Engineering Chemistry All Units

8/8/2019 Engineering Chemistry All Units

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Sources of Water1. Rainwater. It is the purest form of natural water. But unfortunately it

dissolves the toxic gases like CO2, SO2, NO2 etc. and other solids.

2. Sea water. It is the most impure form of water containing about 3.5%dissolved salts of which about 2.6% is NaCl. Other salts present includesulfates, bicarbonates, bromides of sodium, potassium, magnesium etc.

3. River water. The sources of river water are the springs and therainwater. River water while flowing through the land collects lots oforganic matters from falling trees and nearby habitats and also othersoluble and suspended matters from the lands, soils etc.

4. Lake water. It is much purer than river water, dissolved impurities areless but contains lots of organic matter.

5. Underground water. The rainwater and other surface water percolatedown through the soil and rocks and get filtered and finally collected onrocky surface or again come out as spring. Though it contains lesssuspended matter but the dissolved mineral content is quite high and isof high organic purity.

Hardness of Water

Hardness of water is the characteristic of preventing lather formation ofwater with soap. Generally salts like chlorides, bicarbonates and sulfatesof Ca2+, Mg2+ and Fe2+ make water hard.

This hard water on treatment with soap which is stearic or palmitic acidsalts of sodium or potassium causes white precipitate formation ofcalcium or magnesium stearate or palmitate.

Thus the cause of hardness is the precipitation of the soap and henceprevents lathering at first. When the hardness causing ions are removedas insoluble soaps, water becomes soft and forms lather.


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Units of Hardness

Hardness is principally expressed in ppm unit. Other limits include

French degree of hardness, English degree of hardness or Clark, USAdegree of hardness and German degree of hardness.

Scales and Sludges


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PrimingDuring rapid steam production, some liquid water drops arealso carried along with the steam. This wet-steam formation

is called priming.

Priming occurs due to

(i) Presence of large amount of dissolved solids,

(ii) High steam velocities,

(iii)Sudden boiling and

(iv)Sudden increase in steam production.

Priming can be controlled by

(i) Maintaining low water level in boiler,(ii) Avoiding rapid change in steam rate,

(iii)Softening of boiler water and

(iv)Using mechanical device for steam purification.

Foaming

Foaming is the production of bubbles and foams which donot break easily.

Foaming occurs due to the presence of oil in the water.

Foaming can be reduced by

(i) removing oil from boiler-feed water and

(ii) adding anti-foaming agents.

Priming and foaming occur together and they are undesirable

since they wet other mechanical parts of the boiler andreduce their efficiency.

Actual height of the water column cannot be judged due tofoaming hence creating difficulty in the maintenance.


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Lime Soda Process - Reactions of Lime and Soda


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Hot Lime-Soda Process

Advantages Include:

1. the precipitation reaction becomes almost complete.

2. the reaction takes place faster.

3. the sludge settles rapidly.

4. no coagulant is needed.

5. dissolved gases (which may cause corrosion) are removed.

6. viscosity of soft water is lower, hence filtered easily.

7. Residual hardness is low compared to the cold process.

8. Lime soda process is economical.

9. The process improves the corrosion resistance of the water.10.Mineral content of the water is reduced.

11.pH of the water rises, which reduces the content of pathogenicbacteria.


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Ion Exchange resin


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Electroplating

It is a process by which a coating metal is deposited onthe base metal by passing direct current through anelectrolytic solution, containing the soluble salt of thecoating metal.

Electroplating is done both for protecting the metalfrom corrosion and for decorative purposes.

A well cleaned and properly pre treated surface of anymaterial to be electroplated is necessary for obtainingthe coating of long life.

To get good film complexing agents, organic additives,levelers, structure modifiers and wetting agents areadded.

Current density is adjusted to get an adherent film.


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The electrolyte is selected in such a way that it is agood conductor and highly soluble.

It should not undergo hydrolysis, oxidation, reductionand other chemical changes.

It should possess sufficient covering power.

Hence mixture of two or more electrolytes is used forpreparing electrolytic bath.

For a good electrodeposit, the pH of the bath must beproperly maintained.For most plating baths, pHranges from 4 to 8.

Electroplating method depends upon the type of

metal to be electroplated,the size and type of articleto be electroplated, its main objectives andeconomics involved.

Plating bath Normally plating is carried out in rectangular tank

made of wood or steel with a ceramic or polymerlayer inside so as to provide thermal insulation.Thevolume varies between 25 to 2000 L.

Heating if required is provided by heating coils or hotgases. Air sparger or nitrogen sparger is employed tointroduce convection current in the plating bathsolution. DC source is used for electroplating with avoltage of 8-12V and a current density of 1-200mACm-2.

In large plating operations, pumps and filters areemployed to filter out regularly the metallic particle.The anode sludge can be retained by anode in cottonbags.


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Pretreatment of surface of substrate isessential to prepare the surface of the

substrate properly. Removal of organic impurities and grease

from the surface are done by employingorganic solvents like trichloroethylene,methylene chloride and hot aqueous alkalis.pH increase catalyses the hydrolysis of fattyimpurities.

Thin film coatings

Physical Vapour Deposition (PVD)

This is a process of depositing some material

by atom by atom or molecule by molecule or

ion by ion.

The important methods of PVD are thermal

evaporation, sputtering and ion plating.


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Thermal evaporation Oldest and simplest process. In this method the metal to be

coated is placed in a refractory metal boat or crucible.

The boat is heated to melt the content or an electron beam isfocused on the contents to melt.

The contents after melting form an electron cloud in the shapeof ice cream cone with the tip of the cone at the source.

It coats all surfaces in the line of sight of the boat or crucible.This process is widely used to produce decorative coatings onplastic parts those are resembling shiny metal.

Many automobile parts are plastic with a PVD coating ofaluminium. A lacquer coating is applied over the decorativecoating to provide corrosion protection.

Schematic of thermal evaporation deposition


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Sputtering High technology coatings such as ceramics, metal alloys and

organic and inorganic compounds are applied by sputtering.

The substance to be coated is connected to a high voltage dcpower supply.

When the vacuum chamber has been pumped down, acontrolled amount of argon or another gas is introduced toestablish a pressure of about 10-2 to 10-3 torr.

On energizing current supply, plasma is established between

the work and the material to be coated.

The gas atoms are ionized, and they bombard the material tobe coated.

Sputtering

The energy of impinging ions cause atoms of the targetmaterial to be sputtered off, and they are transported throughthe plasma to form a coating.

Direct current sputtering is used when the target is electricallyconductive.Radio-frequency sputtering, which uses a RFpower supply is used when the target is a non conductor suchas polymer.

Sputtered coating processes produce microscopic modules of

diameter of several micrometers and they are called macros.

These macros are undesirable for metal to metal slidingsystems. On the other hand they are usually beneficial tocutting tools.


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Applications

The PVD coatings most widely used in machine design are titanium nitride

(TiN), titanium carbonitride (TiCN) and diamond like carbon.

The latter coatings are also called amorphous hydrogenated carbon coatings,since they have 30% hydrogen as this is produced from hydrocarbon gas.

These coatings are applied to punches, cutting tools etc. These are applied to giveextra hardness to a substrate.

Titanium nitride is gold in colour. The closed coatings are often preferred for many

tool applications.

Chemical Vapour Deposition


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Ion implantation

Basic terms in polymer science

Polymer : Polymers are complex and giant molecules which are

made from joining a large number of small and simple molecules

by primary valency linkage.

Monomer : The individual small and simple molecules from which

the polymer is formed are known as monomer.

Polymerization : The process by which the monomer molecules

are linked to form a big polymer molecule is called polymerization.


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Functionality: The number of bonding sites or active sites in a

monomer is called its functionality.

Degree of polymerization : The number of monomers forming thepolymer chain are called its degree of polymerization

Tacticity : The spatial arrangement of pendent groups of successive

stereocenters (asymmetric carbon) in the main chain is called its

tacticity.

Isotactic Polymer Syndiotactic Polymer

Atactic Polymer

One classification divides polymers in to condensation and

addition polymers and the other divides them in to step and chain

growth polymers.

Depending on their origin polymers are classified into natural

and synthetic polymers.

Depending on kind of atoms constituting backbone of the polymer

they are classified as organic and inorganic polymers.

Depending upon their ultimate use polymers are classified intoplastics, elastomers, fibres and liquid resins.

Classification


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A polymer which can be reshaped into hard and tough utility articles

by applying heat, pressure or both is said to be a plastics.

Examples : Polystyrene, Poly(vinyl chloride), Poly(methyl

methacrylate), polyester etc.

A polymer which can show good strength and elongatoan upon

vulcanization is called an elastomer.

Examples polyisoprene, polyisobutylene, etc.

A polymer which can be drawn into log tilament like material whose

length is at least 100 times at its diameter is called a fibre.

Examples nylon, terylene, polyester, polyacrylonitrile, etc.

A polymer used as adhesives, potting compounds, sealants etc in a

liquid form is called as liquid resin.

Examples epoxy adhesives, poly sulphides, sealants, etc.

Depending on the number of kinds of monomer used in

polymerization they are classified into homopolymer and copolymer.

Homopolymers are one in which only one kind of monomer is used

to prepare the polymer during polymerization.

Examples, polystyrene, polyacrylamide, etc.

Copolymer is one in which more than one kind of monomers are used

to prepare the polymer during polymerization. Examples poly(vinyl

chloride-co-vinyl acetate), poly(styrene-co--butadiene).

Further homopolymers are sub classified into linear, branched and

crosslinked polymer based on their chain configuration (i.e.

depending on their structure).


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Those polymers in which the monomer molecules have been linked

together in one continuous length to form the polymer molecules are

called linear polymers.

Branched polymer molecules are those in which there are side

branches of linked monomer molecules protruding from various

central branch points along the main polymer chain.


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Depending on number of kind of atoms forming three main

chain of the polymer they are classified as homochain polymer

and heterochain polymer.

A polymer in which main chain consist of only one kind of

atom is homochain polymer, while heterochain polymers

consist of more than one kind of atoms in their main chain.

Examples of homochain polymer are PE, PP, PS, PVC, etc.

Examples of heterochain polymer are PEO, pester,

nylon(polyamide) polyurethane, polydimethy isicoxane, etc.

Polymers which can be softened on heating and harden on coolingreversibly, i.e. their hardness is a temporary property subject to

change with rise or fall of temperature are called thermoplastics.

Repeated heating and cooling do not alter the chemical nature of

these materials, because the changes involved are purely of physical

nature.

Examples are polyethylene, polypropylene, poly(vinyl chloride),

polystyrene, nylons, polytetrafluro ethylene (PTFE or Teflon) etc.

Polymers which on heating get hardened and once they have

solidified, they cannot best softened, i.e. they are permanent settingresins are called thermosetting plastics.

Examples are polyesters, Bakelite and epoxy resins etc.


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Due to storing bonds and crosslinking they

are insoluble in almost all organic solvents.

They are usually soluble in some organic

solvents.

They cannot be reclaimed from wastes.These can be reclaimed from wastes.

They are usually, hand, strong and more

brittle.

They are usually soft, weak and less brittle.

They retain their shape and structure even on

heating. Hence they cannot be reshaped and

reversed.

By re-heating to a suitable temperature, they

can be softened, reshaped and thus reversed.

The cross-links and bonds retain their

strength on heating and hence, they do not

soften on heating on prolonged heating

charring of polymers is caused.

They soften on heating readily because

secondary forces between the individual

chain can break easily by heat or pressure.

They have three dimensional networkstructures, joined by strong covalent bonds.

They consist of long chain linear polymerswith negligible cross-links.

They are formed by condensation

polymerization.

They are formed by addition polymerization

only.

ThermosettingThermoplastic

Cellulose Derivatives

Polyethylene (PE)

Polyvinylchloride (PVC) TEFLON

Nylon


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Cotton fiber is mostly cellulose, and

cellulose is made of chains of the sugar,glucose linked together a certain way.


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Cellulose Derivatives

Cellulose derivatives Cellulose is a naturally occurring linear polymer

consisting of -glucose as repeating unit. -glucosehas three free OH groups, which may be fully or

partially substituted by chemical reactions.


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Cellulose acetate

Preparation

Cellulose acetate is obtained by treating cellulose withacetic anhydride or glacial acetic acid in the presenceof H2SO4.

The resulting cellulose triacetate is water insolubleand hence it is partially hydrolysed into cellulosediacetate. Cellulose diacetate is soluble in organicsolvents such as acetone.

[C6H7O2(OH)3]n + 3n CH3COOH [C6H7O2(OCOCH3)3]n +3n H2O

Cellulose triacetate

[C6H7O2(OCOCH3)3]n

Cellulose triacetate

+n H2O [C6H7O2(OH)(OCOCH3)2]n

Cellulose diacetate

+ n CH3COOH

Properties1. They are tough and hard materials

2. They have high tensile and dielectric strength

3. They are not affected by any mineral acids

Uses

1. They are used for the manufacture of radioappliances, windows, combs and non-inflammablecinematographic films, etc


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Addition polymerisation poly(ethylene)

C C

H

H

H

HC C

H

H

H

HC C

H

H

H

HC C

H

H

H

HC C

H

H

H

H

Five ethene monomer units:

C C

H

H

H

H

C C

H

H

H

H

C C

H

H

H

H

C C

H

H

H

H

C C

H

H

H

H

Two single electrons from

the broken double bond

An addition reaction begins when the double bond breaks on each monomer

n

Repeating unit of polymer

The polymer poly(ethylene) with five repeating units shown(n=5)

There are two kinds of polyethylene 1. Low Density Polyethylene (LDPE) 2. High Density Polyethylene (HDPE)


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Addition polymerisation- poly(styrene)

C C

H

C6

H5

H

HC C

H

C6

H5

H

HC C

H

C6

H5

H

HC C

H

C6

H5

H

HC C

H

C6

H5

H

H

Five styrene monomer units:

C C

H

C6H5

H

H

C C

H

C6H5

H

H

C C

H

C6H5

H

H

C C

H

C6H5

H

H

C C

H

C6H5

H

H




New carbon to carbon single bonds are formed joining the monomer unitstogether

n


The polymer poly(styrene) with five repeating units shown(n=5)

Addition polymerisation- poly(propene)

C C

H

CH3

H

H

C C

H

CH3

H

H

C C

H

CH3

H

H

C C

H

CH3

H

H

C C

H

CH3

H

HFive propene monomer units:

C C

H

CH3

H

H

C C

H

CH3

H

H

C C

H

CH3

H

H

C C

H

CH3

H

H

C C

H

CH3

H

H




New carbon to carbon single bonds are formed joining the monomer unitstogether

n


The polymer poly(propene) with five repeating unitsshown( n=5)


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A polymer made form just one monomer is polyethylene. It is themost common plastic you see.

It is used for bottles, buckets, jugs, containers, toys, evensynthetic lumber, and many other things.

Polyvinyl Chloride

Polyvinyl Chloride (PVC)

Preparation: Preparation of PVC involves the following two steps

I step: Vinyl chloride is prepared by treating acetylene with hydrogenchloride at 60-80oC in the presence of metal chloride at catalyst

II step: Polyvinylchloride is obtained by heating water emulsion of vinylchloride in presence of benzoyl peroxide (or) hydrogen peroxide underpressure

Acetylene

H Cl

hydrochloric acid+ Cl

Vinylchloride

Cl

Vinylchloride

Polymerisation

n- CH2 - CH -))

n

Polyvinylchloride

Cl


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Properties:

1. PVC is colourless, odourless and chemically inert powder

2. It is insoluble in inorganic acids and alkalis, but soluble in hot chlorinatedhydrocarbons such as ethylchloride

3. It undergoes degradation in presence of heat (or) light

Uses

1. It is used in the production of pipes, cable insulations, table covers and rain-

coasts etc.,

2. It is also used for making sheets, which are employed for tank-linings, light

fittings, refrigerator components, etc.,

Polyvinylchloride (PVC)

Nylon was discoveredNylon was discovered

in 1935. The namein 1935. The name

nylon is derived fromnylon is derived fromtwo cities where it wastwo cities where it was

discovered namelydiscovered namely

New York (NY) andNew York (NY) and

London (LON).London (LON).


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Diamine, NaOH, in H2O

Adipoyl chloride

in hexane

Nylon 6,6

Preparation of Nylon

We say certain polymers are man-made, but the truth is theymake themselves. Humans only have to get the ingredientsnear each other. The chemicals will assemble themselves.

Two ingredients are mixed and a solid begins to format the junction between the two layers of liquid.

Hot nylon spaghetti can be extracted.


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TEFLON

It is obtained by the condensation

polymerisation of phenol and formaldehyde

in the presence of acid or alkali catalyst

Preparation

The reaction involves the following 3 steps


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OH

Phenol

OH

Monomethylol phenol

CH2OH

CH2OH

CH2OHHOH2C

OH

CH2OHHOH2C

Dimethylol phenol

Trimethylol phenol

OH

HCHO

2HCHO

3HCHO

Methylolation

The first step isthe reactionbetween phenoland formaldehydeforms mono, diand tri-methylolphenols

When methylol phenols are heated with excessof formaldehyde in presence of alkaline catalyst.The methylol phenols condense either through

methylene linkages or through ether linkages to

form resoles. Resole is a low molecular weight linear polymer.

It is completely soluble in alkaline solution


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OH

CH2OHHOH2C

Dimethylol phenol

OH

CH2OHH

+

Monomethylol phenol

OH

CH2HOH2C

OH

CH2OH

Resole

H2O

Alkaline Catalyst

When methylol phenols areheated with excess ofphenol in presence of acidcatalyst, the methylolphenols condense withphenol through methylenelinkages to form novolacs.

Novolac is a highmolecular weight linearpolymer. It is insoluble inalkaline solutions.

OH

CH2OH

OH

H

+

Phenol

OH

CH2

OH

Novolac

H2O

Acid Catalyst

OH

CH2

Monomethylol phenol


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Thermo setting Plastics - Bakelite

Further heating of A-stageresin or B-stage resin orboth in the presence of acuring agent(hexamethylene tetramine)produces hard, rigid,

infusible, cross-linkedpolymer called bakelite

OH

CH2

OHOH

CH2

OH

CH2

OHOH

CH2

CH2 CH2CH2

Bakelite

Cooker with Bakelite Handles


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A Small Segment of Bakelite

EOS

Properties

1. Bakelite is resistant to acids, salts and most organicsolvents, but it is attacked by alkalis because of thepresence of OH groups

2. It possesses excellent electrical insulating property

Uses

1. Bakelite is used as an adhesive in plywoodlaminations & grinding wheels, etc

2. It is also widely used in paints, varnishes

3. It is used for making electrical insulator parts likeplugs, switches, heater handles, etc.,


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O = C

NH2

HCHO

2 HCHONHCH2OH

NH2

O = C

NH2

O = C

NHCH2OH

NHCH2OH

Monomethylol Urea

Dimethylol Urea

Amino Resins Urea Formaldehyde Resins

NHCH2OH

O = C

NHCH2OH

NHCH2OH

O = C

NHCH2OH

NHCH2OH

O = C

NHCH2OH

+ +

NCH2 CH2

C = O

N NCH2

NCH2 CH2NN CH2

C = O C = O

Urea-formald hyde resin


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Properties: Urea-formaldehyde resins give clear,water-white products of good tensile strength, goodelectrical insulation, good chemical-resistance,great hardness, great light-stability and goodabrasion-resistance

Uses

1. For bonding grinding wheels

2. Binder of glass fibres, rockwool etc

3. Bonding plywood

4. Electrical insulator5. Decorative articles like plates, drinking glasses,

dishes, etc.

A polymer is a high molar mass molecular compound made up

of many repeating chemical units.

Naturally occurring polymers

Proteins

Nucleic acids

Cellulose

Rubber

Synthetic polymers

Nylon

Dacron

Lucite

25.1


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Moulding Process

This process involves fabrication ofplastic material into desired shape underthe influence of heat and pressure in aclosed chamber. Some mouldingprocess is given below

1. Compression Moulding

2. Injection Moulding

3. Transfer Moulding4. Extrusion Moulding

Compression Moulding

This method is applied to both thermoplastics andthermosetting plastics

Figure shows a typical method used for compressionmoulding

The mould is made up of two halves, the upper and the lowerhalves.

The lower half usually contains a cavity in the shape of thearticle to be moulded.

The upper half has a projection, which fits into the cavity whenthe mould is closed.

The material to be moulded is placed in the cavity of the

mould. Then the mould is closed carefully under low pressure Finally the mould is heated to 100-200o C and simultaneously

high pressure (100-500 kg/cm2) is applied on the top of themould.

Curing is done either by heating or cooling. After curing themoulded article is taken out by opening the mould parts.


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Compression Moulding

Compression Moulding Machine


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Injection Moulding

1. This method is mainly applicable to thermoplastics.

2. The powdered plastics material is fed into a heated cylinderthrough they hopper (Fig).

3. The plastic material melts under the influence of heat andbecomes fluid.

4. The hot fluid is injected at a controlled rate into a tightly lockedmould by means of a screw arrangement or by a piston

5. The mould is kept cold to allow the hot plastic to cure andbecomes rigid. After curing the mould is opened and the objectis ejected.

6. Telephones, buckets etc., are made by this method.

Advantages

1. Low mould cost2. Low finishing cost3. Low loss of materials4. High speed production

Injection Moulding


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Extrusion Moulding

Extrusion Moulding Machine


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Transfer Moulding1. This method is used for thermosetting plastics

2. The principle is same as that of the injection moulding

3. The powdered moulding materials is taken in a heatedchamber, maintained at low temperature, at which thematerial just begins to become plastic.

4. This plastic is then injected through an orifice into the mouldby a plunger working at high pressure (Fig)

5. Due to the great friction developed at the orifice duringejection, the temperature of the material rises to such anextent that the moulding powder becomes almost liquid. Sothat it flows quickly and easily into the mould.

6. Then the mould is heated upto the curing temperaturerequired for setting. Finally the moulded article is ejectedfrom the mould.

Transfer Moulding


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Transfer Moulding Machine

Advantages of Transfer Moulding

1. More complicated shapes can be fabricatedby this method

2. Less expensive

3. Blisters can be eliminated

4. Shrinkage and distortion are minimum

5. Very delicate articles can be made by thismethod


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Degradable Polymers

Biodegradable Polymers

The biodegradation of polymer proceeds by hydrolysis andoxidation.

The presence of hydrolysable and oxidizable linkages in thepolymer main chain, the presence of suitable substituents, correctstereoconfiguration, balance of hydrophobicity andhydrophilicity and conformational flexibility contribute to thebiodegradability of the polymer.

Biodegradable polymers may be divided into three classes.

They are natural polymers (eg.Protein),biosynthetic polymers(eg. Poly-hydroxy alkanoates) and synthetic polymers(polycaprolacyone and polylactic acid).


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It is a complex constituent of wood connecting cellulose fibers together.

It is brown in colour and largely responsible for the strength and rigidity of

plants.

It is amorphous, aromatic biopolymer and it is obtained from almost all types ofnatural wood based resources.

It is byproduct of pulp and paper mills and is conveniently treated as waste materialhaving low economical usage.

It is separated from the cellulose conventionally either by strong alkaline or acidic

solutions or by high pressure steam treatment followed by solvent extraction.

Unmodified lignin has poor solubility and a thermoplastic melt having flow

characteristic like cellulose.

One method adopted for improving the properties and use of lignin as athermoplastic polymer is esterification.

Lignin was found to be useful as a mould lubricant.

Moreover, lignin and its derivatives are bio degradable.

Lignin

Conducting Polymers

Schematic of PANi/PCL interpolymeric complex,

with PHBSA as the bifunctional linker


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Structural formula of undoped PANi (EB)

PANi exists in three forms of oxidation states:

Leucoemeraldine (fully reduced or only benzenoid amine structures),

Emeraldine (neutral or partially reduced and partially oxidized), and

Pernigraniline (fully oxidized or only quinoid imine structures).

Only doped EB PANi is conductive among the three oxidation states.

The emeraldine-based form of PANi is also the most stable of the

three states because leucoemeraldine is easily oxidized when exposed

to air and pernigraniline is easily degraded.

Pure PANi, in the undoped state, is a poor semiconductor with conductivity of about 10-8 S/cm.

However, once it is doped, its conductivity could increase by a factor of 10 S/cm or more

depending on the dopant.

Doping with acid such as PHBSA can increase conductivity because doping forms a

polaron/bipolaron structure that will increase PANi's charge due to increased relocalization.


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Bipolaron structure of PANi EB

Polaron structure of PANi EB

APPLICATIONS OF CONDUCTING POLYMERS


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Group 1 Group 2Electrostatic materials Molecular electronics

Conducting adhesives Electrical displaysElectromagnetic shielding Chemical, biochemical and thermal sensors

Printed circuit boards Rechargeable batteries and solid electrolytesArtificial nerves Drug release systems

Antistatic clothing Optical computers

Piezoceramics Ion exchange membranes

Active electronics(diodes, transistors) Electromechanical actuators

Aircraft structures 'Smart' structures and Switches

APPLICATIONS OF CONDUCTING POLYMERS

The first group utilizes their conductivity as its main property.

The second group utilizes their electroactivity


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Fuels and Combustion Fuel is a combustible substance, which on combustion

produces a large amount of heat, which can be used forvarious domestic and industrial purposes.

The process of combustion involves oxidation of carbon,hydrogen etc. of the fuels to CO2, H2O, and the difference inthe energy of reactants and the products are liberated as largeamount of heat energy which is utilized.

Fuel + O2 Products + Heat

The primary or main source of fuels are coal and petroleumoils, the amounts of which are dwindling day by day. Theseare stored fuels available in earth's crust and are generallycalled "fossil fuels".

Classification of fuels


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Calorific value of fuels

The most important property of fuel to be taken into

account is its calorific value or the capacity to supply

heat. The calorific value of a fuel can be defined as

"the total quantity of heat liberated when a unit mass or

volume of the fuel is burnt completely".

Units of heat

1. Calorie - Calorie is the amount of heat

required to raise the temperature of one gram

of water through one degree centigrade.

2. Kilocalorie (or) kilogram centigrade unit -

This is the unit of metric system and is equal

to 1000 calories. This may be defined as

"the quantity of heat required to raise thetemperature of one kilogram of water

through one degree centigrade".

Thus 1 kcal = 1000 cal.


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Units of heat

3. British thermal unit (B. Th. U.) - This is

defined as "the quantity of heat required toraise the temperature of one pound of waterthrough one degree Fahrenheit". This isEnglish system unit.

1 B. Th. U. = 252 cal = 0.252 k cal.

1 k cal = 3.968 B. Th. U.

4. Centigrade Heat Unit (C. H. U.) - This is the"quantity of heat required to raise the

temperature of one pound of water throughone degree centigrade".

Thus, 1 k cal = 3.968 B. Th. U. = 2.2 C. H. U.

Higher or Gross Calorific Value (HCV or GCV)

Usually, all fuels contain some hydrogen and when thecalorific value of hydrogen containing fuel is determinedexperimentally, the hydrogen is converted to steam.

If the products of combustion are condensed to roomtemperature (15C or 60F), the latent heat of condensationof steam also gets included in the measured heat, which isthen called "higher or gross calorific value".

So gross or higher calorific value may be defined as "thetotal amount of heat produced when one unit mass/volumeof the fuel has been burnt completely and the products ofcombustion have been cooled to room temperature".


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Lower or Net Calorific Value

In actual use of fuel, the water vapour and moisture etc arenot condensed and escapes as such along with hotcombustion gases. Hence a lesser amount of heat isavailable. So, net or lower calorific value may be defined as"the net heat produced when unit mass / volume of the fuel isburnt completely and the products are permitted to escape".

Net or lower calorific value can be found from GCV value

NCV = GCV - Latent heat of water vapour formed

= GCV - Mass of hydrogen x 9 x latent heat of steam

1 part by mass of hydrogen produces 9 parts by mass ofwater. The latent heat of steam is 587 k cal / kg or 1060 B.Th. U. /lb of water vapour formed at room temperature. (ie15C).

Determination of Calorific Value

Bomb calorimeter


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Corrections

Dulongs Formula

The approximate calorific value of a fuel can be determined by knowing theamount of constituents present:

Gross or higher calorific value (HCV) from elemental constituents of a fuel.

H = 34500 kcal/kg; C = 8080 kcal/kg; S = 2240 kcal/kg

Oxygen present in the fuel is assumed to be present as water (fixed

hydrogen).

Available Hydrogen = Total hydrogen - Fixed hydrogen

= Total hydrogen - 1/8 mass of oxygen in fuel.

Dulongs formula for calorific value from the chemical composition of fuel

is,


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Knocking

In an internal combustion engine, a mixture of gasoline vapour and air

is used as a fuel.

After the initiation of the combustion reaction by spark in the

cylinder, the flame should spread rapidly and smoothly through the

gaseous mixture, thereby the expanding gas drives the piston down

the cylinder.

The ratio of the gaseous volume in the cylinder at the end of the

suction-stroke to the volume at the end of compression-stroke of the

piston is known as the 'compression ratio'.

The efficiency of an internal combustion engine increases with the

compression ratio.

Compression ratio (CR) is defined as the ratio of the cylinder volume(V1) at the end of the suction stroke to the volume (V2) at the end ofthe compression stroke of the piston.

V1 being greater than V2, the CR is >1.

The CR indicates the extent of compression of the fuel-air-mixture bythe piston.

However, successful high compression ratio is dependent on thenature of the constituents present in the gasoline used.

In certain circumstances, due to the presence of some constituents in

the gasoline used, the rate of oxidation becomes so great that the lastportion of the fuel-air mixture gets ignited instantaneously producingan explosive violence, known as 'knocking'.

The knocking results in loss of efficiency, since this ultimatelydecreases the compression ratio.


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Chemical structure and knocking

The tendency of fuel constituents to knock is in the

following order. Straight - chain paraffins > branched - chain

paraffins (isoparaffin) > olefins > cyclo paraffins (naphthenes)

> aromatics.

Octane number

The most common way of expressing the knocking

characteristics of a combustion engine fuel is by 'octane

number', introduced by Edger. It has been found that n-

heptance, knocks very badly and hence, its anti-knock value

has arbitrarily been given zero.


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Octane number is equal to the percentage by volume of iso-

octane (2,2,4-trimethyl pentane) in a mixture of n-heptane andiso-octane having the same knocking tendency compared to thesample of gasoline being tested;

Iso-octane has the best antiknocking properties and assigned anoctane number of 100 whereas n-heptane has poor antiknockingproperty and assigned an octane number of zero.

The hydrocarbons present influence the knocking properties ofgasoline which vary according to the series:

straight chain paraffin > branched chain paraffin > olefin >

cycloparaffin > aromatics.

The fuel which has same knocking tendency with the mixturehaving 80% iso-octance has octane number 80.

Octane number

C CCCCC CH H

H HHHHHH

H HHHHHH

n-heptane

CCH3

CH3

CH3

CH2

CH3

CH CH3

Isooctane


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Improvement of anti-knock characteristics of a fuel

The octane number of many otherwise poor fuels can be raised by theaddition of tetra ethyl lead (C2H5)4Pb or TEL and diethyl telluride

(C2H5)2Te. In motor spirit (Motor fuel) about 0.5ml and in aviation fuel1.0 - 1.5ml of TEL is added per litre of petrol.

TEL is converted into a cloud of finely divided lead and lead oxideparticles in the cylinder and these particles react with any hydrocarbonperoxide molecules formed, thereby slowing down the chain oxidationreaction and thus decreasing the chances of any early detonation.

However deposit of lead oxide is harmful to the engine life. In order tohelp the simultaneous elimination of lead oxide formed from theengine, a small amount of ethylene dibromide (or ethylene dichloride)is also added to petrol.

The added ethylene dibromide removes lead oxide as volatile leadbromide along with the exhaust gases. The presence of sulphurcompounds in petrol reduces the effectiveness of the TEL. TEL ismore effective on saturated hydrocarbons than on unsaturated ones.

Other additives

Oxidation inhibitors - 2,4 - ditertiary butyl - 4 - methyl phenol.

Rust inhibitors - Organic compounds of phosphorus or

antimony.

Ignition control additives - tricresyl phosphate which

suppresses pre-ignition of the fuel due to glowing deposits on

spark plug or a hot spot on the cylinder wall.


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Diesel Engine Fuels

Characteristics of an ideal diesel oil

It should have low spontaneous ignition

temperature.

It should have very little sulphur, aromatic and

ash content.

The ignition lag should be as short as possible.

Knocking In a diesel engine, the fuel is exploded not by a spark, but by the

application of heat and pressure. In the cycle of operations of adiesel engine, air is first drawn into the cylinder and compressed.

Towards the end of the compression stroke, the fuel (diesel oil)is injected as a finely-divided spray into air in the cylinder heatedto about 500C by compression.

The oil absorbs the heat from the air and if it attains its ignitiontemperature the oil ignites spontaneously. The pressure of thegases is further increased by the heat accompanying the ignitionof the oil.

The piston is pushed by the expanding gases and this constitutesthe power stroke.

Fuel feed and ignition continue during this down stroke. Thefuel injection stops at the exhaust stroke.


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The combustion of fuel in a diesel engine is not instantaneous and theinterval between the start of fuel injection and its ignition is called the'ignition delay' and is an important quality of the diesel fuel.

This delay is due to the time taken for the vaporization of the individualdroplets and raising of the vapour to its ignition temperature.

It depends on the engine design, efficiency of mixing of the spray andair, the injector design and mostly on the chemical nature of the fuel.

The ignition delay is shorter for paraffinic fuels than for olefinic,naphthenic and aromatic fuels. Fuels with low carbon residue aredesirable.

Long ignition delays lead to fuel accumulation in the engine evenbefore the ignition and when ignited, an explosion results as thecombined effect of increased temperature and pressure. This isresponsible for the diesel knock.

The diesel fuel should have a spontaneous ignition temperature less thanthe temperature produced by compression.

Diesel engine fuels consist of longer chain hydrocarbons than internal

combustion engine fuels.

The main characteristic of diesel engine fuel is that it should easilyignite below compression temperature.

There should be as short an induction lag as possible. This means thatit is essential that the hydrocarbon molecules in a diesel fuel should beas far as possible the straight-chain ones with a minimum admixture ofaromatic and side-chain hydrocarbon molecules.

The suitability of a diesel fuel is determined by its cetane value, whichis the percentage of hexadecane in a mixture of hexadecane and 2-

methyl naphthalene, which has the same ignition characteristics as thediesel fuel sample, under the same set of conditions.

The cetane number of a diesel fuel can be raised by the addition ofsmall quantity of certain "pre-ignition dopes" like alkyl nitrites such asethyl nitrite, iso-amyl nitrite, acetone peroxide.


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CH3

H HH

H HH

H C CC H14

2-Methyl naphthalene (cetane No.=0)

n-hexadecane (cetane No.=100)

Ignition quality decreases among

hydrocarbons is as follows

n-alkanes > naphthalenes > alkenes > branches

alkanes > aromatics

Cetane number decreases

Ignition quality decreases

Ignition delay increases


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Diesel - Index

On API (American petroleum Institute) scale, the

quality of a diesel fuel is, sometimes, indicated by

diesel-index number which is

Specific gravity (API) X Aniline Point (F)

D. I= -----------------------------------------

100

The higher the diesel-index number the better isthe diesel fuel. D. I. = Cetane number + 3.

Liquified Petroleum Gas (LPG) or Bottled

Gas or Refinery Gas

It is obtained as a by-product during the cracking of heavy oils orfrom natural gas.

LPG is dehydrated desulphurised and traces of odorous organicsulphides (mercaptans) are added to give warning of gas leak.

LPG is supplied under pressure in containers under the trade namelike Indane, Bharat gas.

Calorific value - 27,800 k.cal/m3

Composition - n-butane, iso-butane, butylene and propane with littleor no propylene and ethane.

Uses - Domestic, Industrial fuel, Motor fuel.


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Natural Gas Natural gas is generally associated with petroleum deposits and is

obtained from wells dug in the oil-bearing regions.

A natural gas containing mainly methane but not higherhydrocarbons is said to be lean or dry gas whereas that containingappreciable amounts of propane, butane and other liquidhydrocarbons like pentane, hexane etc is called rich or wet gas.

Harmful H2S gas if present in natural gas, is removed by squbbingwith monoethanolamine

(NH2CH2CH2OH) 2HO.CH2.CH2.NH2 + H2S

(HO CH2 CH2 NH2)2.H2S , on heating H2S gas is liberated.

Calorific value- 12,000 - 14,000 k.cal/m3

Composition - CH4 = 70 - 90%

C2H6 = 5 - 10%

H2 = 3%

CO + CO2 = rest

Uses - It is an excellent domestic fuel, used in manufacture of

a number of chemicals by synthetic processes.It is the raw

material for the manufacture of carbon black and hydrogen.Synthetic proteins have been obtained by microbiological

fermentation of methane.

Natural Gas


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Biogas Aquatic plants, organic wastes from domestic, agricultural and

industrial sectors withhigh B.O.D. value (Feed stock) are digested

anaerobically to produce biogas.

The biogas is totally used as fuel. The chief constituent of biogas ismethane, so the process is also called biomethanation.

Composition of biogas is given below:

Component Volume%

CH4 52-95

CO2 9-45

H2S 0.001-2

H2 0.01-2

N2 0.1-4O2 0.02-6.5

CO 0.001

NH3 Small

Conditions for Biomethanation

Temperature = 35

pH = 6.8-8.2

Anaerobic condition.

Trace elements = Na+, Co+3, Ni+2 etc.

Arrangements for Biomethanation

Feed stock is mainly cowdung.

Biogas


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Biodiesel

What is bio-diesel?

Biodiesel refers to any diesel-equivalent Biofuel madefrom renewable biological materials such as vegetableoils (Vegetable Oil) or animal fats (Animal Fats).

While there are numerous interpretations being applied tothe term biodiesel, the term Biodiesel usually refers to anester, or an oxygenate, made from the oil and methanol(in other words, the name biodiesel can be applied toany transesterified vegetable oil that makes it suitable foruse as a diesel fuel).

Technically, as mentioned earlier, biodiesel is vegetable oil methylester or in general the biodiesel consists of mono alkyl-esters.

It is usually produced by a Transesterification and esterificationreaction of vegetable or waste oil respectively with a low molecularweight alcohol, such as Ethanol and methanol.

During this process, the triglyceride molecule from vegetable oil isremoved in the form of glycerin (soap). Once the glycerin isremoved from the oil, the remaining molecules are, to a dieselengine, somewhat similar to those of petroleum diesel fuel.

While the petroleum and other fossil fuels contain sulfur, ringmolecules & aromatics, the biodiesel molecules are very simplehydrocarbon chains, containing no sulfur, ring molecules oraromatics.

Biodiesel is thus essentially free of sulfur and aromatics. Biodiesel ismade up of almost 10% oxygen, making it a naturally "oxygenated"fuel.


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Electrochemical energy

systems

Batteries

An Electrochemical cell

An electrochemical cell is a device in which aredox reaction is utilized to get electricalenergy.

Commonly referred to as voltaic or galvaniccell.

The electrode where oxidation occurs iscalled anode while the electrode wherereduction occurs is called cathode.


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Nernst equation

Batteries

Batteries use a chemical reaction to do work on charge andproduce a voltage between their output terminals.

The basic element in a battery is called an electrochemical celland makes use of an oxidation/reduction reaction.

An electrochemical cell which produces an external current iscalled a voltaic cell. Voltages generated by such cells havehistorically been referred to as emf (electromotive force).

Batteries are devices where several electrochemical systemsare connected together in series.

And can store chemical energy for later release as electricity

It is a source of direct electric current at a constant voltage.


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Types of

batteries

Primary battery (Primary cells) in which the cell reaction is notreversible. When all the reactants have been converted toproduct, no more electricity is produced and the battery isdead.

Secondary battery (secondary cells) in which cell reactionscan be reversed by passing electric current in the oppositedirection. Thus it can be used for a large number of cycles.

Flow battery and fuel cell in which materials (reactants,products, electrolytes) pass through the battery, which issimply an electrochemical cell that converts chemical toelectrical energy.

Primary batteries

Dry or lachlanche cell

Alkaline battery

Lithium batteries


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DRY(or LACLANCHE) CELL

The venerablecarbon-zinc cellor Lechlanche' cellwas invented in1866 by Georgeslachlanche andwas the mostcommon smallbattery throughoutmost of the 20th

century

Dry cell contains Zn, NH4Cl, ZnCl2 and MnO2

Anodic reaction Zn(s) -> Zn2+(aq) + 2e-

Cathodic reaction

2NH4+(aq) + 2MnO2(s) + 2e- -> Mn2O3(s) + H2O(l) + 2NH3(aq)

Some of the complexity of this reaction comes from the fact that thereduction of the ammonium ion produces two gaseous products

2NH4+(aq) + 2e- -> 2NH3(g) + H2 (g)

which must be absorbed to prevent the buildup of gas pressure.

ZnCl2 (aq) + 2NH3 (g) -> Zn(NH3)2Cl2 (s) 2MnO2 (s) + H2(g) -> Mn2O3(s) + H2O(l)


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Applications and

disadvantages

Used in flash lights, transistor radios, calculators etc

Disadvantages of dry cell

The voltage of this cell is initially about 1.5 volts, but decreases as energyis taken from the cell. Due to the accumulation of the products onelectrodes. It also has a short shelf life and deteriorates rapidly in coldweather.

Oxidation of the zinc wall eventually causes the contents to leak out, sosuch batteries should not be left in electric equipment for long periods.

While these batteries have a long history of usefulness, they are decliningin application since some of their problems are overcome in ALKALINEBATTERIES.

ALKALINE DRY

CELLS

Alkaline cells overcome some of the problems

with carbon-zinc batteries by using potassium

hydroxide in place of ammonium chloride in the

electrolyte.

Potassium hydroxide is a base or alkaline material,

hence "alkaline" batteries. The active materialsused are the same as in the Leclanch cell zinc

and manganese dioxide.


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Chemistr

y The zinc anode does not form the container; it is in the form of a

powder instead, giving a large surface area. The following half-cellreactions take place inside the cell:

At the anode: Zn + 2OH Zn(OH)2 + 2e

At the cathode:

2MnO2 + H2 O + 2eMn2 O3 + 2OH

Overall: Zn + 2MnO2 + H2 O Zn(OH)2 + Mn2 O3

Construction : This cell is inside out compared to the Leclanch cell

These cells have much longer shelf life and perform better under drainand in cold weather.

They avoid the use of the zinc-corroding ammonium ions and do notproduce any gaseous products.


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Advantages and Uses

Zinc does not dissolve as readily in alkaline

medium

Long life

Used in calculators and watches

Lithium batteries

Main attractions of lithium as an anode material is its position asthe most electronegative metal in the electrochemical seriescombined with its low density, thus offering the largest amount ofelectrical energy per unit weight among all solid elements.

Li cannot be used with the traditional aqueous electrolytes due tothe very vigorous corrosive reaction between Li and water withflammable hydrogen as the product.

In the 1980s progress was made in the use of Li as an anodematerial with MnO2, liquid SO2 or thionyl chlorides as the

cathode, and hexaflurophosphate dissolved in propylene carbonateas a typical organic electrolyte.

Li cells are generally properly sealed against contact with air andmoisture Whilst the primary lithium battery has been wellestablished for nearly two decades,


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SystemNominal Cell Voltage (V) AdvantagesDisadvantages Applications

Li/SOCl2 3.60 High Energy density; long shelf life. Onlylow to moderate rate applications. Memory devices; standbyelectrical power devices

Li/SO2 3.00 High energy density; best low-temperatureperformance; long shelf life. High-cost pressurized system,Military and special industrial needs

Li/MnO2 3.00 High energy density; good low-temperatureperformance; cost effective. Small in size, only low-drainapplications, Electrical medical devices; memory circuits;

Chemistr

y The cell is represented asLi/Li+(nonaqueous)/KOH(paste)/MnO 2,Mn(OH)2,C.

The anode is lithium. The cathode is carbon in contact withmanganese (III), Manganese(IV) electrode. The electrolyte is a pasteof aqueous KOH

At anode

Li --- Li+ + e-

At cathode

MnO2+2H2O+ e- ---Mn(OH)2 + OH-

The overall reaction is

Li + MnO2+ 2H2O -- Li+ + Mn(OH)3 + OH-


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Advantages and uses

High electron density

Long shelf life

Low self discharge

Need less maintenance

Can provide very high current

Uses

Used in auto focus cameras

RECHARGEABLE

BATTERIES Lead-acid batteries, invented in 1859 by French

physicist Gaston Plante, are the oldest type of

rechargeable battery.


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Lead-Acid Battery

Batteries use a chemical reaction to do work on charge and produce a

voltage between their output terminals.

Discharging


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When the storage cell is operating as a voltaic cell it issaid to be discharging

In the discharged state both electrodes turn into lead(II)sulfate (PbSO4) and the electrolyte loses its dissolvedsulfuric acid and becomes primarily water.

Due to the freezing-point depression of water, as thebattery discharges and the concentration of sulfuric aciddecreases, the electrolyte is more likely to freeze duringwinter weather.

In the charged state, each cell contains electrodes ofelemental lead (Pb) and lead (IV) dioxide (PbO2) in anelectrolyte of approximately 33.5% v/v (6 Molar)sulfuric acid (H2SO4).

Charging the Lead-Acid Battery

The discharge reaction can be reversed by applying a voltage

from a charging source.

During discharge process the concentration of sulfuric acid

decreases and while charging the concentration may attain their

initial values.

In the charged state, each cell contains electrodes of elemental

lead (Pb) and lead (IV) dioxide (PbO2) in an electrolyte ofapproximately 33.5% v/v (6 Molar) sulfuric acid (H2SO4).


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Overcharging

Because of the open cells with liquid electrolytein most lead-acid batteries, overcharging with

high charging voltages will generate oxygen and

hydrogen gas by electrolysis of water, forming

an explosive mix. The acid electrolyte is also

corrosive.

Uses

For constant power supply for electrical vehicles,

gas engine ignition,

in telephone exchangers,

trains,

mines,

laboratories,

hospitals,automobiles and in power stations


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Nickel Cadmium battery

The nickel-cadmium battery (commonlyabbreviated NiCd or nicad) is a type ofrechargeable battery using nickel oxidehydroxide (NiOOH) and metallic cadmium (Cd)as electrodes and an alkaline potassiumhydroxide KOH electrolyte.

The first NiCd battery was created by ofSweden in 1899. But only introduced in the early1960's

Characteristics

1.4 Volt

Energy density about double that of lead acid

batteries.

Their small size and high rate discharge capacity

made portable tools and other consumerapplications practical for the first time.


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Chemistry

At anode

Cd + 2OH- --Cd(OH)2 +2e-

At cathode

2NiO(OH) +2H2O +2e- --2Ni(OH)2+2OH-

Net reaction

2NiO(OH) + Cd +2H2O-

Cd(OH)2 + 2Ni(OH)2

Advantages

Constant voltage(1.4V)

No gaseous products

Wide temperature range (Up to 70C)

Charging process is strongly endothermic-the battery coolsduring charging. This makes it possible to charge veryquickly

Rapid charge typically 2 hours, but can be as low as 10 to

15 minutes.

The sealed nickel-cadmium cell can be stored in thecharged or discharged state without damage.

Available in a large variety of sizes and capacities


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Applications

Motorised equipment

Power tools

Transistors

Electronic calculators

Commercial and industrial portable products

Medical instrumentation

Emergency lighting

Toys

Cordless and wireless telephones

Disadvantages

NiCad batteries are also prone to damage byovercharging.

Low cell voltage of 1.4 Volts compared with primaryalkaline cells 1.5 Volts and only quarter of the capacityof the alkaline cells.

Self re-sealing safety vents must be incorporated to

prevent damage due to overheating and pressure buildup.

The use of Cadmium in consumer products is nowdeprecated on environmental grounds.


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Problem with NiCad Overcharging must be considered in the design of most

rechargeable batteries. In the case of NiCds, there are twopossible results of overcharging:

If the negative electrode is overcharged, hydrogen gas isproduced.

If the positive electrode is overcharged, oxygen gas isproduced.

NiCd cells are vented, with seals that fail at high internal

gas pressures. The sealing mechanism must allow gas toescape from inside the cell, and seal again properly whenthe gas is expelled. This complex mechanism, unnecessaryin alkaline batteries, contributes to their higher cost.

Nickel Metal Hydride Batteries

Their basic structure is identical to that of Ni-Cd

The NiMH battery was patented in 1986 by StanfordOvshinsky.

Now NiMH batteries have begun to find use in highvoltage automotive applications. The energy density ismore than double that of Lead acid and 40% higher than

that of NiCads


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Anode

Instead of cadmium, hydrogen is used as the

active element at a hydrogen-absorbing negative

electrode (anode).

This electrode is made from a metal hydride

usually alloys of Lanthanum and rare earths that

serve as a solid source of reduced hydrogen that

can be oxidized to form protons.

Components

The components of NiMH batteries include acathode of Nickel-hydroxide, an anode ofHydrogen absorbing alloys

Potassium-hydroxide (KOH) electrolyte.

They are more expensive than Lead-acid andNiCd batteries, but they are considered better

for the environment. The electrolyte is alkaline potassium hydroxide.

Cell voltage is 1.2 Volts


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Advantages

High energy density

Rapid charge possible in 1 hour

overcharging can cause deterioration of the battery. Chargersshould therefore incorporate a timer to prevent overcharging.

Because of potential pressure build up due to gassing theyusually incorporate a re-sealable vent valve

Reconditioning is possible.

Environmentally friendly (No cadmium mercury or lead)

Applications

Low cost consumer applications

Electric razors

Cameras

Mobile phones

Pagers

Medical instruments and equipment

Automotive batteries


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Lithium batteriesLithium batteries

Lithium batteriesLithium batteries The main attractions of lithium as an anode

material is It is the most electronegative metal in the

electrochemical series It has very low density, Means, the largest amount of electrical energy

per unit weight

But Li cannot be used with the traditionalaqueous electrolytes due to the very vigorous corrosive reaction

between Li and water with flammable hydrogen as the product.


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Li batteriesLi batteries

In the 1980s progress was made in the use of Lias an anode material with MnO2, liquid SO2 orthionyl chlorides as the cathode, andhexaflurophosphate dissolved in propylenecarbonate as a typical organic electrolyte.

Li cells are generally properly sealed againstcontact with air and moisture

Cathode materials

The most common compounds used forcathode materials are LiCoO2, LiNiO2 andLiMn2O4.

Of these, LiCoO2 has the bestperformance but is very high in cost, istoxic and has a limited lithium contentrange over which it is stable.

LiNiO2 is more stable, however the nickelions can disorder.

LiMn2O4 is generally the best value formoney, and is also better for theenvironment.


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Anode material andelectrolyte

The anode material is carbon based, usually withcomposition Li0.5C6. This lithium content is lower than would be ideal,

however higher capacity carbons pose safety issues.

Electrolyte Since lithium reacts violently with water, and the cell

voltage is so high that water would decompose, anon-aqueous electrolyte must be used.

A typical electrolyte is LiPF6 dissolved in an ethylene

carbonate and dimethyl carbonate mixture.

Discharging

The following reactions take place upon discharge: At the anode: LixC6 xLi+ + 6C + xe-

At the cathode: xLi+ + Mn2O4 +xe- LixMn2O4 Overall: LixMn2O4 + 6C LixC6 + LixC6


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Chemistry and construction Anode here is a non-metallic compound, e.g. carbon,

which can store and exchange lithium ions.

A lithium ion-accepting material, for example CoO2,

is then used as the cathode material, and lithium ions

are exchanged back and forth between the two during

discharging and charging. These are called

intercalation electrodes.

This type of battery is known as a rocking chairbattery as the ions simply rock back and forth

between the two electrodes.


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Charging Discharging

Lithium ion Cells

Applications: Laptops, cellular phones, electric vehicles

Anode: lithium ions in the carbon material

Cathode: lithium ions in the layered material (lithium compound)

Cathode

LiCoO2+ Cn Li1-XCoO2 + CnLix

Anode

Li1-XCoO2+ CnLix LiCoO2 + Cn

The lithium ion moves from the anode to the cathode during

discharge and from the cathode to the anode when charging.

Exploded laptop

Lithium Polymer batteries are better than Lithium ion batteries

Li-ion batteries use organic solvents to suspend the lithium ions.

In situations where the structure of the battery is compromised, that

solvent can ignite and vent from the pressurized battery. The result is a dangerous explosion

The main advantage of Li-poly batteries that has been discussed inthe press recently is their reluctance to explode under duress


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Lithium Polymer BatteryLithium Polymer Battery

Electrolyte is a polymer

Lithium polymer (Poly-Carbon Monofluoride)

batteries Charging


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Lithium battery-battery of the

future

Imagine your electrical equipment powered byLithium ion battery refuse to die out of charge.

Imagine your 2 hour battery backup of yourlaptop increases to 20 hours.

6 days of standby time of cell phones increases to60 days i.e 2 months ( charging a mobile just 6

times a year !!!

Now !! stop imagining because its going to happenvery soon. scientists from Stanford University have

found a way to use silicon nanowires to produces 10

times the amount of electricity of existing lithium-

ion, known as Li-ion, batteries that power laptops,

iPods, video cameras, cell phones, and countless

other devices. A laptop that now runs on battery

for two hours could operate for 20 hours.


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Solid Oxide Fuel Cells

Anode Reaction:2 H2 + 2 O2- => 2 H2O + 4 e-

Cathode Reaction:O2 + 4 e- => 2 O2-

Overall Cell Reaction:2 H2 + O2 => 2 H2O


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Engineering Chemistry All Units

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Transcript of Engineering Chemistry All Units