All chapters of engineering chemistry

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EN 14 104 Engineering Chemistry (Common for all branches)

Module I

Organo Metallic Compounds – Definition – Classification based on the nature of Metal-Carbon

bond. Metal carbonyls – 18 electron rule – Mononuclear and polynuclear carbonyls (give e.gs of Fe, Co, Ni)

Bio-Inorganic chemistry – Metal ions in biological system – Trace and Bulk metal ions – Hemoglobin

and myoglobin (elementary idea only)

Green Chemistry – Goals of green chemistry – Limitations Twelve principles of green chemistry

with their explanations and examples – Designing a green synthesis – Prevention of waste / byproducts – Atom economy (maximum incorporation of materials used in the process) – Minimization of hazardous / toxic products – prevention of chemical accidents – Green synthesis

Module II

Polymers – Classification – Types of polymerization → Addition, Condensation,

Coordination polymerization and Co-polymerisation – Polymerisation techniques → Bulk, Solution, Suspension and Emulsion – Concept of Tg and Factors affecting Tg – Crystallinity in polymers – Physical and Mechanical properties → Density, Tensile, Tear, Abrasion resistance and Resilience

Lubricants – Theories of friction – Mechanism of lubrication → Thick film, Thin film and Extreme

pressure – Classification → Solid, Liquid, Semisolid – Properties → Viscosity, Flash point, Fire point, Cloud and Pour point, Aniline point and Corrosion stability

Fuels – Classification – Calorific Value – Cracking and Reforming – Petrol Knock and Octane

number – Diesel knock and Cetane number – Bio-Diesel

Module III

Electrochemistry – Single electrode potential – Helmholtz double layer –

Nernst equation – Derivation – Types of electrodes (S.H.E, Calomel, Quinhydrone, glass electrode), pH measurements using glass electrode, Electrochemical cells, Concentration cells - salt bridge – emf measurement – Poggendorf’s compensation method – Electrochemical series – Applications

Storage cells – Lead acid accumulator – Alkaline cells – Nickel Cadmium –Fuel cells – H2/O2 fuel

cell – Solar cells

Module IV

Corrosion and its control – Theories of corrosion – Dry corrosion and Wet corrosion – Galvanic

series - Corrosion of iron in acidic, neutral and basic conditions – Differential aeration corrosion, Stress corrosion – Galvanic corrosion – Factors influencing corrosion. Corrosion control methods – Protection by sacrificial anode – Impressed current - self protecting corrosion products – Pilling Bed worth rule

Coatings – Organic (Paints and Polymers) Inorganic – Metallic (galvanizing, tinning, electroplating,

cementation) – Nonmetallic (phosphate, chromate, anodising, chemical oxide)

Water – Hardness – alkalinity – Determination of hardness (EDTA method) –

Softening → Soda Lime and Ion exchange methods – Purification of water for domestic use Water pollution – BOD, COD, DO

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Theory Internal Continuous Assessment

(Maximum Marks - 50)

Practical Internal Continuous Assessment (Maximum Marks - 50)

% of Marks Marks % of Marks Marks 60% - Tests (minimum 2) 30 50% - Practical and Record 25

30% - Assignments (minimum 2) 15 40% - Test 20

10% - Attendance 05 10% - Attendance 05

Theory University Examination Pattern (Maximum Marks - 100)

� Part A: Analytical/problem solving short questions Candidates have to answer eight questions out of ten. There shall be minimum of two and maximum of three questions from each module with total ten questions.

8 x 5 marks =

40 marks

� Part B: Analytical/Problem solving descriptive questions Two questions from each module with choice to answer one question

4 x 15 marks =

60 marks

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Organo Metallic CompoundsOrgano Metallic CompoundsOrgano Metallic CompoundsOrgano Metallic Compounds

Introduction

Organometallic chemistry, the chemistry of compounds containing metal-carbon bonds,

it encompasses a wide variety of chemical compounds and their reactions, including compounds

containing both σ and π bonds between metal atoms and carbon; many cluster compounds, containing

one or more metal-metal bonds. Aside from their intrinsically interesting nature, many organometallic

compounds form useful catalysts and consequently are of significant industrial interest.

Cr(CO)6 and [Ni(H2O)6]2+ example, are both octahedral. Both CO and H2O are σ donor ligands;

in addition, CO is a strong π acceptor. Other ligands that can exhibit both behaviors include CN-, PPh3,

SCN-, and many organic ligands. Cyclic organic ligands containing delocalized π systems can team up

with metal atoms to form sandwich compounds. A characteristic of metal atoms bonded to organic

ligands, especially CO, is that they often exhibit the capability to form covalent bonds to other metal

atoms to form cluster compounds. These clusters may contain only two or three metal atoms or as

many as several dozen; there is no limit to their size or variety. They may contain single, double,

triple, or quadruple bonds between the metal atoms and may in some cases have ligands that bridge

two or more of the metals.

The first organometallic compound to be reported was synthesized in 1827 by Zeise,

who obtained yellow needle-like crystals after refluxing a mixture of PtC14 and PtC12 in ethanol,

followed by addition of KC1. It is an ionic compound (Zeise's salt) of formula K[Pt(C2H4)Cl3]H2O.

In 1890, Mond reported the preparation of Ni(CO)4, a compound that became commercially useful for

the purification of nickel. Reactions between magnesium and alkyl halides, performed by Barbier in

1898 and 1899, and subsequently by Grignard led to the synthesis of alkyl magnesium complexes now

known as Grignard reagents. Kealy and Pauson reacted the Grignard reagent cyclo-CSH5MgBr with

FeC3, using anhydrous diethyl ether as the solvent. This reaction yield an orange solid of formula

(C5H5)2Fe, ferrocene. The structure of ferrocene consist of an iron atom sandwiched between two

parallel C5H5 rings.

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Classification based on the nature of Metal-Carbon bond

The following five types of organometallic compounds can be distinguished depending

upon the nature of metal-carbon bond; [1] Ionic organometallic compounds [2] Organometallic compounds

containing metal-carbon sigma bond [3] Ylides [4] Organometallic compounds with multicentre bonds [5]

Organometallic compounds with pi bonded ligands.

[1] Ionic organometallic compounds: Most of the organometallic compounds of alkali metals fall in this

category. They have short life because of their high reactivity. Examples are Na+(CH2=CH-CH2)-, Na+(CH2-

C6H5)-, Na+(C6H5)

-, Na+(C5H5)-, etc

[2] Organometallic compounds containing metal-carbon sigma bond: Metallic elements of Group II, III, IV

and V as well as transition metals form organometallic compounds in which the metal atoms are bonded to

carbon atoms by sigma bond.

OMC of Group II

Represented as R2M

OMC of Group III

Represented as R3M

OMC of Group IV

Represented as R4M

OMC of Group V

Represented as R3M

(CH3)2Hg, (CH3)2Cd,

(CH3)2Zn, (CH3)2Mg,

(CH3)3Ga, (C6H5)3Ga,

(CH3)3In,(CH3)3Tl,

(CH3)4Si, (CH3)4Ge,

(CH3)4Sn, (CH3)4Pb,

(CH3)3P, (CH3)3As,

(CH3)3Sb, (CH3)3Bi,

Organometallic compounds of transition metals: Very few examples of alkyl compounds of transition

metals are known because of their greater reactivity. But organic ligand does not contain any β hydrogen will

form stable complex, e.g., [CH3-CH2-Rh(NH3)5]. The alkynyl compounds [M-C≡C-R] are more stable than

alkyl or aryl complexes. The reason is that alkynyl group acts as σ donor as well as π acceptor. Similar

situation occurs in alkenyl compounds [M-CH=CR2]. Some other compounds are σ cyclopentadienyl

complexes containing (η1-C5H5)M linkage.

[3] Ylides: These are the compounds in which the metal is doubly bonded with the carbon atom of the

ligand. Such compounds are formed by the main group elements as well as by the transition elements.

The example is Wittig reagent, Ph3P=CH2.

[4] Organometallic compounds with multicentre bonds: Organometallic compounds which are

loosely called electron-deficient and thus occur in polymeric forms fall under this category.

Examples are (Li-CH3)4, [Be(CH3)2]n, [Al(CH3)3]2, etc. These compounds are considered as intermediate

between ionic organometallic compounds of alkali metals and σ bonded organometallic compounds of Si, Sn,

Pb, etc. Elements which have highest tendency to form this complex are Li, Be, Mg, B and Al.

[5] Organometallic compounds with pi bonded ligands: This category includes organometallic compounds of

alkenes, alkynes and some other carbon-containing compounds having electrons in their π molecular orbitals.

Overlapping of these π orbitals with the vacant orbitals of the metal atom gives rise to an arrangement in which

the metal atom gets bound to all the carbon atoms over which the π molecular orbital of the organic ligand is

spread. The most important compound of this category is ferrocene or (bis-cyclopentadienyl)iron, represented

as (η5-C5H5)2Fe. It is known to have a ‘sandwitch’ structure

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The 18-Electron Rule or Inert Gas Rule or EAN Rule

A complex compound in which the central metal atom appears to have acquired the

configuration of an inert gas by sharing of electrons tends to be more stable, this generalization is

known as inert gas rule. The total number of electrons which the central metal atom appears to

possess in the complex including those gained by it in bonding is called Effective Atomic Number of

the central metal atom.

The effective number of electrons in the (n-1)d, ns and np orbitals of metal (valence shell) in its

complex should be equal to 10 + 2 + 6 = 18, so called 18-electron rule. The total number of electrons

gained by the metal through bonding plus the number of original electrons already present in

(n-1)d, ns and np orbitals of metal should be equal to 18 in any of the stable complexes of the metal.

There are many exceptions to the 18-electron rule, but the rule nevertheless provides some useful

guidelines to the chemistry of many organometallic complexes.

Electrons in the complex may be counted by Donor Pair Method. This method considers

ligands to donate electron pairs to the metal. To determine the total electron count, we must take into

account the charge on each ligand and determine the formal oxidation state of the metal.

Cr(C0)6: A Cr atom has 6 electrons outside its noble gas core. Each CO is considered to act as

a donor of 2 electrons. The total electron count is represented in Table 1. Cr(CO)6 is therefore

considered an 18-electron complex. It is thermally stable; for example, it can be sublimed without

decomposition. Cr(CO)5, a 16-electron species, and Cr(CO)7, a 20-electron species, on the other hand,

are much less stable and are known only as transient species.

Table 1

Table 2

(η5-C5H5)Fe(CO)2Cl: Pentahapto-C5H5 is considered by this method as C5H5-,

a donor of 3 electron pairs; it is a 6-electron donor. As in the first example, CO is counted

as a 2-electron donor. Chloride is considered C1-, a donor of 2 electrons. This complex is formally

an iron(II) complex. Iron(II) has 6 electrons beyond its noble gas core. This electron count

is represented in Table 2.

Carbonyl (CO) Complexes

Carbon monoxide is the most common ligand in organometallic chemistry. It serves as the only

ligand in binary carbonyls such as Ni(CO)4, W(CO)6, and Fe2(CO)9 or, more commonly,

in combination with other ligands, both organic and inorganic. CO may bond to a single metal or

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it may serve as a bridge between two or more metals. In this section, we will consider the bonding

between metals and CO, the synthesis and some reactions of CO complexes, and examples of the

various types of CO complexes.

Two features of the molecular orbitals of CO deserve attention. First, the highest energy

occupied orbital (the HOMO) has its largest lobe on carbon. It is through this orbital, occupied by an

electron pair, that CO exerts its a-donor function, donating electron density directly toward an

appropriate metal orbital (such as an unfilled d or hybrid orbital). Carbon monoxide also has two

empty π* orbitals (the lowest unoccupied, or LUMO); these also have larger lobes on carbon than on

oxygen. A metal atom having electrons in a d orbital of suitable symmetry can donate electron density

to these π* orbitals. These σ-donor and π-acceptor interactions are illustrated in Figure. The overall

effect is synergistic. CO can donate electron density via a σ-orbital to a metal atom; the greater the

electron density on the metal, the more effectively it can return electron density to the π* orbitals of

CO. The net effect can be strong bonding between the metal and CO; however, as will be described

later, the strength of this bonding depends on several factors, including the charge on the complex and

the ligand environment of the metal.

The metal-carbon bond in metal carbonyls possess both s and p character. The M–C σ bond is

formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the

metal. The M–C π bond is formed by the donation of a pair of electrons from a filled d orbital of metal

into the vacant antibonding π* orbital of carbon monoxide. The metal to ligand bonding creates a

synergic effect which strengthens the bond between CO and the metal

Bridging modes of CO

Although CO is most commonly found as a terminal ligand attached to a single metal atom,

many cases are known in which CO forms bridges between two or more metals. In cases in which CO

bridges two metal atoms, both metals can contribute electron density into π* orbitals of CO to weaken

the C - O bond. Ordinarily, terminal and bridging carbonyl ligands can be considered 2-electron

donors, with the donated electrons shared by the metal atoms in the bridging cases. For example, in the

complex the bridging CO is a 2-electron donor overall, with a single electron donated to each metal.

The electron count for each Re atom according to method B is

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9

Bioinorganic ChemistryBioinorganic ChemistryBioinorganic ChemistryBioinorganic Chemistry

Introduction

When one considers the chemistry of biological processes, the boundary between inorganic and

organic chemistry is blurred. The bulk biological elements that are essential to all life include

C, H, N, O (the four most abundant elements in biological systems) along with Na, K, Mg, Ca, P, S

and Cl. The fundamental elements that make up the building blocks of biomolecules (e.g. amino acids,

peptides, carbohydrates, proteins, lipids and nucleic acids) are C, H, N and O, with P playing its part

in, for example, ATP and DNA and S being the key to the coordinating abilities of cysteine residues in

proteins. The roles of the less abundant, but nonetheless essential, elements include osmotic control

and nerve action (Na, K and Cl), Mg2+ in chlorophyll, Mg2+ containing enzymes involved in phosphate

hydrolysis, structural functions of Ca2+ (e.g. bones, teeth, shells) and triggering actions of

Ca2+ (e.g. in muscles). The trace metals are V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Mo, while trace non-

metals comprise B, Si, Se, F and I.

So the chemical elements essential to life forms can be divided into the following

(i) Bulk elements: C, H, N, O, P, S (ii) Macrominerals and ions: Na, K, Mg, Ca, Cl, PO43-, SO4

2-

(iii) Trace elements: Fe, Zn, Cu (iv) Ultratrace elements comprises of (a) Non-metals: F, I, Se, Si,

As, B (b) Metals: Mn, Mo, Co, Cr, V, Ni, Cd, Sn, Pb, Li

Na+ and K+ Most important free intra- and extracellular cations. Regulation of the osmotic pressure, membrane potentials, enzyme activity, signalling.

Mg2+ Chlorophyll; anaerobic energy metabolism (ATP > ATP).

Ca2+

Signalling, muscle contraction, enzyme regulation. Main inorganic part of the endoskeletons (bones, teeth, enamel: hydroxyapatite; Ca5(PO4)3(OH)). Exoskeletons of mussels, shells, corals, sea urchins etc: aragonite or calcite; CaCO3)

VIV/V, MoIV/VI, WIV/VI,

MnII/III/IV, FeII/III,

NiI/II/III, CuI/II

Active centres in electron-transport (redox) enzymes, oxygenases, dismutases.

Fe and Cu Transport of oxygen Fe3+ Iron-storage proteins (ferritins)

Fe2+ + Fe3+ Orientation of magnetobacteria, pigeons, bees in Earth’s magnetic field

Co Synthases and isomerases (cobalamines, e.g. vitamin-B12); methylation of inorganics

Zn2+

In the active centre of hydrolases, carboanhydrase, alcohol dehydrogenase, synthases; genetic transciption (zinc fingers), stabilisation of tertiary and quartary structures of proteins; repair enzymes

SiIV (“silicate“) Involved in the built-up of bones. In the form of SiO2/silica-gels as support in monocotyledonous plants (like grass) and the shells of diatoms

PV (phosphate)

Constituent in hydroxi- and fluorapatite (Ca5(PO4)3(OH/F)); energy metabolism (ATP), NADPH, activation of organic substrate; phospholipids in cell membranes; phosphate esters (DNA, RNA,…).

Se2- Selenocystein in special enzymes (e.g. glutathionperoxidase) F- Fluorapatite (Ca5(PO4)3F) in dental enamel Cl- Along with hydrogencarbonate the most important free anion. I Constituent of thyroid hormones (such as thyroxine).

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V Accumulated by a few organisms, and has been shown to be essential for growth in rats and chicks

Mn, Fe, Cu, Ni, Zn Essential to all organisms Co Essential to mammals and many other organisms Mo Essential to all organisms although green algae may be an exception B Essential to green algae and higher plants, but its role is unknown

Si Exoskeletons of marine diatoms composed of hydrated silica, but its role in other biological systems is less well defined

Se Essential to mammals and some higher plants F Its role is not fully established but its deficiency causes dental caries I Essential to many organisms.

The average amount of iron in the human body (70 kg) is ca. 5 g; iron is thus the most abundant

transition metal in our organism. About 70% of this amount is used for oxygen transport and storage

(haemoglobin, myoglobin), almost 30% are stored in ferritins (iron storage proteins), and about 1% is bound to

the transport protein transferrin and to various iron dependent enzymes; cf. the rough classification to the right.

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Porphyrins

One of the most important groups of compounds is the porphyrins, in which a metal ion is

surrounded by the four nitrogens of a porphine ring in a square-planar geometry and the axial sites are

available for other ligands. Different side chains, metal ions, and surrounding species result in very

different reactions and roles for these compounds.

Porphyrins are found in many metalloenzyme

Enzyme Function Fe-porphyrin Cytochrome Electron transfer Fe-porphyrin Hemoglobin & Myoglobin Dioxygen carrier Mg-porphyrin Chlorophyll Photosynthesis

Oxygen transport

In the pulmonary alveoli, O2 is taken up by haemoglobin (Hb) and 1 L of blood can dissolve

200 ml of oxygen. Simultaneously, hydrogencarbonate is converted to carbonic acid, which in turn is

catalytically degraded into CO2 und H2O (by the zinc enzyme carbonic anhydrase):

After transport of O2 by haemoglobin in the blood stream, the oxygen is transferred to tissue

myoglobin (Mb). Mb has a higher affinity to O2 than Hb.

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Iron Porphyrins

Hemoglobin and Myoglobin: The best known iron porphyrin compounds are hemoglobin and

myoglobin, oxygen transfer and storage agents in the blood and muscle tissue, respectively.

Each of us has nearly 1 kg of hemoglobin in our body, picking up molecular oxygen in the lungs and

delivering it to the rest of the body. Each hemoglobin molecule is made up of four globin protein

subunits, two α and two β. In each of these, the protein molecule partially encloses the heme group,

bonding to one of the axial positions through an imidazole nitrogen. The other axial position is vacant

or has water bound to it (the imidazole ring from histidine is too far from the iron atom to bond).

When dissolved oxygen is present, it can occupy this position, and subtle changes in the conformation

of the proteins result. As one iron binds an oxygen molecule, the molecular shape changes to make

binding of additional oxygen molecules easier. The four irons can each carry one O2, with generally

increasing equilibrium constants: In hemoglobin, the Fe(I1) is about 70 pm out of the plane of the

porphyrin nitrogens in the direction of the imidazole nitrogen bonding to the axial position. When

oxygen bond to the sixth position, the iron becomes coplanar with the porphyrin, oxygen bonds at an

angle of approximately 130°, also with considerable back π bonding (as nearly that of Fe(II1) - O2-).

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As soon as some oxygen has been bound to the molecule, all four irons are readily oxygenated.

In a similar fashion, initial removal of oxygen triggers the release of the remainder and the entire load

of oxygen is delivered at the required site. This effect is also favored by pH changes caused by

increased CO2 concentration in the capillaries. As the concentration of CO2 increases, formation of

bicarbonate causes the pH to decrease and the increased acidity favors release of O2 from the

oxyhemoglobin, called the Bohr effect.

Myoglobin has only one heme group per molecule and serves as an oxygen storage molecule in

the muscles. The myoglobin molecule is similar to a single subunit of hemoglobin. Bonding between

the iron and the oxygen molecule is similar to that in hemoglobin, but the equilibrium is simpler

because only one oxygen molecule is bound: When hemoglobin releases oxygen to the muscle tissue,

myoglobin picks it up and stores it until it is needed. The Bohr effect and the cooperation of the four

hemoglobin binding sites make the transfer more complete when the oxygen concentration is low and

the carbon dioxide concentration is high; the opposite conditions in the lungs promote the transfer of

oxygen to hemoglobin and the transfer of CO2 to the gas phase in the lungs. Myoglobin binds O2 more

strongly than the first O2 of hemoglobin.

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Green Chemistry

Introduction

Green Chemistry is the use of chemistry techniques and methodologies that reduce or eliminate

the use or generation of feedstock, products, by-products, solvents, reagents, etc., that are hazardous to

human health or the environment. Green Chemistry is an approach to the synthesis, processing and use

of chemicals that reduces risks to humans and the environment. This approaches include

new synthesis and processes as well as new tools for instructing aspiring chemists how to do

chemistry in a more environmentally benign (caring, kindly, gentle or compassionate) manner.

In order to evaluate the greenness of a particular process attention must be paid in the first

instance to issues related to safety, health and protection of the environment, due to reactants

(substrates and reagents), auxiliaries (mainly solvents) and waste. While all elements of the lifecycle

of a new chemical or process may not be environmentally benign, it is however important to improve

those stages where improvements can be made.

Definition

The term Green Chemistry is defined as -“The invention, design and application of chemical

products and processes to reduce or to eliminate the use and generation of hazardous substances”.

Green Chemistry is defined as environmentally benign chemical synthesis. Goal of Green Chemistry is

to create better, safer chemicals while choosing the safest, most efficient ways to synthesize them and

to reduce wastes. Green chemistry is the sustainable practice of chemical science and manufacturing

within a framework of industrial ecology in a manner that is sustainable, safe, and non-polluting,

consuming minimum amounts of energy and material resources while producing virtually no wastes.

The key notion of Green Chemistry is ‘‘efficiency’’, including material efficiency,

energy efficiency, man-power efficiency, and property efficiency (e.g., desired function vs. toxicity).

Any ‘‘wastes’’ aside from these efficiencies are to be addressed through innovative Green Chemistry

means. ‘‘Atom-economy’’ and minimization of auxiliary chemicals, such as protecting groups and

solvents, form the pillar of material efficiency in chemical productions.

Principles of Green Chemistry

Green Chemistry aims to eliminate hazards right at the chemical design stage, then throughout

the design, production, use/reuse and disposal processes. Practitioners of Green Chemistry try hard to

invent new chemical methods that do not pollute and that minimize the consumption of energy and

natural resources. In 1998, two US chemists, Dr. Paul Anastas and Dr John Warner outlined

Twelve Principles of Green Chemistry to demonstrate how chemical production could respect human

health and the environment while also being efficient and profitable.

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1. It is better to prevent waste than to treat or clean up waste after it is formed:

It is most appropriate to carry out a synthesis by following a pathway so that formation of waste is

minimum or absent. One type of waste product common and often avoidable is the starting material or

reagent that remains unreacted. The well known saying “Prevention is better than cure should be

followed”.

2. Synthetic methods should be designed to maximize the incorporation of all the materials

used in the process into the final product: A synthesis may generate significant amount of waste or by

product, such a synthesis, even though gives 100% yield, is not considered to be green synthesis.

In order to find, if a particular reaction is green, the concept of atom economy was developed by Berry

Trost of Stanford University. This considers the amount of stating materials incorporated into the

desired final product. Thus by incorporation of greater amounts of the atoms contained in the starting

materials (reactants) in to the formed products, fewer waste by products are obtained. In this way,

using the concept of atom economy along with ideas of selectivity and yield, “greener” more efficient

synthesis can be developed. The atom economy for a reaction can be calculated using the equation,

3. Whenever practicable synthetic methodologies should be designed to use and generate a

substance that poses little or no toxicity to human health and the environment:

Wherever practicable, synthetic methodologies should be designed to use and generate substances that

pose little or no toxicity to human health and the environment. Redesigning existing transformations to

incorporate less hazardous materials is at the heart of Green Chemistry.

4. Chemical products should be designed to preserve efficiency of function while reducing

toxicity: The designing of safer chemical is now possible since the understanding of chemical toxicity.

It is now fairly understood that a correlation exist between chemical structure (presence of functional

groups) and the existence of toxic effects. The idea is to avoid the functionality related to

the toxic effect.

5. The use auxiliary substances should be made unnecessary wherever possible and

innocuous when used: An auxiliary substance (e.g. solvents, separating agents) are used in the

manufacture, processing at every step, it is one that helps in manufacture of chemical, but does not

become an integral part of the chemical. Major problem with many solvents is their volatility that may

damage human health and the environment. The problem of solvents has been overcome by using such

solvents which do not pollute the environment. Such solvents are known as green solvents.

Examples include liquid supercritical CO2, use near-critical water at higher temperatures where water

behaves more like organic solvent, ionic liquid water, or many new ionic liquids have been developed

with a broad range of properties. Even reactions have been conducted in solid state.

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Microwave technology can be used in some reactions to provide the heat energy required to make the

transformation go to completion .With microwave technology, reactions can take place with less toxic

reagents and in a shorter time, with fewer side reactions, all goals of Green Chemistry. Microwave

technology has also been used to create supercritical water that behaves more like an organic solvent

and could replace more toxic solvents in carrying out organic reactions. Another Green Chemistry

approach is the use of a catalyst which facilitates transformations without the catalyst being consumed

in the reaction and without being incorporated in the final product. Therefore, use of catalyst should be

preferred whenever possible.

6. Energy requirements should be recognized for their environmental and economic impacts

and should be minimized: Energy generation, as we know has a major environmental effect.

The requirement of energy can be kept to a base minimum in certain cases by the use of a catalyst.

It is now possible that the energy to a reaction can be supplied by using microwaves, by sonication or

photo chemically.

7. A raw material or feedstock should be renewable rather than depleting, whenever

technically and economically practicable: Non reversible or depleting sources can exhaust by their

continual use. So these are not regarded as sustainable from environmental point of view. The starting

materials which are obtained agricultural or biological processes are referred to as renewable starting

materials. Substances like carbon dioxide (generated from natural sources or synthetic routes like

fermentation) and methane gas (obtained from natural sources such as marsh gas, natural gas, etc)

are available in reasonable amounts and so are considered as renewable starting material.

Methane, a constituent of biogas and natural gas can easily be converted into acetylene by partial

combustion. Acetylene is a potential source of number of chemicals such as ethyl alcohol,

acetaldehyde, vinyl acetate, etc.

8. Unnecessary derivatization (blocking group, protection, deportation,

temporary modification of physical/chemical processes) should be avoided whenever possible:

A commonly used technique in organic synthesis is the use of protecting or blocking group.

These groups are used to protect a sensitive moiety from the conditions of the reaction, which may

make the reaction to go in an unwanted way if it is left unprotected.

9. Catalytic reagents (as selective as possible are superior to stoichiometric reagents:

The catalyst as we know facilitates transformation without being consumed or without being

incorporated into the final product. Catalysts are selective in their action in that the degree of reaction

that takes place is controlled, e.g. mono addition v/s multiple addition. In addition to the benefits of

yield and atom economy, the catalysts are helpful in reducing consumption of energy.

Catalysts carry out thousands of transformation before being exhausted.

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10. Chemical products should be designed so that at the end of their function they

do not persist in the environment and break down into innocuous degradation products:

It is extremely important that the products designed to be synthesized should be biodegradable.

They should not be persistent chemicals or persistent bio accumulators. It is now possible to place

functional groups in a molecule that will facilitate its biodegradation. Functional groups which are

susceptible to hydrolysis, photolysis or other cleavage have been used to ensure that products will be

biodegradable. It is also important that degradation products do not possess any toxicity and

detrimental effects to the environment.

11. Analytical methodologies need to be further developed to allow for real time,

in process monitoring and control prior to the formation of hazardous substances:

Methods and technologies should be developed so that the prevention or minimization of generation of

hazardous waste is achieved. It is necessary to have accurate and reliable reasons, monitors and other

analytical methodologies to assess the hazardous that may be present in the process stream.

These can prevent any accidents which may occur in chemical plants.

12. Substances and the form of a substance used in a chemical process should be chosen

so as to minimize the potential for chemical accidents, including releases, explosions and fires:

The occurrence of accidents in chemical industry must be avoided. It is well known that the incidents

in Bhopal (India) and Seveso (Italy) and many others have resulted in the loss of thousands of life.

It is possible sometimes to increase accidents potential inadvertently with a view to minimize the

generation of waste in order to prevent pollution. It has been found that in an attempt to recycle

solvents from a process (for economic reasons) increases the potential for a chemical accident or fire.

The principles of green chemistry and some examples of their applications to basic and applied

research are illustrated below:

Prevention of Waste: It is better to prevent waste than to treat or clean up waste after it is

formed. The ability of chemists to redesign chemical transformations to minimize the generation of

hazardous waste is an important first step in pollution prevention.

Maximize Atom Economy: Atom Economy is a concept that evaluates the efficiency of a

chemical transformation, and is calculated as a ratio of the total mass of atoms in the desired product

to the total mass of atoms in the reactants. Choosing transformations that incorporate most of the

starting materials into the product are more efficient and minimize waste. The examples are

[1] Diels–Alder reaction is 100%. Atom Economy reaction as all the atoms of the reactants are

incorporated in the cycloadduct. [2] Disinfection of water by chlorination. Chlorine oxidizes the

pathogens there by killing them, but at the same time forms harmful chlorinated compounds.

18

A remedy is to use another oxidant, such as O3 or supercritical water oxidation. [3] Production of allyl

alcohol CH2=CHCH2OH. Traditional route: Alkaline hydrolysis of allyl chloride, which generates the

product and hydrochloric acid as a by-product. CH2=CH-CH2-Cl + H2O → CH2=CH-CH2-OH +HCl

Greener route, to avoid chlorine: Two-step using propylene (CH2=CHCH3), acetic acid (CH3COOH)

and oxygen (O2). Step I. CH2=CH-CH3 + CH3-COOH +��O2 → CH2=CH-CH2-O-CO-CH3 + H2O

Step II. CH2=CH-CH2-O-CO-CH3 + H2O → CH2=CH-CH2-OH + CH3-COOH.

Added benefit: The acetic acid produced in the 2nd reaction can be recovered and used again for the

1st reaction, leaving no unwanted by-product.

Less Hazardous Chemical Syntheses: Synthetic methodologies should be designed to use and

generate substances that possess little or no toxicity to human health and environment.

Some toxic chemicals are replaced by safer ones for a green technology, when reagent choices exist

for a particular transformation. This principle focuses on choosing reagents that pose the least risk and

generate only benign by-products. Production of styrene (=benzene ring with CH=CH2 tail).

Traditional route: Two-step method starting with benzene, which is carcinogenic) and ethylene to

form ethylbenzene, followed by dehydrogenation to obtain styrene. Greener route: To avoid benzene,

start with xylene (cheapest source of aromatics and environmentally safer than benzene).

Another option, still under development, is to start with toluene. Phosgene, COCl2, is commonly used

as a starting material for polycarbonate. Phosgene is a highly toxic substance, and the by-products of

many of its reactions are undesirable. A superior alternative might be dimethyl carbonate

Designing Safer Chemicals for Accident Prevention: New products can be designed that are

inherently safer for the target application. Pharmaceutical products often consist of chiral molecules,

and the difference between the two forms can be a matter of life and death – for example, racemic

thalidomide when administered during pregnancy, leads to horrible birth defects in many new borns.

Evidence indicates that only one of the enantiomers has the curing effect while the other isomer is the

cause of severe defects. That is why it is vital to be able to produce the two chiral forms separately.

Catalysts that can catalyse important reactions that produce only one of the two mirror image forms

are developed. Design chemicals and their forms (solid, liquid, or gas) to minimize the chemical

accidents including explosions, fires and releases to the environment, e.g., manufacture of gold atom

nano particles used diborane (highly toxic and bursts into flame near room temperature) and cancer-

causing benzene. Now, diborane has been replaced by environmentally benign NaBH4 which also

eliminates the use of benzene. Nanoscience and nanotechnology is another important contribution to

green chemistry. Nanotechnology provides huge savings in materials by development of microscopic

and submicroscopic electronic and mechanical devices.

Atom Economy

Designing Safer Chemicals for Accident Preventionvery toxic compounds. It is this production that led to the disaster in Bophal in

Classical way of synthesis

Designing a Green Synthesis

Using “green catalyst” is that its action mimics nature in respect that all natural synthesis is

enzyme catalyzed reactions. This not only helps in designing a highly stereo specific, stereo selective

and enantio - selective product but also these reaction

Generally chloroform, DMC, carbon tetra chloride etc are used as a solvent in organic synthesis

which is not only costly but is very harmful for those who is handling them also. Among them carb

tetrachloride is worst solvent as it is highly environment destructive. In green chemistry an attempt has

been made to minimize or eliminate these effects by using water as a solvent. Not only this super

critical carbon dioxide which are obtained by ele

its critical temperature and pressure. Other super critical fluids used in green chemistry are ethane,

ethene, water, xenon etc.

A great attempt has been made to shift the usage of petroleum based produc

forms 95% of cases as a starting material for various chemicals required in tons per year.

On this basis shifting to biomass for such a vast need is the call of an hour. Researchers are now

finding new ways of converting biomass into starting material. E.g. converting D

acid using certain enzymes helps us to prepare aliphatic compounds from lactic acid.

Less Hazardous Chemical Syntheses

Polycarbonate Synthesis: Phosgene Process [phosgene is highly toxic,

A superior alternative might be dimethyl carbonate

Designing Safer Chemicals for Accident Prevention: The classical way of synthesis of carbarylIt is this production that led to the disaster in Bophal in

Alternative reaction routes

Designing a Green Synthesis



selective product but also these reactions takes place under ambient conditions.


which is not only costly but is very harmful for those who is handling them also. Among them carb



critical carbon dioxide which are obtained by elevating the temperature and pressure of the gas above


A great attempt has been made to shift the usage of petroleum based produc



converting biomass into starting material. E.g. converting D


19

Less Hazardous Chemical Syntheses

Polycarbonate Synthesis: Phosgene Process [phosgene is highly toxic, corrosive]

A superior alternative might be dimethyl

The classical way of synthesis of carbaryl involves It is this production that led to the disaster in Bophal in India in 1984.

Alternative reaction routes



s takes place under ambient conditions.


which is not only costly but is very harmful for those who is handling them also. Among them carbon



vating the temperature and pressure of the gas above


A great attempt has been made to shift the usage of petroleum based products which currently



converting biomass into starting material. E.g. converting D-glucose into lactic


20

On a similar basis E-coli converts D-glucose to catechol which acts as a starting material for aromatic

compounds. On the other hand isomaltulose which is widely available in biomass can be converted

into glucosylmethyl furfural which can be used for production of many heterocyclic compounds.

Besides biomass cash crops is a new hope as ethanol from sugarcane has been derived successfully

and now scientists are trying to use this “bio alcohol” as a source of vehicle for future.

Exhaust from Corn plant has been successfully utilized for preparing bio-degradable plastic.

In a chemical reaction the major concern is the percent yield of desired product.

If the yield in a chemical reaction is satisfactory we hardly bother about the formation of

bi-products. An attempt has been made to further maximize the yield of the desired products by

developing such reactions which are catalyzed reactions whose catalyst can be extracted and further

utilized for other reactions. These reactions are further focused to undergo addition reactions,

rearrangements or pericyclic reaction where a single product is obtained which further increases the

atom efficiency. . For reactions whose desired product is a chiral compound, it is advisory to design

such reactions which eliminates the formation of racemic mixtures. Hence these type of synthesis

should always be either highly stereo-specific or either highly stereo-selective.

Minimizing the energy requirements of industries by maximizing the efficiency of chemical

conversion and decreasing the activation energy of the reactions by using recyclable catalysts can cut

off the energy requirement of industries by half or even more. Eliminating the use of energy

consuming steps like distillation, crystallization, sublimation, ultra filtration etc. and incorporation of

microwave energy which aims to achieve a high temperature at much faster rates and also utilization

of ultrasonic energy for certain reaction can eventually solve this problem.

Besides the above mentioned “pillars of green chemistry” some other points that can also been

incorporated as the supports of green chemistry are as follows: (a) Use of “light” as a carrier of

electrons which can eventually reduce the usage of other chemical agents which act as a carrier of

electron and is obtained as waste products at the end of a redox reaction. (b) Eliminating the un-

necessary use of protection- deprotection methodologies. (c) Replacement of soluble Lewis acids by

mesoporous solids containing bound sulphonates in green synthesis.(d) Utilization of milder reaction

conditions for carrying out a chemical reaction.

Green chemistry is not a solution to all environmental problems but the most fundamental

approach to preventing pollution. Green chemistry is more effective, efficient, elegant, safe and better

chemistry.

21

Polymers

The word ‘Polymer’ is coined from two Greek words: poly means many and mer means unit or

part. The term polymer is defined as very large molecules having high molecular mass. These are also

referred to as macromolecules, which are formed by joining of repeating structural units on a large

scale. The repeating structural units are derived from some simple and reactive molecules known as

monomers and are linked to each other by covalent bonds. This process of formation of polymers from

respective monomers is called polymerization. The transformation of ethene to polythene and

interaction of hexamethylene diamine and adipic acid leading to the formation of Nylon 6, 6 are

examples of two different types of polymerisation reactions.

The functionality of a monomer is the number of sites it has for bonding to other monomers

under the given conditions of the polymerization reaction. Thus, a bifunctional monomer, i.e.,

monomer with functionality two, can link to two other molecules under suitable conditions.

A polyfunctional monomer is one that can react with more than two molecules under the conditions of

the polymerization reactions.

The number of repeating units (n) in the chain so formed is called the

‘degree of polymerization’ (DP = n). Polymers with a high degree of polymerization are called

‘high polymers’ and those with low degree of polymerization are called oligopolymers

(short chain polymers or oligomers). Polymers do not exhibit strength for n < 30 and

that the optimum strength of most of the polymers is obtained at n around 600.

The useful range of n is from 200 to 2000.

22

There are several ways of classification of polymers based on some special considerations.

The following are some of the common classifications of polymers: [1] by Source [2] by Backbone of

the chain [3] by Structure [4] by Compostion [5] by Mode of Polymerization [6] by Molecular force

Classification Based on Source: [1] Natural Polymers: These polymers are found in plants

and animals. Examples are proteins, cellulose, starch, resins and rubber.

[2] Semi-synthetic Polymers: Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate,

etc. are the usual examples of this sub category. [3] Synthetic Polymers: A variety of synthetic

polymers as plastic (polythene), synthetic fibres (nylon 6,6) and synthetic rubbers (Buna - S) are

examples of man-made polymers.

Classification Based on Backbone of the polymer chain: Organic and Inorganic Polymers:

A polymer whose backbone chain is essentially made of carbon atoms is termed as organic polymer.

The atoms attached to the side valencies of the backbone carbon atoms are, however, usually those of

hydrogen, oxygen, nitrogen, etc. The majority of synthetic polymers are organic. On the other hand,

generally chain backbone contains no carbon atom is called inorganic polymers. Glass and silicone

rubber are examples of it.

Classification Based on Structure of Polymers: [1] Linear Polymers: These polymers consist

of long and straight chains. The examples are high density polythen, PVC, etc. Linear polymers are

commonly relatively soft, often rubbery substances, and often likely to soften (or melt) on heating and

to dissolve in certain solvent. [2] Branched Polymers: These polymers contain linear chains having

some branches, e.g., low density polythene. [3] Cross-linked Polymers: These are usually formed from

bi-functional and tri-functional monomers and contain strong covalent bonds betweenvarious linear

polymer chains, e.g. vulcanized rubber, urea-formaldehyde resins, etc. Cross linked polymers are hard

and do not melt, soften or dissolve in most cases.

Linear Polymers Branched Polymers Cross-linked Polymers

Classification Based on Composition of Polymers: [1] Homopolymer: A polymer resulting

from the polymerization of a single monomer; a polymer consisting substantiallyof a single type of

repeating unit. [2] Copolymer: When two different types of monomers are joined in the same polymer

chain, the polymer is called a copolymer.

23

Let's imagine now two monomers (A and B) made into a copolymer in many different ways.

In an alternating copolymer, the two monomers are arranged in an alternating fashion.

In a random copolymer, the two monomers may followin any order. In a block copolymer,

all of one type of monomers are grouped together, and all of the other are grouped together.

In graft copolymer, a block copolymer can be thought of as two homopolymers joined together at the

ends: branched copolymers with one kind of monomers in their main chain and another kind of

monomers in their side chains.

Alternating

Random

Block

Graft

Copolymerization: A heteropolymer or copolymer is a polymer derived from two

(or more) monomeric species, as opposed to a homopolymer where only one monomer is used.

Copolymerization refers to methods used to chemically synthesize a copolymer. Commercially

relevant copolymers include ABS plastic, SBR, Nitrile rubber, styrene-acrylonitrile,

styrene-isoprene-styrene (SIS) and ethylene-vinyl acetate.

Classification Based on Mode of Polymerisation: Polymers can also be classified

on the basis of mode of polymerisation into two sub groups; (a) Addition Polymers and

(b) Condensation Polymers.

Addition Polymers: The addition polymers are formed by the repeated addition of monomer

molecules possessing double or triple bonds, e.g., the formation of polythene from ethene and

polypropene from propene. However, the addition polymers formed by the polymerisation of a single

monomeric species are known as homopolymer, e.g., polythene.

The polymers made by addition polymerisation from two different monomers are termed as

copolymers, e.g., Buna-S, Buna-N, etc.

24

Condensation Polymers: The condensation polymers are formed by repeated condensation

reaction between two different bi-functional or tri-functional monomeric units. In these polymerisation

reactions, the elimination of small molecules such as water, alcohol, hydrogen chloride, etc. take

place. The examples are terylene (dacron), nylon 6, 6, nylon 6, etc. For e.g., nylon 6, 6 is formed by

the condensation of hexamethylene diamine with adipic acid.

It is also possible, with three functional groups (or two different monomers at least one of

which is tri-functional), to have long linkage sequences in two (or three) dimensions and

such polymers are distinguished as cross linked polymers.

Classification Based on Molecular Forces: The mechanical properties of polymers

are governed by intermolecular forces, e.g., van der Waals forces and hydrogen bonds,

present in the polymer. These forces also bind the polymer chains. Under this category,

the polymers are classified into the following groups on the basis of magnitude of intermolecular

forces present in them. They are (i) Elastomers (ii) Fibers (iii) Liquid resins

(iv) Plastics [(a) Thermoplastic and (b) thermosetting plastic].

Elastomers: These are rubber – like solids with elastic properties. In these elastomeric

polymers, the polymer chains are held together by the weakest intermolecular forces.

These weak binding forces permit the polymer to be stretched. A few ‘crosslinks’ are introduced in

between the chains, which help the polymer to retract to its original position after the force is released

as in vulcanised rubber. The examples are buna-S, buna-N, neoprene, etc.

Fibers: If drawn into long filament like material whose length is at least 100 times its diameter,

polymers are said to have been converted into ‘fibre’. Fibres are the thread forming solids which

possess high tensile strength and high modulus. These characteristics can be attributed to the strong

intermolecular forces like hydrogen bonding. These strong forces also lead to close packing of chains

and thus impart crystalline nature. Examples are polyamides (nylon 6, 6), polyesters (terylene), etc.

Liquid Resins: Polymers used as adhesives, potting compound sealants, etc. in a liquid form

are described liquid resins. Examples are epoxy adhesives and polysulphide sealants.

Plastics: A polymer is shaped into hard and tough utility articles by the application

of heat and pressure; it is used as a ‘plastic’. Typical examples are polystyrene, PVC

and polymethyl methacrylate. They are two types (a) thermoplastic and (b) thermosetting plastic.

25

Thermoplastic Polymers: Some polymers soften on heating and can be converted into any

shape that they can retain on cooling. The process of heating, reshaping and retaining the same on

cooling can be repeated several times. Such polymers, that soften on heating and stiffen on cooling,

are termed ‘thermoplastics’. These are the linear or slightly branched long chain molecules capable of

repeatedly softening on heating and hardening on cooling. These polymers possess intermolecular

forces of attraction intermediate between elastomers and fibres. Polyethylene, PVC, nylon and sealing

wax are examples of thermoplastic polymers.

Thermosetting Polymers: Some polymers, on the other hand, undergo some chemical change

on heating and convert themselves into an infusible mass. They are like the yolk of egg,

which on heating sets into a mass, and, once set, cannot be reshaped. Such polymers, that become

infusible and insoluble mass on heating, are called ‘thermosetting” polymers. These polymers are

cross linked or heavily branched molecules, which on heating undergo extensive cross linking in

moulds and again become infusible. These cannot be reused. Some common examples are bakelite,

urea-formaldelyde resins, etc.

26

Types of PolymerizationTypes of PolymerizationTypes of PolymerizationTypes of Polymerization

There are four types of polymerisation reactions; (a) Addition or chain growth polymerisation

(b) Coordination polymerisation (c) Condensation or step growth polymerisation and

(d) Copolymerization

Addition Polymerisation: In this type of polymerisation, the molecules of the same monomer

or different monomers add together on a large scale to form a polymer. The monomers normally

employed in this type of polymerization contain a carbon-carbon double bond (unsaturated

compounds, e.g., alkenes and their derivatives) that can participate in a chain reaction.

A chain reaction consists of three stages, Initiation, Propagation and Termination.

In the Initiation step an initiator molecule is thermally decomposed or allowed to undergo a

chemical reaction to generate an "active species." This "active species," which can be

a free radical or a cation or an anion, then initiates the polymerization by adding to the monomer's

carbon-carbon double bond. The reaction occurs in such a manner that a new free radical or cation or

anion is generated. The initial monomer becomes the first repeat unit in the incipient polymer chain.

In the Propagation step, the newly generated "active species" adds to another monomer in the same

manner as in the initiation step. This procedure is repeated over and over again until the final step of

the process, termination, occurs. In the Termination step, the growing chain terminates through

reaction with another growing chain, by reaction with another species in the polymerization mixture,

or by the spontaneous decomposition of the active site. Under certain conditions, anionic can be

carried out without the termination step to generate so-called "living" polymers.

The following are several general characteristics of addition polymerization:

[1] Once initiation occurs, the polymer chain forms very quickly [2] The concentration of active

species is very low. Hence, the polymerisation mixture consists of primarily of newly-formed polymer

and unreacted monomer [3] Since the carbon-carbon double bonds in the monomers are, in effect,

converted to two single carbon-carbon bonds in the polymer, so energy is released making the

polymerization exothermic with cooling often required.

The mechanism of addition polymerisation can be divided broadly into two main classes,

free radical polymerization and ionic polymerization, although there are some others.

Ionic polymerisation was probably the earliest type to be noted, and is divided into

cationic and anionic polymerisations.

Free radical polymerization: A variety of alkenes or dienes and their derivatives are

polymerised in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide,

acetyl peroxide, tert-butyl peroxide, etc. A free radical may be defined as an intermediate compound

27

containing an odd number of electrons, but which do not carry an electric charge and are not free ions.

For example, the polymerization of ethene to polythene consists of heating or exposing to light a

mixture of ethene with a small amount of benzoyl peroxide initiator.

The first stage of the chain reaction is the initiation process; this process starts with the

addition of phenyl free radical formed by the peroxide to the ethene double bond thus generating a

new and larger free radical.

The second stage of the chain reaction is the propagation process, the radical reacts with

another molecule of ethene, and another bigger sized radical is formed. The repetition of this sequence

with new and bigger radicals carries the reaction forward and the step is chain propagating step

The final stage of the chain reaction is the termination process; the product radical formed

reacts with another radical to form the polymerised product.

Ionic Polymerisation: The addition polymerization that takes place due to

ionic intermediate is called ionic polymerization. Based on the nature of ions used for the initiation

process ionic polymerization classified into two types; (a) Cationic polymerization and

(b) Anionic polymerization

Cationic polymerization depends on the use of cationic initiators which include reagents

capable of providing positive ions or H+ ions. Typical examples are aluminium chloride with water

(AlCl 3+H2O) or boron trifluoride with water (BF3+H2O). They are effective with monomers

containing electron releasing groups like methyl (-CH3) or phenyl (-C6H5) etc.

They include propylene(CH3CH=CH2) and the styrene (C6H5CH=CH2).

i) Chain Initiation : Decomposition of the initiator is shown as BF3 + H2O →

H+ + BF3(OH–). The proton (H+) adds to C – C double bond of alkene to form stable carbocation.

28

ii) Chain Propagation: Carbocation add to the C – C double bond of another monomer molecule to

from new carbocation.

iii) Chain Termination: Reaction is terminated by combination of carbocation with negative ion (or)

by loss of proton

Anionic polymerization depends on the use of anionic initiators which include reagents

capable of providing negative ions. Typical catalysts include sodium in liquid ammonia,

alkali metal alkyls, Grignard reagents and triphenylmethyl sodium [(C6H5)3C-Na].

They are effective with monomers containing electron withdrawing groups like nitrile (–CN) or

chloride (-Cl), etc. They include acrylonitrile [CH2=C(CN)], vinyl chloride [CH2=C(Cl)],

methyl methacrylate [CH2=C(CH3)COOCH3], etc.

i) Chain Initiation : Potassium amide (K+NH2-) adds to C – C double bond of alkene to form stable

carbanion.

where W is electron withdrawing group

ii) Chain Propagation: Carbanion adds to the C – C double bond of another monomer molecule to

from new carbanion.

iii) Anionic polymerization has no chain termination reaction. So it is called living polymerization.

29

Coordination polymerization: It is also a subclass of addition polymerization. It usually

involve transition-metal catalysts. Here, the "active species" is a coordination complex, which initiates

the polymerization by adding to the monomer’s carbon-carbon double bond. The most important

catalyst for coordination polymerization is so-called Ziegler-Natta catalyst discovered to be effective

for alkene polymerization. Ziegler-Natta catalysts combine transition-metal compounds such as

chlorides of titanium with organometallic compounds [TiCl3 with Al(C2H5)3]. An important property

of these catalysts is that they yield stereoregular polymers when higher alkenes are polymerized,

e.g., polymerization of propene produces polypropene with high selectivity. Branching will not occur

through this mechanism since no radicals are involved; the active site of the growing chain is the

carbon atom directly bonded to the metal.

Zeigler-Nata catalysts: These are a special type of coordination catalysts, comprising two

components, which are generally referred to as the catalyst and the cocatalyst. The catalyst component

consists of chlorides of titanium (TiCl3 and TiCl4) and the cocatalysts are organometallic compound

such as triethyl aluminium (Al(C2H5)3).

Triethyl aluminium [A l(R)3] act as the electron acceptor whereas the electron donor is titanium

halides and the combination, therefore, readily forms coordination complexes (Fig. 1).

The complex formed, now acts as the active centre. The monomer is complexed with the metal ion of

the active centre in a way that the monomers attached towards the Ti—C bond (C from the

alkyl group R) in the active centre, when it forms a π complex with the Ti ion(Fig. 2).

Figure. 1 Figure. 2

The bonds between R and Ti opens up producing an electron deficient Ti and

a carbanion at R (Fig. 3). The Ti ion attracts the π electrons pair or the monomer and forms σ bond

(Fig. 4). This transition state now gives rise to the chain growth at the metal carbon bond, regenerating

the active centre (Fig. 5). Repeating the whole sequence, with the addition of second monomer

molecule, we will get the structure of the resultant chain growth as shown in Fig. 6.

Figure. 3 Figure. 4 Figure. 5 Figure. 6

30

Condensation Polymerisation: This type of polymerisation generally involves a repetitive

condensation reaction (two molecules join together, resulting loss of small molecules) between two bi-

functional monomers. These polycondensation reactions may result in the loss of some simple

molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation

polymers. In these reactions, the product of each step is again a bi-functional species and the sequence

of condensation goes on. Since, each step produces a distinct functionalised species and is independent

of each other; this process is also called as step growth polymerisation. The type of end polymer

product resulting from a condensation polymerization is dependent on the number of functional end

groups of the monomer which can react.

Monomers with only one reactive group terminate a growing chain, and thus give end products

with a lower molecular weight. Linear polymers are created using monomers with two reactive end

groups and monomers with more than two end groups give three dimensional polymers

which are cross linked.

Polyester is created through ester linkages between monomers, which involve the functional

groups carboxyl and hydroxyl (an organic acid and an alcohol monomer). The formation of polyester

like terylene or dacron by the interaction of ethylene glycol and terephthalic acid is an example of this

type of polymerisation.

31

Polyamide is created through amide linkages between monomers, which involve the functional

groups carboxyl and amine (an organic acid and an amine monomer). Nylon-6 is an example which

can be manufactured by the condensation polymerisation of hexamethylenediamine with adipic acid

under high pressure and at high temperature.

This type of polymerization normally employs two difunctional monomers that are capable of

undergoing typical organic reactions. For example, a diacid can be allowed to react with a diol in the

presence of an acid catalyst to afford polyester. In this case, chain growth is initiated by the reaction of

one of the diacid's carboxyl groups with one of the diol's hydroxyl groups. The free carboxyl or

hydroxyl group of the resulting dimer can then react with an appropriate functional group in another

monomer or dimer. This process is repeated throughout the polymerization mixture until all of the

monomers are converted to low molecular weight species, such as dimers, trimers, tetramers, etc.

These molecules, which are called oligomers, can then further react with each other through their free

functional groups. Polymer chains that have moderate molecular weight can he built in this manner.

32

The following are several general characteristics of this type of polymerization:

(1) The polymer chain forms slowly, sometimes requiring several hours to several days

(2) All of the monomers are quickly converted to oligomers, thus, the concentration of growing chains

is high (3) Since most of the chemical reactions employed have relatively high energies of activation,

the polymerization mixture is usually heated to high temperatures (4) Step-reaction polymerizations

normally afford polymers with moderate molecular weights, i.e., <100,000 (5) Branching or

crosslinking does not occur unless a monomer with three or more functional groups is used.

Copolymerization: It is a polymerisation reaction in which a mixture of more than one

monomeric species is allowed to polymerize and form a copolymer. The copolymer can be made not

only by chain growth polymerisation but by step growth polymerisation also. It contains multiple units

of each monomer used in the same polymeric chain. For example, a mixture of styrene and

1, 3 – butadiene can form a copolymer called styrene butadiene rubber (SBR).

33

Technology of PolymerizationTechnology of PolymerizationTechnology of PolymerizationTechnology of Polymerization

Monomers may be polymerised by the following methods (1) polymerization in homogeneous

systems (2) polymerization in heterogeneous systems

Polymerization in Homogeneous systems: The homogeneous polymerization techniques

involve pure monomer or homogeneous solutions of monomer and polymer in a solvent.

These techniques can be divided into two methods: (i) the bulk and (ii) the solution polymerizations.

Bulk polymerization: Bulk polymerization is the simplest technique and produces the highest-

purity polymers. Only monomer, a monomer-soluble initiator are used. This method helps easy

polymer recovery and minimum contamination of product. The viscosity of the mixture is low initially

to allow ready mixing, heat transfer, and bubble elimination. This method is used for the preparation

of polyethene, polystyrene, etc. Disadvantages: Reaction medium becomes increasingly viscous as

reaction goes to higher conversion, making stirring, heat removal and processing more difficult.

It leads to uneven polymerization and loss of monomer. Free-radical polymerizations are typically

highly exothermic. An increase temperature will increase the polymerization rate; generate heat

dissipation and a tendency to develop of localized “hot spots”. Near the end of polymerization, the

viscosity is very high and difficult to control the rate as the heat is “trapped” inside. It leads to the

autoacceleration process in which the propagation rate is very higher than that of termination rate. This

method is seldom used in commercial manufacture.

Solution polymerization: This method is used to solve the problems associated with the bulk

polymerization because the solvent is employed to lower the viscosity of the reaction, thus help in the

heat transfer and reduce autoacceleration. It requires the correct selection of the solvents. Both the

initiator and monomer be soluble in each other and that the solvent are suitable for boiling points,

regarding the solvent-removal steps. It is often used to produce copolymers. This method is used for

the preparation of polyvinyl acetate, poly (acrylic acid), and polyacrylamide.

Advantages: (i) Solvent has low viscosity, reaction mixture can be stirred (ii) Solvent acts as a

diluent and aids in removal of heat of polymerization (iii) Solvent reduces viscosity, making

processing easier (iv) Thermal control is easier than in the bulk and (v) “Cheap” materials for the

reactors (stainless steel or glass lined). Disadvantages: (i) Reduce monomer concentration which

results in decreasing the rate of the reaction and the degree of polymerization (ii) Mobility is reduced

and this can affect termination events, so the rate of reaction is increased (iii) Solvent may terminate

the growing polymer chain, leading to low molecular weight polymers (iv) Difficult to remove solvent

from final form, causing degradation of bulk properties (v) Clean up the product with a non solvent or

evaporation of solvent (vi) Small production per reactor volume (vii) Not suitable for dry polymers.

34

Polymerization in heterogeneous systems: Polymerization occurs in disperse phase as large

particles in water or occasionally in another non-solvent (suspension polymerisation), or dispersed as

fine particles. The last-named process is usually known as emulsion polymerisation.

Suspension (Bead or Pearl) polymerisation: Monomer, initiator (must soluble in monomer)

and polymer must be insoluble in the suspension media such as water i.e., the reaction mixture is

suspended as droplets in an inert medium. Suspension polymerization consists of an aqueous system

with monomer as a dispersed phase and results in polymer as a dispersed solid phase. This method is

used for the preparation of polystyrene, polyvinyl chloride, polyvinyl acetate, etc.

A reactor fitted with a mechanical agitator is charged with a water insoluble monomer and

initiator. Droplets of monomer (containing the initiator) are formed. As the polymerization proceeds,

the viscosity of dispersed phase increases and they become sticky. Aggregation of these sticky droplets

is prevented by the addition of a dispersing agent (protective colloid, e.g., water-soluble colloid such

as gum acacia). Near the end of polymerization, the particles are hardened, are the bead or pearl

shaped polymers recovered by filtration, and followed by washing step.

Advantages: (i) Polymerisation to high conversion (ii) Low viscosity due to the suspension

(iii) Easy heat removal due to the high heat capacity of water (iv) Excellent heat transfer because of

the presence of the solvent (v) Solvent cost and recovery operation are cheap

(vi) Polymerization yields finely divided, stable latexes and dispersions to be used directly in coatings,

paints, and adhesives. Disadvantages: (i) Contamination by the presence of suspension and other

additives low polymer purity (ii) Must separate and purify polymer, or accept contaminated product

(iii) Reactor cost may higher than the solution cost

Emulsion polymerisation: An emulsion polymerization consists of water (as the heat-transfer

agent), monomer, initiator (is soluble in water and insoluble in the monomer), a surfactant or

emulsifier (such as sodium salt of long-chain fatty acid). This method is used for the preparation of

polyvinyl acetate, polychloroprene, butadiene/styrene/acrylonitrile copolymers, etc.

A typical recipe for emulsion polymerization consists of water, monomer, fatty acid soap

(emulsifying agent), and water soluble initiator. When a small amount of soap is added to water,

the soap ionizes and the ions move around freely. The soap anion consists of a long oil-soluble portion

(R) terminated at one end by the water-soluble portion. So emulsifier molecules arrange themselves

into colloidal particles called micelles. In water containing a insoluble monomer molecule, the soap

anion molecules orient themselves at the water–monomer interfaces with the hydrophilic ends facing

the water, while the hydrophobic ends face the monomer phase. When the water-soluble initiator

undergoes thermal decomposition to form the water-soluble radicals react with monomer dissolved in

35

interior of the micelle. Emulsion polymerization takes place almost exclusively in the micelles.

As polymerization proceeds, the active micelles consume the monomers within the micelle.

Monomer depletion within the micelle is replenished first from the aqueous phase and subsequently

from the monomer droplets. The active micelles grow in size with polymer formation, to preserve their

stability; these growing polymer particles absorb the soap of the parent micelles.

Advantages: (i) Overcomes many environmental problems: “solvent” is water (ii) If final desired

product is polymer is washed with water to remove the soap phase by coagulation

Bulk Solution Suspension Emulsion

Suspension Emulsion

Concept of Concept of Concept of Concept of Glass Transition Temperature Glass Transition Temperature Glass Transition Temperature Glass Transition Temperature (T(T(T(Tgggg))))

The glass transition temperature (Tg) is the temperature at which the internal energy of the

chains of the polymer increases such as extends that the chains just starts leaving their lattice sites.

It is the temperature at which a hard amorphous polymer becomes soft. It is the transformation from a

rigid material to one that has rubber like characteristics and temperature has large effect on chain

flexibility. Below glass transition temperature (Tg), polymers are usually hard, brittle and glass-like in

mechanical behavior. Above glass transition (Tg), polymers are usually more soft and rubbery (elastic).

Why is that? bond rotations are “freezing” which means chains can’t slip past each other so polymer

becomes brittle.

36

Melting of a crystalline polymer (Tm): Transforming solid with an ordered structure to a

viscous liquid with a highly random structure. It is the temperature at which the crystalline regions of

the polymer melt to become amorphous. More ordered polymers have higher Tm

In the study of polymers and their applications, it is important to understand the concept of the

glass transition temperature Tg. The glass transition is a phenomenon observed in linear amorphous

polymer. It occurs at fairly well defined temperature when the bulk material ceases to be brittle and

glassy in character and become less rigid and more rubbery. The knowledge of Tg is essential in the

selection of materials for various applications.

Many Physical properties change profoundly at the glass transition temperature, including

mechanical properties and electrical properties. All of these are dependent on the relative degree of

freedom for molecular motion within a given polymeric material and each can be used to monitor the

point at which the glass transition occur.

37

Factors affecting glass transition temperature: Any structural features or externally imposed

conditions that influence chain mobility will also affect the value of Tg. Some of these structural

factors include chain flexibility; stiffness, including steric hindrance, polarity, or interchain attractive

forces; geometric factors; copolymerization; molecular weight, branching; cross-linking; and

crystallinity. External variables are plasticization, pressure, and rate of testing.

1. Chain Flexibility: Chain flexibility is determined by the ease with which rotation occurs about

primary valence bonds. Polymers with low hindrance to internal rotation have low Tg values.

Long-chain aliphatic groups — ether and ester linkages — enhance chain flexibility, while rigid

groups like cyclic structures stiffen the backbone.

2. Geometric Factors: Geometric factors, such as the symmetry of the backbone and the presence of

double bonds on the main chain, affect Tg. Polymers that have symmetrical structure have lower Tg

than those with asymmetric structures. Additional groups near the backbone for the symmetrical

polymer would enhance steric hindrance and consequently raise Tg. Another geometric factor affecting

Tg is cis–trans configuration. Double bonds in the cis form reduce the energy barrier for rotation of

adjacent bonds, “soften” the chain, and hence reduce Tg.

3. Interchain Attractive Forces: Intermolecular bonding in polymers is due to secondary attractive

forces. Consequently, it is to be expected that the presence of strong intermolecular bonds in a

polymer chain, i.e., a high value of cohesive energy density, will significantly increase Tg.

The steric effects of the groups like CH3, –Cl, and –CN are similar, but the polarity increases,

consequently, Tg is increased. The same effect of increased Tg when one considers going from the

intermolecular forces in poly(methyl acrylate), an ester, through the strong hydrogen bonds in

poly(acrylic acid) to primary ionic bonds in poly(zinc acrylate). Secondary bonding forces are

effective only over short molecular distances. Therefore, any structural feature that tends to increase

the distance between polymer chains decreases the cohesive energy density and hence reduces Tg.

4. Copolymerization: It is desirable to be able to control Tg, however, this is often impossible.

Polymer chemists have circumvented this problem to some extent by copolymerization.

A copolymer system may be characterized by the arrangement of the different monomers

(random, alternating, graft, or block). The increased disorder resulting from the random or

alternating distribution of monomers enhances the free volume and consequently reduces Tg.

For block or graft copolymers in which the component monomers are incompatible, phase separation

will occur. Two separate glass transition values will be observed, each corresponding to the Tg of the

homopolymer.

38

5. Molecular Weight: Since chain end segments are restricted only at one end, they have relatively

higher mobility than the internal segments, which are constrained at both ends. At a given temperature,

therefore, chain ends provide a higher free volume for molecular motion. As the number of chain ends

increases (which means a decrease in molecular weight), the available free volume increases,

and consequently there is a depression of Tg.

6. Cross-Linking and Branching: By definition, cross-linking involves the formation intermolecular

connections through chemical bonds. This process necessarily results in reduction in chain mobility.

Consequently, Tg increases. For lightly cross-linked systems like vulcanized rubber,

Tg shows a moderate increase over the uncross-linked polymer. For highly cross-linked systems like

phenolics and epoxy resins, the glass transition is virtually infinite. This is because the molecular chain

length between cross-links becomes smaller than that required for cooperative segmental motion.

Like long and flexible side chains, branching increases the separation between chains, enhances the

free volume, and therefore decreases Tg.

7. Crystallinity: In semicrystaline polymers, the crystallites may be regarded as physical cross-links

that tend to reinforce or stiffen the structure. Viewed this way, it is easy to visualize that Tg will

increase with increasing degree of crystallinity.

8. Plasticization: Plasticity is the ability of a material to undergo plastic or permanent deformation.

Consequently, plasticization is the process of inducing plastic flow in a material. In polymers, this can

be achieved in part by the addition of low-molecular-weight organic compounds referred to as

plasticizers. Plasticizers are usually nonpolymeric, organic liquids of high boiling points. Plasticizers

are miscible with polymers and, in principle, should remain within the polymer. Addition of

plasticizers to a polymer, even in very small quantities, drastically reduces the Tg of the polymer.

Plasticizers function through a solvating action by increasing intermolecular distance, thereby

decreasing intermolecular bonding forces.

Crystallinity in polymersCrystallinity in polymersCrystallinity in polymersCrystallinity in polymers

Linear and branched polymers do not form crystalline solids because their long chains prevent

efficient packing in a crystal lattice. Most polymer chains have crystalline regions and amorphous

regions: Ordered crystalline regions, called crystallites, are places where sections of the polymer chain

lie in close proximity and are held together by intermolecular interactions. Ordered regions of

polyethylene, are held together by van der Waals interactions, whereas ordered regions of nylon chains

are held together by intermolecular hydrogen bonding. Amorphous regions are places where the

polymer chains are randomly arranged, resulting in weak intermolecular interactions.

Crystalline regions impart toughness to a polymer, while amorphous regions impart flexibility.

39

The greater the Crystallinity of a polymer—that is, the larger the percentage of ordered regions—the

harder the polymer. Branched polymers are generally more amorphous and, since branching prevents

chains from packing closely, they are softer, too.

Although it may at first seem surprising, polymers can form crystal structures (all we need is a

repeating unit, which can be based on molecular chains rather than individual atoms).

Some parts of structure align during cooling to form crystalline regions (chains align alongside each

other). Around crystallites get amorphous regions. Most real polymers contain both amorphous and

crystalline regions, called semicrystaline.

The morphology of most polymers is semi-crystalline. That is, they form mixtures of small

crystals and amorphous material and melt over a range of temperature instead of at a single melting

point. The crystalline material shows a high degree of order formed by folding and stacking of the

polymer chains. An amorphous solid is formed when the chains have little orientation throughout the

bulk polymer. The amorphous or glass-like structure shows no long range order, and the chains are

tangled as illustrated below. There are some polymers that are completely amorphous, but most are a

combination with the tangled and disordered regions (amorphous regions) surrounding the crystalline

areas called semicrystaline. Such a combination is shown in the following diagram.

Crystalline Amorphous Crystalline and Amorphous regions

Semicrystaline

40

In the crystallization process, it has been observed that relatively short chains organize

themselves into crystalline structures more readily than longer molecules. Therefore, the degree of

polymerization (DP) is an important factor in determining the Crystallinity of a polymer. Polymers

with a high DP have difficulty organizing into layers because they tend to become tangled.

The cooling rate also influences the amount of Crystallinity. Slow cooling provides time for

greater amounts of crystallization to occur. Fast rates, on the other hand, such as rapid quenches, yield

highly amorphous materials. Subsequent annealing (heating and holding at an appropriate temperature

below the crystalline melting point, followed by slow cooling) will produce a significant increase in

Crystallinity in most polymers, as well as relieving stresses.

Low molecular weight polymers (short chains) are generally weaker in strength. Although they

are crystalline, only weak Van der Waals forces hold the lattice together. This allows the crystalline

layers to slip past one another causing a break in the material. High DP (amorphous) polymers,

however, have greater strength because the molecules become tangled between layers. In the case of

fibers, stretching to 3 or more times their original length when in a semi-crystalline state produces

increased chain alignment, Crystallinity and strength. In most polymers, the combination of crystalline

and amorphous structures forms a material with advantageous properties of strength and stiffness.

Also influencing the polymer morphology is the size and shape of the monomers' substituent

groups. If the monomers are large and irregular, it is difficult for the polymer chains to arrange

themselves in an ordered manner, resulting in a more amorphous solid. Likewise, smaller monomers,

and monomers that have a very regular structure (e.g. rod-like) will form more crystalline polymers.

% Crystallinity depends on several factors: Rate of cooling (faster cooling – less Crystallinity).

Type of polymer (simple structures – more Crystallinity, Copolymers – less Crystallinity).

Linear polymers more easily form crystals. Degree of Crystallinity ranges from 5 - 95% -

The higher % Crystallinity → higher strength. When polymers are crystallized they form spherical

structures called spherulites.

Physical properties of polymersPhysical properties of polymersPhysical properties of polymersPhysical properties of polymers

41

Mechanical properties of polymers Mechanical properties of polymers Mechanical properties of polymers Mechanical properties of polymers (Density, Tensile, Tear, Abrasion resistance, Resilience)

42

Engineering applications of polymers are governed to a great extent by strain hardening

considerations. The designer using polymeric materials must, therefore, understand their mechanical

behaviour with respect to the maximum permissible strains to avoid failure. As for most materials, a

simple tensile stress-strain curve provides a good start towards understanding the mechanical

behaviour of a particular polymer. This curve is usually established by continuously measuring the

force developed as the sample is elongated at constant rate of extension until it breaks.

Portions of the curve in Fig.1 represent the stress-strain behaviour of any polymer and are used to

define several useful quantities.

The initial slope provides a value for Young's modulus (or the modulus of elasticity) which is a

measure of stiffness. The curve also gives yield stress, strength and elongation at break.

The area under the curve or work to break is a rough indication of the toughness of the polymeric

material. The stress at the knee in the curve (known as the yield point) is a measure of the strength of

the material and resistance to permanent deformation. The stress at the breaking point, commonly

known as ultimate strength, is a measure of the force required to fracture the material completely.

Hard and Brittle material such as an amorphous polymer far below its Tg, usually has an

initial slope indicative of very high modulus, moderate strength, a low elongation at break, and a low

area under the stress-strain curve. Polymeric materials showing hard brittle behaviour at room

temperature or below are polystyrene, poly(methyl methacrylate) and phenol-formaldehyde resins.

Hard and Strong polymers have high modulus of elasticity, high strength, and elongation at

break of approximately 5 percent. The shape of the curve often suggests that the material has broken

where a yield point might be expected. This type of curve is characteristic of thermoplastics such as

poly(vinyl chloride) formulations and polystyrene polyblends.

Hard and Tough polymers have high yield points, high modulus, high strength and large

elongations. This behaviour is shown by polymers such as fibers like rayon, dacron and nylons.

Soft and tough polymers have low yield points, low modulus, moderate strength at break,

and very high elongation ranging from 20 to 100 percent. This behaviour is shown by polymers such

as elastomers like NR, SBR, and NBR

43

Hardness: The ability of a polymer to resist scratching, abrasion, cutting, or penetration. It is measured by its ability to

absorb energy under impact loads. It is also a measure of the wearing quality of a material and it is an indication of machinability

qualities of the polymer. Toughness: It is the amount of energy a polymer can absorb before actual fracture or failure takes place.

The ability of a polymer to withstand shock and vibrations. It is related to impact strength, i.e., resistance to shock loading.

It is the ability of a polymer to withstand both plastic and elastic deformation. Stiffness: The resistance of a polymer to elastic

deformation, i.e., a polymer which suffers slight deformation under load has a high degree of stiffness.

[1] Density: Mass per unit volume (at defined temperature). Relative Density is the mass of the

polymer with the mass of equal volume of a specific (reference) substance most often water.

Density is frequently measured as a quality control parameter. A specimen, with smooth surfaces

from crevices and dust, is weighed in air (W1) and then in freshly boiled water (W2),

then ρ�� = � ��

ρ�� [2] Tensile Strength: The strength of a polymer is its capacity to withstand destruction under

the action of loads. It determines the ability of a polymer to withstand stress without failure.

Tensile strength or ultimate strength is the stress corresponding to the maximum load reached before

rupturing the polymer, TensilestrengthorStress = ��!��"�#$%&��'(��)*��+++!�$�,

[3] Abrasion Resistance: It is defined as the ability of a polymer to withstand mechanical action

(such as rubbing, scrapping, or erosion) that tends progressively to remove material from its surface.

Abrasion is closely related to frictional force, load and true area of contact. An increase in any one of

the three results in greater abrasion or wear. Abrasion process also creates oxidation on the surface

from the build up of localized high temperatures.

[4] Resilience: It is the capacity of a polymer to absorb energy elastically. When a body is loaded,

it changes its dimension, and on the removal of the load it regains its original dimensions.

In fact, the polymer behaves perfectly like a spring. So long as it remains loaded, it has stored energy

in itself. On removal of the load, the energy stored is given off exactly as in a spring when the load is

removed. Resilience gives capacity of the polymer to bear shocks and vibrations.

[5] Wear and Tear: It occurs when a steady rate of increase in the use of polymers in bearing

applications and in situations where there is sliding contact e.g. gears, piston rings, seals, cams, etc.

Wear and tear is characterized by fine particles of polymer being removed from the surface or

the polymer becomes overheated to the extent where large troughs of melted polymer are removed.

The wear and tear of polymers is extremely complex subjects which depend markedly on the nature of

the application and the properties of the material. Hence it is characterized by adhesion and

deformation which results in frictional forces that are not proportional to load but rather to speed.

The mechanism of wear and tear is complex; the relative rates may change depending on specific

circumstance.

44

Lubricants

In all types of machines, the surfaces of moving or sliding or rolling parts rub against each

other. Due to the mutual rubbing of one part against another, a resistance is offered to their movement.

This resistance is known as friction . It causes a lot of wear and tear of surfaces of moving parts.

Any substance introduced between two moving/sliding surfaces with a view to reduce the friction

(or frictional resistance) between them, is known as a lubricants. The main purpose of a lubricant is

to keep the moving/sliding surfaces apart, so that friction and consequent destruction of material is

minimized. The process of reducing friction between moving/sliding surfaces, by the introduction of

lubricants in between them, is called lubrication.

Function of Lubricants: (1) It reduces wear and tear of the surfaces by avoiding direct metal

to metal contact between the rubbing surfaces, i.e. by introducing lubricants between the two surfaces

(2) It reduces expansion of metal due to frictional heat and destruction of material

(3) It acts as coolant of metal due to heat transfer media (4) It provides smooth relative motion

(5) It reduces maintenance cost (6) It also reduces power loss in internal combustion engines

Theories of Friction: (1) Welding theory: All metal surfaces, regardless how much finely

finished they are, appear as a series of peaks (or asperites) and valleys.

So when two solid surfaces are pressed one over the other, only the peaks of the two surfaces come in

real contact. Under the action of a load, the local pressure at the peaks becomes sufficiently great to

cause deformation of the peaks to create weld junctions between them.

(2) Mechanical Interlocking: When one surface moves over another, the peaks and valleys present

on the surface undergo interlocking; restrict the movement of one surface over the other.

This accounts for static friction. (3) Molecular Attraction: Atoms of one material are plucked out of

the attractive range of their counterparts on the mating surface, lead to the friction.

(4) Electrostatic Attraction: When stick-slip phenomenon takes place between rubbing metal surfaces,

a net flow of electrons takes place producing clusters of charges of opposite polarity at the interface.

These charges are responsible for holding the surfaces together by electrostatic attraction.

Mechanism of Lubrication: The phenomenon of lubrication can be explained

with the help of the following mechanism; (a) Thick-Film lubrication (Fluid- Film

or hydrodynamic lubrication) (b) Thin Film lubrication (Boundary lubrication) and

(c) Extreme Pressure lubrication

45

(a) Thick-Film lubrication: In this, moving/sliding surfaces are separated from each other by

a thick film of fluid (at least 1000 A° thick), so that direct surface to surface contact and welding of

welding of junctions rarely occurs. The lubricant film covers/fills the irregularities of moving/sliding

surfaces and forms a thick layer between them, so that there is no direct contact between the material

surfaces. This consequently reduces friction.

The lubricant chosen should have the minimum viscosity (to reduce the internal resistance

between the particles of the lubricant) under working conditions and at the same time,

it should remain in place and separate the surfaces.

Hydrocarbon oils (mineral oils which are lower molecular weight hydrocarbons with about

12 to 50 carbon atoms) are considered to be satisfactory lubricants for thick-film lubrication.

In order to maintain the viscosity of the oil in all seasons of year, ordinary hydrocarbon lubricants are

blended with selected long chain polymers.

(b) Thin Film lubrication: This type of lubrication is preferred where a continuous film of

lubricant cannot persist. In such cases, the clearance space between the moving/sliding surfaces is

lubricated by such a material which can get adsorbed on both the metallic surfaces by either physical

or chemical forces. This adsorbed film helps to keep the metal surfaces away from each other at least

up to the height of the peaks present on the surface.

46

Vegetable and animal oils and their soaps can be used in this type of lubrication because they

can get either physically adsorbed or chemically react in to the metal surface to form a thin film of

metallic soap which can act as lubricant. Although these oils have good oiliness, they suffer from the

disadvantage that they will break down at high temperatures. On the other hand, mineral oils are

thermally stable and the addition of vegetable/animal oils to mineral oils, their oiliness can also be

brought up. Graphite and molybdenum disulphide are also suitable for thin-film lubrication.

(c) Extreme Pressure lubrication: When the moving/sliding surfaces are under very high

pressure and speed, a high local temperature is attained under such conditions, liquid lubricants fail to

stick and may decompose and even vaporize. To meet these extreme pressure conditions, special

additives are added to minerals oils. These are called extreme pressure additives. These additives form

more durable films (capable of withstanding very high loads and high temperatures) on metal surfaces.

Important additives are organic compounds having active radicals or groups such as chlorine

(as in chlorinated esters), sulphur (as in sulphurized oils) or phosphorus (as in tricresyl phosphate).

These compounds react with metallic surfaces, at existing high temperatures, to form metallic

chlorides, sulphides or phosphides.

Classification of Lubricants: Lubricants are classified on the basis of their physical state,

as follows; (a) Liquid lubricants or Lubricating Oils, (b) Semi-solid lubricants or Greases and

(c) Solid lubricants.

(a) Liquid lubricants or Lubricating oils: Lubricating oils also known as liquid lubricants.

The characteristics of good lubricating oils are: (1) high boiling point (2) low freezing point

(3) adequate viscosity for proper functioning in service (4) high resistance to oxidation and heat

(5) non-corrosive properties and (6) stability to decomposition at the operating temperatures.

Lubricating oils are classified into three categories; (i) Animal and Vegetables oils,

(ii) Mineral or Petroleum oils and (iii) blended oils.

(i) Animal and Vegetables oils: Animal oils are extracted from the crude fat and vegetables oils

such as cotton seed oil and caster oils. These oils possess good oiliness and hence they can stick on

metal surfaces effectively even under elevated temperatures and heavy loads. But they suffer from the

disadvantages that they are costly, undergo easy oxidation to give gummy products and hydrolyze

easily on contact with moist air or water. Hence they are only rarely used these days for lubrication.

But they are still used as blending agents in petroleum based lubricants to get improved oiliness.

(ii) Mineral or Petroleum oils: These are basically lower molecular weight hydrocarbons with

about 12 to 50 carbon atoms. As they are cheap, available in abundance and stable, hence they are

widely used. But the oiliness of mineral oils is less, so the addition of higher molecular weight

compounds like oleic acid and stearic acid increases the oiliness of mineral oil.

47

(iii) Blended oils: No single oil possesses all the properties required for a good lubricant and

hence addition of proper additives is essential to make them perform well.

Such additives added lubricating oils are called blended oils. Examples: The addition of higher

molecular weight compounds like oleic acid, stearic acid, palmetic acid, etc or vegetables oil like

coconut oil, castor oil, etc increases the oiliness of mineral oil.

(b) Semi-solid Lubricants or Grease: A semi-solid lubricant obtained by combining

lubricating oil with thickening agents is termed as grease. Lubricating oil is the principal component

and it can be either petroleum oil or a synthetic hydrocarbon of low to high viscosity. The thickeners

consist primarily of special soaps of Li, Na, Ca, Ba, Al, etc. Non-soap thickeners include carbon black,

silica gel, polyureas and other synthetic polymers, clays, etc. Grease can support much heavier load at

lower speed. Internal resistance of grease is much higher than that of lubricating oils; therefore it is

better to use oil instead of grease. Compared to lubricating oils, grease cannot effectively dissipate

heat from the bearings, so work at relatively lower temperature.

(c) Solid lubricants: They are preferred where (1) the operating conditions are such that a

lubricating film cannot be secured by the use of lubricating oils or grease

(2) contamination (by the entry of dust particles) of lubricating oils or grease is unacceptable

(3) the operating temperature or load is too high, even for grease to remain in position and

(4) combustible lubricants must be avoided. They are used either in the dry powder form or with

binders to make them stick firmly to the metal surfaces while in use.

They are available as dispersions in non-volatile carriers like soaps, fats, waxes, etc

and as soft metal films.

The most common solid lubricants are graphite, molybdenum disulphide,

tungsten disulphide and zinc oxide. They can withstand temperature upto 650° C and

can be applied in continuously operating situations. They are also used as additives to mineral oils and

greases in order to increase the load carrying capacity of the lubricant.

Other solid lubricants in use are soapstone (talc) and mica.

Graphite: It is the most widely used of all the solid lubricants and can be used either in the

powdered form or in suspension. It is soapy to touch; non-inflammable and stable upto a temperature

of 375° C. Graphite has a flat plate like structure and the layers of graphite sheets are arranged one

above the other and held together by weak van der Waal’s forces. These parallel layers which can

easily slide one over other make graphite an effective lubricant. Also the layer of graphite has a

tendency to absorb oil and to be wetted of it.

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Molybdenum Disulphide: It has a sandwich-like structure with a layer of molybdenum atoms

in between two layers of sulphur atoms. Poor interlaminar attraction helps these layers to slide over

one another easily. It is stable upto a temperature of 400° C.

Properties of Lubricants: (1) Viscosity (2) Flash Point and Fire Point

(3) Cloud Point and Pour Point (4) Aniline Point and (5) Corrosion Stability

(1) Viscosity: It is the property of liquid by virtue of which it offers resistance to its own flow

(the resistance to flow of liquid is known as viscosity). The unit of viscosity is poise. It is the most

important single property of any lubricating oil, because it is the main determinant of the operating

characteristics of the lubricant. If the viscosity of the oil is too low, a liquid oil film cannot be

maintained between two moving/sliding surfaces. On the other hand, if the viscosity of the oil is too

high, excessive friction will result. Effect of temperature on viscosity: Viscosity of liquids decreases

with increasing temperature and, consequently, the lubricating oil becomes thinner as the operating

temperature increases. Hence, viscosity of good lubricating oil should not change much with change in

temperature, so that it can be used continuously, under varying conditions of temperature.

The rate at which the viscosity of lubricating oil changes with temperature is measured by

an arbitrary scale, known as Viscosity Index (V. I). If the viscosity of lubricating oil falls rapidly

as the temperature is raised, it has a low viscosity index. On the other hand, if the viscosity of

lubricating oil is only slightly affected on raising the temperature, its viscosity index is high.

(2) Flash Point and Fire Point: Flash point is the lowest temperature at which the lubricant oil

gives off enough vapours that ignite for a moment, when a tiny flame is brought near it;

while Fire point is the lowest temperature at which the vapours of the lubricant oil burn continuously

for at least five seconds, when a tiny flame is brought near it. In most cases, the fire points are

5° C to 40° C higher than the flash points. The flash and fire do not have any bearing with lubricating

property of the oil, but these are important when oil is exposed to high temperature service.

A good lubricant should have flash point at least above the temperature at which it is to be used.

This safeguard against risk of fire during the use of lubricant.

(3) Cloud Point and Pour Point: When the lubricant oil is cooled slowly,

the temperature at which it becomes cloudy or hazy in appearance, is called its cloud point;

while the temperature at which the lubricant oil cease to flow or pour, is called its pour point.

Cloud and pour points indicate the suitability of lubricant oil in cold conditions.

Lubricant oil used in a machine working at low temperatures should possess low pour point;

otherwise solidification of lubricant oil will cause jamming of machine. It has been found that

presence of waxes in the lubricant oil raise pour point.

49

(4) Aniline Point: Aniline point of the lubricant oil is defined as the minimum equilibrium

solution temperature for equal volumes of aniline and lubricant oil samples. It gives an indication of

the possible deterioration of the lubricant oil in contact with rubber sealing; packing, etc.

Aromatic hydrocarbons have a tendency to dissolve natural rubber and certain types of synthetic

rubbers. Consequently, low aromatic content in the lubricant oil is desirable. A higher aniline point

means a higher percentage of paraffinic hydrocarbons and hence, a lower percentage of aromatic

hydrocarbons. Aniline point is determined by mixing mechanically equal volumes of the lubricant oil

samples and aniline in a test tube. The mixture is heated, till homogenous solution is obtained.

Then, the tube is allowed to cool at a controlled rate. The temperature at which the two phases

(the lubricant oil and aniline) separate out is recorded at the aniline point.

(5) Corrosion Stability: Corrosion stability of the lubricant oil is estimated by carrying out

corrosion test. A polished copper strip is placed in the lubricant oil for a specified time at a particular

temperature. After the stipulated time, the strip is taken out and examined for corrosion effects.

If the copper strip has tarnished, it shows that the lubricant oil contains any chemically active

substances which cause the corrosion of the copper strip. A good lubricant oil should not effect the

copper strip. To retard corrosion effects of the lubricant oil, certain inhibitors are added to them.

Commonly used inhibitors are organic compounds containing P, As, Cr, Bi or Pb.

Essential requirements or characteristics of a good lubricant are as follows:

[1] It should have a high viscosity index [2] It should have flash and fire points higher than the

operating temperature of the machine [3] It should have high oiliness [4] The cloud and pour points of

a good lubricant should always be lower than the operating temperature of the machine

[5] The volatility of the lubricating oil should be low [6] It should deposit least amount of carbon

during use [7] It should have higher aniline point [8] It should possess a higher resistance towards

oxidation and corrosion [9] It should have good detergent quality

Syllabus

Lubricants ––– Theories of friction ––– Mechanism of lubrication (thick film,

thin film and extreme pressure) ––– Classification (solid, liquid and semisolid) –––

Properties (viscosity, flash point and fire point, cloud and pour point, aniline point,

and corrosion stability)

50

Fuels

CLASSIFICATION OF FUEL

Fuels are classified as follows: [1] Primary fuels which occur in nature, e.g. coal, petroleum

and natural gas [2] Secondary fuels which are derived from the primary fuels, e.g. coke, gasoline and

coal gas. Both primary and secondary fuels may be further classified based upon their physical state as

(a) solid fuels, (b) liquid fuels and (c) gaseous fuels

CHARACTERISTICS OF A GOOD FUEL

The following are the characteristics of a good fuel: [1] A good fuel should be cheap and

readily available [2] It should have a high calorific value [3] A good fuel should have a moderate

ignition temperature. If the ignition temperature is low, the fuel can catch fire easily and the risk of fire

hazards is high. If the ignition temperature is very high, it is very difficult to ignite the fuel.

Ignition temperature is defined as the minimum temperature at which the substance ignites and bums

without further addition of external heat [4] The moisture content of the fuel should be very low

because the moisture content reduces the calorific value [5] A good fuel should have low non-

combustible matter content or ash content because the ash content reduces the calorific value or

heating value of the fuel. The disposal of ash is also a big problem and it increases the cost of

operation [6] The products of combustion should not pollute the atmosphere. Gases like CO, SO2 and

H2S are some of the harmful gases [7] Combustion should be easily controllable, i.e. combustion of

the fuel should start easily or stop when required [8] It should not undergo spontaneous combustion

[9] It should be safe, convenient and economical for storage and transport.

51

ADVANTAGES AND DISADVANTAGES OF SOLID, LIQUID AND G ASEOUS FUELS

Solid Fuels: The following are the advantages of solid fuels: [1] Solid fuels occur widely and

they are cheap [2] They can be handled and transported very easily [3] No complex mechanism is

required for their burning [4] They can be stored conveniently without any problem like explosion

[5] They have a moderate ignition temperature.

The disadvantages of solid fuels are as follows: [1] Solid fuels form a lot of ash during burning

and disposal of ash is very difficult [2] A lot of labour is required to transport solid fuels

[3] The burning process of solid fuels is not as clear as that of liquid and gaseous fuels

[4] A large space is required for storage of solid fuels and sometimes they may undergo spontaneous

ignition. [5] Since a lot of air is necessary for complete combustion of solid fuels, the thermal

efficiency is not so high.

Liquid Fuels: The advantages of liquid fuels are as follows: [1] Liquid fuels occupy less storage

space than solid fuels [2] As compared to solid fuels; they have a high calorific value

[3] They can be easily transported through pipes. 4. Liquid fuels do not yield any ash or residue during

burning [5] The burning process of liquid fuels is clear [6] The combustion is uniform and very easily

controllable [7] For complete combustion of liquid fuels, less air is required than that of the solid fuels

and hence their thermal efficiency is high [8] They can be used in IC engines, boilers and gas turbines

[9] They do not undergo spontaneous combustion.

The disadvantages of liquid fuels are as follows: [1] When the liquid fuels undergo incomplete

combustion, they give unpleasant odour [2] In comparison with solid fuels they are costly

[3] Risk of fire hazards is more in the case of inflammable and volatile liquid fuels. Thus, they should

be stored and transported more carefully [4] Some amount of liquid fuels may escape due to

evaporation during storage [5] Special type of burners and sprayers are required for effective

combustion.

Gaseous Fuels: The following are the advantages of gaseous fuels: [1] Gaseous fuels on

burning do not produce any ash or smoke [2] They can be very easily transported to any place as they

can flow through the pipes [3] They have a high calorific value and produce high temperature

[4] Burning of fuels can be controlled and the nature of flame can be easily made oxidizing or

reducing [5] Gaseous fuels can be produced by the poorest quality of solid fuels [6] Gaseous fuels can

be used in IC engines, boilers and gas turbines [7] Compared to solid and liquid fuels, they have a high

thermal efficiency.

The following are the disadvantages of gaseous fuels: [1] Since gases occupy a large volume,

they require large storage tank s[2] Gaseous fuels are highly inflammable and the Possibility of fire

hazards is very high

52

CALORIFIC VALUE

The efficiency of a fuel is judged by its calorific value. The calorific value of a fuel is defined

as the total amount of heat liberated by the complete combustion of a unit mass of the fuel.

The quantity of heat can be measured by the unit, Calorie. The calorie is defined as the amount

of heat required to raise the temperature of I gram of water through 1°C (l5-16°C).

The calorific values of solid and liquid fuels are expressed in cal/g. The calorific values of gaseous

fuels are expressed in cal/m3.

Gross calorific value and Net calorific value: When a fuel containing hydrogen is burnt,

the hydrogen present in the fuel undergoes combustion and is converted into steam. As the products of

combustion are cooled to room temperature, the steam gets condensed into water and latent

heat is evolved. Thus, the latent heat of condensation of steam is also included in the calorific

value determination which is called higher calorific value or gross calorific value (GCV).

GCV is defined as the heat liberated when a unit quantity of the fuel is completely burnt and the

products of combustion are cooled to room temperature.

In actual combustion practice, the products of combustion are not cooled to room temperature

and are allowed to escape into the atmosphere with the result that only a lesser amount of heat is

available. The amount of heat so available is called lower or net calorific value (NCV).

The net calorific value is defined as the net heat produced when a unit quantity of the fuel is

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

Net calorific value = gross calorific value - latent heat of condensation of water vapour

produced = GCV - mass of hydrogen per unit weight of the fuel burnt x 9 x latent heat of condensation

of water vapour

53

54

55

Fixed Bed Catalytic Cracking Moving Bed Catalytic Cracking

Reforming is a process used to improve the anti-knock characteristics of a gasoline by bringing about structural modifications in the components of gasoline either thermally or in the presence of catalyst. The main reactions in the reforming process are as follows: [1] Dehydrogenation [2] Hydrocracking [3] Isomerization

Dehydrogenation Hydrocracking Isomerization

56

Knocking

Knocking is a kind of explosion due to rapid pressure rise occurring in an IC engine.

In a petrol engine, a mixture of gasoline vapour and air (l: 17) is used as a fuel. The air and gasoline

vapours are compressed and ignited by an electric spark. The chemical reaction taking place is the

oxidation of hydrocarbons. The products of the oxidation reaction drive the piston down the cylinder.

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

circumstances, the oxidation is sudden and the mixture detonates and produces an explosive sound

called engine knock which results in the loss of power. Knocking not only results in a decreased power

output but can also cause mechanical damage by overheating of the cylinder parts.

A good gasoline should resist knocking. It was recognized that chemical structures of the fuel

hydrocarbons largely determine their knocking tendency. The tendency to knock decreases in the

following order: straight chain paraffins > branched chain paraffins > cyclo-paraffins > olefins >

aromatics.

Edgar introduced the octane number to express the knocking chracteristics of a combustion

engine fuel. It has been found that n-heptane knocks very badly and hence its antiknock value is

arbitrarily given as zero. On the other hand, isooctane gives very little knocking and so its anti-knock

value has been given as 100. Thus, the octane number is defined as the percentage of isooctane in the

n-heptane-isooctane blend which has the same knocking characteristics as the gasoline sample, under

the same set of conditions. Since isooctane has good anti-knock properties, it is clear that greater the

octane number, greater is the resistance to knocking.

Chemical structure and knocking: The knocking tendency decreases with increase in

compactness of the molecules, double bonds and cyclic structure. With normal paraffins, the antiknock

properties decrease with the increase in length of the hydrocarbon chain. Thus, the octane numbers of

n-butane, n-pentane, n-hexane and n-heptane are 90, 60, 29 and 0 respectively.

Branched chain paraffins have higher anti-knock properties than their normal isomers.

Olefins have higher anti-knock properties than the corresponding paraffins. Aromatic hydrocarbons

such as benzene and toluene have high octane numbers.

Leaded Petrol

The anti-knock properties of a gasoline can be improved by the addition of suitable additives.

Tetraethyl lead (TEL) is added to petrol and is called leaded petrol. This addition process is called

doping. This addition was first proposed by Thomas Midgley. TEL reduces the knocking tendency of

hydrocarbons. Knocking is a free radical mechanism leading to a chain reaction 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 the knocking

57

process and thus stop the chain growth. When this leaded petrol is used as a fuel, lead and lead oxide

vapours formed may contaminate the atmosphere. To avoid this, ethylene dibromide is added along

with TEL. This ethylene dibromide reacts with Pb and PbO to give PbBr2 which will escape into the

atmosphere.

Improving the octane number of a fuel: The octane number of a fuel may be improved by the

following: [1] The addition of anti-knock compounds like TEL [2] Low octane petrol is blended with

high octane compounds like alcohol, e.g. straight· run petrol is mixed with reformed petrol,

benzol and alcohol [3] Reforming.

Diesel Oil

Diesel oil is a fraction obtained between 2S0-320°C and is a mixture of C15H32 and C18H38

hydrocarbons, Its calorific value is about 11000 kcal/kg. It is used as a diesel engine fuel.

Diesel knock: In a diesel engine, air is first drawn into the cylinder and compressed,

this compression is accompanied by a rise in temperature to about 500°C. Near the completion of the

compression stroke, oil is sprayed into the heated air; Droplets of the oil in the atomized form get

vaporized and ignited. This raises temperature as well as pressure, the piston is pushed by the

expanding gases and this constitutes a power stroke.

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

start of fuel injection and its ignition is called ignition delay and is an important quality of the diesel

fuel. This delay is due to the time taken for the vaporization of individual droplets and rising of the

vapour to its ignition temperature. Long ignition delays lead to accumulation of more vapours in the

engine and when ignited an explosion results as the combined effect of increased temperature and

pressure. This is responsible for diesel knock. In order to avoid diesel knock, the ignition delay period

should be as short as possible. The cetane number decreases in the following order: straight chain

paraffins > cycloparaffins > olefins > brandied paraffins > aromatics

The diesel fuels are graded by means of cetane rating. Cetane, i.e. n-hexadecane

[CH3(CH2)14CH3] having a very short ignition delay is given the value of 100 in the rating scale.

α-methylnaphthalene having a longer ignition delay represents zero of the scale,

The percentage of cetane in the cetane-α-methylnaphthalene mixture which has the same ignition

delay as the fuel under test is the cetane number of the fuel. High cetane number fuels eliminate diesel

knock. The cetane number of a diesel fuel may be increased by the addition of ethyl nitrite,

amyl nitrite, etc.

58

59

Biodiesel

There has been an increase in efforts to reduce the reliance on petroleum fuel for energy

generation. Among the alternative fuel, biodiesel has received much attention for diesel engines due to

their advantages as the renewable, domestically produced energy resources and they are

environmentally friendly (biodegradable spills, reduction of unburned hydrocarbon, CO and

particulate matter). Biodiesel can be produced from animal or vegetable oils via base catalyzed trans-

esterification process. The term biodiesel refers to mixtures of alkyl esters of long chain fatty acids.

The reaction scheme for the production of biodiesel is as follows:

(1) Trans-esterification: Vegetable oils (Triacylglycerols are triesters) on reaction with methanol

(renewable resource and does not raise toxicity) in the presence of catalyst like NaOH or KOH,

produce glycerol and methyl ester of fatty acid (Biodiesel).

Vegetable Oil + Methanol + Catalyst ⇌ Biodiesel + Glycerol

where R = CH3-(CH2)16- , hence composition of biodiesel is CH3-(CH2)16-COOCH3

Biodiesel contains no petroleum; it can be blended at any level with petroleum or diesel.

A 100% biodiesel fuel is referred as B100. Biodiesel blends are referred as BXX, where XX indicates

the amount of biodiesel in the blend (For example, B20 blend is 20% by volume biodiesel and

80% by volume petrol.

Biodiesel has higher cetane number; this oil reduces the emission of CO2 by 78%, CO by 44%,

sulphate by 100%, polycyclic aromatic hydrocarbon by75-85% and particulate matter. Vegetable oil

is produced from soybean and tallow (USA), rapeseed (Europe), Jatropa curcas and Karanja (India).

Syllabus

FuelsFuelsFuelsFuels – Classification – Calorific Value – Cracking and Reforming – Petrol Knock and Octane

number – Diesel Knock and Cetane number – Bio-Diesel

60

ElectrochemistryElectrochemistryElectrochemistryElectrochemistry

What are half reactions?

Let us consider the reaction 2Na + Cl2 → 2Na+ + 2Cl–. It occurs by the transfer of electrons

from Na to Cl. Na loses an electron and is said to be oxidized to Na+ ion. At the same time,

Cl gains an electron and is reduced to Cl– ion. Such a reaction which is brought about by loss of

electrons (oxidation) and gain of electrons (reduction) simultaneously is called an

Oxidation-Reduction reaction or Redox reaction in brief. It may be noted that in the overall redox

reaction no free electrons are generated.

The redox reaction can be considered as made up of two reactions. For example, the redox

reaction 2Na + Cl2 → Na+ + 2Cl– is composed of two half-reactions: 2Na → 2Na+ + 2e– (oxidation)

and Cl2 + 2e– → 2Cl– (reduction)

Each of the two reactions shows just its oxidation or just the reduction portion of the overall

redox reaction. Being half components of the redox reaction, these reactions are called Half-reactions.

The first half-reaction that proceeds by oxidation is often referred to as the Oxidation half-reaction.

The second half-reaction that occurs by reduction, is referred to as the Reduction half-reaction.

When the two half-reactions are added together, the sum is the net redox reaction.

Electrochemical cells

A device for producing an electrical current from a chemical reaction (redox reaction) is called

an electrochemical cell. How a Redox reaction can produce an electrical current? When a bar of zinc

is dipped in a solution of copper sulphate, copper metal is deposited on the bar (See Figure 1).

The net reaction is Zn + Cu2+ ⎯⎯→ Zn2+ + Cu. This is a redox reaction and the two half-reactions are:

Zn ⎯⎯→ Zn2+ + 2e– and Cu2+ + 2e– ⎯⎯→ Cu

In this change, Zn is oxidized to give Zn2+ ions and Cu2+ ions are reduced to Cu atoms.

The electrons released in the first half-reaction are used up by the second half-reaction.

Both the half reactions occur on the zinc bar itself and there is no net charge.

Now, let the two half-reactions occur in separate compartments which are connected by a wire

(See Fig. 2) The electrons produced in the left compartment flow through the wire to the other

compartment. However the current will flow for an instant and then stop. The current stops flowing

because of the charge build up in the two compartments. The electrons leave the left compartment and

it would become positively charged. The right compartment receives electrons and becomes negatively

charged. Both these factors oppose the flow of electrons (electrical current) which eventually stops.

61

Fig. 2: Separate half-reactions cause flow of electrons (Current) in the wire connecting them

This problem can be solved very simply. The solutions in the two compartments may be

connected, say, by a salt bridge. The salt bridge is a U-tube filled with an electrolyte such as NaCl,

KCl, or K2SO4. It provides a passage to ions from one compartment to the other compartment without

extensive mixing of the two solutions. With this ion flow, the circuit is complete and electrons pass

freely through the wire to keep the net charge zero in the two compartments.

Cell diagram or Representation of a Cell

A cell diagram is an abbreviated symbolic depiction of an electrochemical cell. For this

purpose, we will consider that a cell consists of two half-cells. Each half-cell is again made of a metal

electrode contact with metal ions in solution.

(1) A single vertical line (|) represents a phase boundary between metal electrode and ion solution

(electrolyte). Thus the two half-cells in a voltaic cell are indicated as

62

It may be noted that the metal electrode in anode half-cell is on the left, while in cathode

half-cell it is on the right of the metal ion.

(2) A double vertical line (||) represents the salt bridge, porous partition or any other means of

permitting ion flow while preventing the electrolyte from mixing.

(3) Anode half-cell is written on the left and cathode half-cell on the right.

(4) In the complete cell diagram, the two half-cells are separated by a double vertical line

(salt bridge) in between. The zinc-copper cell can now be written as

(5) The symbol for an inert electrode, like the platinum electrode is often enclosed in a bracket.

(6) The value of emf of a cell is written on the right of the cell diagram. Thus a zinc-copper cell has

emf 1.1V and is represented as

Voltaic Cells

A Voltaic cell, also known as a galvanic cell is one in which electrical current is generated by a

spontaneous redox reaction. A simple voltaic cell is shown in Fig. 3. Here the spontaneous reaction of

zinc metal with an aqueous solution of copper sulphate is used. Zn + Cu2+ ⎯⎯→ Zn2+ + Cu

A bar of zinc metal (anode) is placed in zinc sulphate solution in the left container. A bar of

copper (cathode) is immersed in copper sulphate solution in the right container. The zinc and copper

electrodes are joined by a copper wire. A salt bridge containing potassium sulphate solution

interconnects the solutions in the anode compartment and the cathode compartment.

The oxidation half-reaction occurs in the anode compartment. Zn ⎯⎯→ Zn2+ + 2e–

The reduction half-reaction takes place in the cathode compartment. Cu2+ + 2e– ⎯⎯→ Cu

Fig. 3: A simple voltaic (galvanic) cell

63

When the cell is set up, electrons flow from zinc electrode through the wire to the copper

cathode. As a result, zinc dissolves in the anode solution to form Zn2+ ions. The Cu2+ ions in the

cathode half-cell pick up electrons and are converted to Cu atoms on the cathode. At the same time,

SO42– ions from the cathode half-cell migrate to the anode half-cell through the salt bridge.

Likewise, Zn2+ ions from the anode half-cell move into the cathode half-cell. This flow of ions from

one half-cell to the other completes the electrical circuit which ensures continuous supply of current.

The cell will operate till either the zinc metal or copper ion is completely used up.

Daniel Cell

It is a typical voltaic cell. It was named after the British chemist John Daniel. It is a simple zinc

copper cell like the one described above. In this cell the salt-bridge has been replaced by a porous pot.

Daniel cell resembles the above voltaic cell in all details except that Zn2+ ions and SO42– ions flow to

the cathode and the anode respectively through the porous pot instead of through the salt-bridge.

Inspite of this difference, the cell diagram remains the same.

Cell reaction

The flow of electrons from one electrode to the other in an electrochemical cell is caused by the

half-reactions taking place in the anode and cathode compartments. The net chemical change obtained

by adding the two half-reactions is called the cell reaction. Thus, for a simple voltaic cell described

above, we have (a) Half-reactions: Zn(s) ⎯⎯→ Zn2+ (aq) + 2e– and Cu2+ (aq) + 2e– ⎯⎯→ Cu(s)

(b) Cell reaction by adding up the half-reactions: Zn(s) + Cu2+ (aq) ⎯⎯→ Zn2+ (aq) + Cu(s)

64

Cell potential or emf

In a Zn-Cu voltaic cell, electrons are released at the anode and it becomes negatively charged.

The negative electrode pushes electrons through the external circuit by electrical repulsions.

The copper electrode gets positive charge due to the discharge of Cu2+ ions on it. Thus electrons from

the outer circuit are attracted into this electrode. The flow of current through the circuit is determined

by the ‘push’, of electrons at the anode and ‘attraction’ of electrons at the cathode.

These two forces constitute the ‘driving force’ or ‘electrical pressure’ that sends electrons through the

circuit. This driving force is called the electromotive force (abbreviated emf) or cell potential.

The emf of cell potential is measured in units of volts (V) and is also referred to as cell voltage.

Calculating the emf of a cell

The emf of a cell can be calculated from the half-cell potentials of the two cells

(anode and cathode) by using the following formula, /0122 = /0345671 − /39671 = /: − /; where ER and EL are the reduction potentials of the right-hand and left-hand electrodes respectively.

It may be noted that absolute values of these reduction potentials cannot be determined. These are

found by connecting the half-cell with a standard hydrogen electrode whose reduction potential has

been arbitrarily fixed as zero.

Measurement of emf of a cell [Poggendorf’s compensation method]

The emf of an unknown cell can be measured with the help of a potentiometer (Fig. 4).

It consists of a wire AB which is about one metre long. The two ends of this wire are connected to a

working battery W. A standard cell C1 (i.e., a cell of known emf) is connected to the end A.

At the other end, the cell C1 is connected to a galvanometer through a key K1. The galvanometer is

then joined to a sliding contact that moves on the wire AB. The cell C2 whose emf is to be measured is

similarly connected to the key K2, the galvanometer and then the sliding contact. By using the key K1,

the cell C1 is put into the circuit and the contact is moved to and fro along AB. When no current flows

through the galvanometer, the point of contact X1 is recorded. Then by using the key K2, the cell C2 is

put into the circuit and the procedure is repeated to find the corresponding point X2. The emf of the

cell C2 is calculated by using the following equation:

Fig. 4: Measuring the emf of a cell with a potentiometer

65

Potential differences at interfaces

The transition region between two phases consists of a region of charge unbalance known as the

electric double layer [Helmholtz double layer]. As its name implies, this consists of an inner

monomolecular layer of adsorbed water molecules and ions, and an outer diffuse region that

compensates for any local charge unbalance that gradually merges into the completely random

arrangement of the bulk solution. In the case of a metal immersed in pure water, the electron fluid

within the metal causes the polar water molecules to adsorb to the surface and orient themselves so as

to create two thin planes of positive and negative charge. If the water contains dissolved ions,

some of the larger (and more polarizable) anions will loosely bond (chemisorb) to the metal, creating a

negative inner layer which is compensated by an excess of cations in the outer layer.

Electrochemistry is the study of reactions in which charged particles (ions or electrons) cross

the interface between two phases of matter, typically a metallic phase (the electrode) and a conductive

solution, or electrolyte. A process of this kind can always be represented as a chemical reaction and

is known generally as an electrode process. Electrode processes take place within the double layer and

produce a slight unbalance in the electric charges of the electrode and the solution.

Single electrode potential

An electrochemical cell consists of two half-cells. With an open-circuit, the metal electrode

in each half-cell transfers its ions into solution. Thus an individual electrode develops a potential

with respect to the solution. The potential of a single electrode in a half-cell is called the

Single electrode potential. Thus in a Daniel cell in which the electrodes are not connected externally,

the anode Zn/Zn2+ develops a negative charge and the cathode Cu/Cu2+, a positive charge.

The amount of the charge produced on individual electrode determines its single electrode potential.

The single electrode potential of a half-cell depends on: (a) concentration of ions in solution;

(b) tendency to form ions; and (c) temperature.

Standard emf of a cell [E°]

The emf generated by an electrochemical cell is given by the symbol E. It can be measured with

the help of a potentiometer. The value of emf varies with the concentration of the reactants

and products in the cell solutions and the temperature of the cell. When the emf of a cell is determined

under standard conditions, it is called the standard emf. The standard conditions are (a) 1 M solutions

of reactants and products; and (b) temperature of 25°C. Thus standard emf may be defined as:

the emf of a cell with 1 M solutions of reactants and products in solution measured at 25°C.

Standard emf of a cell is represented by the symbol E°. With gases 1 atm pressure is a standard

condition instead of concentration. For a simple Zn-Cu voltaic cell, the standard emf, E°, is 1.10 V.

This means that the emf of the cell operated with [Cu2+] and [Zn2+] both at 1 M and 25°C is 1.10 V.

66

Determination of emf of a half-cell

By a single electrode potential, we also mean the emf of an isolated half-cell or its half-reaction.

The emf of a cell that is made of two half-cells can be determined by connecting them to a voltmeter.

However, there is no way of measuring the emf of a single half-cell directly. A convenient procedure

to do so is to combine the given half-cell with another standard half-cell. The emf of the newly

constructed cell, E, is determined with a voltmeter. The emf of the unknown half-cell,

E° can then be calculated from the expression, /<13=>?17 = /: − /; If the standard half-cell acts as anode, the equation becomes, /: = /<13=>?17

On the other hand, if standard half-cell is cathode, the equation takes the form,/; = −/<13=>?17

The standard hydrogen half-cell or Standard Hydrogen Electrode (SHE) is selected for

coupling with the unknown half-cell. It consists of a platinum electrode immersed in a 1 M solution of

H+ ions maintained at 25°C. Hydrogen gas at one atmosphere enters the glass hood and bubbles over

the platinum electrode. The hydrogen gas at the platinum electrode passes into solution,

forming H+ ions and electrons. H2 ⎯⎯→ 2H+ + 2e–

Fig. 6: SHE Fig. 7: The Zn electrode coupled with H electrode

The emf of the standard hydrogen electrode is arbitrarily assigned the value of zero volts.

So, SHE can be used as a standard for other electrodes. The half-cell, whose potential is desired, is

combined with the hydrogen electrode and the emf of the complete cell determined with a voltmeter.

The emf of the cell is /0122@ = /:AB54@ − /;1C4@

For example, it is desired to determine the emf of the zinc electrode, Zn | Zn2+. It is connected

with the SHE as shown in Fig. 7. The complete electrochemical cell may be represented as:

67

The emf of the cell is /0122@ = /:@ − /;@ = /DE/@ − /F9/F9HI@ = @ − @. KL = −@. KLM

Similarly, the emf of the copper electrode, Cu2+ | Cu can be determined by pairing it with the

SHE when the electrochemical cell can be represented as:

The emf of this cell has been determined to be 0.34 V which is the emf of the copper half-cell.

/0122@ = /:@ − /;@ = /N>/N>HI@ − /DE/@ = @. OP − @ = @. OPM

The two situations are shown in Fig. 8

Fig. 8: SHE can act both as cathode and anode when joined with another half-cell

When it is placed on the right-hand side of the Zinc electrode, the hydrogen electrode reaction

is 2H+ + 2e– ⎯⎯→ H2. The electrons flow to the SHE and it acts as the cathode. When the SHE is

placed on the left hand side, the electrode reaction is H2 ⎯⎯→ 2H+ + 2e–. The electrons flow to the

copper electrode and the hydrogen electrode as the anode. Evidently, the SHE can act both as anode

and cathode and, therefore can be used to determine the emf of any other half-cell electrode.

IUPAC convention places the SHE on the left-hand side: The electrons flow from left-to-

right and the given half-cell electrode gains electrons (reduction). The observed emf of the combined

electrochemical cell is then the emf of the half-cell on the right-hand. Such emf values of half-cells,

or half reactions, are known as the Standard reduction potentials or Standard potentials.

68

Electrochemical Series

The standard electrode potentials (reduction) of a number of electrodes are given in Table.

These values are said to be on the hydrogen scale since in these determinations, the potential of the

standard hydrogen electrode used as the reference electrode has been taken as zero. When the various

electrodes are arranged in the order of their increasing values of standard reduction potentials on the

hydrogen scale, then this arrangement is called electrochemical series.

Applications of Electrochemical Series or Electrode Potentials: The following are the

applications of electrochemical series:

[1]The standard emf of a cell can be calculated if the standard electrode potential values are

known./0122@ = /:AB54@ − /;1C4@

[2] The relative tendencies of metals to go into solution can be noted by the help of electrochemical

series. Metals on the top are more easily ionized into solution.

[3] The metal with negative reduction potential will displace hydrogen from an acid solution:

Zn + H2SO4 → ZnSO4 + H2 EZn0 = -0.76 volt

The metal with positive reduction potential will not displace hydrogen from an acid solution.

Ag + H2SO4 → no reaction EAg0 = +0.80 volt

[4] Metals which lie higher in the series can displace from the solution those elements which lie below

them in the series. Thus, zinc can displace copper from the solution.

[5] The anodic or more active metals in the series are more prone to corrosion. The cathodic or more

noble metals are less prone to corrosion. The galvanic series is used for this purpose.

Galvanic Series: the galvanic series is used to provide sufficient information in predicting the

corrosion behaviour in a particular set of environmental conditions. Oxidation potential measurements

of various metals and alloys have been made using the standard calomel electrode as the reference

electrode and immersing the metals and alloys in sea water. These are arranged in decreasing order of

activity and this series is known as the galvanic series. The galvanic series gives more practical

information on the relative corrosion tendencies of the metals and alloys. The speed and severity of

corrosion depends upon the difference in potential between the anodic and cathodic metals in contact.

69

Using Electrochemical Series or Electrode Potentials: In Table the standard reduction

potentials (E°) are arranged in the order of increasing potentials. The relative position of electrodes

(M/M+) in the table can be used to predict the reducing or oxidising ability of an electrode.

The electrodes that are relatively positive indicate that reduction reaction involving addition of

electrons, M+ + e– ⎯⎯→ M is possible. In case of relatively negative potential involving loss of

electrons, M ⎯⎯→ M+ + e– is indicated. It also follows that the system with higher electrode potential

will be reduced by the system with lower electrode potential.

Predicting the Oxidising or Reducing Ability: (1) The more positive the value of E°, the

better the oxidising ability (the greater the tendency to be reduced) of the ion or compound, on moving

upward in the Table. (2) The more negative the value of E° the better the reducing ability of the ions,

elements or compounds on moving downward in the Table. (3) Under standard conditions, any

substance in this Table will spontaneously oxidise any other substance lower than it in the Table.

Predicting cell emf: The standard emf, E°, of a cell is the standard reduction potential of right-

hand electrode (cathode) minus the standard reduction potential of the left-hand electrode (anode).

E!��R = ES$TU�R − E&)�R = CathodePotential– AnodePotential Let us predict the emf of the cell

by using the E° values from the TableE!��R = ES$TU�R − E&)�R = 0.80 − ^−0.763b = 1.563V

Predicting Feasibility of Reaction: The feasibility of a redox reaction can be predicted with

the help of the electrochemical series. The net emf of the reaction, Ecell can be calculated from the eqn

Predicting whether a metal will displace another metal from its salt solution or not:

The metals lying higher up in the series are strong oxidising agents and their ions are readily reduced

to the metal itself. In general we can say that a metal lower down the electrochemical series can

precipitate the one higher up in the series.

Predicting whether a metal will displace hydrogen from a dilute acid solution: Any metal

lying below hydrogen is a stronger reducing agent than hydrogen and will convert H+ to H2.

This explains why Zn lying below hydrogen reacts with dil H2SO4 to liberate H2, while Cu lying above

hydrogen does not react.

70

The Nernst Equation

We know experimentally that the potential of a single electrode or half-cell varies with the

concentration of ions in the cell. In 1889 Walter Nernst derived a mathematical relationship which

enable us to calculate the half-cell potential, E, from the standard electrode potential, E°, and the

temperature of the cell. This relation known as the Nernst equation can be stated as

/ = /@ − H. O@O:f9g 26Bh

Where E° = standard electrode potential, R = gas constant, T = Kelvin temperature,

n = number of electrons transferred in the half-reaction, F = Faraday of electricity and

K = equilibrium constant for the half-cell reaction as in equilibrium law

Derivation: Consider a reversible cell, e.g. Daniel cell. The overall cell reaction occurring in the

cell is represented by the equationF9 + N>Hj ⇌ F9Hj + N>. In general, for a reversible cell the

equation is k + l ⇌ N + m. The electrical energy of a reversible cell can be measured by the free

energy decrease (-∆G) of the reaction taking place in the cell. In the cell, if the reaction involves the

transfer of n number of electrons, then n Faradays of electricity will flow. If E is the emf of the cell,

then the total electrical energy produced in the cell is -∆G = nFE where -∆G = decrease in free energy.

At standard conditions, -∆G0 = nFE0 where -∆Go = standard free energy change.

Standard free energy change is the change in free energy when the concentration of reactants

and products are unity. EJ is the standard emf of the cell in which the reactants and products are kept

at 1 molar concentration at 25°C, ∆G and ∆G0 are related as ∆G = ∆G0 + RT ln K where K is the

equilibrium constant of the reaction defined as the ratio of the concentration of the products to the

concentration of the reactants.

Divide the above equation by –nF

At 25°C, T = 298 K, R = 8.314 11K/mol, F = 96500 coulombs. Therefore,

This is known as the Nernst equation. This equation may be used to calculate the emf of cells

when concentration of reactants and products of the cell reactions are known.

71

Applications of the Nernst Equation

[1] Calculation of Half-cell potential: For an oxidation half-cell reaction when the metal

electrode M gives Mn+ ion, M → Mn+ + ne-. The Nernst equation takes the form

/ = /@ −H. O@O :f

9g26B

no9jp

nop

The concentration of solid metal [M] is equal to zero. Therefore, the Nernst equation can be written as

/ = /@ −H. O@O :f

9g26Bno9jp

Substituting the values of R, F and T at 25°C, the quantity 2.303 RT/F comes to be 0.0591.

Thus the Nernst equation can be written in its simplified form as

/ = /@ −@. @qrs

926Bno9jp

This is the equation for a half-cell in which oxidation occurs. In case it is a reduction reaction,

the sign of E will have to be reversed.

[2] Calculation of Cell potential: The Nernst equation is applicable to cell potentials as well. Thus,

/0122 = /0122@ −

@. @qrs

926B h

K is the equilibrium constant of the redox cell reaction.

[3] Calculation of Equilibrium constant for the cell reaction: The Nernst equation for a cell is

/0122 = /0122@ −

@. @qrs

926B h

At equilibrium, the cell reaction is balanced and the potential is zero. The Nernst equation becomes

@ = /0122@ −

@. @qrs

926B h , 51901 26B h =

9/0122@

@. @qrs

Concentration Cells

A concentration cell is an electrochemical cell which produces electrical energy by the transfer

of material from a system of higher concentration to a system of lower concentration. The difference

in concentration may be due to the difference in concentration of the electrodes or electrolytes. Based

on this, concentration cells are divided into two categories; (i) Electrode Concentration Cells and

(ii) Electrolyte Concentration Cells

Electrode Concentration Cells

In electrode concentration cells, two electrodes of the same metal with different concentrations

are dipped in the same solution of the electrolyte, e.g. amalgam concentration cells. Amalgam

electrodes are produced by mixing various proportions of any metal and mercury. Depending on the

amount of metal taken during the preparation of amalgam, the concentration of electrodes will vary.

72

Electrolyte Concentration Cells

Cell potentials depend on concentration of the electrolyte. Thus a cell can be constructed by

pairing two half-cells in which identical electrodes are dipping in solution of different concentrations

of the same electrolyte. Such a cell called Electrolyte concentration cell. It may be described as:

a cell in which emf arises as a result of different concentrations of the same electrolyte in the

component half-cells.

The electrode dipped in the solution of lower concentration is an anode and the electrode

dipped in solution of higher concentration is a cathode. The overall reaction involves the transfer

of material from higher concentration to lower concentration. The two solutions are connected

together by a salt bridge. A typical concentration cell is shown in Figure. It consists of two silver

electrodes, one immersed in 0.1 M silver nitrate solution and the other in 1 M solution of the same

electrolyte. The two solutions are in contact through a membrane (or a salt bridge). When the

electrodes are connected by a wire, it is found experimentally that electrons flow from the electrode in

more dilute (0.1M) solution to that in the more concentrated (1 M) solution.

Explanation: The concentration of Ag+ ions in the left compartment is lower (0.1M) and in the

right compartment it is higher (1M). There is a natural tendency to equalize the concentration of

Ag+ ions in the two compartments. This can be done if the electrons are transferred from the

left compartment to the right compartment. This electron transfer will produce Ag+ ions in

the right compartment by the half-cell reactions: Ag → Ag+ + e- (Left compartment) and

Ag+ + e- → Ag (Right compartment)

Fig. 9: A typical Electrolyte Concentration Cells Emf of Concentration Cell

Suppose the concentrations in the two half-cells are C1 and C2 at 25°C, C2 being greater than

C1. Then emf, E, of the concentration cell will be given by the difference between the two electrode

potentials. In terms of Nernst equation

/ = u/o + @. @qrs9 26B NHv − u/o + @. @qrs

9 26BNsv = @. @qrs9 26B NHNs

EM is the standard electrode potential of the metal M and n is the valence of the ions in contact with it.

73

Types of Types of Types of Types of ElectrodesElectrodesElectrodesElectrodes (S.H.E, Calomel, Quinhydrone, glass electrode)

[1] Gas electrode {Pt/H2, H+}: It contains an inert electrode like platinum dipped in a solution

containing ions (H+ ions) to which the gas (H2 gas) is bubbled continuously.

The electrode is represented as Pt (s)/H2 (P = x atm), H+ (aq). The electrode reaction can be written as

2H+ (aq) + 2e- → H2 (g). The electrode potential is /w4/EH/EI =/w4/EH/EI@ − :f9g xy

NEHzNEI{

H

Example is Standard Hydrogen Electrode (SHE). It consists of a platinum electrode immersed

in a 1 M solution of H+ ions maintained at 25°C. Hydrogen gas at one atmosphere enters the glass

hood and bubbles over the platinum electrode.

[2] Metal-metal insoluble salt electrode {M (s) /MX (s)/X- (aq)}: It consists of a pure metal

[M (s)] coated by a layer of its sparingly soluble salt [MX (s)] and kept immersed in a solution

containing a common anion [X- (aq)]. It is represented as M (s) /MX (s)/X- (aq). The electrode reaction

can be written as MX (s) + ne- → M (s) + X- (aq). Electrode potential is

/o/o|/|} =/o/o|/|}@ − :f9g xy

No. N|}No|

Example is Calomel electrode: The standard calomel electrode, SCE, consists of a wide glass-

tube with a narrow side-tube. It is set up as illustrated in Fig. 10. A platinum wire is dipping into

liquid mercury covered with solid mercurous chloride (Hg2Cl2, calomel). The tube is filled with a 1 M

solution of KCl (or saturated KCl solution). The side-tube containing KCl solution provides the salt

bridge which connects the electrode to any other electrode. The calomel electrode is represented as

The half-cell reaction is Hg2Cl2 + 2e- → 2Hg + 2Cl-

Calomel electrode Fig. 10: The calomel electrode coupled with zinc electrode

74

[3] Quinhydrone Electrode: It is a widely used secondary standard electrode.

It involves the redox reaction between quinine (Q) and hydroquinone (QH2), represented in Figure 1.

The hydroquinone half-cell consists of a platinum strip immersed in a saturated

solution of quinhydrone at a definite H+ ion concentration (buffered solution).

Quinhydrone is a molecular compound which gives equimolar amounts of quinone and hydroquinone

in solution represented in Figure 2. The potential developed is measured against a hydrogen electrode

or calomel electrode. The emf with respect to a standard hydrogen electrode is 0.2875 V at 25°C.

The electrode system may be represented in Figure 3

Figure 1

Figure 2

Figure 3

[4] Glass electrode: It consists of a glass tube having a thin-walled bulb at the lower end.

The bulb contains a 1M HCl solution. Sealed into the glass-tube is a silver wire coated with silver

chloride at its lower end. The lower end of this silver wire dips into the hydrochloric acid, forming

silver-silver chloride electrode. The glass electrode may be represented as

When placed in a solution, the potential of the glass electrode depends on the H+ ion

concentration of the solution. The potential develops across the glass membrane as a result of a

concentration difference of H+ ions on the two sides of the membrane. This happens much in the same

way as the emf of a concentration cell develops. A commonly used secondary standard electrode is

the so-called glass electrode. Its emf is determined by coupling with a standard calomel electrode

(SCE). The glass electrode provides one of the easiest methods for measuring the pH of a given

solution. A simple type of glass electrode is shown Fig. 11.

Fig. 11: Glass electrode

75

The pH of solutions

A knowledge of the concentration of hydrogen ions (more specifically hydronium ions) is of the

greatest importance in chemistry. Hydrogen ion concentrations are typically quite small numbers.

Therefore, chemists report the hydrogen ion concentration of a solution in terms of pH. It is defined as

the negative of the base-10 logarithm (log) of the H+ concentration. Mathematically it may be

expressed as pH = – log [H+] where [H+] is the concentration of hydrogen ions in moles per litre.

Alternative and more useful forms of pH definition are:

The pH concept is very convenient for expressing hydrogen ion concentration.

It was introduced by Sorensen in 1909. It is now used as a general way of expressing other quantities

also, for example,

pH Scale: In order to express the hydrogen ion concentration or acidity of a solution, a pH

scale was evolved. The pH is defined as

The hydrogen ion concentration of different acidic solutions were determined experimentally.

These were converted to pH values using the above relations. Then these pH values were computed on

a scale taking water as the reference substance. The scale on which pH values are computed is called

the pH scale.

Thus the H+ ion and OH– ion concentrations in pure water are both 10–7 mol l– 1 at 25°C and it

is said to be neutral. In acidic solution, however, the concentration of H+ ions must be greater than

10–7 mol l–1. Similarly in a basic solution, the concentration of OH– ions must be greater than

10–7 mol l–1. Thus we can state:

Expressing the [H+] in terms of pH for the different solutions cited above, we get what we call

the pH scale. On this scale the values range from 0 to 14. Since pH is defined as – log [H+] and the

hydrogen ion concentration of water is 10– 7, the pH of water is 7. All solutions having pH less than 7

are acidic and those with pH greater than 7 are basic. pH decreases with the increase of [H+].

The lower the pH, higher is the [H+] or acidity.

In any aqueous solution, the product of [H+] and [OH–] is always equal to Kw. This is so

irrespective of the solute and relative concentrations of H+ and OH– ions. However, the value of Kw

depends on temperature. At 25°C it is 1.0 × 10– 14. Thus,

76

Determination of pH of a solution using glass electrode: A half-cell is set up with the test

solution as electrolyte. The emf of the cell depends on the concentration of H+ ions or pH of the

solution. The emf of the half-cell is determined by coupling it with another standard half-cell and

measuring the emf of the complete cell. The commonly used standard electrode is the glass electrode.

A glass electrode is immersed in the solution of unknown pH. It is coupled with a standard calomel

electrode (SCE) as shown in Figure. The emf of the complete cell can be determined experimentally.

Glass electrode A glass electrode coupled with standard calomel electrode for determining pH

Calomel electrode

Calculations: The potential of the glass electrode, EG, at 25°C is given by equation

The value of the potential of calomel electrode is known while Ecell can be found

experimentally. Therefore, we can find pH of a given solution if E°G is known. It can be determined by

using a solution of known pH in the cell and measuring Ecell. This value of E°G is constant for a

particular glass electrode and can be used for any subsequent determinations of pH of unknown

solutions with the help of equation. The potential of the cell, Ecell, cannot be measured using ordinary

potentiometer or voltmeter as the resistance of the glass membrane is very high and the current small.

Therefore, an electronic voltmeter is required which reads pH directly.

Merits and demerits of Glass electrode: A glass electrode is universally used because

[1] It is simple to operate. [2] It is not easily poisoned. [3] Its activity is not affected by strong

oxidising and reducing agents. [4] Since E°G depends on a particular glass electrode used, it is not a

universal constant and also changes with time. Hence a glass electrode only compares pH values while

the hydrogen electrode measures pH absolutely.

77

Batteries

Introduction

Batteries are considered as storehouses for electrical energy on demand. An electrochemical

power source or battery is a device which converts the chemical energy derived from a chemical

reaction into electrical energy.

The chemical reaction involved in a battery is a redox reaction and some of these reactions are

reversible. The reverse reaction can be brought out by supplying energy, i.e. by applying the current to

the system from an external source. This process is called charging of the battery. Such batteries in

which the cell reactions can be reversed by passing direct electric current in opposite direction are

called secondary batteries. Lead-acid accumulators, nicad battery, etc. are secondary batteries.

Batteries that cannot be charged in this manner, because the cell reactions are irreversible, are called

primary batteries. Primary batteries are the most common batteries available today because they are

cheap and simple to use. Carbon-zinc dry cells, alkaline batteries, mercury batteries, lithium batteries,

etc. are examples for primary batteries. Table shows the differences between primary and secondary

batteries. In the following sections we will discuss in detail primary and secondary batteries.

Primary Batteries

Lechlanche Cell or Zinc-Carbon Dry Cell: The zinc-carbon cell uses a zinc anode, a

manganese dioxide cathode and an electrolyte of ammonium chloride dissolved in water. Powdered

carbon is mixed with manganese dioxide to improve conductivity and retain moisture.

In the dry cell which is a modified Lechlanche cell consists of a zinc anode shaped as a

container for the electrolyte and a graphite rod as a cathode surrounded by MnO2 and a paste of NH4CI

and ZnCl2 as an electrolyte. The dry cell is so called since there is no fluid phase presents (Figure 1).

The voltage of the cell is about 1.5 volt. It is commonly used in flash lights and radios.

78

Chemistry: As the cell is discharged zinc is oxidized and manganese dioxide is reduced:

Figure 1: Interior section of a dry cell

Drawbacks of dry cells: The following are the drawbacks of dry cells:

[1] Dry cells do not have an indefinite life period. Since the electrolyte medium is acidic,

zinc metal dissolves slowly there by reducing the life time even if it is not in use.

[2] When current is rapidly drawn from the cell, due to the building up of products on the electrodes

there is a drop in voltage.

Alkaline Battery: An alkaline battery is an improved form of the dry cell.

In this cell, the electrolyte is a concentrated solution of potassium hydroxide (35-52%).

As the electrolyte in this cell is an alkali, it is called alkaline battery. The cathode is made from a

mixture of manganese dioxide and carbon. The anode mix consists of alkaline electrolyte, zinc powder

and small quantity of gelling agent (starch) to immobilize the electrolyte and suspend the zinc powder.

The alkaline cell derives power from the oxidation of zinc anode and the reduction of MnO2 cathode.

79

The emf of the cell is 1.5 volt. The advantages of this battery over a dry battery are as follows:

[1] As potassium hydroxide is used as an electrolyte, zinc does not readily dissolve.

[2] Since there is no corrosion of zinc, the life of alkaline battery will be longer.

[3] Its output capacity is high.

Secondary Batteries or Storage Batteries or Accumulators

Lead-acid Accumulator or Acid Battery: A storage cell is one which can operate both as a

voltaic cell and as an electrolytic cell. When it acts as a voltaic cell it supplies electrical energy. On

recharging it acts as an electrolytic cell.

An acid battery consists of a negative electrode of porous lead (lead sponge) as the anode and a

positive electrode of lead dioxide as the cathode. A number of such electrode pairs are immersed in an

aqueous solution of 20% sulphuric acid (specific gravity: 1.15 at 25°C) which is the electrolyte

(Figure 2).

Figure 2: Lead-acid storage cell

Discharging: The electrode reactions that occur during the discharge of the cell, i.e. when

current is drawn from the cell are as follows:

Anode Cathode

The overall cell reaction is as follows:

The PbSO4 formed gets precipitated on the cathode and in the solution.

80

Charging: When both anode and cathode become covered with PbSO4, the cell stops its

functioning. Recharging is done by applying a voltage across the electrodes that is slightly higher than

the voltage that the battery can deliver. The net reaction during discharging is as follows:

During the discharge process the consumption of sulphuric acid is replaced by an equivalent

quantity of water and the sulphuric acid concentration decreases. On charging the reverse reaction

takes place. During the reverse reaction water is consumed and sulphuric acid is regenerated. Hence

the original strength of acid is restored. Since both of these changes are associated with variations in

the specific gravity of the acid, the extent of charge or discharge of the cell at any time can be

determined by testing the specific gravity of the acid.

Uses: The following are the uses of lead-acid storage cells: [1] Lead-acid storage cells are used

in automobiles, hospitals, telephone exchanges, etc. [2] As it is rechargeable it is used in UPS

(uninterrupted power supply) a power system which maintains current flow without even a momentary

break in the event of current failure.

Maintenance: If the lead-acid batteries are properly maintained in the following ways,

they can function for long periods: [1] Avoid over discharging of the battery. [2] Maintain the

electrolyte at the proper level by adding water (whenever required). [3] Keep the battery clean.

[4] Avoid overheating of the battery.

Nickel-Cadmium Battery or Nicad Battery: A nickel-cadmium battery is a type of alkaline

storage battery. This battery consists of a cadmium anode, nickel oxyhydroxide cathode and an

alkaline electrolyte (potassium hydroxide).

During discharge cadmium metal oxidizes to cadmium hydroxide at the anode:

By accepting the electrons, nickel oxyhydroxide [NiO(OH)] is reduced to nickel hydroxide [Ni(OH)2]

at the cathode:

The overall cell reaction is:

The emf of the cell is 1.3 volt. This is a rechargeable battery. When the discharged battery is

connected to an external voltage source, the cell reaction is reversed.

81

Advantages: Nickel--cadmium cells are characterized by long life, relatively high rates of

discharge and charge and ability to operate at low temperatures.

Uses: Nicad cells are used in calculators, electrical shavers, etc. The good low

temperature performance has lead to wide use of nickel-cadmium batteries in aircraft and space

satellite power systems.

Fuel cells

A fuel cell is an electrochemical cell in which the chemical energy of the fuel-oxidant system is

directly converted into electrical energy. It is an energy conversion device or electricity generator.

A fuel cell operates like a galvanic cell with the exception that the reactants are supplied from

outside. It is an example for a primary cell. This is capable of supplying current as long as it is

provided with the supply of reactants.

Hydrogen-Oxygen Fuel Cell: A fundamental and important example of a fuel cell is the

hydrogen/oxygen cell. Like an electrochemical cell, the fuel cell is also having two electrodes and an

electrolyte. The two electrodes are made up of porous graphite admixed with nickel powder.

The electrolyte used is potassium hydroxide solution maintained at 200°C and 20-40 atmospheres.

Hydrogen and oxygen gases are bubbled through the anode and cathode compartments respectively.

Hydrogen is oxidized at the anode whereas the oxygen gets reduced at the cathode (Figure 3).

Figure 3: Hydrogen-oxygen fuel cell

82

The cell reaction is the same as combustion of hydrogen in air or oxygen. Generally a large

number of these cells are stacked together in series to make a battery called fuel cell battery

or fuel battery.

In the fuel cells, gaseous fuels used are hydrogen, alkanes and co. Among the liquid fuels

methanol, ethanol, etc. are very important. Oxygen, air, hydrogen peroxide, etc. are some of the

oxidants used.

Advantages of fuel cells: The following are the advantages of fuel cells:

[1] The energy conversion efficiency is very high (75-83%). [2] They are used as power sources in

spacecrafts. [3] The product of a hydrogen-oxygen fuel cell is pure water which can be used for

drinking purpose. [4] Noise and thermal pollution are very low. [5] The maintenance cost is very low.

[6] It saves fossil fuels.

Limitations: The following are the limitations of fuel cells: [1] The cost of power from a fuel

cell is high as result of the cost of electrodes and pure hydrogen gas. [2] As the fuels used are gases,

they have to be stored in big tanks under high pressures

Solar Cells or Photovoltaic Cells

Photovoltaic cells convert solar energy directly into electrical energy.

Description of solar cells: The conventional solar cells made up of p-type doped semiconductor

(i.e., silicon doped with boron) and n-type doped semiconductor (i.e., silicon doped with phosphorus)

is shown in Figure 4

Figure 4: Solar cell

83

The surface layer is made up of n-type semiconductor and it is extremely thin (~ 0.2 µm) so

that sun light can penetrate through it. The bottom layer is made up of p-type semiconductor. The

electrodes made of Ti-Ag solder are attached to both the sides to provide electrical contact. The

electrode on the top surface is in the form of a metal grid with fingers which permit sun light to pass

through. On the back side, the electrode completely covers the surface. An anti-reflection coating of

silicon oxide having a thickness of 0.1 µm is also put on the top surface.

Working: When p-type and n-type semiconductors are placed together, electrons from

n-type side diffuse across p-n junction to combine with holes present in p-type semiconductor side.

As a result positive ions (on n-type side) and negative ions (on p-type side) are created near

the junction to certain thickness. The separation of charges produces an electric field across

p-n junction which is about 0.6-0.7 volt. This potential at p-n junction prevents the charges moving

across it further.

When light strikes on the p-n junction, electron-hole pair is produced. As the potential barrier

resists the flow of charge carriers across it, they flow through the conductor/load connected externally

to produce electric current. Since the emf of a single solar cell is about 0.6 V, they are arranged into

larger groupings called arrays in solar panels.

Applications: The following are the application of solar cells: [1] Solar cells are used to

provide power supply for space satellites. [2] They are used for the distillation of water to get pure

drinking water. [3] Solar cells provide thermal energy for solar cookers, solar furnaces, etc.

[4] They provide electricity for street lighting in remote areas, and to run water pumps and radios in

desert areas. [5] Solar cells provide electric power to light houses.

Chemical sensors

A chemical sensor is a transducer which provides direct information about the chemical

composition of its environment. It can warn occupants of potentially toxic agents in air. It consists of a

physical transducer and a chemically selective layer.

Actually, it contains an array of tiny micro-hot-plates in conjunction with thin metalized films

such as tin, titanium, or zinc oxides. Both the hot plates and sensing films are incorporated into an

integrated circuit device. Sensitive films create a sensitive surface for detecting ambient chemicals.

If a specific chemical of interest is present, the resistance of the device changes. It produces a type of

“signature” for a specific chemical that can be matched up against a library of chemical signatures to

identify both the type and concentration of the gas in the ambient air. Metal-oxide semiconductors

(MOS) structures are of particular interest for chemical sensors.

84

CorrosionCorrosionCorrosionCorrosion

Introduction

Metals and alloys are used as construction and fabrication materials in engineering. If the metal

or alloy structure is not properly maintained, they deteriorate slowly by the action of atmospheric

gases, moisture and other chemicals. This phenomenon of metals and alloys to undergo destruction by

the act of environment is known as corrosion.

Corrosion is defined as the gradual eating away or deterioration of a metal by chemical or

electrochemical reactions with its environment.

Due to corrosion the useful properties of a metal such as malleability, ductility and electrical

conductivity are lost. The most familiar example of corrosion is rusting of iron when exposed to

atmospheric conditions. During this, a layer of reddish scale and powder of oxide (Fe3O4) is formed

and the iron becomes weak. Another example is the formation of green film or basic carbonate

[CuCO3 + Cu(OH)2] on the surface of copper when exposed to moist air containing CO2.

It has been roughly assessed that the amount of iron wasted due to corrosion is one fourth of

world production. The direct loss due to corrosion in India amounts to Rs. 200 crore/annum while the

money spent annually in controlling corrosion is of the order of Rs. 50 crore. It is better to control

rather than to prevent corrosion, since it is impossible to eliminate corrosion.

Cause of Corrosion

In nature, most metals are found in a chemically combined state known as an ore. All the metals

except gold, platinum and silver exist in nature in the form of their oxides, carbonates, sulphides,

sulphates, etc. These combined forms of the metals represent their thermodynamically stable state (low

energy state). The metals are extracted from these ores after supplying a large amount of energy.

Metals in the uncombined condition have a higher energy and are in an unstable state. It is their natural

tendency to go back to the low energy state, i.e., combined state by recombining with the elements

present in the environment. This is the main reason for corrosion.

Effects of corrosion: The following are the effects of corrosion: [1] Lost production during a

shut down [2] Replacement of corroded equipment [3] High maintenance costs such as repainting

[4] Loss of efficiency [5] Contamination of the product.

Theories of Corrosion: [1] Direct chemical attack or Chemical or Dry corrosion

[2] Electrochemical theory or Wet corrosion [3] Differential aeration or

Concentration cell corrosion

85

Theories of CorrosionTheories of CorrosionTheories of CorrosionTheories of Corrosion

[1] Direct Chemical Attack or Chemical or Dry Corrosion

Whenever corrosion takes place by direct chemical attack by gases like' oxygen, nitrogen and

halogens, a solid film of the corrosion product is formed on the surface of the metal which protects the

metal from further corrosion. If a soluble or volatile corrosion product is formed, then the metal is

exposed to further attack. For example, chlorine and iodine attack silver generating a protective film of

silver halide on the surface. On the other hand, stannic chloride formed on tin is volatile and so

corrosion is not prevented.

Oxidation corrosion is brought about by direct action of oxygen at low or high temperatures on

metals in the absence of moisture. Alkali metals (Li, Na, K, etc.) and alkaline earth metals

(Mg, Ca, Sn, etc.) are readily oxidized at low temperatures. At high temperatures, almost all metals

except Ag, Au and Pt are oxidized. Alkali and alkaline earth metals on oxidation produce oxide

deposits of smaller volume. This results in the formation of a porous layer through which oxygen can

diffuse to bring about further attack of the metal. On the other hand, aluminium, tungsten and

molybdenum form oxide layers of greater volume than the metal from which they were produced.

These non-porous, continuous and coherent oxide films prevent the diffusion of oxygen and hence the

rate of further attack decreases with increase in the thickness of the oxide film.

The protective or non-protective nature of the oxide film is determined by a rule known as the

Pilling-Bedworth rule. The ratio of the volume of the oxide formed to the volume of the metal

consumed is called the Pilling-Bedworth rule. According to it, if the volume of the oxide layer is

greater than the volume of the metal, the oxide layer is protective and non-porous. On the other hand,

if the volume of the oxide layer formed is less than the volume of the metal, the oxide layer is non-

protective and porous.

[2] Electrochemical Theory or Wet Corrosion

According to the electrochemical theory, the corrosion of a metal in aqueous solution may be a

two-step process, one involving oxidation and another reduction. It is known that two metals having

different electrode potentials form a galvanic cell when they are immersed in a conducting solution.

The emf of the cell is given by the difference between the electrode potentials. When the electrodes are

joined by a wire, electrons flow from the anode to the cathode. The oxidation reaction occurs at the

anode, i.e. at the anode the metal atoms lose their electrons to the environment and pass into the

solution in the form of positive ions. Fe → Fe2+ + 2e-. Thus, there is a tendency at the anode to

destroy the metal by dissolving it as ions. Hence corrosion always occurs at anodic areas. The

electrons released at the anode are conducted to the cathode and are responsible for various cathodic

reactions such as electroplating (deposition of metals), hydrogen evolution and oxygen absorption:

86

(i) Electroplating: The metal ions at the cathode collect the electrons and deposit on the cathode

surface. Cu2+ + 2e- → Cu

(ii) Liberation of hydrogen: In an acid solution, (in the absence of oxygen) hydrogen ions

accept electrons and hydrogen gas is formed. 2H+ + 2e- → H2

In a neutral or alkaline medium, (in the absence of oxygen) hydrogen gas is liberated with the

formation of OH- ions. 2H2O + 2e- → H2 + 2OH-

(iii) Oxygen absorption: In the presence of dissolved oxygen and in an acid medium, oxygen

absorption reaction takes place. 4H+ + O2 + 4e- → 2H2O

In the presence of dissolved oxygen and in a neutral or weakly alkaline medium, OH- ions are

formed. 2H2O + O2 + 4e- → 4OH- Thus it is clear that the essential requirements of electrochemical

corrosion are as follows: (a) Formation of anodic and cathodic areas. (b) Electrical contact between the

cathodic and anodic parts to enable the conduction of electrons. (c) An electrolyte through which the

ions can diffuse or migrate. This is usually provided by moisture.

[3] Differential Aeration or Concentration Cell Corrosion

Anodic and cathodic areas may be generated even in a perfectly homogeneous and pure metal

due to different amounts of oxygen reaching different parts of the metal which form oxygen

concentration cells. In such circumstances, those areas which are exposed to greater amount of air

become cathodic while the areas which are little exposed or not exposed to air become anodic and

suffer corrosion. Figure shows the part of a metal surface covered with dirt which is less accessible to

air than the rest of the metal.

Hence, the area covered with dirt becomes anodic and suffers corrosion. The anodic

reaction is Fe → Fe2+ + 2e-

The most common reactions taking place at the cathode are

which is further oxidized to Fe(OH)3. Since the anodic area is small and the cathodic area is large,

corrosion is more concentrated at the anode. Thus, a small hole is formed on the surface of the metal.

This type of intense local corrosion is called pitting. Figure shows a wire fence in which the areas

where the wires cross are less accessible to air than the rest of the fence and hence corrosion takes

place at the wire crossings which are anodic.

87

In a similar way, iron corrodes under drops of water or salt solution. Areas covered by droplets,

having less access of oxygen become anodic with respect to the other areas which are freely exposed

to air. Differential aeration corrosion occurs when one part of metal is exposed to a different air

concentration from the other part. This causes a difference in potential between differently aerated

areas. It is experimentally found that poor oxygenated parts are anodic. Consequently, differential

aeration of a metal causes a flow of current.

Factors influencing corrosionFactors influencing corrosionFactors influencing corrosionFactors influencing corrosion: The rate and extent of corrosion depend mainly on

(a) the nature of the metal and (b) the nature of the environment.

Nature of the Metal

[I] Position in the galvanic series: The extent of corrosion depends upon the position of the

metal in the galvanic series. Greater the oxidation potential, the greater is the rate of corrosion.

When two metals are in electrical contact, the metal higher up in the galvanic series becomes anodic

and suffers corrosion. Further, the rate and severity of corrosion depend upon the difference in their

positions in the galvanic series. The greater is the difference, the faster is the corrosion

of anodic metal.

88

[2] Relative areas of the anode and cathode: The rate of corrosion is more when the area of the

cathode is larger. When the cathodic area is larger, the demand for electrons will be more,

and this results in increased rate of dissolution of metals at anodic regions.

[3] Purity of the metal: The impurities present in a metal create heterogeneity and thus galvanic

cells are set up with distinct anodic and cathodic areas in the metal. The higher the percentage of

impurity present in a metal, the faster is the rate of corrosion of the anodic metal. For instance,

impurities such as Pb and Fe in zinc lead to till formation of tiny electrochemical cells at the exposed

part of the impurity and the corrosion of zinc around the impurity takes place due to local action.

It is evident that the corrosion resistance of a metal may be improved by increasing impurity.

[4] Physical state of the metal: Metal components subjected to unevenly distributed stresses are

easily corroded. Even in a pure metal, the areas under stress tend to be anodic and suffer corrosion.

Caustic embrittlement takes place in stressed parts such as bends, joints and rivets in boilers.

[5] Nature of the oxide film: Metals such as Mg, Ca and Ba form oxides whose volume is less

than the volume of the metal. Hence, the oxide film formed will be porous, through which oxygen can

diffuse and bring about further corrosion. On the other hand, metals like AI, Cr and Ni form oxides

whose volume is greater than that of the metal and the nonporous oxide film so formed will protect the

metal from further corrosion.

[6] Solubilities of the products of corrosion: Solubility of the corrosion product formed is an

important factor in corrosion. If the corrosion product is soluble in the corroding medium, the

corrosion of the metal will proceed faster. On the other hand, if the corrosion product is insoluble, then

the protective film formed tends to suppress corrosion.

Nature of the Environment

[1] Temperature: The rate of chemical reactions and the rate of diffusion of ions increase with

temperature. Hence, corrosion increases with temperature. A passive metal may become active at a

higher temperature.

[2] Humidity: Atmospheric corrosion of iron is slow in dry air but increases rapidly in the

presence of moisture. This is due to the fact that moisture acts as the solvent for the oxygen in the air

to furnish the electrolyte essential for setting up a corrosion cell. Rusting of iron increases when the

relative humidity of air reaches from 60 to 80%.

[3] Effect of pH: The possibility of corrosion with respect to pH of the solution and the

electrode potential of the metal can be correlated with the help of a Pourbaix diagram.

The Pourbaix diagram for iron in water is shown in Figure.

89

.

The diagram shows clearly the zones of corrosion, immunity and passivity.

In the diagram x is a point where pH is 7 and the electrode potential is -0.4 V.

It is present in the corrosion zone. This shows that iron rusts in water under those conditions.

This is noticed to be true in actual practice also.

From figure, it is seen that the rate of corrosion can be altered by shifting the point x into

immunity or passivity regions. The iron would be immune to corrosion if the potential is changed to

about -0.8 V and this can be achieved by applying external current. On the other hand, the corrosion

rate of iron can also be reduced by moving into the passivity region by applying positive potential. The

diagram clearly indicates that the corrosion rate can also be reduced by increasing the pH of the

solution by the addition of alkali without disturbing the potential.

[4] Nature of the electrolyte: The nature of the electrolyte also influences the rate of corrosion.

If the electrolyte consists of silicate ions, they form insoluble silicates and prevent further corrosion.

On the other hand, if chloride ions are present, they destroy the protective film and the surface is

exposed for further corrosion. If the conductance of electrolyte is more, the corrosion current is easily

conducted and hence the rate of corrosion is increased.

[5] Concentration of oxygen and formation of oxygen concentration cells:

The rate of corrosion increases with increasing supply of oxygen. The region

where oxygen concentration is lesser becomes anodic and suffers corrosion.

Corrosion often takes place under metal washers where oxygen cannot diffuse readily.

Similarly, buried pipelines and cables passing from one type of soil to another suffer corrosion due to

differential aeration, e.g. lead pipeline passing through clay and then through sand. Lead pipeline

passing through clay get corroded because it is less aerated than sand.

90

Types of Corrosion: Types of Corrosion: Types of Corrosion: Types of Corrosion: The following are the different types of corrosion:

[1] Galvanic corrosion: Galvanic corrosion is a type of electrochemical corrosion in which two

different types of metals in contact are jointly exposed to corrosive atmosphere. Here the metal with

more negative electrode potential will become the anode and goes into solution or corrode, e.g. (a)

zinc and copper metals and (b) steel pipe connected to copper plumbing. This corrosion can be

minimized by (a) avoiding galvanic couples and (b) providing an insulating material between the two

metals.

[2] Stress corrosion cracking: Corrosion of metals is also influenced by some physical

differences like internal stresses in the metals. Such differences result during manufacture, fabrication

and heat treatment. Metal components are subjected to unevenly distributed stresses during their

manufacturing process. The electrode potential thus varies from one point to another. Areas under

great stress act as the anode while areas not under stress act as the cathode. Various treatments of

metals and alloys such as cold working or quenching, bending and pressing introduce uneven stress

and lead to stress corrosion. Corrosion takes place so as to minimize the stress. Most of the time it

ends in breaking of the components into pieces. Corrosion of head and point portions of a nail

indicates that they have been acting as anode to the middle portion. Actually the head and the point

portions were put under stress during their manufacture. In the case of iron-wire hammered at the

middle, corrosion takes place at the hammered part and results in breaking of the wire into two pieces.

Caustic embrittlement takes place in stressed parts such as bends, joints and rivets in boilers.

Corrosion Control: Corrosion Control: Corrosion Control: Corrosion Control: Corrosion can be controlled by the following ways:

[1] By selection of the material: Selection of the right type of the material is the main factor for

corrosion control. Thus, noble metals are used for surgical instruments and ornaments as they are most

immune to corrosion. [2] By using pure metals: Pure metals have higher corrosion resistance. Even

minute amount of impurities may lead to severe corrosion, e.g. 0.02% iron in aluminium decreases its

corrosion resistance. [3] By alloying: Both corrosion resistance and strength of many metals can be

improved by alloying, e.g. stainless steels containing chromium produce a coherent oxide film which

protects the steel from further attack. [4] By annealing: Heat treatment like annealing helps to reduce

internal stresses and reduces corrosion. [5] By eliminating galvanic action: If two metals have to be in

contact, they should be so selected that their oxidation potentials are as near as possible. Further, the

area of the cathode metal should be smaller than that of the anode, e.g. copper nuts and bolts on large

steel plate. The corrosion can also be reduced by inserting an insulating material between the two

metals. [6] By cathodic protection: The principle involved in cathodic protection is to force the metal

behave like a cathode. Since there will not be any anodic area on the metal, corrosion does not occur.

91

There are two types of cathodic protection. (a) Sacrificial anodic protection. (b) Impressed current

cathodic protection.

(a) Sacrificial anodic protection: In this technique, a more active metal is connected to the

metal structure to be protected so that all the corrosion is concentrated at the more active metal and

thus saving the metal structure from corrosion. This method is used for the protection of sea going

vessels such as ships and boats. Sheets of zinc or magnesium are hung around the hull of the ship. Zinc

and magnesium being anodic to iron get corroded. Since they are sacrificed in the process of saving

iron (anode) they are called sacrificial anodes. The corroded sacrificial anode is replaced by a fresh

one, when consumed completely. Important applications of sacrificial anodic protection are as follows:

[i] Protection from soil corrosion of underground cables and pipelines (Figure a). [ii] Magnesium

sheets are inserted into domestic water boilers to prevent the formation of rust water (Figure b).

Sacrificial anodic protection

Figure a Figure b

(b) Impressed current cathodic protection: In this method, an impressed current is applied in

an opposite direction to nullify the corrosion current and converting the corroding metal from anode to

cathode. This can be accomplished by applying sufficient amount of direct current from a battery to an

anode buried in the soil and connected to the corroding metal structure which is to be protected.

The anode is in a backfill (composed of gypsum) so as to increase the electrical contact with the soil.

Since in this method current from an external source is impressed on the system, this is called

impressed current method.

92

[8] By modifying the environment: The corrosion rate can be reduced by

modifying the environment. The environment can be modified by the following:

(a) Deaeration: The presence of increased amounts of oxygen is harmful since it increases the

corrosion rate. Deaeration aims at the removal of dissolved oxygen. Dissolved oxygen can be removed

by deaeration or by adding some chemical substances like Na2SO3 (b) Dehumidification: In this

method, moisture from air is removed by lowering the relative humidity of surrounding air. This can

be achieved by adding silica gel which can adsorb moisture preferentially on its surface.

(c) Inhibitors: In this method, some chemical substances known as inhibitors are added to the

corrosive environment in small quantities. These inhibitors substantially reduce the rate of corrosion.

(i) Anodic inhibitors: Anodic inhibitors include alkalis, molybdates, phosphates. chromates, etc.

When these inhibitors are added, they react with the ions of the anode and produce insoluble

precipitates. The so formed precipitate is adsorbed on the anode metal forming a protective film

thereby reducing corrosion. (ii) Cathodic inhibitors: In an electrochemical corrosion, the cathodic

reactions are of two types depending on the environment. In acidic solution, the cathodic reaction is

2H+ + 2e- → H2. In acidic solution; the corrosion can be controlled by slowing down the diffusion of

H+ ions through the cathode. This can be done by adding organic inhibitor like amines and pyridine.

They adsorb over the cathodic metal surface and act a protective layer. In neutral solution,

the cathodic reaction is H2O + 1/2O2 + 2e- → 2OH- The formation of OH- ions is only due to the

presence of oxygen. By elimination the oxygen from the medium, the corrosion rate can be reduced.

Oxygen can be removed by adding some reducing agents (Na2SO3) or by deaeration.

(iii) Vapour phase inhibitors: Vapour phase inhibitors (VPIs) are organic inhibitor which readily

sublime and form a protective layer on the metal surface, e.g., dicyclohexylammoniumnitrite. They are

used in the protection of machinery, sophisticated equipment, etc. which are sent by ships. The

condensed inhibitor can be easily wiped off from the metal surface.

[9] By Passivation: Passivation is a phenomenon of converting an active surface of a metal into

passive i.e. more corrosion resistant by forming a thin, non-porous and highly protective film over the

surface. When the metals like aluminium and tin are exposed to the atmosphere or to the oxidizing

environment, their surfaces rapidly get converted into oxides. The non-porous nature of these oxide

layers prevents further corrosion. In other words, the metals are passivated. Similarly, metals which

are susceptible to corrosion are made passive by alloying with one or more metals which are passive or

resist corrosion. For example, iron is rendered passive by alloying it with any of the transition metals

such as chromium, nickel and molybdenum.

[10] By applying protective coating: The surface of engineering material can be protected from

corrosion by covering it by a protective coating. This coating may be of organic or inorganic material.

93

Protective CoatingsProtective CoatingsProtective CoatingsProtective Coatings

Introduction

Protective coatings are used to protect the metals from corrosion. The main types of protective

coatings are classified as follows.

The protective coatings must be chemically inert to the environment and also sufficiently thick.

Besides protection from corrosive conditions, such coatings can also give decorative appearance to the

base metals. To be more effective, these coatings should adhere well to the surface.

Pretreatment of the surface or preparation of materials for coating: The outermost surface of

the base metal (which is to be protected) usually contains impurities like rust, scale and grease.

These substances, if present at the time of coating, will give porous and discontinuous coatings.

In order to get a uniform, smooth and coherent protective coating, these substances are removed by the

following methods.

Degreasing: Oil and grease may be removed by cleaning with organic solvents such as

chloroform, toluene and acetone. Immersion in hot alkaline solutions is the most commonly used

cleaning technique. For example, sodium carbonate and sodium hydroxide are used for this purpose.

Removal of Oxide Scales or Descaling: Removal of the oxide scales and corrosion products

(rust) from the surface is called descaling. In this process, the base metal is dipped inside the acid

solution at higher temperatures. The acid penetrates through cracks and pores of the scales and then

their dissolution takes place. Acids like sulphuric acid, hydrochloric acid and nitric acid are used under

dilute conditions.

Mechanical Cleaning: Oxide scales, rust and corrosion products are also removed by abrasion

such as grinding, wire brushing and polishing.

Electrochemical Method: The electrochemical method is used to remove oxide scales which are

not removed by other methods. The base metal is made either anode or cathode with an electrolyte

(acid or base). At the anode the oxide scale is dissolved in the electrolyte and leaves the base metal,

whereas at the cathode the metal oxides are reduced to metal.

94

Metallic CoatingsMetallic CoatingsMetallic CoatingsMetallic Coatings

Surface coatings made up of metals are known as metallic coatings. These coatings separate the

base metal from the corrosive environment and also function as an effective barrier for the protection

of base metals. Metallic coatings are mostly applied on iron and steel because they are cheap and

commonly used. Metallic coatings are usually imparted by the following methods.

Hot Dipping: In the process of hot dipping, the metal to be coated is dipped in the molten bath

of the coating metal and the thickness of the coating is adjusted by squeezing out the excess of the

coating metal with rollers. Such hot dip coatings are generally non-uniform. The common examples of

hot dip coatings are galvanizing and tinning.

[1] Galvanizing: The process of coating a layer of zinc on steel is called galvanizing.

The steel article is first pickled with dilute sulphuric acid to remove traces of rust, dust, etc, at 60-

90°C for about 15-20 minutes. Then this metal is dipped in a molten zinc bath maintained at 430°C.

The surface of the bath is covered with ammonium chloride flux to prevent oxide formation on the

surface of molten zinc. The coated base metal is then passed through rollers to correct the thickness of

the film. It is used to protect roofing sheets, wires, pipes, tanks, nails, screws, etc.

[2] Tinning: The coating of tin on iron is called tin plating or tinning. In tinning, the base is

first pickled with dilute sulphuric acid to remove surface impurities. Then it is passed through molten

tin covered with zinc chloride flux. The tin coated article is passed through a series of rollers

immersed in a palm oil bath to remove the excess tin. Tin-coated utensils are used for storing

foodstuffs, oils, etc. Galvanizing is preferred to tinning because tin is cathodic to iron, whereas zinc is

anodic to iron. So, if the protective layer of the tin coating has any cracks, iron will corrode. If the

protective layer of the zinc coating has any cracks, iron being cathodic does not get corroded. The

corrosion products fill up the cracks, thus preventing corrosion.

Cementation: In cementation, the base metal is heated with the coating metal in the form of

fine powder in order to promote the diffusion of the coating metal into the base metal.

The coatings obtained are of uniform thickness. The base metal is generally steel and the coating

metals used is zinc, chromium and aluminium. When the coating metal is zinc, the process is called

sherardizing. When the coating metal is chromium, the process is called chromizing. When the coating

metal is aluminium, the process is called calorizing.

[1] Sherardizing: Cementation with zinc powder is called sherardizing. The base metal is

heated with zinc dust in a metal drum maintained at a temperature of 350-370°C. The drum is closed

tightly and rotated with constant heating for two to three hours. During this process zinc gets diffused

into iron forming an alloy of Fe-Zn on the surface. Sherardized coatings are used for protecting small

steel parts such as nuts and bolts against atmospheric corrosion.

95

[2] Chromizing: The base metal is heated with a powdered mixture of 55 per cent chromium

and 45 per cent alumina at a temperature of 1300-1400°C for about 3-4 hours in a closed drum.

The purpose of using alumina is to prevent the coalescing of chromium particles. The outermost

surface of the base metal is converted into a chrome alloy which protects the metal against corrosion.

This method is used to protect gas turbine blades. [3] Calorizing: Here the base metal is heated with a

powdered mixture of aluminium and alumina in a drum at a temperature of 840-930°C for 4-6 hours.

Electroplating or Electrodeposition: Electroplating is a process in which metals are deposited

or plated on base metals from solutions containing metallic ions by means of electrolysis.

The objectives of electroplating are as follows: (1) To obtain improved resistance to corrosion and

chemical attack (2) To get better appearance (3) To get increased hardness (4) To change the surface

properties of metals and non-metals. In the electroplating process, the freshly cleaned base metal

which is to receive the coat is made the cathode in a suitable electrolyte bath containing (a) a solution

of the salt of the metal 10 be electrodeposited, (b) buffer solution to control the pH, and (c) additional

reagents to enhance conductivity and to aid the formation of smooth, dense and coherent coating.

The concentration of the salt solution is maintained by the addition of the metal salt at regular intervals

or by the use of continuously dissolving anode of the metal. The plating is usually done at a high

current density. The nature of the deposit depends upon the current density, pH and the concentration

of the bath. A typical electroplating process is shown in Figure.

Organic Coatings: Organic Coatings: Organic Coatings: Organic Coatings: Organic coatings are inert organic barriers applied to the surface of

base metals for corrosion resistance and decoration. Paints, varnish, lacquers and enamels are the main

organic coatings. Paints: Paint is a viscous suspension of finely divided solid pigment in a fluid

medium which on drying yields an impermeable film having considerable hiding power.

When paint is applied to a metal surface, the thinner evaporates, while the drying oil slowly oxidizes

forming a dry pigmented film. Requirements of a good paint: A good paint should essentially have

the following: [1] A good paint should fonn a good impervious and uniform film on the metal surface.

[2] It should have a high hiding (covering) power. [3] The film should not crack on drying.

[4] A good paint should adhere well to the surface. [5] It should spread on the metal surface easily.

[6] It should give a glossy film. [7] It should be corrosion resistant. [8] A good paint should give a

stable and decent colour on the metal surface.

96

Constituents of paint and their functions: The important constituents of paint are as follows:

(1) Pigments (2) Vehicle or drying oils or medium (3) Thinners (4) Driers (5) Fillers or extenders

(6) Plasticizers (7) Anti-skinning agents

(1) Pigments: A pigment is a solid and colour-producing substance in the paint.

(2) Vehicle or drying oils or medium: The liquid portion of the paint in which the pigment is

dispersed is called a medium or vehicle. This is the film forming constituent of the paint. Vehicles are

high molecular weight fatty acids present in animal and vegetable oils, e.g. linseed oil, dehydrated

castor oil, soyabean oil and fish oil. (3) Thinners: Thinners are added to paints to reduce the

consistency or viscosity of the paints so that they can be easily applied to the metal surface. Thinners

are volatile in nature and evaporate easily after application of the paint, e.g. turpentine and petroleum

spirit (4) Driers: Driers are used to accelerate or catalyze the drying of the oil film by oxidation,

polymerization and condensation, e.g. naphthenates, borates and tungstales of lead, cobalt and

manganese. (5) Fillers or extenders: Fillers are used to reduce the cost and increase the durability of

the paint, e.g. talc, china clay, calcium sulphate and calcium carbonate. (6) Plasticizers: Plasticizers

are chemicals added to paints to give elasticity to the film and to prevent cracking of the film, e.g.

triphenyl phosphate and tricresyl phosphate. (7) Anti-skinning agents: They are chemicals added to

the paint to prevent skinning of the paint, e.g. polyhydroxy phenols.

Mechanism of drying oils: Drying oils are the film forming constituent of the paint.

These are fatty oils which are extracted from plants or animals. The fatty oils are tri-esters of glycerol.

The oil film after it has been applied to the protected surface absorbs oxygen from air at the double

bonds forming peroxides which isomerizes, polymerize and condense to form a tough and highly cross

linked macromolecular film.

Failure of paint: Paint may fail due to several reasons: (1) Chalking: Chalking is the gradual

powdering of paint. It is caused by continuous destructive oxidation of the oil after the original drying

of paint. (2) Erosion: It is very quick chalking. (3) Flaking: Flaking is caused due to poor adherence

of a paint film to the surface because of the presence of grease on the surface. (4) Checking: It is a

very fine type of surface cracking. (5) Alligatoring: The centre portion of the film remains attached to

the surface, whereas the portion around the centre peels off.

The failure of paint can be prevented by the following ways: (1) Carefully preparing the

surface before application of paint. (2) Applying a suitable primer coat. (3) Applying the paint evenly.

(4) Allowing each paint coat to dry sufficiently before the next coat is applied.

97

Water and Its Treatment

Rain water is almost pure (may contain some dissolved gases from the atmosphere).

Being a good solvent, when it flows on the surface of the earth, it dissolves many salts. Presence of

calcium and magnesium salts in the form of bicarbonate, chloride and sulphate [Cations are Ca2+ ion

and Mg2+ ion and Anions are HCO3- ion, Cl- ion, and SO4

2- ion] in water makes water ‘hard’ .

Hard water does not give ready and permanent lather with soap. Water free from soluble salts of

calcium and magnesium is called Soft water. It gives lather with soap easily.

Hardness of Water: Hardness was originally defined as the soap consuming capacity of a

water sample. Soap is the sodium salt of higher fatty acids, e.g. sodium stearate. The sodium salt is

soluble in water, but the corresponding calcium and magnesium ions are insoluble in water.

When soap is added to soft water, it dissolves and lathers readily.

On adding soap solution to a sample of hard water which contains calcium or magnesium ions,

soap is precipitated as insoluble salts which prevent the formation of lather. This reaction causes the

loss of soap. No lather is obtained until all the ions are removed. So, large amount of soap is consumed

unnecessarily before lather is formed.

2C17H35COONa (sodium stearate) + CaCl2 → (C17H35COO)2Ca [ppt of calcium stearate] + 2NaCI

The hardness of water is of two types: (i) Temporary Hardness, and (ii) Permanent Hardness.

Temporary Hardness is due to the presence of magnesium and calcium bicarbonates

[Ca(HCO3)2 and Mg(HCO3)2]. It is also called carbonate hardness (CH). It can be removed by

Boiling. During boiling, the soluble Mg(HCO3)2 is converted into insoluble Mg(OH)2 and Ca(HCO3)2

is changed to insoluble CaCO3. These insoluble precipitates can be removed by filtration.

Filtrate thus obtained will be soft water.

Mg^HCO�b� ��$,T�⎯⎯⎯⎯� MgÔHb� ↓ +2CO� ↑ and Ca^HCO�b� ��$,T�⎯⎯⎯⎯� CaCO� ↑ +CO� ↑ +H�O

Permanent Hardness is due to the presence of soluble salts of magnesium and calcium in the

form of chlorides and sulphates in water (CaCl2, CaSO4, MgCl2 and MgSO4). Permanent hardness is

not removed by boiling. It is also called non-carbonate hardness (NCH).

Total Hardness: Temporary hardness and permanent hardness constitute the

total hardness which is also expressed as the sum of the concentration of calcium and magnesium ions.

Total Hardness = Temporary Hardness + Permanent Hardness or = [Ca2+] + [Mg2+] Units of hardness: The following are the common units used in hardness measurements.

[1] Parts per million (ppm): It is defined as the number of parts by weight of CaCO3 present in million parts by weight of water. 1 ppm = 1 part of CaCO3

equivalent hardness in 106 parts of water, so 1ppm = ��)U��',++�R��+�)�� . Both temporary and permanent hardness is expressed in ppm as CaCO3. The choice of CaCO3

because is due to the fact that it is the most insoluble salt in water.

[2] Milligram per litre: It is defined as the number of milligrams of CaCO3 present in one litre of water.1 mg/l = 1 mg of CaCO3 equivalent hardness in one

litre of water Since weight of 1 litre of water = 1 kg = 1000 g = 1000 x 1000 mg = 106 mg so 1 mg/l = 1 mg of CaCO3 per 106 mg of water = 1 part of CaCO3 per

106 parts of water = 1 ppm. Thus, mathematically both units are equal.

98

Determination of Total Hardness of Water by Complexometric Titration

[EDTA Method]: Eriochrome Black-T [EBT] is the indicator used in the determination of hardness

by complexometric titration with EDTA. Here, Eriochrome Black-T is a complex organic compound

[sodium-1-(1-hydroxy 2-naphthylato)-6-nitro-2-naphthol-4-sulphonate] and EDTA is a hexadentate

ligand [disodium salt of ethylenediamine tetraacetic acid].

OH

N N

NO2

HO

SO3Na

Eriochrome Black-T [EBT]

NH2C

H2CN

HOOCH2C

NaOOCH2C CH2COOH

CH2COONa EDTA

[Disodium salt of ethylenediamine tetraacetic acid]

Principle: Estimation of hardness by EDTA method is based on the principle that EDTA forms

metal complexes with hardness producing metal ions in water. These complexes are stable when the

pH is maintained between 8 and 10 and also the indicator becomes effective only at this pH range.

In order to maintain the pH, buffer solution (NH4Cl and NH4OH mixture) is added. The completion of

the complexation reaction is indicated by Eriochrome Black-T indicator. When this indicator is added

to the sample water it forms indicator-metal complexes of wine red colour.

Ca2+/ Mg2+ + EBT → [Ca2+/ Mg2+ EBT]

from hard water Wine red coloured unstable complex

Now when this wine red-coloured solution is titrated against EDTA solution,

EBT in the unstable complex is replaced by EDTA to form a stable metal-EDTA complex and

liberates the free Eriochrome Black-T. At this point, the colour of the solution changes from wine red

to original blue colour which showing the end point of the titration.

[Ca2+/ Mg2+ EBT] + EDTA → [Ca2+/ Mg2+ EDTA] + free EBT

Wine red coloured Stable metal-EDTA complex Blue colour unstable complex (Colourless)

The temporary hardness is removed by boiling and after the removal of precipitate by filtration;

the permanent hardness in the filtrate is determined by titration with EDTA as before.

Therefore, total hardness - permanent hardness - temporary hardness

Experimental procedure: A known volume of the sample of hard water (V ml) is treated with

about 10 ml of a buffer solution and 5 drops of Eriochrome Black-T indicator. This solution is then

titrated against the standard EDTA reagent (standardized such that 1 ml of the reagent corresponds to

1 mg of CaCO3). The end point is the colour change from wine red to blue.

99

A known volume (V ml) of sample water is taken in a beaker and boiled for 15 minutes.

After cooling the mixture, it is filtered and thoroughly washed. The filtrate is collected and made up to

a known volume (V ml). This solution is titrated against EDTA as before. The volume of EDTA

consumed is V2 ml. Then,

Hence, (Total hardness) - (Permanent hardness) gives the temporary hardness.

Calculation steps

1 ml of Standard hard water = 1 mg CaCO3

V1 ml of EDTA solution = 20 ml of Standard hard water

= 20 mg CaCO3

1 ml of EDTA solution, = 20V� mgCaCO�

= …………..mg CaCO3

20 ml of given water sample = V2 ml of EDTA solution

= V� × �20V� mgCaCO��

1 ml of given water sample = V� × �20V� mgCaCO�� ×120

1000 ml of given water sample = V� × �20V� mgCaCO�� ×120 × 1000

= V�V� ×

2020 × 1000mgCaCO�

= V�V� × 1000mgCaCO�

Total Hardness of given water sample = V�V� × 1000ppm = ⋯……… . ppm

100

Problem: 0.5 g of CaCO3 was dissolved in dil. HCl and diluted to 500 ml. 50 ml of this

solution required 48 ml of EDTA solution for titration. 50 ml of a hard water sample required

15 ml of the same EDTA solution for titration. Calculate the total hardness of water.

500 ml of CaCl2 solution = 0.5 g of CaCO3

= 0.5 x 1000 mg of CaCO3

= 500 mg of CaCO3 hence 1 ml of CaCl2 solution = 1 mg of CaCO3

Standardization of EDTA

1 ml of CaCl2 solution = 1 mg of CaCO3

50 ml of CaCl2 solution = 50 mg of CaCO3

48 ml of EDTA solution = 50 mg of CaCO3

1 ml of EDT A solution = �R�� × 1 = 1.04 mg of CaCO3

Total hardness

1 ml of EDTA solution = 1.04 mg of CaCO3

15 ml of EDTA solution = 1.04 X 15 = 15.6 mg of CaCO3

This amount of hardness is present in 50 ml of the water sample.

So total hardness present in one litre = ��.��R × 1000 = 312 ppm of CaCO3 equivalent

101

102

103

104

105

Alkalinity : The alkalinity of water is due to the presence of a wide variety of salts of weak

acids such as carbonates, bicarbonates, phosphates, etc., and also due to the presence of weak and

strong bases (due to contamination with industrial wastes). The major portion of alkalinity in natural

water is caused by the presence of bicarbonates that are formed when water containing free carbon

dioxide percolates through soils containing calcium carbonate and magnesium carbonate.

CaCO3 + CO2 + H2O → Ca(HCO3)2

The alkalinity of natural water may be taken as an indication of the concentration of hydroxides,

carbonates and bicarbonates.

Caustic Embrittlement: It is the phenomenon during which the boiler material becomes brittle

due to the accumulation of caustic substances. It is a very dangerous form of stress corrosion occurring

in mild steel boiler metals exposed to alkaline solution at high temperatures, resulting in the failure of

the metal. Stressed parts like bends, joints and rivets are severely affected. Boiler water usually

contains a small proportion of Na2CO3. In high pressure boilers, this breaks up to give NaOH and

makes the boiler water more alkaline: Na2CO3 + H2O → 2NaOH + CO2

This very dilute alkaline boiler water flows into the minute hair cracks and crevices by capillary

action. There the water evaporates and the concentration of caustic soda increases progressively.

The concentrated alkali dissolves iron as sodium ferroate in crevices, cracks, etc. where the metal is

stressed. This causes brittlement of boiler parts particularly stressed parts like: bends, joints and rivets

causing even failure of the boiler. Highly alkaline water may lead to caustic embrittlement.

Caustic embrittlement can be avoided (a) by using sodium phosphate as softening reagent instead of

Na2CO3 and (b) by adding tannin or lignin to boiler water which blocks the hair cracks.

Dissolved Oxygen (D.O): Oxygen is poorly soluble in water. The solubility of oxygen of air in

fresh water varies from 7.5 - 14.5 mg/Lit. Dissolved oxygen is needed for living organism

to maintain their biological process. It is an important factor in corrosion. Dissolved oxygen in water is

mainly responsible for the corrosion of a boiler. The dissolved oxygen in water attacks the boiler

material at high temperatures.

2Fe + 2H2O + O2 → 2Fe(OH)2

4Fe(OH)2 + O2 + 2H2O → 2[Fe2O3·3H2O] (rust)

Determination of Dissolved Oxygen present in a given Water Sample by Iodometric Method

(Winkler’s Method): The principle involved in the determination of dissolved oxygen is to bring about

the oxidation of potassium iodide (KI) to iodine (I2) with the dissolved oxygen present in the water

sample after adding MnSO4, KOH and KI, the basic manganic oxide formed act as an oxygen carrier

to enable the dissolved oxygen in the molecular form to take part in the reaction.

106

MnSO� + 2KOH ⟶ MnÔHb� + K�SO� 2MnÔHb� + O� ⟶ 2MnOÔHb� Basic manganic oxide which on acidification gives

MnOÔHb� + H�SO� ⟶MnSO� + 2H�O + nOp 2KI + H�SO� + nOp ⟶ K�SO� + H�O + I�

The liberated iodine (I2) is titrated against standard sodium thiosulphate (Na2S2O3) solution using

starch as indicatornStarch + I� ⟶ Bluecolouredcomplexp. I� + 2Na�S�O� ⟶ Na�S�O� + 2NaI

Procedure: Take 100 ml of given water sample into a conical flask, and titrate slowly against

N/50 standard sodium thiosulphate solution (taken in the burette). When the colour of the solution is

very light yellowish add about 2 ml of freshly prepared starch solution, so the colour of the solution

turned into blue. Continue the titration till the disappearance of blue colour of the solution and note

down the volume of the titrant used. The titration is repeated until a concordant volume is obtained.

Normality of standard Na2S2O3 solution, N1 = 150 = 0.02N

Volume of standard Na2S2O3 solution, V1 = ……………ml

Volume of given water sample, V2 = ………….....ml

Normality of given water sample, N2 can be calculated from the normality formula,

i.e., N1 x V1 = N2 x V2

Normality of given water sample, N2 = N� × V�V�

= ………………..N

Amount of Dissolved Oxygen = N� × Eq.wtofOxygen = N� × 8 g/Lit

= ………………g/Lit

Amount of Dissolved Oxygen in ppm = ………… . .× 1000mg/Lit

= ………………..ppm

Removal of Dissolved O2: Dissolved oxygen can be removed from water by chemical

and mechanical means. Sodium sulphite (Na2SO3), hydrazine (N2H4), etc. are some of the chemicals

used for removing oxygen.

2Na2SO3 + O2 → 2Na2SO4

N2H4 + O2 → N2 + 2H2O

Hydrazine is found to be an ideal compound for removing dissolved oxygen since the products are

water and inert N2 gas. It removes oxygen without increasing the concentration of dissolved salts.

107

Impurities in water: The impurities present in water may be broadly classified as follows.

[1] Dissolved impurities: The dissolved impurities are mainly the carbonates, bicarbonates, chlorides

and sulphates of calcium, magnesium, iron, sodium and potassium. The presence of these salts imparts

hardness to water. The dissolved impurities also include dissolved gases like oxygen

and carbon dioxide.

[2] Suspended impurities: The following are the types of suspended impurities:

(a) Inorganic: Clay and sand (b) Organic: Oil globules, vegetable and animal matter.

The above suspended impurities impart turbidity, colour and odour to water.

[3] Colloidal impurities: They are finely-divided silica and clay, organic waste products,

complex protein amino acids, etc.

[4] Microorganisms: They are algae, fungi and bacteria.

Potable Water (water for domestic supply): Municipalities have to supply potable water,

i.e., It is a water of sufficiently high quality that it can be consumed or used without risk of immediate

or long term harm. The following are characteristics of potable water

[1] It should be sparking clear, soft, pleasant in taste, perfectly cool and odourless

[2] Its turbidity should not exceed 10 ppm [3] Its alkanity should not be high (pH = 8.0)

[4] Its dissolved solids should be less than 500 ppm [5] It should be free from objectionable minerals

such as lead, arsenic, chromium and manganese salts and also free from objectionable dissolved gases

like hydrogen sulphide [6] It should be free from disease producing micro-organisms.

Drinking water comes from two major sources: (a) Surface water such as lakes, rivers, and

reservoirs, but it requires both filtration and disinfection in order to use as drinking water.

(b) Ground water, which is pumped from wells, it is considered to be the purest source of water.

Rivers, lakes and wells are the most common sources of water used by municipalities.

The actual treatment methods depend directly on the impurities present. For removing various types of

impurities the following treatment processes are employed.

108

Treatment Processes for Removal of Impurities

Screening: It is the process of removing floating materials like wood pieces and leaves from

water. Raw water is allowed to pass through a screen having a large number of holes which removes

the small and large floating matter.

Sedimentation: It is the process of removing suspended impurities by allowing the water to

stand undisturbed for 2-6 hours in big tanks. Due to force of gravity, most of the suspended particles

settled down at the bottom and they are removed. Sedimentation removes only 70-75% of the

suspended matter.

Coagulation: Finely-divided silica, clay, etc. do not settle down easily and hence cannot be

removed by sedimentation. Most of these are in colloidal form and are negatively charged and hence

do not coalesce because of mutual repulsion. Such impurities are removed by coagulation method.

Here, certain chemicals like alum and Al2(SO4)3 are added to water. When Al2(SO4)3 is added to water,

it hydrolyzes to form a gelatinous precipitate of Al(OH)3 The gelatinous precipitate of Al(OH)3 entraps

finely divided and colloidal impurities, settles to the bottom and can be removed easily.

Filtration: For removing bacteria, colour, taste, odour, fine suspended particles, etc. and to

produce clear water, filtration is used. In this process, water is passed through beds of fine sand, coarse

sand and other granular material. The porous material used is the filtering medium and the equipment

used for filtration is known as filter, e.g. slow sand filter.

A typical slow sand filter is shown in Figure 1.5. It consists of a tank containing thick beds of

fine sand (at the top), coarse sand, fine gravel and coarse gravel (at the bottom).

When the water passes through the filtering medium, it flows through the various beds slowly due to

gravity. The rate of filtration slowly decreases due to the blockage of impurities in the pores of the

sand bed. When the rate of flow becomes very slow, filtration is stopped and the bed is cleaned by

scraping of a smaller layer of the sand bed (top layer) and replacing it with the clean sand.

Bacterias are partly removed by this filtration process.

109

Sterilization: The complete removal of harmful bacteria is known as sterilization.

The following sterilizers are generally used for sterilizing water.

[1] Sterilization by chlorine or bleaching powder: Chlorine is the most common sterilizing

agent in water treatment. Chlorine may be added in the form of bleaching powder or directly as a gas

or in the form of concentrated solution in water.

When bleaching powder is added to water, HOCI which acts as a powerful germicide is

produced. It is believed that HOCI reacts with bacteria and inactivate the enzymes present in the cells

of bacteria. These enzymes are responsible for the metabolic activities of microorganisms.

Since these enzymes are inactivated, microorganisms become dead.

CaOCl2 + H2O → Ca(OH)2 + Cl2; Cl2 + H2O → HCI + HOCI; HOCI + Bacteria → Bacteria are killed

[2] Sterilization by ultraviolet radiations: Ultraviolet radiations emanating from electric mercury

vapour lamp is capable of sterilizing water. This process is particularly useful for sterilizing swimming

pool water. This process is highly expensive.

[3] Sterilization by ozone: Ozone is a powerful disinfectant and is readily absorbed by water.

Ozone is highly unstable and decomposes to give nascent oxygen which is capable of destroying the

bacteria. O3 → O2 + [O]. This process is relatively expensive.

Break point chlorination

In break point chlorination a sufficient amount of chlorine is added to oxidize (a) organic matter,

(b) reducing substances (Fe2+, H2S, etc.) and (c) free ammonia in raw water leaving behind mainly free chlorine

which destroys pathogenic bacteria. When chlorine is added to water, initially it reacts with ammonia and there

will formation of chloramines (See equations). Thus the amount of combined residual chlorine (chloramines)

increases with increasing dosage. Then the oxidation of chloramines and other impurities start and there is a

fall in combined chlorine content. Thus combined residual chlorine decreases to a minimum at which oxidation

of chloramines and other organic compounds complete. This minimum is the breaking point of chlorine (Fig)

The reason for such behaviour is due to the fact that some organic compounds which defy oxidation at

lower chlorine concentration get oxidized when the break point chlorine concentration is reached.

Since, it is these organic compounds which are generally responsible for bad taste and odour in water, it is clear

that break point chlorination eliminates bad taste and odour. Further chlorination increases the free residual

chlorine (CI2, HOCI, OCI-). Hence, it use chlorine as a good disinfectant, the chlorine dosage has to be given

more than the break point.

110

111

112

Module IV Module IV Module IV Module IV ---- WaterWaterWaterWater NoNoNoNo QuestionsQuestionsQuestionsQuestions MarksMarksMarksMarks

1 Mention the chemical species causing alkanity in water? 5

2 What is the hardness of water? How is it expressed? 5

3 Distinguish between Hard water and Soft water.

What is break point chlorination? 7

4 Why buffer is added during titration of hard water against EDTA solution?

Write the structure of EDTA and EBT 8

5

0.30 g of CaCO3 was dissolved in HCl and the solution made on to one litre

with distilled water. 100 ml of this solution required 30 ml of EDTA solution

on titration. 100 ml of hard water sample required 55 ml of EDTA solution on

titration. After boiling 100 ml of this water, cooling, filtering and then titration

required 10 ml of EDTA solution. Calculate the temporary and permanent

hardness of water

8

6

20 ml of std hard water (containing 15 g CaCO3/litre) required 25 ml of EDTA

solution for end point 100 ml of water sample required 18 ml of EDTA

solution while the same water after boiling required 12 ml EDTA solution.

Calculate total hardness of water

8

7 How is water softened by lime soda process? 7

8 With a neat diagram, discuss the demineralization of water

using ion exchange method 8

9 Enumerate the various stages involved in the purification of water for

domestic use 8

10 What are the important sources of water pollution? 5

11 With relevant chemical equations, outline the estimation of dissolved

oxygen 7

12 Among BOD and COD, which is greater? Why? 5

13 Distinguish between BOD and COD, How it can be determined? 8

14 Write a note on experimental determination of BOD of a polluted water

sample 5

Module I Module I Module I Module I –––– Bioinorganic ChemistryBioinorganic ChemistryBioinorganic ChemistryBioinorganic Chemistry

NoNoNoNo QuestionsQuestionsQuestionsQuestions MarksMarksMarksMarks

1 What is the importance of bulk and trace metal ions in biological systems 5

2 What roles of iron and copper play in biology? 5

3 Give elementary idea about haemoglobin and myoglobin 8

113

Module II Module II Module II Module II ---- PolymerPolymerPolymerPolymer NoNoNoNo QuestionsQuestionsQuestionsQuestions MarksMarksMarksMarks

1 What are the classifications of polymers?

Give one example for addition and condensation polymers 8

2 How would you classify polymers based on source and applications? 5

3 Differentiate between thermoplastic and thermosetting plastic.

Give two examples of each type 5

4 Discuss the various types of polymerization with suitable examples 5

5 Distinguish between polymerization by addition and condensation

processes 5

6 Discuss the mechanism of addition polymerization 8

7 Explain free radical, cationic and anionic mechanism of polymerization 8

8 Discuss the mechanism of coordination [or Ziegler-Natta] polymerization 5

9 Briefly explain the techniques of polymerization 8

10 What is glass transition temperature? Discuss the factors affecting Tg.

What is its significance? 8

11 Write notes about crystallinity in polymers and the factors affecting it 7

Module II Module II Module II Module II ---- LubricantsLubricantsLubricantsLubricants


1 What are lubricants? Discuss the classification of lubricants with examples 7

2 Explain the classification of liquid lubricants 5

3 What are lubricating oils? Give their properties and uses 5

4 Write a note on grease as a lubricant 5

5 Write a note on the term solid lubricants 5

6 Draw and explain the structure of graphite and molybdenum disulphide.

How are they used as lubricants? 7

7 Outline the mechanism of lubrication 8

8 Discuss in detail the properties of lubricants highlighting their importance 8

9 How do viscosity and viscosity index influence the selection of lubricants for

a particular purpose? 5

10 Write short notes on: (i) Solid lubricants (ii) Flash and Fire point

(iii) Aniline point (iv) Cloud and Pour point 10

114

University Questions

Module II Module II Module II Module II ---- FuelsFuelsFuelsFuels


1 Distinguish between Gross (or higher) and Net (or lower) calorific value of fuel 5

2 What is cracking and what for it used? What are the types of cracking? 8

3 Write informative notes on Reforming 5

4 How does knocking occur in I.C engines? How can it be prevented? 10

5 What is meant by knocking in a petrol engine and what is it due to? 5

6 What is octane number of petrol? How is petrol knocking related to chemical

structure of the constituents of petrol and how can it be reduced? 7

7 How do you explain knocking in a diesel engine? How can it be controlled?

What is cetane number? 7

8 Establish the relationship between knocking in I.C engines and molecular

structure of the constituents in petrol and diesel fuels 7

9 Differentiate Octane number and Cetane number 8

10 Write informative notes on Biodiesel 5

Module I Module I Module I Module I –––– Green ChemistryGreen ChemistryGreen ChemistryGreen Chemistry

1 What is green chemistry? How is it important? 5

2 What are the goals of green chemistry and its limitations? 5

3 Explain the twelve principles of green chemistry with examples 10

4 Discuss any four principles of green chemistry 8

5 Write short notes on atom economy 5

6 What are the steps involved in minimization of hazardous/toxic products? 5

7 Write notes on any two of the synthetic methods used in green chemistry 10

8 Discuss the designing of green synthesis 8

Module I Module I Module I Module I –––– Organometallic CompoundsOrganometallic CompoundsOrganometallic CompoundsOrganometallic Compounds

1 What are organometallic compounds? How are they classified 5

2 Write a note on structure and bonding in organometallic compounds 5

3 Discuss the 18-electron rule with any two examples 5

4 What are pi-acceptor ligands?

Discuss in details the nature of bonding involved in metal carbonyls 8

5 What are metal carbonyls?

Discuss mononuclear and polynuclear carbonyls with examples 8

6 Write short note on carbonyls of iron and nickel 7

115

Module III Module III Module III Module III ---- ElectrochemistryElectrochemistryElectrochemistryElectrochemistry


1 Define oxidation potential and reduction potential 5

2 Explain single electrode potential. Derive the expression for it 5

3 What is standard electrode potential? Give its importance 5

4 What is reference electrode? Give examples 5

5 What is a hydrogen electrode? 5

6 Write a descriptive account on different types of electrodes 8

7 How will you determine pH of a solution using glass electrode? 8

8 Describe an experiment to determine the electrode potential of an electrode 7

9 What is an electrochemical cell?

How EMF is measured by using hydrogen electrode and glass electrode? 8

10 How is EMF of an electrochemical cell determined through Poggendorf’s

compensation method? 5

11 What is electrochemical series? What are its applications? 5

12 Derive the Nernst equation for electrode potential 5

13 What is the effect of electrolyte concentration on electrode potential 5

14 Differentiate between electrochemical cell and concentration cell 5

15 Derive the expression for EMF in concentration cells

Module III Module III Module III Module III –––– Storage CellsStorage CellsStorage CellsStorage Cells


1 Write the redox reactions taking place in the lead acid accumulators 5

2 Describe the construction of lead-acid battery with the reactions 8

3 Describe the construction and functioning of lead acid accumulators and

Nickel-Cadmium cells 10

4 Write a descriptive account on Fuel cells and Nickel-Cadmium cell 8

5 Define fuel cell. Explain the construction and working of H2-O2 fuel cells 5

6 Write a descriptive account on Fuel cells and Solar cells 8

7 Discuss the usage of solar cells 5

116

Module IV Module IV Module IV Module IV –––– CorrosionCorrosionCorrosionCorrosion


1 How is corrosion caused? What are the conditions of dry corrosion? 5

2 Outline the different types of corrosion 8

3 Describe the various factors influencing the corrosion 5

4 What is direct corrosion? 5

5 Explain the mechanism of dry corrosion.

Explain the role of oxide film in dry corrosion and classify them 8

6 Explain the mechanism of wet corrosion. Give details of corrosion protection

through sacrificial anodic method and impressed current method 8

7 What is meant by differential aeration corrosion?

Illustrate with suitable examples 8

8 Write a note on galvanic corrosion 5

9 Briefly discuss on galvanic series and galvanic corrosion 7

10 Write a note on electrochemical corrosion 5

11 Distinguish between chemical and electrochemical corrosion 5

12 Give the mechanism of rusting of iron 5

13 State and explain Pilling-Bedworth rule 5

Module IV Module IV Module IV Module IV –––– Corrosion Control Corrosion Control Corrosion Control Corrosion Control NoNoNoNo QuestionsQuestionsQuestionsQuestions MarksMarksMarksMarks

1 Give details of corrosion protection through sacrificial anodic method and

impressed current method 7

2 Mention the essential ingredients of paint. What are their functions?

Give examples 7

3 How are metals protected from corrosion by electroplating? 5

4 Explain the process of electroplating 5

5 What is cathodic and anodic protection for controlling corrosion?

Discuss their merits and demerits 8

6 What is anodizing? Explain anodisation of aluminium 5

7 Write a note on galvanizing and anodisation of aluminium 5

8 Discuss the different types of inorganic coatings 8

9 Describe the electrolytic bath for phosphate and chromate coatings 5

117

Important Portions

Module IModule IModule IModule I Module IIModule IIModule IIModule II

Organometallic

Compounds

Classification

Polymer

Free radical and Ionic 18-electron rule Techniques Bonding in metal carbonyls Tg and Factors

Bioinorganic Roles of Bulk and Trace ions

Lubricants

Mechanism of lubrication Hemoglobin & Myoglobin Solid Lubricants

Green

Chemistry

Twelve Principles Properties Atom Economy

Fuels

Cracking Less toxic chemical synthesis Reforming Avoid chemical accidents Knocking Green Synthesis Cetane & Octane Number

Module IModule IModule IModule IIIIIIIII Module IVModule IVModule IVModule IV

Electro

chemistry

Nernst equation

Corrosion

Dry and Wet corrosion Electrochemical series and its applications

Differential aeration Sacrificial anodic method

Different types of electrodes Impressed current method pH by using glass electrode Electroplating

Storage Cells

Lead acid accumulators Anodisation Nickel-Cadmium cells

Water

EDTA titration method Fuel cells Water for domestic use Solar cells BOD and COD

Theory Internal Continuous Assessment (Maximum Marks - 50)

Practical Internal Continuous Assessment (Maximum Marks - 50)

% of Marks Marks % of Marks Marks 60% - Tests (minimum 2) 30 50% - Practical and Record 25

30% - Assignments (minimum 2) 15 40% - Test 20

10% - Attendance 05 10% - Attendance 05

University Theory Examination Pattern (Maximum Marks - 100) � Part A: Analytical/problem solving short questions

Candidates have to answer eight questions out of teneight questions out of teneight questions out of teneight questions out of ten. There shall be minimum of two and maximum of three questions from each module with total ten questions.

8 x 5 marks =

40 marks

� Part B: Analytical/Problem solving descriptive questions Two questions from each module with choice to answer one question

4 x 15 marks =

60 marks

118

COMBINED FIRST AND SECOND SEMESTER B.TECH. (ENGINEERING)

MODEL EXAMINATION, APRIL – 2016

(2014 Scheme)

EN 14 104 Engineering Chemistry

Time: Three hours Maximum: 100 marks

Part A Answer any eight questions

Each question carries 5 marks

I [1] Discuss the 18-electron rule with two examples

[2] What is the importance of bulk and trace metal ions in biological systems

[3] Discuss about green synthesis

[4] Write about the concept of Tg and crystallinity in polymers

[5] Write a note on the term solid lubricants

[6] What is cracking and what for it used? What are the types of cracking?

[7] What is electrochemical series? What are its applications?

[8] How pH is measured by using glass electrode?

[9] Describe the various factors influencing the corrosion

[10] Among BOD and COD, which is greater? Why?

Part B Answer one full question from each module

Module I

II (A) (i) What are organometallic compounds? Explain the classification of

organometallic compounds with examples {8 marks}

(ii) Explain the twelve principles of green chemistry {7 marks}

Or

(B) (i) Give elementary idea about hemoglobin and myoglobin{7 marks}

(ii) What are metal carbonyls? Discuss the nature of bonding involved in it and

write examples for mononuclear and polynuclear carbonyls {8 marks}

119

Module II

III (A) (i) Explain free radical, cationic and anionic mechanism of polymerization

{8 marks}

(ii) Outline the mechanism of lubrication {7 marks}

Or

(B) (i) How does knocking occur in I.C engines? How can it be prevented? {8 marks}

(ii) Briefly explain the techniques of polymerization {7 marks}

Module III

IV (A) (i) Derive the Nernst equation for electrode potential {7 marks}

(ii) Describe the construction and functioning of lead acid accumulators and

Nickel –Cadmium cells {8 marks}

Or

(B) (i) How is EMF of an electrochemical cell determined through Poggendorf’s

compensation method? {7 marks}

(ii) Write a descriptive account on Fuel cells and Solar cells {8 marks}

Module IV

V (A) (i) Give details of corrosion protection through sacrificial anodic method and

impressed current method {8 marks}

(ii) Enumerate the various stages involved in the purification of water for domestic use

{7 marks}

Or

(B) (i) How is hardness of water determined experimentally by EDTA titration method?

Write the structure of EDTA, EBT and necessary calculations {8 marks}

(ii) What is meant by differential aeration corrosion? Illustrate with suitable examples

{7 marks}

120

COMBINED FIRST AND SECOND SEMESTER B.TECH. (ENGINEERING)

MODEL EXAMINATION, APRIL – 2016

(2014 Scheme)

EN 14 104 Engineering Chemistry

Time: Three hours Maximum: 100 marks

Part A Answer any eight questions

Each question carries 5 marks

I [1] What are organometallic compounds? How are they classified?

[2] Write short notes on atom economy

[3] Distinguish between polymerization by addition and condensation processes

[4] Explain the classification of lubricants

[5] Write informative notes on biodiesel

[6] Differentiate between electrochemical cell and concentration cell?

[7] Explain the construction and functioning of solar cells

[8] Distinguish between chemical and electrochemical corrosion

[9] Discuss the phosphate and chromate coatings

[10] How is water softened by soda lime process?

Part B Answer one full question from each module

Module I

II (A) (i) What are metal carbonyls? Discuss the nature of bonding involved

in it and write examples for mononuclear and polynuclear carbonyls {8 marks}

(ii) Discuss about the prevention of waste, minimization of toxic products and

prevention of chemical accidents in green chemistry {7 marks}

Or

(B) (i) Give elementary idea about hemoglobin and myoglobin{7 marks}

(ii) Explain the twelve principles of green chemistry {8 marks}

121

Module II

III (A) (i) Distinguish between free radical and ionic mechanism of polymerization

{8 marks}

(ii) Discuss in detail the properties of lubricants {7 marks}

Or

(B) (i) Discuss about petrol knock and diesel knock. Write notes about

octane number and cetane number {8 marks}

(ii) Explain the concept of Tg of polymer and the factors affecting Tg {7 marks}

Module III

IV (A) (i) Write a descriptive account on different types of electrodes {7 marks}

(ii) Write a descriptive account on Fuel cells and Nickel-Cadmium cell { 8 marks}

Or

(B) (i) How pH is measured by using glass electrode? { 7 marks}

(ii) Discuss about the concentration cells {8 marks}

Module IV

V (A) (i) Outline the galvanic corrosion, stress corrosion and corrosion control {8 marks}

(ii) With relevant chemical equations, outline the estimation of dissolved oxygen

{7 marks}

Or

(B) (i) With a neat diagram, discuss the demineralization of water using ion exchange

method {8 marks}

(ii) Discuss about the metallic coatings {7 marks}

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