % H2 = 3%

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SCE Dept. of S & H

Uses: CNG is a cheapest, clearest & the least polluting fuel for automobiles instead of petrol

or diesel.

(ii) Liquid Petroleum Gas: (LPG)

It is obtained as a byproduct during the cracking of heavy oil.

Its composition is

Butane

= 27%

Isobut

ene =

25%

Butyle

nes =

43%

Propan

e = 5%

Its calorific value is 27,800 kcal /m3.

LPG is marketed under the trade names like Indane, HP, Bharat gas in steel cylinders

under high pressure.

A small amount of Ethyl mercaptan is added during filling of cylinders to help in

detecting leakage of gas.

LPG ensures complete combustion with no smoke & causes the least

environmental pollution.

Uses: 1. It is used as a domestic fuel.

2. It is used as a fuel in vehicles (i.e) motor fuel.

3. It is used in industries & laboratories

8. Explain Water gas with reaction.

It is a mixture of CO & H2 with small amount of N2.

Its calorific value is about 2800kcal/m3.

Composition: CO = 41%, H2 = 51%, N2 = 4% & CO2 = 4%.

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SCE Dept. of S & H

The water gas producer consists of a tall steel vessel, lined with refractory bricks.

It is provided with cup & cone feeder at the top & side opening for water gas exists. At the

bottom it is provided with 2 inlet pipes for passing air & steam.

When steam & little air is passed alternatively over a red hot coke maintained at

about 900-1000°C in a reactor, water gas is produced.

Two steps of reaction in production of water gas: Step 1: C + H2O CO + H2. ENDOTHERMIC

Coke steam water gas

Here the steam is passed through red hot coke, where CO & H2 gases are produced. The

reaction is endothermic.

Step 2: C + O2 CO2. EXOTHERMIC.

Uses: 1. It is used for preparation of power alcohol.

2. For the production of H2 & in synthesis of NH3.

3. To manufacture synthesis petrol in Fischer – Tropsh process.

9. Explain Producer gas with reaction.

It is a mixture of CO & N2 with small amount of

H2. Its calorific value is about 1300 kcal/m3.

Composition: CO = 30%, H2 = 10-15%, N2 = 51-56% & others = rest.

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SCE Dept.of S&H

The producer gas producer consists of a tall steel vessel, lined inside with refractory bricks. It

is provided with cup & cone feeder at the top & side opening for producer gas exit.

At the bottom it is provided with an one inlet for passing air & steam.

When a mixture of air & steam is passed over a red hot coke at 1100°C in a reactor, the

producer gas is produced.

C+O2 +N2 CO + N2.

Coke from air producer gas

Uses: It is a used as a reducing agent in metallurgical operations.

Used for heating muffle furnaces & open- hearth furnaces.

10. Describe flue gas analysis by Orsat’s apparatus method.

Flue gas analysis is carried out by Orsat apparatus method.

A mixture of gases like CO2, CO, O2 & N2 etc coming out from the combustion

chamber is called flue gases.

If the flue gas contains,

CO -- It indicates incomplete combustion &

O2 --It indicates excess supply of air used in combustion.

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SCE Dept.of S&H

Apparatus: It consists a horizontal tube. At one end this tube, U tube containing fused

CaCl2 is connected through 3 way stop cock. The other end is connected with a graduated

burette. The horizontal tube is also connected with 3 different absorption bulbs I, II, III for

absorbing CO2, CO and O2.

I bulb: It contains KOH solutions & it absorbs CO2 only.

II bulb: It contains Alkaline pyrogallol solution & it absorbs CO2, & O2

III bulb: It contains Ammoniacal cuprous chloride solution & it absorbs CO2,CO & O2

The 3 way stop cock is connected with flue gas supply & it is sucked into the burette & it is

adjusted by 100cc. then the 3 way stop cock is closed. In bulb I, co2 is absorbed by KOH

solution & I is closed & II stopcock is opened, O2 is absorbed by alkaline pyrogallol

solution. Now II is closed & III is opened. CO is absorbed by ammonical cuprous chloride.

The decrease in volume of the flue gas in the burette indicates the volume of I CO2, II

O2,

III CO respectively.

Significance: It gives an clear idea about the complete or incomplete process. 156


SCE Dept.of S&H

Anna university Questions

1. How is flue gas analysed by orsat method? Explain with neat diagram (Apr.

Dec-06)

2. Define gross and net calorific values of a fuel. How are they related? (June-06)

3. Calculate the gross and net calorific value of coal having the following

compositions

C = 85%, H = 8%, S = 1%, N = 2% ash = 4% (May-07).

Exercise

1. A sample of coal was found to contain the following. C = 81%, H = 4%,

O=2% N = 1% the remaining being ash. Estimate the quantity of minimum air

required for the complete combustion of 1 kg of the sample, if 40% of the

excess air is supplied (Ans: 14.95kg).

2. Calculate the minimum amount of air required for complete combustion of

50kg of fuel containing 80% carbon, 6% hydrogen 2% sulphur and the rest

nitrogen by weight. (Ans: 572 kg).

3. A coal sample on analysis gives C = 75%, H2 = 52%, O2 = 12.8%, S = 2% and

the rest ash.Calculate the amount of air needed for the complete combustion, if

1kg of the coal is bunt with 30% excess air (Ans: 13kg).

4. A fuel contains C = 75%, H = 4%, O = 5%, S = 7% remaining ash. Calculate

the minimum quantity of air required for the complete combustion of 1kg of

fuel. (Ans: 10.18 kg).

5. What is the volume of air required for complete combustion of 1m3 of mixture

containing 80% CH4 and 20% C2H6 (Ans: 10.95m3)

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