UNIT I - ELECTROCHEMISTRY - Vidyarthiplus

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UNIT – I WATER TECHNOLOGY

PART - A

1. What is hardness of water? What are its types? (Jan 2011, June 2012) The property of water which makes it not lather with soap is called hardness. It is mainly because of

hardness producing salts like MgCO3, CaSO4, Mg(HCO3)2, CaCO3 etc.,

Types - i) Temporary hardness ii) Permanent hardness

2. What are the disadvantages of using hard water in boiler? (Nov. 2010) Water used for steam generation should be free from hardness, otherwise it will cause

boiler problems like scale and sludge formation, priming and foaming, boiler corrosion,

etc. Hence, water is softened before feeding it to boiler.

3. What are the salts responsible for the temporary and permanent hardness of water? Temporary hardness: Carbonates and Bicarbonates of Ca and Mg (e.g) CaCO3, Mg(HCO3)2. Permanent

hardness: Sulphates, Chlorides of Ca, Mg (e.g) CaCl2, MgCl2, CaSO4, Mg SO4

4. How the hardness of water is expressed? What are the units practiced for expressing hardness of

water? (Apr. 1995) (June 2009) The hardness is, usually, expressed in terms of equivalent amount of CaCO3. The choice of CaCO3 in

particular due to:

a) Its molecular weight is 100 (equivalent weight = 50) which makes calculation easy.

b) It is the most insoluble salt that can be precipitated in water treatment.

Hardness can be expressed by any of the following units

ppm - mg/l - degree Clarkes - Degree French

5. How is hardness of water detected? (or)Give the test to detect hardness of water. (Apr 1994) a) Eriochrome Black – T indicator gives wine red colour in hard water.

b) With soap, hard water gives a scummy(dirty white) precipitate.

2C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl

(Sodium stearate) (Scummy precipitate)

6. What are ion-exchange resins? (May 2008); (May 2011) Ion-Exchange Resins are long chain, insoluble, cross linked, organic polymers which are capable of

exchanging its ions with water. They are of 2 types.

1) Cation exchange resins – RH+ (e.g) Sulphonated coals , RSO3H

2) Anion exchange resins – R’OH- (e.g) Ureaformaldehyde, Amines R-NH2

7. Distinguish between soft water and demineralized water. (June 2011)

Soft water Demineralized water It does not contain hardness producing

calcium and magnesium ions, but it may

contain other ions like K+, Na+, Cl- etc.

Demineralized water does not contain

any ions including hardness producing

ions.

Softening involves removal of only

hardness causing ions.

Demineralization involves removal of

all the ions present in water. 8. What are Boiler troubles? How are they caused? (May 2007) Sludge and Scale formation - Caustic embrittlement - Boiler corrosion - Priming and foaming are

collectively known as boiler troubles. They are caused by the hardness causing salts present in boiler feed

water.

9. What is caustic embrittlement? How can it be prevented? (June 2012, June 2014) It is the inter-crystalline cracking of boiler due to NaOH.. NaOH content is increased due to the dissolved

salts like Na2CO3 which is added during internal treatment. To prevent caustic embrittlement: i.)Sodium

phosphate can be used as softening agent instead of Na2CO3 ii) The hair line cracks can be sealed by waxy

materials like tannin and lignin.

10. What are scales? How can it be prevented? Mention its disadvantages.(June 2009)

Scales are hard, thick and adherent precipitate deposited on boilers due to salts like CaSO4 , Ca(HCO3)2

present in water.

Disadvantages: Wastage of fuel - Lowering of boiler safety - Decrease in efficiency of boilers - Danger of

explosion

Prevention methods: 1) Internal treatment methods (eg) Calgon conditioning

2) External treatment methods (eg) Ion-exchage method


3) By using mechanical scrubber

11. What are priming and foaming? (Nov 2005) Priming: When boiler is producing steam rapidly, some particles of the liquid water are carried along-with

steam. This process of wet steam formation is called priming.

Foaming: It is the production of persistent foam or stable bubbles in boilers, which do not break easily.

12. What is meant by ‘internal conditioning of water’? Name any two boiler compounds used in internal

conditioning of boiler feed water. (Apr. 1994) (Oct. 1996) The residual salts which are not removed by external methods can be removed by adding some chemicals

directly into the boiler water. These chemicals are known as ‘Boiler compounds’. This method is known as

internal conditioning of water’

Eg : Carbonate conditioning – Sodium carbonate, Phosphate conditioning – Sodium phosphate.

13. What is the role of phosphates in the internal treatment of water?(May 2005)

(or) Name an internal method used for high pressure boiler and explain. (Dec 2011) Phosphate salts are used as internal boiler compounds for high pressure boilers. Phosphates precipitate the

soluble Ca and Mg salts as their insoluble phosphates. They are also helpful in maintaining the pH of the

boiler water.

3CaSO4 + 2 Na3PO4 Ca3(PO4)2 + 3 Na2SO4

Three types of Phosphate salts are used:

S.N

o

Salt Name of the salt Used for treating

1 Na3PO4 Tri sodium Phosphate highly acidic water 2 Na2HPO4 Di sodium hydrogen Phosphate slightly acidic water 3 NaH2PO4 Sodium di hydrogen phosphate highly alkaline water

14. Calgon treatment prevents scale formation in boilers. Give reason. (Jan 2010, June 2010,Jan 2011,

June 2014) Addition of calgon (sodium hexa meta phosphate) to boiler feed water interacts with calcium ions in the

CaSO4 forming a soluble complex, thus preventing the precipitation of scale and sludge forming salt. 2CaSO4 + Na2[Na4(PO3)6] Na2[Ca2(PO3)6] + 2Na2SO4.

Soluble complex.

15. What do you understand by demineralization of water ?What are the advantages and disadvantages

of demineralization process? ?(Oct. 1996) During this process cations and anions of water are completely removed. It uses two columns of cation -

exchange column and anion- exchange column filled with resins

Advantages: i) The process can be used to soften highly acidic or alkaline waters.

ii) It produces water of very low hardness (2ppm).

iii) It is very good for treating water for use in high-pressure boilers.

Disadvantages: i) The equipment is costly and more expensive.

ii) The turbidity must be below 10 ppm.

16. How is the exhausted zeolite softener bed regenerated? (Jan 2009) Exhausted zeolite can be regenerated by using 10% brine (NaCl) solution.

CaZe + 2NaCl Na2Ze + CaCl2

17. What is coagulation? Coagulation is the process of removing fine suspended and colloidal impurities by the addition of requisite

amount of chemicals (called coagulants) to water before sedimentation. Ex: Al2(SO4)3 + 3 Ca(HCO3)2 2 Al(OH)3 + 3 CaSO4 + 6 CO2

18. What is reverse osmosis? Mention some of its advantages. (Nov. 2003, Dec 2011) When we apply an excess and opposite Hydrostatic pressure to overcome the osmotic pressure, then higher

concentrated solvent will flow to the lower one. This is known as reverse osmosis.

Advantages:

i) It removes ionic and non-ionic, colloidal and high molecular weight organic matter.

ii) It removes colloidal silica, which is not removed by demineralization

iii) The life time of membrane is quite high, about 2 years. The membrane can be replaced within a few

minutes, thereby providing uninterrupted water supply.

19. What is Desalination? Name the different methods of desalination. (May 2011)


Removal of common salt (NaCl) from water is called ‘Desalination’. Various methods of desalination:

Reverse Osmosis, Distillation, Electro- dialysis, Freezing, Solar distillation, etc.

20. Distinguish between carbonate and non carbonate hardness.

S.N

o

Carbonate hardness/temporary hardness Non-carbonate hardness/permanent

hardness 1. This due to the presence of bicarbonates of

calcium, magnesium and other heavy metals

present in water.

This due to the presence of chlorides

and sulphates of calcium and

magnesium 2. This can be removed by boiling This cannot be removed by boiling

21.What are the requirements(or) requisites of boiler feed water? Water used in boilers known as boiler feed water must be free from

(i) Hardness producing salts (Ca2+, Mg2+ ions)

(ii) Suspended impurities like silica, oil, etc.

(iii)Dissolved gases like O2, CO2, etc.

22. What are boiler compounds? Give examples. Boiler compounds are chemicals added inside the boilers to remove scale forming

substances. Egs: Sodium carbonate, Calgon, Sodium phosphate.

23. Give the chemical formula of zeolite. Wht is its importance? Zeolite is hydrated sodium alumino silicate. Chemical formula of sodium zeolite is

Na2O. Al2O3.xSiO2. YH2O, where X= 2-10, Y = 2-6. Zeolites are capable of exchanging reversibly their

sodium ions for hardness producing ions in water.

24. Zeolite softener cannot be used for softening turbid water. Why? The suspended matters in turbid water clogs the pores of the zeolite bed and restrict the

water flow.

25. Zeolite softeners cannot be used for softening brackish water. Why? The softening of brackish water involves the removal of sodium ions.Since sodium zeolite is

a cation exchanger,the exchange of same cation does not produce any softening.

26. What are the merits of ion -exchange process? (i) Highly acidic and alkaline water can be treated.

(ii) Residual hardnesss of the water is 0-2 ppm. So it is very good for use in high

pressure boilers .

(iii) Resins can be regenerated.

27. How will you regenerate the column in demineralization process? The exhausted cation exchange resin is regenerated by passing dil.HCl or dil.H2SO4.

The exhausted anion exchange resin is regenerated by passing dil. NaOH.

28. What are the merits and demerits of zeolite process?

Merits: (i) The softened water has hardness between 15-50ppm.

(ii) Requires less time for softening

(iii) No sludge is formed during this process.

Demerits: (i)The treated water contains more sodium salts which cannot be used as boiler feed

water.

(ii) Highly turbid water cannot be treated by this method.

(iii) Zeolite plant occupies more space.

29. What is meant by colloidal conditioning? Formation of scale in boilers can be avoided by adding organic substances like kerosene, tannin, agar-

agar, etc. these substances get coated over the scale forming precipitates thereby yielding non-sticky deposits

which can be removed.

30. What is boiler corrosion? Boiler corrosion is the decay of boiler material by a chemical or electrochemical attack. It is due

to the presence of dissolved gases such as CO2, O2 and the salt like MgCl2.

Mechanical deaeration is used to remove dissolved gases such as CO2 and O2.

31. Distinguish between Zeolite process and demineralization process.

S.N

o.

Zeolite process Demineralization process


1. Exchange only cations Exchange cations as well as anions. 2. Acidic water cannot be treated since acid

decomposes the zeolite

Acidic water can be treated.

3. Zero hardness cannot be achieved Zero hardness can be achieved 4. The treated water contains large amount of

dissolved salts which leads to caustic

embritttlement in boilers.

The treated water does not contain any ions

and therefore it can be safely used in boilers.

32. What is blow down operation?

It is a process in which a portion of concentrated water containing large amount of dissolved salts are

replaced by fresh water frequently during steam production.

33. What are the differences between sludges and scales.

S.N

o.

Sludges Scales

1. It is a soft, loose precipitate formed inside

the boiler walls during steam production

It is hard, adherent coating formed on the

inner walls of the boiler during steam

production 2. Sludges are produced due to the presence of

temporary hardness causing compounds like

Ca(HCO3)2, Mg(HCO3)2

Scales are produced due to the presence

of permanent hardness causing

substances like

CaCl2,CaSO4,MgCl2,MgSO4 3. It is also produced due to the presence of

suspended and colloidal impurities.

It is produced due to the presence of

dissolved salts. PART –B 1. What are ion exchange resins? How are they useful in removing hardness of water? (June 2014)

Ion-Exchange Resins are long chain, insoluble, cross linked, organic polymers which are

capable of exchanging its ions with water. They are of 2 types.

1) Cation exchange resins – RH+

(e.g) Sulphonated coals , RSO3H

2)Anion exchange resins – R‟OH- (e.g) Ureaformaldehyde, Amines R-NH2

A.Ion exchange method ( Demineralisation)

Working:

1. Here all the cations and anions are completely removed. It uses two column of cation exchange

column and anion exchange column filled with resins.

2. Resins are long chain, insoluble, cross linked, organic polymers. There are 2 types. i)

Cation exchange resins – RH+


ii) Anion exchange resins . R‟OH- (e.g) Ureaformaldehyde, Amines R-NH2

These resins are prepared by copolymerization of styrene and di vinyl benzene.

3. The water is fed into cylinder –I where all the cations are replaced by RH2

Resins.

2RH+

+ Ca2+ R2 Ca

2+ + 2 H

+

4. The cation free water is fed to cylinder II, where all the anions are replaced.


2R‟OH-

+ SO42- R2‟ SO4

2- + 2 OH

-

5. So, the resultant water is free from all types of ions.

Diagram:

Regeneration:

On prolonged use, as all the resins are exhausted, there will be no H+

or OH –

ions

to exchange the unwanted ions. So, they have to be regenerated.

Cation resins are regenerated by HCl and anion resins by NaOH.


Advantages of Ion exchange method:

(i) Can be used for high pressure boilers also.

(ii) It can treat highly acidic or alkaline water.

(iii) We can get pure water as hardness as low of 2 ppm.

Drawbacks of Ion exchange method:

i) Expensive

ii) Fe, Mn cannot be removed as they form complexes with resins

iii) Cannot be used for turbid water as they clog the resins.

2. Explain the demineralization of water by Ion-exchange process. How is the exhausted cation

and anion exchange resins regenerated?

A.Ion exchange method ( Demineralisation)

Working:

1. Here all the cations and anions are completely removed.It uses two column of cation exchange column

and anion exchange column filled with resins.

2. Resins are long chain, insoluble, cross linked, organic polymers. There are 2 types.

i) Cation exchange resins – RH+


ii) Anion exchange resins . R‟OH- (e.g) Ureaformaldehyde, Amines R-NH2

iii) The resins are prepared by copolymerization of styrene and di vinyl benzene.

3. The water is fed into cylinder –I where all the cations are replaced by RH2

Resins. 2RH+

+ Ca2+ R2 Ca

2+ + 2 H

+

4. The cation free water is fed to cylinder II, where all the anions are replaced.

2R‟OH-

+ SO42- R2‟ SO4

2- + 2 OH

-

5. So, the resultant water is free from all types of ions.

Diagram:


Regeneration:

On prolonged use, as all the resins are exhausted, there will be no H+

or OH –

ions to exchange the

unwanted ions. So, they have to be regenerated.

Cation resins are regenerated by HCl and anion resins by NaOH.

Advantages of Ion exchange method:

i) Can be used for high pressure boilers also.

ii) It can treat highly acidic or alkaline water.

iii) We can get pure water as hardness as low of 2 ppm.

Drawbacks of Ion exchange method:

i) Expensive

ii) Fe, Mn cannot be removed as they form complexes with resins

iii) Cannot be used for turbid water as they clog the resins.

3. What is meant by reverse osmosis? Explain the desalination of brackish water by reverse osmosis.

Reverse Osmosis Method: (Desalination): Hyperfiltration/ Super filtration/ brackish water treatment

1. Removal of common salt (NaCl) from water is called „ Desalination‟.

2. Various methods:

Reverse Osmosis, Distillation, Electro dialysis, Freezing, Solar distillation, etc.,

3. Brackish water: Water containing dissolved salts with a peculiar salty taste.

4. Osmosis: When two different concentrated solutions are separated by a semi permeable membrane, due to

osmotic pressure, low concentrated solvent flows to higher one. This is known as osmosis.

5. But when we apply an excess and opposite Hydrostatic pressure(15-40kg/cm2) to overcome the osmotic

pressure, then higher concentrated solvent will flow to the lower one. This is known as reverse osmosis.

6. During this RO process, only the water flows across the membrane and it prevents the salt migration.

So, this method is also called as „ Super filtration‟.


7. The membrane is madeup of cellulose acetate, cellulose butyrate, polymethacrylate

Advantages of Reverse Osmosis:

1.High life time

2.Removes ionic, non-ionic and colloidal silica impurities , which can not be removed by

demineralization method.

3. Low capital cost.

4. Simple operational procedure.

5. The membrane can be replaced within a few minutes, thereby providing uninterrupted water supply.

4. How is scale formed in boilers? What are its drawbacks?

Sludge and scale:

Definition: If the water contains hardness causing salts like MgSO4, MgCl2 ,CaSO4 , Ca(HCO3)2 . on

evaporation, the salts are precipitated . If they form loose, slim , non-adherent precipitate, It is known as sludge. If

they form hard, thick , adherent precipitate, it is known as scale.

Reasons for sludge and scale: Sludge: MgCl2 , MgSO4, CaCl2

Scale: CaCO3 , MgCO3. Ca(HCO3)2

Disadvantages of scales and sludges:

a) Wastage of fuel:

Scales have low thermal conductivity. So, the heat transfer from boiler to inside water may not be sufficient.

In order to provide steady supply of heat to water, over heating is to be done which causes wastage of fuel. The

wastage of fuel depends on the thickness and nature of the scale which is shown in the table.

b) Decrease in efficiency:

Scales sometimes deposit in the valves and condensers of the boilers and heat exchangers and cause choking.

This results in decrease in efficiency of the boiler and heat exchangers.

c) Boiler explosion:

Sometimes due to overheating the thick scales may crack which causes the sudden contact of high heated boiler

material with water. This causes formation of a large amount of steam and it develops high pressure. This leads

to explosion.

Examples of Scale deposits:

1. CaCO3 Deposit: It is formed due to decomposition of calcium bicarbonate. It has lower solubility and forms

tenacious scale. The liberated CO2 produces carbonic acid and produces boiler corrosion.

2. CaSO4 deposit: At high temperature and high pressure boilers, It forms harder and denser deposit.


3. Mg(OH)2 deposit: It is formed due to decomposition of magnesium bicarbonate. It reacts with water and

liberates corrosive HCl.

4. SiO2 deposit: It forms hard, porcelain coating on the boiler surface.

5. Fe deposit: It cause dark coloured, magnetic deposits.

Prevention of Scales:

1. At earlier stage, scraper, wire brush, mechanical scrubber can be used to remove scales.

2. Thermal shocks are used to remove brittle scales.

3. By external treatment methods (eg) Ion exchange demineralization, zeolite softening

4. By using boiler compounds in internal treatment (eg) Carbonate, phosphate, calgon, EDTA

5. By acid treatment

5. What are zeolites? Explain the softening of water by zeolite process mentioning its advantages, disadvantages

and regeneration methods. (Jan 2009, June 2014)

ZEOLITE (PERMUTIT) SOFTENING PROCESS

i)Hydrated sodium alumino silicates available in nature are known as zeolite.

ii) These natural zeolites are green sand . They are non porous in nature.

iii) Zeolites are having the general formula Na2O . Al2O3. xSiO2 . yH2O (x = 2 to 10; y = (2 to6)

iv) The porous and gel structured synthetic zeolites are known as permutit.

v) These zeolites and permutits are used for water softening.

vi) Synthetic zeolite is represented as Na2Ze.

Principle

i) The sodium ions are loosely held in these zeolites. They are easily replaced by Calcium and

magnesium ions present in the water.

ii) When hard water is passed through a bed of sodium zeolite kept in a cylinder, it exchanges its sodium

ion with Ca2+

and Mg2+

ions present in the water to from Calcium and Magnesium zeolites.


iii) Zeolite softeners may be of pressure type or gravity type.

Reactions:

The outcoming water is enriched with large amount of sodium salts which do not cause any

hardness. But in cannot be used in boiler.

Regeneration:

i) On prolonged use, all the zeolite sodium ions are exhausted.

ii) There will be no sodium ions to exchange the Calcium and magnesium ions. So, they have to be

regenerated.

iii) The exhausted zeolite is regenerated by treating with 10% solution of NaCl.

Regeneration reaction:

The regeneration step comprises of a) backwashing b) salting c) rinsing before reuse

CaZe + 2 NaCl → Na2Ze + CaCl2

MgZe + 2 NaCl → Na2Ze + MgCl2

Advantages of zeolite process:

1. The outlet water will have least hardness around 1 – 2 ppm only.

2. As the zeolite can be regenerated, the method is cheap.

3. Operation is easy.

4. The space requirement for this setup is minimum .

5. No sludge is formed during this process.

Disadvantages of zeolite process:

1. Turbid water cannot be treated as it blocks the pores of the zeolite bed.

2. Acidic water cannot be treated as it decomposes the structure of zeolite.

3. Water containing Fe, Mn cannot be treated as the regeneration is difficult.

4. Brackish water cannot be treated because it contains Na+ ions. So, the ion exchange reaction will not

occur.

5. When the softened water contains more dissolved sodium salts, it may result in boiler corrosion and caustic

embrittlement.


6. What are boiler compounds? Describe briefly the various methods of internal conditioning of Boiler

feed water (Jan 2009, June 2014)

INTERNAL TREATMENT BY BOILER COMPOUNDS:

The residual salts that are not removed by external methods can be removed by adding some

chemicals directly into the boiler water. These chemicals are known as Boiler compounds. This method

is known as Internal treatment.

E.g) Carbonate conditioning, Phosphate conditioning , Calgon conditioning, etc.,

a) Carbonate conditioning:

Used for low pressure boilers. Here the salts like CaSO4 are converted to easily removable CaCO3. But sometimes it produces NaOH, CO2 and hence Carbonic acid. So it is less preferred.

CaSO4 + Na2CO3 → CaCO3 + Na2SO4

b)Phosphate conditioning:

Used for high pressure boiler. No risk of CO2 l iberation.

3CaSO4 + 2 Na3PO4 → Ca3(PO4)2 + 3 Na2SO4

Three types of Phosphate salts are used:

S.No Salt Name Used for treating

1 Na3PO4 Tri sodium Phosphate highly acidic water

2 Na2HPO4 Di sodium hydrogen Phosphate slightly acidic water

3 NaH2PO4 Sodium di hydrogen phosphate highly alkaline water

c) Calgon conditioning:

Calgon is the trade name of sodium hexa meta phosphate- Na2 [ Na4 (PO3)6].

With calcium ions it forms a soluble complex and prevents scale and sludge formation. It is used for high

and low pressure boilers.

2CaSO4 + Na2[ Na4 (PO3)6] → Na2 [Ca2(PO3)6] + 2 Na2SO4

d) Colloidal conditioning

Formation of scale in boilers can be avoided by adding organic substances like kerosene, tannin, agar-agar, etc.

these substances get coated over the scale forming precipitates thereby yielding non- sticky deposits which can

be removed 7. What are boiler troubles? How are they caused? Suggest steps to minimize the boiler troubles. (June 2014)

Definition:

The water fed into the boiler for the production of steam is called “Boiler feed water”.

Requirements for boiler water

S.No Requirements for boiler water If not, it will cause

1 Free from hardness causing salts Sludge and scale

2 Free from oil and greases Foaming

3 Free from dissolved salts, suspended impurities Caustic embrittlement

4 Free from dissolved gases, suspended salts Boiler corrosion

Boiler troubles (OR) Disadvantages of using hard water in boiler

1. Scale and Sludge formation


2. priming and foaming (Carry over)

3. caustic embrittlement (Inter crystalline cracking)

4. boiler corrosion.

Caustic Embrittlement:

It is the intercrystalline cracking of boiler due to Na2CO3. In high pressure, Na2CO3 undergoes hydrolysis to

produce NaOH. This makes water caustic. The NaOH contenting water flows into the minute hair-cracks.

Na2CO3+ H2O 2 NaOH + CO2

This NaOH occupies the hair line cracks of boiler metal and converts the insoluble Fe into soluble Sodium

Ferroate. Thus it makes the cracks bigger in bents, joints and crevices.

Fe + 2 NaOH Na2FeO2 + H2 ↑

(Insoluble) (soluble)

Prevention of caustic embrittlement:

1. As softening agent, we can use sodium phosphate instead of sodium carbonate.

2. The hair line cracks can be sealed by waxy materials like Tannin and Lignin.

Sludge and scale:

Definition: If the water contains hardness causing salts like MgSO4, MgCl2 ,CaSO4

Ca(HCO3)2 . on evaporation, the salts are precipitated . If they form loose, slim , non-adherent precipitate, it is known as sludge. If they form hard, thick , adherent precipitate, it is known as scale.




a) Wastage of fuel:

Scales have low thermal conductivity. So, the heat transfer from boiler to inside water may not be sufficient. In

order to provide steady supply of heat to water, over heating is to be done which causes wastage of fuel. The

wastage of fuel

depends on the

thickness and

nature of the scale which is shown in the table.


Scales sometimes deposit in the valves and condensers of the boilers and heat exchangers and cause

choking. This results in decrease in efficiency of the boiler and heat exchangers.


Sometimes due to overheating the thick scales may crack which causes the sudden contact of high heated

boiler material with water. This causes formation of a large amount of steam and it develops high

pressure. This leads to explosion.

Thickness of scale (mm) 0.325 0.625 1.25 2.5 12

Wastage of fuel (%) 10 15 50 80 150

S.No Name of the deposit Properties 1 CaCO3 Deposit It is formed due to decomposition of calcium bicarbonate. It has

lower solubility and forms tenacious scale. The liberated CO2

produces carbonic acid and produces boiler corrosion.

2 CaSO4 deposit At high temperature and high pressure boilers, It forms harder

and denser deposit.


Examples of Scale deposits:

Prevention of Scales:

1. At earlier stage, scraper, wire brush, mechanical scrubber can be used to remove

scales.

2. Thermal shocks are used to remove brittle scales.

3. By external treatment methods (eg) Ion exchange demineralization, zeolite

softening

4. By using boiler compounds in internal treatment (eg) Carbonate, phosphate,

calgon, EDTA

5. By acid treatment.

Priming and Foaming: (Carry Over)

1) Due to rapid boiling, the steam may carry some water droplets along with it. This is

called wet steam .The process of wet steam production is called Priming. It can reduce the

heat of the steam and cause corrosion in the pipelines.

Priming is due to:

a) Improper design of boiler

b) High water level

c) High velocity of steam

d) Uneven boiling

Priming can be controlled by

i)Proper boiler design

ii)Maintaining proper water level

iii)Proper boiling

2) If oils and greases are present, they produce stable bubbles on the water surface. This will

increase the wet steam production. This is known as “Foaming”.

Foaming is prevented by adding

i) Anti foaming agents (e.g.) synthetic poly amides, castor oil

ii) Coagulants (e.g.) Aluminium hydroxide

3) Foaming and priming are collectively known as „ Carry over”.

Boiler Corrosion

It may be due to three major reasons:

i)Dissolved oxygen

ii) Dissolved CO2

iii) Dissolved salts like MgCl2

Corrosion Due to dissolved oxygen:

Dissolved oxygen in presence of water, causes corrosion. The dissolved oxygen in water attacks

the boiler material at higher temperature.

4Fe + 6 H2O + 3O2 4 Fe (OH)3

(Rust)

3 Mg(OH)2 deposit It is formed due to decomposition of magnesium bicarbonate. It

reacts with water and liberates corrosive HCl.

4 SiO2 deposit It formsa hard, porcelain coating on the boiler surface.

5 Fe deposit It cause dark coloured, magnetic deposits.


Prevention from oxygen:

a) Chemical method -

i)Adding Sodium Sulphite: 2 Na2SO3 + O2 2 Na2SO4

This method results in other precipitates which can have some side effects. So this

method is less preferred.

ii)Adding Hydrazine: N2H4 + O2 N2 + 2 H2O

This method results in inert gas and pure water, and has no side effects. So it is preferred.

b) Mechanical deaeration method:

1. This is based on the principle that at high t e m p e r a t u r e , l o w pressure and high exposed

area, the solubility of gases in water is decreased. So, the gases can be expelled easily.

2. Here, the water is fed into the mechanical deaerator which is provided with vacuum pump, heaters and

perforated plates.

3. The out coming water will be free from dissolved gases.

Corrosion due to CO2

Salts like Calcium bicarbonate on heating produces CO2 . CO2 dissolves in water to form carbonic acid

which corrodes the boiler metal.

∆

Ca (HCO3)2 CaCO3 + H2O + CO2

H2O + CO2 H2CO3

Prevention from CO2

1. Chemical method: By adding calculated amount of ammonium hydroxide

2NH4OH + CO2 (NH4)2CO3 + H2O

2. Mechanical deaeration method ( similar to oxygen method)

Corrosion due to Dissolved salts like MgCl2

Dissolved salts like MgCl2 cause acid formation. This will be prevented by alkali neutralisation.

MgCl2 + 2 H2O Mg(OH)2 + 2 HCl (Corrosive acid)

Neutralisation:

Excess acidic nature is neutralized by adding alkalis and vice versa.

HCl + NaOH NaCl + H2O

8. Define boiler feed water with its requirements. What are the disadvantages of using hard water in boilers

Mention its prevention methods.


Boiler troubles (OR) Disadvantages of using hard water in boiler

1. Scale and Sludge formation

2. priming and foaming (Carry over)

3. caustic embrittlement (Inter crystalline cracking)

4. boiler corrosion are collectively known as boiler troubles.

Caustic Embrittlement: (Inter crystalline cracking of boiler metal)

It is the intercrystalline cracking of boiler due to Na2CO3. In high pressure, Na2CO3 undergoes

hydrolysis to produce NaOH. This makes water caustic. The NaOH contenting water flows into the

minute hair-cracks.

Na2CO3 + H 2 O →2 NaOH + CO2

This NaOH occupies the hair line cracks of boiler metal and converts the insoluble Fe into soluble

Sodium Ferroate. Thus it makes the cracks bigger in bents, joints and crevices.


( Insoluble) (Soluble)




Sludge and scale:

Definition: If the water contains hardness causing salts like MgSO4, MgCl2 ,CaSO4 , Ca(HCO3)2 .

on evaporation, the salts are precipitated . If they form loose, slimy , non-adherent precipitate, It is known as sludge. If they form hard, thick, adherent precipitate, it is known as scale.




a) Wastage of fuel:

Scales have low thermal conductivity. So, the heat transfer from boiler to inside water may not

Be sufficient. In order to provide steady supply of heat to water, over heating is to be done which


causes wastage of fuel. The wastage of fuel depends on the thickness and nature of the scale which is

shown in the table.


Wastage of fuel (%) 10 15 50 80 150


Scales sometimes deposit in the valves and condensers of the boilers and heat exchangers and

cause choking. This results in decrease in efficiency of the boiler and heat exchangers.


Sometimes due to overheating the thick scales may crack which causes the sudden contact of high

heated boiler material with water. This causes formation of a large amount of steam and it

develops high pressure. This leads to explosion.

Examples of Scale

deposits:





and denser deposit.



4 SiO2 deposit It forms a hard, porcelain coating on the boiler surface.


Prevention of

scales


scales.

2. Thermal shocks are used to remove brittle

scales.


softening

4. By using boiler compounds in internal treatment (eg) Carbonate, phosphate, calgon,

EDTA





Priming is due to:



b) High water level


d) Uneven boiling




iii)Proper boiling




i) anti foaming agents (e.g.) synthetic poly amides , castor oil



Boiler Corrosion


i)Dissolved oxygen ii) Dissolved CO2



Dissolved oxygen in presence of water, causes corrosion. The dissolved oxygen in water attacks the

boiler material at higher temperature.

4Fe + 6 H2O + 3O2 4 Fe (OH)3

(Rust)




This method results in other precipitates which can have some side effects. So this method is less

preferred.




1. This is based on the principle that at high temperature , low pressure and high exposed area, the

solubility of gases in water is decreased. So, the gases can be expelled easily.


perforated plates.



.




∆

Ca(HCO3)2 CaCO3 + H2O + CO2

H2O + CO2 H2CO3

Prevention from CO2



2. Mechanical deaeration method ( similar to oxygen method) Corrosion due

to Dissolved salts like MgCl2



Neutralisation:



9. Explain in detail the scale and sludge formation and priming and foaming.

Sludge and scale:

Definition: If the water contains hardness causing salts like MgSO4, MgCl2 ,CaSO4 , Ca (HCO3)2 . on evaporation, the salts are precipitated . If they form loose, slim , non-adherent precipitate, It is known as sludge. If

they form hard, thick , adherent precipitate, it is known as scale.

Reasons for sludge and scale:


Sludge: MgCl2 , MgSO4, CaCl2



a) Wastage

of fuel:

Scales have low thermal conductivity. So, the heat transfer from boiler to inside water may

not be sufficient. In order to provide steady supply of heat to water, over heating is to be done which causes wastage of fuel. The wastage of fuel depends on the thickness and nature of the scale which is shown in the table.


Wastage of fuel (%) 10 15 50 80 150

b) Decrease in

efficiency:

Scales sometimes deposit in the valves and condensers of the boilers and heat

exchangers and cause choking. This results in decrease in efficiency of the boiler and heat

exchangers.

c) Boiler

explosion:

Sometimes due to overheating the thick scales may crack which causes the sudden contact of

high heated boiler material with water. This causes formation of a large amount of

steam and it develops high pressure. This leads to explosion.

Examples of Scale

deposits:





and denser deposit.



4 SiO2 deposit It formsa hard, porcelain coating on the boiler surface.


Prevention of

Scales:


scales.

2. Thermal shocks are used to remove brittle

scales.


softening

4. By using boiler compounds in internal treatment (eg) Carbonate, phosphate, calgon,


EDTA





Priming is due to:


b) High water level


d) Uneven boiling




iii) Proper boiling




i)Anti foaming agents (e.g.) synthetic poly amides , castor oil



10. Explain in detail about caustic embrittlement and boiler corrosion.

Caustic Embrittlement: (Inter crystalline cracking of boiler metal)

It is the intercrystalline cracking of boiler due to Na2CO3. In high pressure, Na2CO3 undergoes

hydrolysis to produce NaOH. This makes water caustic. The NaOH contenting water flows into the

minute hair-cracks.

Na2CO3 + H2O 2 NaOH + CO2

This NaOH occupies the hair line cracks of boiler metal and converts the insoluble Fe into soluble

Sodium Ferroate. Thus it makes the cracks bigger in bents, joints and crevices.


( Insoluble) (Soluble)




Boiler Corrosion


i) Dissolved oxygen


ii) Dissolved CO2



Dissolved oxygen in presence of water, causes corrosion. The dissolved oxygen in water attacks the

boiler material at higher temperature.

4Fe + 6 H2O + 3O2 4 Fe (OH)3

(Rust)




This method results in other precipitates which can have some side effects. So this method is less

preferred.




1. This is based on the principle that at high temperature , low pressure and high exposed area, the

solubility of gases in water is decreased. So, the gases can be expelled easily.


perforated plates.





∆

Ca(HCO3)2 CaCO3 + H2O + CO2

H2O + CO2 H2CO3

Prevention from CO2



2. Mechanical deaeration method ( similar to oxygen method) Corrosion due

to Dissolved salts like MgCl2



Neutralisation:




UNIT II –ELECTROCHEMISTRY AND CORROSION

PART A

1. What are the conditions for a cell to be reversible? A cell will be reversible if it satisfies the following conditions,

i) If Eappl =Ecell , no chemical reaction takes place

ii) If Eappl > Ecell , chemical reaction will take place in reverse direction

2. Define standard electrode potential. (June 2014)

The measure of tendency of a metallic electrode to lose or gain electrons when in contact with a solution of its own salt

of 1M concentration at 25 ° C .

3. Give the Nernst equation. For an electrode reaction, Mn+

(aq) + n e- M (s)

The Nernst’s equation is, E Mn+/ M = E0 Mn+

/ M +2.303RT/nF log [ Mn+ ]

4. Significance of Nernst equation? i. Used to calculate the single electrode potential.

ii. Polarity of the electrode in an electrochemical cell can be determined.

5. Calculate the reduction potential of Cu/Cu2+ (0.5 M) at 25oC. given that Eo= 0.337: (Cu 2+= 0.5M)

E=Eo + n

0591.0log [Cu2+] ; E=0.337 +

2

3010.00591.0 x ; E= 0.3281 V

6. What is single electrode potential? (Nov.2010), (May 2011), (May 2005), (Nov. 2002), (Apr. 1996).

It is the tendency of a metallic electrode to lose or gain electrons when it is in contact with its own ions in solution.

7. Define EMF of a cell. (Oct. 1996), (May 2005) Electromotive force is defined as, "the difference of potential which causes flow of current from one

electrode of higher potential to the other electrode of lower potential. Thus, the emf of a galvanic cell can be calculated

using the following relationship.

E°cell = E°right − E°left 8. Distinguish between reversible and irreversible cells giving one example for each. (Nov. 2004), (Jan.2005)

S.No Reversible Cells

Irreversible Cells

1. A cell which obeys the three conditions

of thermodynamic reversibility are

called reversible cell.

Cells which do not obey the conditions of

thermodynamic reversibility are called

irreversible cells. 2. Cell reaction is reversed when external

potential greater than cell potential is

applied.

The cell reaction is not completely reversed.

3. Daniel cell, secondary batteries

(rechargeable batteries).

Zinc – silver cell, Dry cell (Primary Cells)

9. Suggest a method to determine the electrode potential of zinc.(May. 2005) The saturated calomel electrode is coupled with another Zn electrode, the potential of which is to be determined. Since

the reduction potential of the coupled Zn electrode is less than E° of calomel electrode (+ 0.2422 V), the calomel electrode

will act as cathode and the reaction is

Ecell = E°right − E°left

Ecell = E°cal − E°Zn

E°Zn = E°cal − Ecell

= + 0.2422 − 1.0025

E°Zn = − 0.7603 volt

10. What are reference electrodes? Give two examples. (Nov. 2010), (May 2010), (Nov. 2006) The electrode potential is found out by coupling the electrode with another reference electrode, the potential

of which is known or arbitrarily fixed as zero. Primary reference electrode: standard hydrogen electrode &

Secondary reference electrode: calomel electrode.

11. Mention any two application of EMF measurement. (Nov. 2005)


1. Determination of standard free energy change and equilibrium constant.

(i) The standard free energy change of a reaction can be calculated as follows− ΔG° = nFE°

(ii) The equilibrium constant of a reaction can be calculated as follows.

E° = Standard emf of the cell; K = Equilibrium constant

2. Determination of pH by using a standard hydrogen electrode.

A hydrogen electrode is introduced into the solution, pH of which is to be determined. It is then coupled with a

standard hydrogen electrode through the salt bridge and the emf of the cell is measured. If E is the emf of the cell,

pHnF

2.303RTE

From the above equation the hydrogen ion concentration or the pH of the solution can be calculated.

12. Write down the formulation of the standard hydrogen electrode. (Nov. 1994) Hydrogen electrode consists of platinum foil, that is connected to a platinum wire and sealed in a glass tube.

Hydrogen gas is passed through the side arm of the glass tube. This electrode, when dipped in a 1N HCl and hydrogen

gas at 1 atmospheric pressure is passed forms a standard hydrogen electrode. The electrode potential of SHE is zero at

all temperatures.

It is represented as,Pt, H2 (1 atm)/H+ (1 M); E° = 0 V .

13. Distinguish between electrochemical series and galvanic series. (May 2004)

Sl.No. Electrochemical series Galvanic series 1

When the various electrodes (metals)

are arranged in the order of their

increasing values of standard reduction

potential on the hydrogen scale, then

the arrangement is called

electrochemical series.

The metal electrodes are placed in sea

water under std. conditions and the

electrode potentials are measured. When

these electrode potentials are arranged in

the decreasing order, a serious produced

known as galvanic series.

2 This series comprises of metals and

non-metals.

The series comprises metals and alloys

14. What is a secondary reference electrode? Give one example with its electrode potential

value. (Jan. 2006) Calomel electrode consists of a glass tube containing mercury at the bottom over which mercurous chloride is placed.

The remaining portion of the tube is filled with a saturated solution of KCl. The bottom of the tube is sealed with a

platinum wire. The side tube is used for making electrical contact with a salt bridge. The electrode potential of the

calomel electrode is + 0.2422 V. It is represented as, Hg, Hg2Cl2(s), KCl (sat.solution);E° = 0.2422V

15. Give two limitations of H2 electrode. (Nov.2009) i. It requires hydrogen gas and is difficult to set up and transport.

ii. It requires considerable volume of test solution.

iii. The solution may poison the surface of the platinum electrode.

iv. The potential of the electrode is altered by changes in barometric pressure.

16. Can we use a nickel spatula to stir a solution of copper sulphate? Given that E0Ni2+/Ni= - 0.23

& E0Cu2+/Cu= + 0.34V. (Nov. 2003)

No, since nickel has negative reduction potential, it has the tendency to go as ions into solution.


17. A zinc rod is placed in 0.1M ZnSO4 solution at 2980K. Write the electrode reaction and

Calculate the potential of the electrode. E0 Zn2+/Zn = - 0.76V. (Nov. 2004) Electrode reaction is, Zn2+ +2e- → Zn

]log[MnF

2.303RTE E no

= - 0.76 – 0.03 V = - 0.79V.

18. Zinc reacts with dilute H2SO4 to give hydrogen but silver does not. Explain given that

E0 Ag+/Ag = + 0.080V and E0 Zn2+/Zn = - 0.76V. Since zinc has negative reduction potential and is placed above hydrogen in electrochemical

series, it liberates hydrogen but silver has positive reduction potential and is placed below

hydrogen in emf series. So, Ag does not liberate hydrogen.

19. What is an electrochemical cell? An electrochemical cell is a device used to convert chemical energy in to electrical energy.

20. What is electrochemical corrosion? When two dissimilar metals are in contact with each other in the presence of aqueous solution or moisture wet

corrosion occurs. This is otherwise known as electrochemical corrosion. 21. What is Pilling Bed worth ratio? The ratio of the volume of oxide film formed to the volume of metal consumed is called Pilling Bed worth Ratio.

22. What is Hydrogen embrittlement ? Accumulation of hydrogen gases in the voids develops very high pressure, which causes cracks and blisters on metal.

This process is called hydrogen embrittlement.

23. What is Decarburization? Carbon content of steel decreases when heated with hydrogen.This process is called decarburization.

24. What is dry Corrosion? Give Examples (June 2014)

Dry corrosion involves direct chemical attack of the metal by atmospheric oxygen, Carbon dioxide, hydrogen sulphide

etc. Eg. (i) Tarnishing of silver in H2S gas. (ii) Action of dry HCl in iron surface.

25. Mention two differences between dry and wet corrosion? 1. Dry corrosion involves direct chemical attack of the metal by atmospheric oxygen. Wet corrosion involves setting

up of tiny galvanic cells.

2. Dry corrosion follows adsorption mechanism. Wet corrosion follows electrochemical reaction mechanism.

26. What is pitting corrosion? Pitting is a localized attack which results in the formation of a hole around which the metal is relatively unattacked.

27. What is pipe line corrosion? Pipe line corrosion occurs when buried pipes or cables pass from one type of soil to another. Eg. From clay to sand

particles. As clay is less aerated than sand particles, differential aeration corrosion takes place.

28. Zinc is readily corroded when coupled with copper than with lead. Why? Since the difference of zinc- copper is very far as compared zinc-lead in the electrochemical series, zinc gets corroded

vigorously when coupled with copper than with lead.

29. Explain Cathodic Protection? Cathodic protection is prevention of corrosion by making the metallic structure act as cathode in the electrolytic cell.

30. State the basic design rules in controlling corrosion. (i) Avoid galvanic corrosion

(ii) Drainage affects corrosion, so complete drainage is important

(iii) Avoid sharp corners and bends

(iv) Avoid crevices

(v) Cathodic protection

31. What are corrosion inhibitors?Give two examples. Corrosion inhibitors are inorganic or organic substances which when added to the corrosive

environment decrease the rate of corrosion.

32. How will you control corrosion by modifying the environment?

Corrosion can be controlled by modifying the environment via, deareation, dehumidification, deactivation, alkaline

neutralization and using corrosion inhibitors.

] Log [ 0.1

nF

0.0591

-0.76 E


33. What is the role of pigment in paint? Give two examples. Pigment provides desired colour to the paint. Green-chromium oxide.

Blue – prussion blue.

34. What is electroless plating? It is a technique of depositing a metal from its salt solution on a catalytically active surface of the metal to be

protected by using a suitable reducing agent without using electrical energy.

35. What is electroplating? Electroplating is the process in which the coating metal is deposited on the base metal by passing a direct current

through an electrolytic solution containing the soluble salt of the coating metal.

36. What is the effect of pH of the conducting medium on corrosion of metals?(June 2009) Rate of corrosion is more in the acidic medium than alkaline or neutral medium.

37. What is the principle involved in impressed cathodic current method of prevention of

corrosion?(June 2009)

In the impressed cathodic current method, an impressed current is applied in the opposite direction of the corrosion

current to nullify it and the corroding metal is converted from anode to cathode .Thus the metal is protected from

corrosion.

38. Explain Galvanic corrosion with an example. ( Dec 2009 & Nov 2009)

When two dissimilar metals are in contact with each other in the presence of the aqueous solution or moisture or

electrolyte, the metal higher in electro chemical series undergoes corrosion. This type of corrosion is called galvanic

corrosion. Eg., Zinc and Copper .Here Zinc act as anode and undergoes corrosion.

39. Using chemical equation , state the mechanism of corrosion of iron in weakly alkaline solution.(June 2010) Iron dissolves as Fe 2+ ions with the liberation of electrons.

Fe →Fe2+ + 2 e

At cathode : the liberated electrons flow from anode to cathode through metal, where the electrons are gained by

dissolved oxygen to form OH- ions. ½ O2 + H2O + 2 e → 2 OH-

40. What is the effect of H2S gas when it comes in contact with iron metals?(Dec 2010)

Iron reacts with H2S and liberate atomic Hydrogen.

Fe + H2S → FeS + 2 H

This atomic Hydrogen diffuses ready on the metal surface and collects in the voids, where it combines to form

molecular hydrogen.

H + H → H2

Collections of these gases in the voids develop very high pressure which causes cracks and blisters on the metal.

41. Name two metals in which the specific volume of its oxides are greater than that of the metals.(June 2011)

Lead, Aluminium , Copper, Tin

42. What is rust? What is its chemical formula?(June 2011)

Rust is ferric oxide, Fe2O3.3H2O

43. What are the advantages of electroless plating over electroplating? - No electricity is required and can be easily plated on insulators. (Plastics, Glass)

- Complicated parts can be easily plated.

- It produces hot surface and good wear resistance.

- Electroless coating possesses good mechanical, chemical and magnetic properties.

44. What are the main objectives of electroplating? (i) To give corrosion protection to the base metal

(ii) To increase commercial value of inferior metals

(iii)To increase the electrical and thermal conductivity

(iv) To increase the strength and wear resistance.

45. What are the characteristics of good paint? (i) A good paint should have a good spreading power

(ii) It should form durable, tough and resistant to wear film on drying.

(iii) Colour of paint should not change

(iv) It should not crack on drying. It should dry quickly

46. What is concentration cell or differential aeration corrosion? Give an example. Concentration cell type corrosion occurs when a metal is exposed to an electrolyte with varying amount of oxygen.

The metal part immersed in conducting liquid or partially buried in soil is less aerated acts as anode. At anode

corrosion occurs. The rest of metal part exposed to higher concentration of oxygen acts as cathode.eg Pipeline

corrosion, Pitting corrosion.


PART- B

1. Derive the Nernst equation. Discuss in detail any two applications. (June 2014)

Nernst equation for electrode potential

Consider the following redox reaction

Mn+

+ ne− ↔ M

For such a redox reversible reaction, the free energy change (∆G) and its equilibrium constant (K) are inter

related as

∆G = − RT ln K + RT ln [ Product ]

[ Reactant ]

= ∆G° + RT ln [ Product ] ........ (1)

[ Reactant ]

where,

∆G° = Standard free energy change

The above equation (1) is known as Van’t Hoff isotherm. The decrease in free energy (− ∆G) in the above

reaction involves transfer of ‘n’ number of electrons, then ‘n’ faraday of electricity will flow. If E is the emf of the cell,

then the total electrical energy (nEF) produced in the cell is

− ∆G = nEF

(or)

− ∆G° = nE°F ........ (2)

where,− ∆G = decrease in free energy change.

(or) − ∆G° = decrease in standard free energy change. Comparing equation 1 and 2, it becomes

− nEF = − nE°F + RT ln[ M ]

[ Mn+

] . ....... (3)

Dividing the above equation (3) by – nF

[ the activity of solid metal [M] = 1 ]

E = E° − RTln 1

nF [ Mn+

]

In general, E = E° − RTln [Product] ....... (4)

nF [Reactant]

(or)

E = E° + RTln [ Mn+

]

nF

When, R = 8.314 J/K/mole; F = 96500 coulombs;

T = 298 K (25°C), the above equation becomes

]log[MnF

2.303RTE E no

(or)

]log[Mn

0.0591E E n

redo

....... (5)

In general,

C logn

0.0591E E oxi

o

Similarly for oxidation potential


]log[Mn

0.0591E E n

oxio

....... (6)

The above equation 5 & 6 are known as “Nernst equation for single electrodepotential”.

Applications of Nernst equations

1. Nernst equation is used to calculate electrode potential of unknown metal.

2. Corrosion tendency of metals can be predicted.

2. What is electrochemical series? Give its applications.

The arrangement of various metals in increasing order of their standard reduction potentials is called as

electrochemical series or electromotive force series

Significance of emf series (or) Applications of electrochemical series (or) Applications of Nernst equation

1. Calculation of standard emf of the cell

The standard emf of a cell (E°) can be calculated if the standard electrode potential values are known using the

following relation.E°cell

= E°R.H.E

− E°L.H.E

2. Relative ease of oxidation (or) reduction

Higher the value of standard reduction potential (+ve value) greater is the tendency to get reduced. (i.e. Metals on the

top (–ve value) are more easily ionised) (oxidised).


(a) The fluorine has higher positive value of standard reduction potential (+ 2.87 V), and shows higher tendency towards

reduction.

(b) The lithium has highest negative value (− 3.01 V) and shows higher tendency towards oxidation.

3. Displacement of one element by the other

Metals which lie higher in the emf series can displace those elements which lie below them in the series. For

example, we may know whether Cu will displace Zn from the solution or vice-versa. We know that standard reduction

potential of Cu & Zn.

i.e., E°Cu2+

/Cu = + 0.34 V and E°Zn2+

/Zn = − 0.76 V.

So, Cu2+

has a great tendency to acquire Cu form, than Zn2+

has for acquiring Zn form.

So, Cu2+

has a great tendency to acquire Cu form, than Zn2+

has for acquiring Zn form.

4. Determination of equilibrium constant for the reaction

Standard electrode potential can also be used to determine the equilibrium constant (K) for the reaction. We know

that,From the value of E°, the equilibrium constant for the cell reaction can be calculated.

5. Hydrogen Displacement Behaviour

Metals with negative reduction potential (i.e., the metals placed above H2 in the emf series) will displace the

hydrogen from an acid solution.

Example:

Zinc reacts with dil H2SO

4 to give H

2 but Ag does not, why?

Zn + H2SO

4 −−−>ZnSO

4 + H

2 ↑E°

Zn = − 0.76 volt

The metal with positive reduction potential (ie., the metals placed below H2 in the emf series) will not

displace the hydrogen from an acid solution.

Ag + H2SO

4 → No reactionE

o

Ag = + 0.80 volt

6. Predicting Spontaneity of Redox Reactions

Spontaneity of redox reaction can be predicted from the emf (E°) value of the complete cell reaction.

(i) If the E° of the cell is positive, the reaction is spontaneous.

(ii) If the E° of the cell is negative, the reaction is not feasible.

In general, an element having lower reduction potential can displace another metal having higher

reduction potential from its salt solution spontaneously.


3. Differentiate between electrolytic cell and electrochemical cell.

4. Explain how single electrode potential is determined by using SCE or SHE.

Measurement of single electrode potential of Zn

using saturated calomel electrode


The saturated calomel electrode is coupled with another Zn electrode, the potential of which is to be determined

(Fig. 1.4). Since the reduction potential of the coupled Zn electrode is less than E° of calomel electrode (+ 0.2422

V), the calomel electrode will act as cathode and the reaction is

Hg2Cl

2(s) + 2e− ↔ 2Hg

(l) + 2Cl

−

Ecell

= E°right

− E°left

Ecell

= E°cal

− E°Zn

E°Zn

= E°cal

− Ecell

= + 0.2422 − 1.0025

E°Zn

= − 0.7603 volt .

5. Differentiate chemical and electrochemical corrosion with suitable examples.

6. Describe the mechanism of chemical and electrochemical corrosion. (June 2014)

DRY CHEMICAL CORROSION

Oxidation Corrosion (or) Corrosion by Oxygen

Oxidation corrosion is brought about by the direct attack of oxygen at low or high temperatures on

metal surface in the absence of moisture. Alkali metals (Li, Na, K, etc.)

Alkaline-earth metals (Mg, Ca, Sn, etc.) are rapidly oxidised at low temperature. At high temperature,

almost all metals (except, Ag, Au and Pt) are oxidised.

Mechanism of Dry Corrosion

(i) Oxidation occurs first at the surface of the metal resulting in the formation of metal ions

(M

2+

), which occurs at the metal / oxide interface.

M −−−−−> M2+ + 2e-

(ii) Oxygen changes to ionic form (O2-) due to the transfer of electron from metal, which occurs at the oxides

film / environment interface

½ O2 + 2e- −−−−−> O2-


(iii) Oxide ions reacts with the metal ion to form the metal-oxide film.

M + ½ O2 −−−−−> M2+ + O2- ≡ MO

(Metal-oxide film)

Once the metal surface is converted to a monolayer of metal-oxide, for further corrosion (oxidation) to

occur, the metal ion diffuses outward through the metal-oxide barrier.

Thus the growth of oxide film commences perpendicular to the metal surface (Fig. 2.2).

WET ELECTROOCHEMICAL CORROSION

Wet corrosion occurs under the following conditions.

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

solution or moisture.

(ii) When a metal is exposed to varying concentration of oxygen or any electrolyte.

(a) Hydrogen Evolution Type Corrosion

“All metals above hydrogen in the electrochemical series have a tendency to get dissolved in acidic

solution with simultaneous evolution of hydrogen gas”


When iron metal contacts with non-oxidising acid like HCl, H2 evolution occurs.

At Anode

Iron undergoes dissolution to give Fe2+ ions with the liberation of electrons.

Fe −−−−−> Fe+ + 2e- (Oxidation)

At Cathode

The liberated electrons flow from anodic to cathodic part, where H+ ions get reduced to H2.

2H+ + 2e- −−−−−> H2↑ (Reduction)

The surface of iron is usually, coated with a thin film of iron oxide. However, if the oxide film develops,

some crack will come and anodic areas are created on the surface while the remaining part acts as cathode

When iron metal contacts with a neutral solution of an electrolyte in presence of oxygen, OH−

ions are

formed.

(b) Absorption of oxygen (or) Formation of hydroxide ion type corrosion

Iron dissolves as Fe2+ with the liberation of electrons.

At Anode

Fe −−−−−> Fe2+ + 2e- (oxidation)`

At Cathode

The liberated electrons flow from anodic to cathodic part through metal, where the electrons are taken up

by the dissolved oxygen to form OH−

ions.


½ O2 + H2O + 2e- −−−−−> 2OH-

Thus, the net corrosion reaction is

Fe2+ + 2OH-−−−−−>Fe(OH)2↓

If enough O2 is present Fe(OH)2 is easily oxidised to Fe(OH)3, a rust (Fe2O3H2O)

4Fe(OH)2 + O2 + 2H2O −−−−−> 4Fe(OH)3

7. Describe the sacrificial anode and impressed current cathodic protection methods of corrosion control.

By Cathodic Protection

The principle involved in the cathodic protection is to force the metal to behave like a cathode.

The important cathodic protections are

(i) Sacrificial anodic protection.

(ii) Impressed current cathodic protection

(i) Sacrificial Anadic Protection Method

In this method, the metallic structure to be protected is made cathode by connecting it with more active

metal (anodic metal). So that all the corrosion will concentrate only on the active metal. The artificially made

anode thus gradually gets corroded protecting the original metallic structure. Hence this process is otherwise

known as sacrificial anodic protection.

Aluminium, Zinc, Magnesium are used as sacrificial anodes.

Applications of Sacrificial Anodic Protection

This method is used for the protection of ships and boats. Sheets of Mg or Zn are hung around the hull of

the ship (Fig. 2.17).

Zn or Mg will act as anode compared to iron (ship or boat is made of iron), so corrosion concentrates on

Zn or Mg. Since they are sacrificed in the process of saving iron, they are called sacrificial anodes.


(d) Protection of underground pipelines, cables from soil corrosion (Fig. 2.18(a)).

(c) Insertion of Mg sheets into the domestic water boilers to prevent the formation of rust (Fig. 2.18(b)).

(d) Calcium metal is employed to minimize engine corrosion.

(iii) Impressed Current Cathodic Protection Method

In this method, an impressed current is applied in the opposite direction of the corrosion current to nullify

it, and the corroding metal is converted from anode to cathode.

This can be done by connecting negative terminal of the battery to the metallic structure, to be protected,

and positive terminal of the battery is connected to an inert anode.

Inert anodes used for this purpose are graphite, platinised titanium. The anode is buried in a “back fill”

(containing mixture of gypsum, coke, breeze, sodium sulphate). The “back fill” provides good electrical contact to

anode Fig. 2.19).


Applications of Impressed Current Protection

Structures like tanks, pipelines, transmission line towers, underground water pipe lines, oil pipe lines,

ships, etc., can be protected by this method.

Fig. 2.19 Impressed Current Cathodic Protection

8. What are important factors with influence the rate of corrosion of a metal?

Factors Influencing the Rate of Corrosion

The rate and extent of corrosion mainly depends on

(i) Nature of the metal.

(ii) Nature of the environment.

Nature of the Metal

(a) Position in EMF Series

The extent of corrosion depends on the position of the metal in the emf series. Metals above the hydrogen

in emf series get corroded vigorously.

Lower the reduction potential, greater is the rate of corrosion. When two metals are in electrical contact,

the more active metal (or the metal having high negative reduction potential) undergoes corrosion.

The rate and severity of corrosion depends on the difference in their positions in the emf series.

Greater the difference faster is the corrosion rate.

(b) Relative Areas of the Anode and Cathode

The rate of corrosion will be more, when the cathodic area is larger. When the cathodic area is larger, the

demand for electrons will be more and this results in an increased rate of corrosion (dissolution) of metals at

anodic area.

(c) Purity of the Metal

The 100% pure metal will not undergo any type of corrosion. But, the presence of impurities in a metal

create heterogeneity and thus galvanic cells are set up with distinct anodic and cathodic area in the metal.

Higher the percentage of impurity, faster is the rate of corrosion of the anodic metal. The effect of

impurities on the rate of corrosion of zinc is given below.


(d) Over Voltage or Over Potential

The over voltage of a metal in the corrosive environment is inversely proportional to corrosion rate.

(e) Nature of the surface film

The nature of the oxide film formed on the metal surface decides the extent of corrosion which can be

decided by Pilling-Bedworth rule

In the case of alkali and alkaline earth metals such as Mg, Ca, etc. form oxide, whose volume is less than

the volume of the metal. Hence the oxide film will be porous and non-protective and bring about further

corrosion.

But in heavy metals like Al, Cr, etc. form oxide,

whose volume is greater than that of the metal. Hence the oxide film will be non-porous and protective and

prevents further corrosion.

(f) Nature of the Corrosion Product

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

the corrosion product is volatile (like MoO3 on Mo surface), the corrosion rate will be faster.

9. Discuss the types of electrochemical corrosion with examples.

Galvanic Corrosion

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

galvanic corrosion occurs.

Here, the more active metal (with more negative electrode potential) acts as anode and the less active

metal (with less negative electrode potential) acts as cathode.

Example: Zn – Fe Couple

Fig. 2.5 (a) represents Zn-Fe couple, in which zinc (more active or higher in emf series) dissolves in

preference to iron (less active metal) i.e., Zn acts as anode and undergoes corrosion and Fe acts as cathode.

Example: Cu – Fe Couple

Fig. 2.5 (b) represents Fe − Cu couple, in which iron (more active, when compared to Cu) dissolves in

preference to copper (less active) i.e., Fe acts as anode and undergoes corrosion and Cu acts as cathode.

Examples for Galvanic Corrosion


(i) Steel screw in a brass marine hardware corrodes

This is due to galvanic corrosion. Iron (higer position in electrochemical series) becomes anodic and is

attacked andcorroded, while brass (lower in electrochemical series) acts as cathodic and is not attacked.

(ii) Bold and Nut made of the same metal is preferred

It is preferred in practice, because galvanic corrosion is avoided due to homogeneous metals (no anodic

and cathodicpart)

Prevention

Galvanic corrosion can be minimised by providing an insulating material between the two metals

Differential aeration (or) concentration cell corrosion`

This type of corrosion occurs when a metal is exposed to varying concentration of oxygen or any

electrolyte on the surface of the base metal.

Example:

Metals partially immersed in water (or) conducting solution (called water line corrosion).

Differen

tial Aeration Corrosion

If a metal is partially immersed in a conducting solution the metal part above the solution is more aerated

and hence become cathodic. On the other hand, the metal part inside the solution is less aerated and thus, become

anodic and suffers corrosion.

At anode (less aerated) corrosion occurs M −−−−−> M2+

+ 2e−

At cathode (more aerated part) OH−

ions are produced

½ O2

+ H2

O + 2e−

−−−−−> 2OH−

a) Pitting or localised corrosion.

b) Crevice corrosion.

c) Pipeline corrosion.

d) Corrosion on wire fence.

Examples for Differential Aeration Corrosion


a) Pitting or Localized Corrosion.

Pitting is a localised attack, resulting in the formation of a hole around which the metal is relatively

unattached.

Example:

Metal area covered by a drop of water, sand, dust, scale, etc.

Let us consider a drop of water or aqueous NaCl resting on a metal surface (Fig. 2.7). The area covered

by the drop of water acts as an anode due to less oxygen concentration and suffers corrosion. The uncovered area

(freely exposed to air) acts as a cathode due to high oxygen concentration.

The rate of corrosion will be more, when the area of cathode is larger and the area of anode is smaller.

Therefore, more and more material is removed from the same spot. Thus a small hole or pit is formed on the

surface of the metal.

At Anode

Iron is oxidised to Fe2+ ions

Fe −−−−−> Fe2+ + 2e-

At Cathode

Oxygen is converted to OH- ions.

½ O2 + H2O + 2e- −−−−−> 2OH-

Net reaction is


3

O

2

-2 Fe(OH)(OH) FeOH2Fe

This type of intense corrosion is called pitting.

(b) Crevice Corrosion

If a crevice between different metallic objects or between metal and non-metallic material is in contact

with liquids, the crevice becomes the anodic region and suffers corrosion.

This is due to less oxygen in crevice area. The exposed areas act as the cathode (Fig. 2.8).

(c) Pipeline Corrosion

Differential aeration corrosion may also occur in different parts of pipeline.

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

aerated) to sand (more aerated) may get corroded due to differential aeration.

(d) Corrosion on Wire - Fence

Fig. 2.10 shows a wire fence in which the areas where the wires cross are less aerated than the rest of the

fence and hence corrosion occurs at the wire crossings, which are anodic.


Other Examples for Differential Aeration Corrosion

(i) Corrosion occurring under metal washers, where oxygen cannot diffuse easily.

(ii) Lead pipeline passing through clay to cinders undergo corrosion. Since the pipeline under cinders is

more aerated, it gets corroded easily.

10. How do you control corrosion by proper designing and selection of materials?

Corrosion Control

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

Control of corrosion by Modifying the Metal

1.By selection of the metal

2. By using pure metal



1. By Selection of the Metal

Selection of right type of metal is the main factor for corrosion control. Thus, noble metals are used in

ornaments and in surgical instruments, as they are most immune to corrosion.

2. By using Pure Metal

Pure metals have higher corrosion resistance. Even a small amount of impurity may lead to severe

corrosion.

3. By Alloying

Corrosion resistance of many metals can be improved by alloying. For example, stainless steel containing

chromium produce a coherent oxide film, which protects the steel from further attack.

4. By Proper Design

Some of the important rules for designing, which must be observed are given below.


(i) Avoid galvanic corrosion

(ii) Drainage affects corrosion

(iii) Avoid sharp corners and bends

(iv) Avoid crevices

(i) Avoid Galvanic Corrosion

If two different metals are joined, galvanic corrosion will occur. In such a case galvanic corrosion is prevented by

(a) Selecting the metals as close as possible in the electrochemical series.

(b) Providing smaller area for cathode and larger area for anode.

(c) Inserting an insulating material between the two metals (Fig. 2.12).

Tanks and other containers must be designed in such a way that, the whole of the liquid should be drained off

completely (Fig. 2.13).

(ii) Drainage Affects Corrosion


(iv) Avoid Crevices

Crevices allow moisture and dirt, which results in increased electrochemical corrosion. This can be prevented by

filling the crevices with a filler (Fig. 2.15).

Riveted

joints produce crevice corrosion, so welded joints are preferred.

11. What are the requisites of a good paint? Discuss the ingredients of paint and explain their functions.

(June 2014)

PAINTS

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

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

oxidation forming a pigmented film

Constituents and their functions of a paint

1. Pigments


2. Vehicle (or) drying oil

3. Thinners (or) solvents

4. Extenders (or) fillers

5. Driers

6. Plasticisers

7. Anti-skinning agents

1. Pigments

Pigments are solid and colour producing substances in the paint.

Functions

(i) It gives colour and opacity to the film.

(ii) It also provides strength to the film.

(iii) It protects the film by reflecting the destructive

uv rays.

(iv) It increases weather resistance of the film.

Example:

1. Vehicle (or) Drying Oil

This is a non-volatile portion of a medium and film forming constituent of the paint. These are high

molecularweight fatty acids present in vegetable and animal oils.

Functions

(i) They form a protective film by the oxidation and polymerisation of the oil.

(ii) They hold the pigment particles together on the metalsurface.

(iii) They impart water repellency, toughness and durability to the film.

3. Thinners (or) Solvents

This is a volatile portion of a medium. It easily evaporates after application of the paint.

Functions

(i) It reduces the viscosity of the paint, so that it can be easily applied on the surface.

(ii) It dissolves the oil, pigments, etc. and produces a homogeneous mixture.

(iii) It increases the elasticity of the film.


(iv) It increases the penetrating power of the vehicle.

Example: Linseed oil, Dehydrated Castor Oil

4. Extenders (or) fillers

These are white (or) colourless pigments.

Functions

(i) It reduces the cost of the paint.

(ii) It retards the settling of the pigment in all paints.

(iii) It modifies the shades of the pigments.

(iv) It prevents shrinkage and cracking.

Functions

(i) They act as oxygen-carriers (or) catalysts.

(ii) They provide oxygen, which is essential for oxidation, polymerisation of drying oil.

5. Driers

These are the substances, used to accelerate the process of drying.

These are chemicals added to the paint to provide elasticity to the film and to prevent cracking of the film.

6. Plasticisers

Example: Triphenyl phosphate, tricresyl phosphate, etc.

7. Anti – Skinning Agents

These are chemicals added to the paint to prevent gelling and skinning of the paint.

Example: Polyhydroxy phenol.

12. What is electroplating? Discuss the plating composition, mechanism and applications of copper plating.

Definition

Electroplating is a process in which the coating metal is deposited on the base metal by passing a direct current through an

electrolytic solution containing the soluble salt of the coating metal.

Objectives

To give corrosion protection to the base metal

To increase commercial value of inferior metals

To increase the electrical and thermal conductivity

To increase strength and wear resistance

Components for electroplating

Electrolyte is an aqueous solution of CuSO4

The electronically conducting cathode (Object to be plated)


Impure copper (anode)

A direct current source

Process

The metal object to be plated is first with dilHCl or Dil. H2SO4. The cleaned object is then made cathode of an electrolytic

cell and copper plate as the anode CuSO4 and Dil H2SO4 solutions are taken as the electrolyte. When the current is passed

from the battery through the solution, copper dissolves in the electrolyte and deposits uniformly on the metal object.

Various chemical reactions

CuSO4ionises as

CuSO4→ Cu2+ + SO42-

At Cathode : On passing current Cu2+ ions move to the cathode and get deposited there as Cu metal.

Cu2+ + 2e- → Cu

At Anode: The free sulphate ions migrate to the copper anode and dissolves an equivalent amount of Cu to form CuSO4

Cu + SO42-→ CuSO4

In order to get strong, adherent and smooth deposit certain additives (glue, gelatin.) are added to the electrolytic bath. The

improve the brightness of deposit , brightening agents are added in the electrolytic bath. The favourable conditioning for a

copper electrdepositsareoptimum temperature (40-45oC) optimum current density (30-40mA/ cm2) and low metal ion

concentrations.

13. What is electroless plating? Explain pretreatment of object, bath composition, mechanism and applications of

electroless nickel plating.

Electro less Plating

Electro less plating is a technique of depositing a noble metal (from its salt solution) on a catalytically active

surface of the metal, to be protected, by using a suitable reducing agent without using electrical energy.

Principle


The reducing agent reduces the metallic ions to metal, which gets plated over the catalytically activated surface giving a

uniform thin coating.

Metal ions + Reducing agent −−−−−>Metal +Oxidised product(s)

( Deposited)

Example:

Electroless Nickel Plating

Pretreatment and activation of the surface

Step : 1

The surface to be plated is first degreased by using organic solvents or alkali, followed by acid treatment.

Activation depend on the type of metal (or) alloy (or) non-metal used

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

(ii) The surface of Mg alloy is activated by thin coating of Zn and Cu over it.

(iii) Metals and alloys like Al, Cu, Fe, brass, etc., can be directly Ni − plated without activation.

(iv) Non-metallic articles (like plastics, glass, etc.,) are activated by dipping them in the solution containing SnCl2+

HCl, followed by dipping in palladium chloride solution. On drying a thin layer of Pd is formed on the surface.

Step II

Preparation of Plating bath

The plating bath consists of the following ingredients


Procedure for Plating

Step III

The pretreated object is immersed in the plating bath for the required time. During which the following reduction

reaction will occur and the Ni gets coated over the object.

Various Reactions

Ni2+ + 2e- −−−−−> Ni

At Cathode:

H2PO2

−

+ H2O −−−−−> H2PO3

−

+ 2H

+

+ 2e

-

At Anode:

Net reaction:

Ni

2+

+ H2PO2

−

+H2O −−−−−> Ni + H2PO3

−

+ 2H

+

Applications

1. Electroless Ni–plating is extensively used in electronic appliances.

2. Electroless Ni–plating is used in domestic as well as automotive fields (eg.,jewellery, tops of perfume

bottles.)

3. Electroless Ni–coated polymers are used in decorative and functional works.

4. Electroless Cu & Ni coated plastic cabinets are used in digital as well as electronic instruments.

Advantages of Electroless Plating Over Electroplating

1. No electricity is required.

2. Electroless plating on insulators (like plastics, glass) and semiconductors can be easily carried out.

3.Complicated parts can also be plated uniformly.

4. Electroless coatings possess good mechanical, chemical and magnetic properties.

14. Give the cell reactions of the following cells. (June 2014) (i) Zn(s)/Zn2+(0.01M) //Ni2+(0.5M)/Ni(s)

(ii) Zn(s)/Zn2+(aq)//Ag+(aq)/Ag(s)

(iii) Ni(s)/Ni2+(1M)//Pb2+(1M)/Pb(s)

(iv) Ag(s)/Ag+(aq)//Pt,H2(g)/H2(g)

S.No

Anode Cathode Overall cell reaction


i. Ni −−−−−> Ni

2+

+ 2e

−

Zn2+ + 2 e- Zn Ni + Zn2+−−> Ni

2+

+ Zn

ii. 2Ag −−−−−> 2Ag

1+

+ 2e

−

Zn2+ + 2 e- Zn 2Ag + Zn2+ −−−−−> 2Ag

1+

+ Zn

iii. Pb −−−−−> Pb

2+

+ 2e

−

Ni2+ + 2 e- Ni

Pb + Ni2+ −−−−−> Pb

2+

+ Ni

iv. 2Ag −−−−−> 2Ag

1+

+ 2e

−

H2 + 2 e- 2 H-

2Ag + H2 −−−−−> 2Ag

1+

+ 2 H-


UNIT- III ENERGY SOURCES

PART A

1. How do nuclear changes differ from simple chemical changes? Nuclear changes alter the number of protons and neutrons in the nuclei of the atoms concerned; while simple chemical

changes involve in reorganization of electrons only.

2. What are moderators in nuclear reactor? Moderator is either heavy water or graphite, which slows down the speed of neutrons in the nuclear reactors.

3. State critical mass. Minimum mass of a lump of uranium-235 which will undergo fission in a chain reaction is called critical mass.

4. What is meant by multiplication factor in a fission reaction? Every fission reaction produces two or three neutrons. In a nuclear reactor, the nuclear chain reactions are controlled so

that an equilibrium state is reached, where exactly only one neutron is used for further fission.

K = number of neutrons generated / number of neutrons disappeared

5. What is a breeder reactor?

A fission reactor which produces more fissionable material than is consumed in its operation is called a breeder

reactor.

6. What is a nuclear chain reaction? How it is controlled? Nuclear chain reaction is an autocatalytic reaction in which the number of neutrons keeps on multiplying rapidly till

the whole of fissionable material is disintegrated. Control rods are used control and regulate the number of neutrons

that can cause fission.

7. What is a battery? A battery is a combination of electro-chemical cell connected in series. Any redox reaction occurring at an appropriate

electrode can be employed to generate electricity in such cell.

8. What are primary batteries? Give an example.

It is irreversible cell. Battery which cannot be recharged again by passing external electricity is called primary battery.

Example: Leclanche cell, mercury cell, alkaline battery

9. What are secondary batteries? Give an example.(Jan-2013)

They are reversible cells. Batteries which can be recharged again by passing external electric current are called

“secondary batteries”.

Ex. Ni-Cd battery, lead-acid battery and lithium battery

10. What are the advantages of alkaline battery than dry battery?

(i)Zinc does not dissolve in basic medium.

(ii)Life is longer than dry battery, because there is no corrosion on zinc.

(iii)Alkaline battery maintains its voltage (1.5V), as the current is drawn from it.

11. What are the advantages of Li battery? (Jan-2012, June-2012)

(i)Its cell voltage is high, 3V (ii) Lithium is a light-weight metal

(iii) Li has the more negative E° value and therefore generates a higher voltage than the other types of cells

(iv)All the constituents of the battery are solids and, therefore there is no risk of leakage from the battery.

12. Write the applications of solar cells.

Solar energy is made use in the electrification of rural areas of tropical region where the sunlight is effective during

daytime. Solar cells are useful in refrigerator, water heater, water pump and cooker.

13. What is a fuel cell or flow battery? A fuel cell is a device in which thermal energy is directly converted to electrical energy. In a conventional system,

thermal energy is converted to mechanical energy and the mechanical energy into electrical energy.

14. Write the advantages of fuel cells.

(i) High efficiency of energy conservation (chemical to electrical energy)

(ii) No emission of gases and pollutants.

15. How are anodic and cathodic electroactive materials made in Ni-Cd battery?

At anode: Cadmium is oxidized to Cd2+ and further it combines with OH- ions and to form Cd(OH)2

At cathode: NiO2 gains electrons and it undergoes reduction at the cathode from +4 to +2. Ni2+ ions combine with OH-

ions to form Ni(OH)2

16.Write the uses of lead storage battery. (i)Lead storage cell is used to supply current mainly in automobiles


(ii)It is also used in gas engine ignition, telephone exchanges and power stations etc.

17.Write the advantages of Ni-Cd battery.

(i)It is a portable, rechargeable cell and its cell voltage is fairly constant (about 1.4 V). Like a dry cell, it can be left for

long periods of time without any appreciable deterioration, since no gases are produced during charging (or charging).

(ii)It is used in electronic calculators, electronic flash units, transistors and other battery powered small tools.

18.What are the characteristics of fuel cells?

(i)They do not store chemical energy

(ii)The efficiency of a fuel cell is about twice that of a conventional power plant for generating electricity

(iii)Fuel cell generators are free of the noise, vibration, heat transfer, thermal pollution and other problems normally

associated with conventional power plants

19. What are fissile and fertile nucleides? (Jan-2012, Jan-2013)

Fissile materials: The materials which undergo fission by slow moving neutrons are called as fissile materials.

Examples: U-235, Pu-239, U-233, Pu-241.

Fertile materials: The materials which do not undergo fission easily but may be made by bombardment with fast

moving neutrons are called as fertile materials.

Example: U-238, Th-232.

20. What are non-conventional energy sources? Give two examples. (June-2012) Renewable energy sources also called as non-conventional energy sources that are continuously replenished by natural

processes. Example: solar energy, wind energy.

21. What is Nuclear fission? Give any one nuclear fission reaction.

The process of breaking a heavy nucleus with a slow neutron into two lighter nuclei of almost equal size with the

liberation of large amount of energy is called as nuclear fission or atomic fission.

92U235 + 0n1 → [ 92U236] → 56Ba141 + 36Kr92 + 30n1 + 200.5 MeV ( Energy)

22. What are the characteristics of nuclear fission process?

(i) Breaking of heavy nucleus by aneutron gives two or more lighter nuclei.

(ii) The fission of each nucleus generates 2 or3 neutrons.

(iii) Large amount of energy is liberated during fission.

(iv) All the fission products are radioactive, and emit beta and gamma radiations.

23. Explain the significance of reproduction factor in nuclear chain reaction?

K = number of neutrons generated / number of neutrons disappeared

(i)If K > 1 ,the nucear chain reaction will lead uncontrolled growth of the neutrons and cause an atomic explosion.

(ii)If K = 1 , the nuclear chain reaction is maintained so that at at least only one neutron must be allowed to strike

another nucleus. The chain reaction in most of reactors is controlled by means of control rods such as boron or

cadmium which can absorb neutrons

(iii)If K < 1, no more nuclear chain reaction occurs and the reactor stops working

24. Define the term nuclear energy. The liberation of large amount of energy along with the ejection of 2 or 3 neutrons during nuclear fission reaction is

called nuclear energy or atomic energy.

25. What is nuclear fusion? Give any one nuclear fusion reaction.

Nuclear fusion is the process in which two or more lighter nuclei combine to form one single heavier nucleus.

Eg : The combination of various isotopes of hydrogen to form helium.

1H2 + 1H3 → 2He4 + 0n1 + 17.6 MeV (Energy)

26. Distinguish between nuclear fission and nuclear fusion.

S.No. Nuclear Fission Nuclear fusion

1. A heavy nucleus splits into two nuclei Two lighter nuclei fuse

together

2. Enormous amount of energy is liberated Enormous amount of energy

is liberated

3. The process is possible at room temperature Process is possible only at

very high temperatures

4. It is a chain process. It is not a chain process

27. Explain proton cycle.


The reaction in the proton cycle are supposed to be the conversion of four protons into a helium nucleus and two

positrons with the release of about 17.8 MeV per gram of atom of hydrogen.

1H1 + 1H1 → 1H2 + -1e0 + Energy

21H2 + 21H1 → 21He3 + Energy

22He3 → 2He4 + 1H2 + -1e0 + Energy

41H1 → 2He4 + 2(+1e0) + 17.8 MeV

28. What is Nuclear reactor?

Nuclear reactor is a device in which nuclear fission is produced in a controlled manner to release nuclear energy. The

main purpose of a reactor is to produce electrical power. In nuclear reactors , the heat generated during fission is used

to produce steam. The steam is used to drive the turbines which produce electricity.

29. What are advantages of nuclear power generation?

(i) Nuclear power generation emits relatively lower amounts of green house gases.

(ii) It is possible to generate a high amount of electrical energy in one single plant.

30. Explain the function of control rods ?

Control rods absorb neutrons and regulate the number of neutrons that can cause fission Eg: Boron ,Cadmium.

31.What is the purpose of a coolant in a nuclear reactor ? Name the various coolants used in nuclear reactors.

Coolants are used to carry away the heat produced inside the reactor to heat exchanger where it transfers its heat to

water and produces steam which is utilized for power generation.

Water (light water or heavy water), gases (He, Co, Air), Moltern Na, Molten alloy of Na and K.

32. What is the principle of pressurized water reactor?

In PWR, water at high pressure is pumped to act as both coolant and moderator. The high high pressure water is then

passed through a heat exchanger where heat is transferred to water to generate steam.

33. What is principle of boiling water reactor?

In BWR , water is allowed to boil directly in the reactor core . the boiling water generates steam,which is drawn away

from the reactor and used to rotate the turbine.

34. Compare light water reactor and breeder reactor?

S.No. Light water reactor Breeder reactor

1. Fissile materials like U235 are used as

fuel

Fertile materials like U238 is used as fuel

2. During fission, no radioactive

emission occurs

During fission,gamma and beta emission

occurs.

3. Moderators are used to slow down

the neutrons

Moderators are not used

4. Light water is used as coolant Liquid sodium is used as coolant

34. How is electricity generated from wind?

Wind energy is generated by harnessing the wind with wind turbines. When the wind is passes through the turbine’s

rotor blades, the blades turn and convert the wind energy into kinetic energy this energy in turn spins a rotor inside a

generator where the kinetic energy is converted into electrical energy.

35. Mention the advantages and limitation wind electric power.

Advantages:

(i) Wind energy is the most environment friendly, clean and renewable source of power.

(ii) There is no fuel consumption, hence low operating costs.

Limitations:

(i)Wind towers and turbine blades are subject to dam

(ii)The noise made by rotating wind machine blades can be annoying to nearby neighbors.

36. Explain charging and discharging characteristics of a battery?

Discharging is an electrochemical process by which a battery delivers current to an external circuit at an cost of the

consumption of electrode materials.

Charging is an electrolytic process by which a constant current is passed through a battery in order to regenerate the

active materials back into their original form.

37. What are the advantages of storage batteries?


(i) The secondary batteries have advantages over the primary batteries in that the net cell reaction can be reversed

during the charging process and the current can be drawn during the discharge process.

(ii) Storage batteries have better cycle life and capacity, so that it can be used over and over again.

PART B

1. Explain the essential parts of a light water nuclear reactor with neat diagram. (Jan-2014)

COMPONENTS OF A LIGHT WATER NUCLEAR REACTOR

1. Fuel rods

2. Moderator

3. Control rods

4. Coolant

5. Protective screen

6. Heat exchanger / pressure vessel

7. Turbine

FUEL RODS:

The fissionable material used in the nuclear reactor is enriched U-235. It is used in the form of rods or strips.

Example: U 235

, Pu239

Function: It produces heat energy and neutrons, that neutron starts nuclear chain reaction.

CONTROL RODS:

To control the rate of fission of U-235 , movable rods made of Cd or B are suspended between fuel rods. These rods

absorb the excess neutrons . So the fission reaction proceeds at steady rate. These rods are lowered and raised as of need.

If the rods are deeply inserted inside the reactor, they will absorb more neutrons and the reaction becomes very

slow. If the rods are pushed outwards, they will absorb less neutrons and the reaction will be very fast.

BnB

CdnCd11

5

1

0

10

5

114

43

1

0

113

43

Example: Cadmium, Boron

Function: It controls the nuclear chain reaction and avoids the damage to the reactor. MODERATOR:

The substances used to slow down the neutrons are called moderators.

Example: Ordinary water, Heavy water, graphite, beryllium.

Function: The kinetic energy of fast neutron (1meV) is reduced to slow neutrons (0.25 eV).

COOLANT:

In order to absorb the heat produced during fission reaction, the coolant is circulated in

the reactor core. It enters the base and leaves at the top. The heat carried by outgoing liquid is used to produce steam.

Example: Water (act as coolant and moderator) Heavy water , liquid metal ( Na or K) Function: It cools the fuel

core.


PRESSURE VESSEL:

It encloses the core and also provides the entrance and exit passages for coolant. Function: It withstands

the pressure as high as 200 atm.

PROTECTIVE SHIELD:

The moderator, control rods and fuel element are enclosed in a chamber which has a thick concrete shield(10m

thick).

Function: The environment and the operating persons are protected from destruction in case of leakage of

radiation. HEAT EXCHANGER:

It transfers the heat liberated from the reactor core to boil water and produce steam at about 400Kg/cm2.

TURBINE:

The steam generated in the heat exchanger is used to operate a steam turbine, which drives a generator to

produce electricity.

LIGHT WATER NUCLEAR POWER PLANT

It is the one in which U-235 fuel rods are submerged in water. Here water acts as coolant and moderator.

WORKING

The fission reaction is controlled by inserting or removing the control rods of B10

automatically from the spaces in

between the fuel rods. The heat emitted is absorbed by the coolant (light water) .The heated coolant then goes to the

heat exchanger containing sea water, which is converted to steam. The steam drives the turbines, generating electricity.

2. (i) Write short notes on breeder reactor.

(ii) Explain the characteristics of nuclear fission reactions.

Breeder reactor is the one which converts non-fissionable material (U 238

Th232

)

Into fissionable material(U 235

, Pu239

).

ePunU 2239

94

1

0

239

94

nproductsfissionnPU 1

0

1

0

239

94 3

In breeder reactor, of the three neutrons emitted in the fission of U-235, only one is used in propagating the fission of

U-235.The other two are allowed to react with U-

238. Thus two fissionable atoms Pu-239 are produced

for each atom of U-235 consumed. The breeder

reactor produces more fissionable material than

it uses.


In general

1. The fissionable nucleides such as U-235 and Pu-239 are called fissile nucleides.

2. The non-fissionable nucleides such as U-238 & Th-232 are called fertile nucleides.

CHARACTERISTICS OF NUCLEAR FISSION REACTION

1. Heavy nucleus splits into two or more nuclei.

2. Two or more neutrons are produced by fission of each nucleus.

3. Large quantity of energy is produced during the nuclear fission reaction..

4. All the fission fragments are radioactive in nature, giving off gamma radiations

5. The atomic weights of nuclear fission product ranges from 70 to 160.

6. All the fission reactions are self propagating chain reaction because one of the fission products is neutron.

7. The nuclear reactions can be controlled by absorbing the neutrons using Cd, Boron.

8. Every secondary neutron released in the fission reaction does not strike the nucleus. Some escape into air. Hence a

chain reaction cannot be maintained.

9. The number of neutrons resulting from a single fission is known as multiplication factor. When it is less than 1,

nuclear chain reaction does not take place.

3. ( i) What is reversible battery? Describe the construction and working of lead-acid battery with

reaction occurring during charging and recharging. (Jan-2013)

Batteries which can be recharged again by passing external electric current are called “secondary batteries” or

reversible batteries.

Ex. Ni-Cd battery, lead-acid battery and lithium battery

LEAD ACID STORAGE BATTERY OR ACCUMULATOR It was invented by Gaston Plante in 1859.

It acts both as voltaic cell and electrolytic cell. On supplying electrical energy, this acts as a voltaic cell. On

recharging, the cell acts as an electrolytic cell.


Description:

1. Anode – Lead

2. Cathode – PbO2

3. Electrolyte - dil. H2SO4.

4. Insulator- rubber or glass fiber. 5. Cell representation- Pb /PbSO4 // H2SO4(aq) // / PbSO4/ PbO2

6. Anode reaction - Pb+SO42-

→ PbSO4 + 2e-

E0

anode 0.36V

7. Cathode reaction - PbO2 + 4H+

+ SO42-

+ 2e-

→ PbSO4+2H2O E0

cathode 1.69

8. The net reaction is

Pb( s ) 2H2 SO4( aq) PbO2( s ) 2PbSO4 2H2O E0

cell 2.05V

Uses:

1. It is used to supply current mainly in automobiles such as cars, Buses, trucks,etc

2. It is also used in gas engine ignition, telephone exchanges, hospitals, power stations.

(ii) Write a brief note on alkaline battery.

ALKALINE BATTERIES

1. Anode – Zinc powder

2. Cathode – Manganese dioxide

3. Electrolyte – KOH

4. Cell representation- Zn (s) /KOH (aq)/MnO2(s)

5. Anode – reaction eOHZnOHZn saqs 2)(2 )(2)()(

6. Cathode reaction )()(32)(2)(2 222 aqsls OHOMneOHMnO

7. Te net reaction is )(2)(32)(2)(2)( )(2 sslss OHZnOMnOHMnOZn

8. The cell develops an emf of 1 volt to 1.5 volt

9. 9. ADVANTAGES:

10. It can deliver higher current without severe voltage drop.

4. (i)Write a note on Ni-Cd battery?

(i) Ni-Cd Battery


1. Anode– Cadmium

2. Cathode– A metalgrid containing apaste ofNiO2actingas a cathode.

3. Electrolyte – KOH 4. Cellrepresentation-Cd(s)/ Cd (OH)2// KOH// Ni (OH)2/ NiO2(s)

5. Anode reaction- Cd(s)+2OH-

→ Cd (OH)2+2e-

6. Cathode reaction- NiO2+2H2O(l)+2e-

→ 2OH-

+Ni (OH)2 + energy

7. Thenet reaction(Discharging) is Cd(s) +NiO2 + 2H2O(l) → Cd(OH)2 +Ni(OH)2+energy

8. Recharging: Cd(OH)2 +Ni(OH)2+energy → Cd(s) +NiO2 + 2H2O(l)

9. Diagram:

Advantages:

oIt is lighter and smaller.

oIt has longer lifethan lead storagecell.

oLike a drycell, itcan bepacked in a sealedcontainer.

Disadvantages:

Uses:

It is moreexpensive than lead storagebattery.

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

(ii)LITHIUM BATTERY

Lithium battery is a solid state battery because instead of liquid or paste electrolyte, solid electrolyte is used.

1. Anode – Lithium 2. Cathode – TiS2

3. Electrolyte – Polymer solid electrolyte (permits the passage of ions but not electrons)

4. Cell representation- Li(s)// Polymer// TiS2-

5. Anode reaction - Li(s) → Li+

+ e-

6. Cathode reaction- TiS2 + e-

→ TiS2-

7. The net reaction (Discharging) Li(s) + TiS2 → Li+

+ TiS2-

8. Capacity – 3V per cell

9. Applications- It is used in calculators, transistors, headphones, cordless appliances.

Li Solid TiS2

Other types of secondary lithium batteries

(i) Li/ MnO2 (ii) Li/V2O5 (iii)

Li/MoO2 (iv) Li/Cr3O8


Lithium - Sulphur Battery

1. Anode – Lithium in molten state (Electron donor)

2. Cathode – Sulphur in molten state (Electron acceptor)

3. Electrolyte – Solid β alumina (NaAl11O17)

4. Cell representation- Li(s)// Solid β alumina // S

5. Anode reaction - 2Li(s) → 2 Li+

+2 e-

6. Cathode reaction- S + 2e-

→ S2-

7. The net reaction (Discharging) 2Li(s) + S → 2Li+

+ S2-

8. Recharging: 2Li+

+ S2-

→ 2Li(s) + S

9. S2-

ions polymerized to give poly sulphide ions.

S2-

+ n S (Sn+1)2-

β – Alumina prevents direct contact between Li and S.

10. Capacity – 3.7V per cell

11. Applications- It is used in power tools and electric vehicles.

5. (i) Write a brief note on hydrogen-oxygen fuel cell. (Jan-2013)


6. (i) Give an account of wind energy.

WIND ENERGY

Energy recovered from the force of the wind is called wind energy. o

The wind energy is harnessed by making use of wind mills.

WIND MILLS:

The strike of blowing wind on the blades of the wind mill makes it rotating continuously. The rotational

motion of the blade drives a number of machines like water pump, flour mills and electric generators.

Nowadays windmill uses large sized propeller blades and connected to a generator through a shaft. Wind

mills are capable of generating about 100kW electricity.

WIND FARMS:

When a large number of wind mills are installed and joined together in a definite pattern it forms a wind

farm. The wind farms produce a large amount of electricity.

Condition:

The minimum speed required for satisfactory working of a wind generator is


15Km/hr.

Advantages:

(i) It does not cause any pollution.

(ii) It is very cheap.

(iii) It is renewable.

Disadvantages:

1. Public resists for locating the wind forms in populated areas due to noise generated by the

machines.

2. Wind forms located on the migratory routes of birds will hazards.

(ii) Explain the principle and application of solar cells. (Jan-2013, Jan-2014)

PHOTOGALVANIC CELL

It is the one which converts the solar energy directly into electrical energy. Principle:

The basic principle is based on the photovoltaic effect. When solar rays fall on a two layer of semiconductor

devices, a potential difference between two layer is produced. This potential difference causes flow of electrons and

produces electricity. Construction:

Solar cell consists of a p- type semiconductor (Si doped with B) and n-type semiconductor (Si doped

with P). They are in close contact with each other.

Working:

When solar rays fall on p-type semiconductor, the electrons from the valence band get promoted to the conduction

band and cross the p-n junction into n-type semiconductor. Thereby potential difference is produced which causes flow

of electrons and hence current is generated.

Thus when this p and n layers are connected to an external circuit, electrons flow from n- layer to p-layer and hence

current is generated.


APPLICATIONS OF SOLAR CELLS: 1. Lighting purpose.

2. Solar pumps can be run by solar battery.

3. Used in calculators, electronic watches, radios and TV.

4. Used to drive vehicles.

5. Used in space craft and satellites

Advantages:

Solar cells are nonpolluting and eco-friendly.

7. Write an account of fission and fusion reactions.

Nuclear fission is defined as the process of splitting of heavier nucleus into two or more smaller nuclei

with simultaneous liberation of large amount of energy.

Example: nKrBanU 1

0

92

36

141

56

1

0

235

92 3

MECHANISM OF NUCLEAR FISSION

When U-235 is bombarded by slow moving neutron, unstable U-236 is formed.

This nucleus disintegrates into two equal nuclei with the release of huge amount of energy and few neutrons.

nRbCs

nSrXe

nKrBa

UnU1

0

90

37

144

55

1

0

90

38

144

54

1

0

93

36

140

56

236

92

1

0

235

92

2

2

3

CHARACTERISTICS OF NUCLEAR FISSION REACTION

1. Heavy nucleus splits into two or more nuclei.

2. Two or more neutrons are produced by fission of each nucleus.

3. Large quantity of energy is produced during the nuclear fission reaction..

4. All the fission fragments are radioactive in nature, giving off gamma radiations

5. The atomic weights of nuclear fission product ranges from 70 to 160.


6. All the fission reactions are self propagating chain reaction because one of the fission products is neutron.

7. The nuclear reactions can be controlled by absorbing the neutrons using Cd, Boron.

8. Every secondary neutron released in the fission reaction does not strike the nucleus. Some escape into air. Hence a

chain reaction cannot be maintained.

9. The number of neutrons resulting from a single fission is known as multiplication factor. When it is less than 1,

nuclear chain reaction does not take place.

NUCLEAR FUSION

The process of combination of lighter nuclei to form heavier nuclei, with simultaneous liberation of huge

amount of energy is called as nuclear energy. Example: fusion reaction in sun

energyHeHH 4

2

2

1

2

1

CHARACTERISTICS OF NUCLEAR FUSION REACTION

1. It is the combination of lighter nuclei.

2. It does not emit radioactive rays.

3. It takes place at very high temperature

4. (106

K)

5. The mass number and atomic number of

the

6. fission product is higher than the

starting elements.

7. It does not give rise to chain reaction

8. Positrons are emitted.

9. It cannot be controlled.

8. Describe with the help of a neat sketch, the construction and working of a pressurized water reactor. What are

the advantages and disadvantages?

a) Pressurised Water Reactor (PWR)

Principle and working

In PWR, water at high pressure is pumped to act as both coolant and moderator. The high-pressure water

is then passed through a heat exchanger where heat is transferred to water to generate steam. Where H2O

does not boil in the core hence need separate steam generator.

Components


Fuel: Uranium dioxide enriched to 3.2% U235

Moderator: Light Water

Coolant: Pressurized light water (141atm)

Temperature:317oC

Construction:

Advantages

In PWR, water is used as coolant and moderator which is cheap

In PWR, small number of control rods is used.

Disadvantages

Capital cost of PWR is high since the reactor and primary loop works under

pressure.

Shielding is needed as the coolant becomes radioactive. 9. Explain with neat sketch of a boiling water reactor.

a) Boiling Water Reactor (BWR)

Principle and working

In BWR, water is allowed to boil directly in the reactor core. The boiling water generates steam, which is drawn

away from the reactor and used to rotate the turbine. Where the H2O boils in the core, so no need for steam

generator.

Components

Fuel: Uranium dioxide enriched to 2.4% U235

Moderator: Light Water

Coolant: Pressurized light water (75atm)

Temperature: 285oC

Construction:


Advantages:

BWR is simple and cheap reactor as it eliminates heat exchanger, pressurizer etc.

BWR operates at low pressure.

Disadvantages:

Shielding of pipes and turbine is needed as the steam entering turbine is radioactive.

BWR has lower power density.

UNIT-IV ENGINEERING MATERIALS

PART A

1. What are abrasives? Give two examples each for Natural and Artificial abrasives. Abrasives are hard substances, used for polishing, shaping, grinding operations. They are characterized by high

melting point, high hardness and chemically inactive.

Example: Natural abrasives – Diamond, Emery

Synthetic abrasives – Silicon carbide, Boron carbide

2. What is Moh’s scale? Name the hardest substance known. Moh’s scale is a scale, in which common abrasives are arranged in the order of increasing hardness. The hardest

material is Diamond and its Moh’s scale is 10.

3. What are called soft abrasives? Abrasives having their hardness 1-4 in Moh’s scale are known as soft abrasives.

Example: Talc,Gypsum

4. What is Corundum? Corundum is crystalline aluminum oxide(Al2O3).Its hardness is 9 in Moh‘s scale. It is used in grinding wheels and

glasses.

5. How is silicon carbide or carborundum prepared? It is prepared by heating a mixture of silica (60%) and carbon (40%) with saw dust and a little salt in an electric

furnace to about temperature 1500oC.

SiO2 + 3C SiC + 2CO↑ ΔH= +478.7KJ

6. How is Boron carbide prepared? It is prepared by heating a mixture of Boron oxide(B2O3) and coke (carbon) in an electric furnace to about 2200oC.

2B2O3 + 7C B4C + 6CO↑

7. Arrange norbide, carborundum, corundum, garnet increasing order of hardness? Garnet <Norbide <Corundum <Carborundum

8. Mention some important applications of abrasives. (i) To clean the surface prior to coating abrasive powders are used. Example: Quartz, Garnet.

(ii) To prepare smooth wood, metal and plastic surfaces, abrasive paper is used. Example: Alumina, silicon carbide.

(iii) To remove the scales from iron surfaces, grinding wheels are used.

9. What is emery? It is finely grained opaque massive mineral dark-grey to black in Colour. It consists of 55-75% crystalline alumina,

20-40%magnetite and 12% other mineral of which the major part is tourmaline.

10. What are refractories?


Materials that can withstand high temperature without Softening or undergoing any deformation in shape are called

refractories. Example: Silicon carbide, Zirconia

11. Mention any three characteristics of good refractories. (i) It should be infusible at high operating temperatures.

(ii) It should be chemically inert towards corrosive action of gases, metallic slags and liquids

produced in the furnaces.

(iii) It should resist the abrading action of flue gases, flames etc.

(iv) It should have high refractoriness

12. What is meant by refractoriness of a refractory? It is the temperature withstanding capacity of a material. It refers the ability of a material to withstand very high

temperature without softening or deformation under particular service conditions.

13. What is meant by pyrometric cone equivalent of a refractory? It is the number which indicates the softening temperature of a particular refractory specimen

with standard dimensions(38mm height, and 19mm triangular base) and composition. Silica bricks -PCE number 32,

Alumina bricks - PCE number 37

14. What is thermal spalling? How is it minimized? It is the property of breaking or cracking or peeling off a refractory material under high temperature, especially when

there is a sudden change in temperature. Thermal spalling is due to rapid change in temperature and also due to slag

penetration.

It can be minimized by two ways.

i) By avoiding sudden changes in temperature.

ii) By using high porosity, low coefficient of expansion and good thermal conductivity refractory.

15. What are the important properties of high alumina bricks? Alumina bricks possess very low coefficient of expansion, high porosity, high temperature

load- bearing capacity

16. What is meant by dimensional stability? (June 2014) It is defined as the stability of a refractory in its dimension when it is heated to high temperature over a prolonged

time. The dimensional stability of a material should be high. It may be reversible or irreversible.

17. Define porosity of a refractory. Porosity of a refractory material is given by the ratio of its pores volume to that of its bulk volume.

Porosity (P) = W-D X 100

W-A

Where W= Weight of saturated specimen (with water) in air.

D= Weight of dry specimen

A= Weight of saturated specimen (with water) in water

18. What are the raw materials used for the manufacture of cement. (i). Calcareous materials, CaO (limestone)

(ii). Argillaceous materials, Al2O3 and SiO2 (clay)

(iii). Powdered coal or fuel oil

(iv). Gypsum (CaSO4. 2H2O)

19. Give the appropriate composition of Portland cement. Dicalcium silicate = 25%

Tricalcium silicate = 45%

Tricalcium aluminate = 10%

Tetracalcium alumino ferrite = 9%

Rest = CaO + MgO + CaSO4

20. What is meant by flash set? When the cement is mixed with water, at first, hydration of tricalcium aluminate (C3A) takes place rapidly (within 1

day), and the paste becomes quite rigid within a short time which is known as initial set or flash set.

21. Why should we do not paint the freshly plastered cement surface? Because it blocks the pores or capillaries present on the surface and prevents hydration of C3S and C2S which are

responsible for development of compressive strength of cement.

22. Define the term “setting” and “hardening” of cement. Setting: It is defined as the stiffening of the original plastic mass, due to the formation

of tobermonite gel.

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


23. What is the difference between setting and hardening of lime and cement? Setting and hardening of lime involves dehydration and carbonation reaction.

Ca(OH)2 + CO2 CaCO3 + H2OCaO + CO2 CaCO3

Setting and hardening of cement is mainly due to hydration and hydrolysis reaction of

Bogue compounds with water.

24. What are components of a water proof cement? (June 2014) Water proof cement consists of portland cement clinker with an additive like sodium oleate, Calcium stearate,

aluminium stearate etc. (0.1 – 0.5% by weight)

25. Define the glass? Glass is an amorphous, hard, brittle and transparent or translucent, super-cooled liquid, obtained by fusing a mixture

of a number of metallic silicates. Most commonly silicates of Na, Ca and Pb are used. It possesses no sharp melting-

point, crystalline structure and definite formal.

26. What are general properties of glasses? (i). It is amorphous. (ii). It has no definite melting-point.

(iii). It is very brittle. (iv). It softens on heating.

(v). It is affected by alkalis.

27. Give the composition, properties and uses of hard glass. Composition: The approximate composition is K2O.CaO.6SiO2

Properties: (i).Potash-lime possess high melting point, so it will not fuse easily.

(ii).It is less acted upon by acids, alkali and other solvents than ordinary glasses.

Uses: These glasses are used for manufacturing combustion tubes, chemical apparatus, etc.

28. What are the important uses of Glass wool? (i). It is used for electrical and sound insulation.

(ii). It is used in filtration of corrosive liquids like acids.

(iii). It is also used for manufacturing fibre-glass, by blending with plastic resins.

PART B 1. What are soft abrasives? Give examples. Describe the classification of abrasives with suitable

examples. Explain Moh’s scale of hardness. (June 2014)

Hardness:

It is the ability of an abrasive to grind or scratch away other materials. The harder the abrasive quicker will

be its abrading action. Hardness of the abrasive is measured on Moh’s scale or Vicker’s scale.

Moh’s scale is a scale, in which common abrasive (natural or artificial) are arranged in the order of their

increasing hardness.

Soft abrasives:

Abrasives having their hardness 1-4 in Moh’s scale are known as soft abrasives.

Example: Talc, Gypsum, Calcite, Fluorite

Classification of abrasives:

Abrasives are classified into two types.

1. Natural abrasives

a) Non – siliceous abrasives:

Examples: Diamond, Corundum, Emery

b) Siliceous abrasives:

Examples: Quartz, Garnets

2. Artificial abrasives:

Examples: Carborundum, Norbide, Alundum

2. Write a note on synthetic abrasives.

Synthetic abrasives

1. Carborundum or Silicon carbide

Manufacture:

Silicon carbide is manufactured by heating sand(60%) and coke(40%) with some sawdust and a little salt

in an electric furnance at about 15000C.

Saw dust increases the porosity.


Salt reacts with iron and other similar impurities, present in the raw materials, forming volatile chlorides.

This also increases the porosity.

SiO2 + 3C SiC + 2C

The silicon carbide, removed from the furnance, is then mixed with bonding agent (Clay) and then shaped,

dried and fired.

Properties:

Its Moh’s scale value is 9.

It can withstand up to 16500 C.

It is hard and brittle.

It has thermal conductivity between metals and ceramics.

It high mechanical strength.

Uses:

a. Silicon carbides are used as heating elements in furnaces in the form of rods and bars.

b. They are used in kilns, coke ovens, muffle furnaces and floors of heat treatment furnaces.

c. SiC bonded with tar is used for making high conductivity crucibles.

2. Nor boride or boron carbide (B4C)

Manufacture:

It is prepared by heating a mixture of boron oxide and coke in an electric furnance at about 27000C.

Properties:

Its hardness is 10 in Moh’s scale.

It is light weight and black coloured compound.

It is highly resist to chemical attack and erosion.

It resists oxidation much better than diamond.

Uses:

It is used for cutting and sharpening hard high speed tools.

It is used to prepare scratch and resistant coatings.

3.Alundum (Al2O3)

Manufacture: It is prepared by heating a mixture of calcined bauxite, coke and iron in an electric furnace to

about 4000 °C

Bauxite+3O2→2Al2O3

Properties

1. It is an artificial corundum and is not as hard as carborundum but is less brittle and tougher.

2. It is stable at high temperature.

3. Its hardness is 9 in moh’s scale.

4. It is resistant to acids.

Uses

1. It is used in grinding of hard steels and other materials of high tensile strengths.

2. It is also used in the manufacture of abrasives wheels.

3. Discuss in detail about properties of refractories.

Refractoriness:

1. The ability to withstand very high temperature without undergoing any deformation in shape is called

refractoriness.

2. It is necessary that a material , to be used as refractory, should have a softening temperature much higher

than the operational temperature of the furnace.

3. It is determined by Pyrometric Cone Equivalent (PCE) test.


4. Pyrometric or Seger cones are made up of definite composition and definite dimension. (19 mm base and

38mm height- pyramid shaped).

5. A series of Seger cones are placed in an electric furnace along with our specimen with the same

dimension. The electric plate is heated through 10oC increment. The temperature at which the apex of the

specimen touches the bottom of the table is called refractory temperature. The corresponding Seger cone

number is taken as PCE value. Greater the PCE value, greater will be the refractory temperature. PCE values

ranges from 1 to 40.

6. Seger number 1 corresponds to 1110oC , 2 corresponds to 1120oC and so on.

cone38 ,

cone37 ,

cone 36

7. Objectives of PCE test:

a) Determination of the softening temperature of a refractory material.

b) Classification and testing the purity of refractories.

c) Checking whether a given refractory material can be used at the particular servicing temperature.

Refractoriness under load (RUL):

1. It is essential that refractory materials must also possess high mechanical strengths, even at operating

temperatures, to bear the maximum possible load, without breaking.

2. The ability to withstand very high temperature without any deformation under high pressure and load is

called RUL. It is determined by RUL test.

3. Our test specimen is made up into dimensions of 5 cm2 base and 75cm height.A constant pressure of 1.75

Kg / cm2 is applied on the specimen and the temperature is raised through 10oC .

4. The record of the height of the specimen versus temperature is made by a plot.

5. The temperature at which 10% deformation occurs is known as RUL temperature. RUL temperature is

always less than refractoriness

Porosity:

Porosity is the ratio between pore volume to the bulk volume.

P = Pore volume

Bulk volume

P % = W – D X 100

W – A

Where, W = Weight of saturated specimen

D = Weight of dry specimen

A = Weight of specimen when submerged in water

Depending on the applications, the refractories may have high porosity or low porosity.

Disadvantages of high porosity:

1. It reduces the strength,

No Name PCE value Temperature

1 Silica 32 1710oC

2 Alumina 36 1800oC

3 Magnesite 38 1850oC


2. It causes corrosion,

3. It causes abrasion.

Advantages of high porosity:

1. The air in the pores act as an insulator. So, it reduces the heat loss.

2. Due to high porosity, expansion and shrinkage is maintained in equilibrium. So, it reduces thermal

spalling.

4. Classify refractories and give one example for each type.

Classification of refractories:

A) Based on Chemical Composition

B) Based on Thermal property

No TYPE PCE value Refractoriness

Temp (oC)

Examples

TYPE &

EXAMPLE

RAW

MATERIA

L

BINDER

FIRIN

G

TEMP

PROPERTIES USES

Acidic

i) Alumina

ii) silica

Al2O3 Clay

1700oC 1.Affected by bases

2. Not affected by H20, CO2

3. porosity is 8.3%

4.Low coefficient of

expansion

5. great resistance to slags

6. low spalling

7.Refractriness 1500oC

8. RUL 1350oC

1.Cement industries

2.Aluminium

industries

3. Brass production

4. Lead kilns

Basic

i)Magnesiteii)

Dolomite

CalcinedMg

O

Fe2O3

1500oC 1.Affected by acids

2.25% porosity

3. Undergoes spalling

4. Refractoriness 2000oC

5. RUL 1500oC

6. Poor abrasion resistance

7. Very little shrinkage

1. Steel industries

2.Copper

industries

3.Bessemer

Converter

4.Antimony

convertors

5.Refining

furnaces of gold,

silver and platinum

Neutral

i)Zirconia

ii)Graphit

e

ZrO2 Colloidal

zirco

nia +

MgO

stabil

izer

1770oC 1.Low thermal expansion

2.Affected by H20, CO2

3.Undergoes very low

spalling

4. Refractoriness 2000oC

5. RUL 1900oC

6. Costly

7.Resistant to thermal

shocks

1. High voltage

electric

furnaces.


1 Low heat duty refractories 19 - 28 1520 – 1630 Impure silica

2 Intermediate heat duty ref. 28 – 30 1630 – 1670 Fireclay

3 High heat duty refractories 30 – 33 1670 – 1730 Chromite

4 Super heat duty refractories Above 33 Above 1730 Magnesite

5. Explain the manufacture, properties and uses of alumina and magnesite bricks. (June 2014)

1. Aluminous refractories:

It contains higher percentage of mineral Kaolinite(Al2O3.SiO2.2H2O).

Grinding:

The raw materials clay (mixture of silica and alumina) and grog (calcined, granulated fireclay are ground to

small size in a pug mill.

Mixing:

Then the powdered raw materials are mixed with water (Binding material)

Moulding:

Moulding can be done by machine pressing, tamping, slip casting.

Burning:

Burning can be done using kilns. Various types of kilns are used like continuous kilns, tunnel kilns.

Time required is 6-10 days

Temperature 1200-14000 C

Cooling process takes time of 7- 10 days according to the kilns used.

Care should be taken to avoid cracking of the refractory.

Properties:

1. RUL is 13500C under the load of 3.5 Kg/cm2

2. Porosity is 8.3%

3. Spalling tendency can be minimized by using calcined fireclay (grog).

Uses:

Fireclay bricks are used in lime kils, glass furnances and steel industries.

2. Magnesite bricks:

Grinding:

The raw material magnesite is calcined at about 16000 C and then crushed to fine powder in crushers.

Mixing:

Then the powdered raw materials are mixed with iron oxide (Binding material)

Moulding:

Moulding can be done by hydraulic pressers.

Drying:

It should be done very slowly and carefully.

Burning:

Burning can be done using kilns.

Temperature is 15000 C and the time required is 4 weeks.

Properties:

1. RUL is 15000C under the load of 3.5 Kg/cm2

2. Porosity is 25%

Uses:

1. It is used in the furnaces of gold, silver, platinum, steel industries.

2. It is used in the lining the basic converters.

6. What are the raw materials used for the manufacture of Portland cement? Describe the

manufacture of cement by wet process.

a. Raw materials and functions


The raw material required for the manufacture of Portland cement are

1. Calcareous materials CaO (e.g. Limestone)

2. Argellaceous materials Al2O3 and SiO2 (e.g. Clay)

3. Powdered coal or fuel oil

4. Gypsum (CaSO4. 2H2O)

Functions of the ingredients in cement

1. Lime

Lime is the principle constituent of cement. Its proportion must be properly regulated. Excess of like reduces

the strength of cement, because it makes the cement to expand and disintegrate. Lesser the amount of lime

than required also reduces the strength of cement, because of quick-setting.

2. Silica

It imparts strength to cement

3. Alumina

It is responsible for the setting action of cement. Excess of alumina makes the cement quick-setting

4. Iron oxide

It provides colour, strength and hardness to the cement.

5. Sulphur trioxide

Presence of small amount of SO3 imparts soundness to cement. (The cement is said to be sound, if it resists

the change in volume of already set concrete. The unsoundness of cement causes the disintegration of

concrete.) Excess of SO3 makes the cement unsoundness.

6. Gypsum

It acts as a retarding agent for quick setting of cement.

Manufacture of Portland cement

Manufacture of Portland cement involves the following four major operations.

1. Mixing of raw materials

2. Burning

3. Grinding

4. Storage and packing

1. Mixing of raw materials – Wet process

The calcareous material (limestonea0 is crushed, powdered and stored in a storage tank (called silos). The

argillaceous material (clay) is thoroughly washed with water to remove any adhering organic matter and

stored in a basin. Then the powdered limestone (from silos) and wet-clay (from basin) are led to “grinding

mills”, where they are mixed to form a paste called “Slurry”. The slurry is led to a “Correcting basin” where

its chemical composition may be adjusted. Thus the slurry containing about 38 to40% water is stored in

storage tank

2. Burning

Burning of “slurry” is carried out in rotary kiln. The rotary kiln is a long horizontal steel cylinder (2.5.3.0 m

dia and 90 to 120 m length) lined inside with refractory bricks, which rotates at a speed of 1 r.p.m. The kiln

is set in slightly inclined position of about 5-6 to allow the material to travel slowly from one end to the

another end.


Process

The “corrected slurry” is fed into the kiln from the upper end, while the hot flame is forced into the kiln from

the lower end. Due to slope and slow rotation, the material gradually descends in the kiln into different zones

of increasing temperatures.

i) Drying Zone

The upper part of the kiln is known as drying zone, where the temperature is about 400c. In this zone, most

of the water in the slurry gets evaporated.

ii) Calcination zone

The central part of the kiln is known as calcinations zone where the temperature is about 1000C. In this zone,

limestone gets decomposed into CaO and CO2

CaCO3 → CaO + CO2↑

Limestone Quick lime

iii) Clinkering zone

The lowest part of the kiln is known as clinkering zone, where the temperature is about 1350-1500C. In this

zone lime reacts with clay (containing Al2O3.SiO2.Fe2O3) to form various Bogue compounds (C2S.C3S,C3A,

C4AF)

2CaO + SiO2 → 2CaO.SiO2 (C2S)

3CaO + SiO2 → 3CaO.SiO2 ( C3S)

3CaO + Al2O3 → 3CaO. Al2O3 (C3A)

4CaO + Al2O3 + Fe2O3 → 4 CaO. Al2O3. Fe2O3 (C4AF)

These Bogue compounds fuse together to form small, hard, grayish coloured stone like mass called cement

clinkers.

3. Grinding

The hot clinkers are cooled with atmospheric air and then pulverized together with 2-3% gypsum in ball

mills. Gypsum acts as a retarding agent for quick setting of cement.

4. Storage and packing

The cement coming out of the grinding mill is stored in a concrete storage silos. Then the cement is packed

in jute bags by automatic machine.


7. Explain the chemistry involved in setting and hardening of cement . (June 2014)

When the cement is mixed with water, hydration and hydrolysis reactions of Bogue compounds of cement

begin, resulting in the formation of gel and crystalline products. The insoluble gels and crystals have the

ability to surround inert materials like sand, bricks, crushed stones etc

Setting

It is defined as the stiffening of the original plastic mass, due to the formation of tobermonite gel

Hardening

It is defined as the development of strength due to formation of crystals.

Chemical reactions involved in setting and hardening of cement.

i) When the cement is mixed with water, at first, hydration of tricalcium aluminate (C3A) takes place rapidly

(within 1 day) and the paste becomes quite rigid within a short time which is known as initial set or flash set.

3CaO. Al2O3 + 6H2O → 3CaO. Al2O3.6H2O + 880 KJ/Kg

Tricalciumaluminate Hydrated tricalcium aluminate

(crystalline)

(Or)

C3A + 6H2O →C3A.6H2O

In order to retard the rapid hydration of C3A ( early initial setting), gypsum is added during grinding of

cement clinkers. Gypsum reacts with C3A to give insoluble calcium sulpho aluminate complex (3CaO.

Al2O3CaSO4. 2H2O). which does not possess hydrating property and retards early setting of cement.

C3A + 3CaSO4.2H2O → C3A. 3CaSO4. 2H2O

Calcium sulpho aluminate

ii) After the hydration of C3A, C3S begins to hydrate to give tobermonite gel and crystalline Ca(OH)2. This is

responsible for the development of initial strength of cement. The hydration of C3S gets completed

within7days. It does not contribute much to the strength of cement.

2[3CaO. SiO2] + 6H2O → 3CaO.2SiO2.3H2O + 3Ca(OH)2 + 500 KJ/Kg

Tricalcium silicate Tobermonite gel Crystalline

(or)

2C3S + 6H2O → C3S2. 3H2O + 3Ca(OH)2

Note: Tobermonite gel possesses a very high surface area and very high adhesive property.

iii) Dicalcium silicate (C2S) reacts with water very slowly and gets completed in 7 to 28 days.

2(2CaO.SiO2) + 4H2O → 3CaO.SiO2. 3H2O + Ca(OH)2 + 250 KJ/Kg

Dicalcium silicate Tobermonite gel Crystalline

(Or)

2C2S + 4 H2O → C3S2.3H2O + Ca(OH)2

The increase in strength between 7 to 28 days is due to the formation of tobermonite gel and crystalline

Ca(OH)2 of both C2S and C3S.

iv) Though the hydration of tetracalciumalumino ferrite (C4AF) takes place initially, the hardening takes

place finally through crystallization, along with C2S.

4CaO.Al2O3.Fe2O3 + 7H2O → 3CaO.Al2O3.6H2O + CaO.Fe2O3.H2O + 420 KJ/Kg

Tetracalciumalumino ferrite Crystalline Gel

(Or)

C4AF + 7H2O → C3A.6H2O

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

of Ca(OH)2 and hydrated tricalcium aluminate.

Development of compressive strength of the cement, due to hydration and hydrolysis of Bogue compounds,

are shown in the graph.2


8. What happens when water is added to cement? When the cement is mixed with water, hydration and hydrolysis reactions of Bogue compounds of cement

begin, resulting in the formation of gel and crystalline products. The insoluble gels and crystals have the

ability to surround inert materials like sand, bricks, crushed stones etc

Setting

It is defined as the stiffening of the original plastic mass, due to the formation of tobermonite gel

Hardening

It is defined as the development of strength due to formation of crystals.

Chemical reactions involved in setting and hardening of cement.

i) When the cement is mixed with water, at first, hydration of tricalcium aluminate (C3A) takes place rapidly

(within 1 day) and the paste becomes quite rigid within a short time which is known as initial set or flash set.

3CaO. Al2O3 + 6H2O → 3CaO. Al2O3.6H2O + 880 KJ/Kg

Tricalciumaluminate Hydrated tricalcium aluminate

(crystalline)

(Or)

C3A + 6H2O →C3A.6H2O

In order to retard the rapid hydration of C3A ( early initial setting), gypsum is added during grinding of

cement clinkers. Gypsum reacts with C3A to give insoluble calcium sulpho aluminate complex (3CaO.

Al2O3CaSO4. 2H2O). which does not possess hydrating property and retards early setting of cement.

C3A + 3CaSO4.2H2O → C3A. 3CaSO4. 2H2O

Calcium sulpho aluminate

ii) After the hydration of C3A, C3S begins to hydrate to give tobermonite gel and crystalline Ca(OH)2. This is

responsible for the development of initial strength of cement. The hydration of C3S gets completed

within7days. It does not contribute much to the strength of cement.

2[3CaO. SiO2] + 6H2O → 3CaO.2SiO2.3H2O + 3Ca(OH)2 + 500 KJ/Kg

Tricalcium silicate Tobermonite gel Crystalline

(or)

2C3S + 6H2O → C3S2. 3H2O + 3Ca(OH)2

Note: Tobermonite gel possesses a very high surface area and very high adhesive property.

iii) Dicalcium silicate (C2S) reacts with water very slowly and gets completed in 7 to 28 days.

2(2CaO.SiO2) + 4H2O → 3CaO.SiO2. 3H2O + Ca(OH)2 + 250 KJ/Kg

Dicalcium silicate Tobermonite gel Crystalline


(Or)

2C2S + 4 H2O → C3S2.3H2O + Ca(OH)2

The increase in strength between 7 to 28 days is due to the formation of tobermonite gel and crystalline

Ca(OH)2 of both C2S and C3S.

iv) Though the hydration of tetracalciumalumino ferrite (C4AF) takes place initially, the hardening takes

place finally through crystallization, along with C2S.

4CaO.Al2O3.Fe2O3 + 7H2O → 3CaO.Al2O3.6H2O + CaO.Fe2O3.H2O + 420 KJ/Kg

Tetracalciumalumino ferrite Crystalline Gel

(Or)

C4AF + 7H2O → C3A.6H2O

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

of Ca(OH)2 and hydrated tricalcium aluminate.

Development of compressive strength of the cement, due to hydration and hydrolysis of Bogue compounds,

are shown in the graph.2

9. Explain the process of manufacture of glass by pot furnace process (June 2014)

a. Raw materials used in the manufacture of glass

S.No Name of the element Source of the

element

Name of the glass

produced

1 Sodium(Na) Na2CO3,Na2SO4 Soft glass

2 Potassium(K) Potash,K2CO3,KNO3 Hard glass

3 Calcium(Ca) Lime,limestone

4 Barium(Ba) BaCO3

5 Lead Litharge, red lead Flint glass

6 Zinc ZnO Heat and shock

proof glass

7 Borate Borax,Boric acid Heat and shock

proof glass

8 Silica Sand,Quartz

Colour

i)Yellow Ferric salt

ii)Green Ferrous and chromium salt

iii)Blue Cobalt salt


Manufacture of glass:

It involves four steps

Step 1 Melting:

The raw materials in proper proportions (e.gsand,sodaash,and lime stone for common glass) are mixed and

finely powedered. The homogeneous mixture (known as Batch) is fused with some broken glass caleed

“cullet” in the pot (or) tank of the tank furnace, in which heating is done by burning producer gas and air

mixture over the charge.

Air fuel gas flue gases. The cullet melts at a comparatively low temperature and assists in melting the rest of

the charge. During melting of ordinary soda glass, the following series of reactions occur. The reaction is an

acid – base reaction leading to the formation of various silicates.

CaCO3 + SiO2 → CaSiO3 + CO2↑

Na2CO3 + SiO2 → Na2SiO3 + CO2↑

Step 2 Working of molten glass:

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

between rollers.

Step 3 Annealing:

Glass articles are then allowed to cool gradual to room temperature(sudden cooling must be avoided, because

cracking occurs.) The longer the annealing period, the better is the quality of the glass.

Step 4 Finishing:

The glass articles, after annealing period ,are subjected to finishing process such as cleaning, polishing,

cutting, sand – blasting etc.

10. How are the following glasses made? Write their composition and uses

(i) Flint glass (ii) Pyrex glass (iii) Alumino silicate glass

(i) Lead glass or flint glass

Raw Materials

Lead oxide ( instead of calcium oxide) and silica are fused.

For dense optical glasses, as much as 80% of PbO is incorporated. In addition of K2O is used, instead of

sodium oxide.

Composition

The approximate composition is K2O.PbO. 6SiO2

Properties

1.It is bright, lustrous and possesses high specific gravity (3 to 3.3)

2. It is more expensive to manufacture, than the ordinary lime-soda glass.

3. It has a lower softening temperature than soda-glass.

4. It has higher refractive-index and excellent electrical properties.


Uses

1. Lead glasses are used for high quality table wares, neon sign tubings, optical purposes ( like lenses etc),

electrical insulators.

2. High lead content glasses are used for extra-dense optical glasses for windows and shields to protect

personnel from X-rays and gamma-rays in medical and atomic energy fields respectively.

(ii)Boro Silicate glass or pyrex glass or Jena glass.

Raw materials

Silica, boron with a small amount of alumina and some oxides.

Composition

A typical formula for the glass is

SiO2 (80.5%), B2O3(13%), Al2O3(3%), K2O(3%) and Na2O (0.5%)

Properties

1. The substitution of alkali (Na2O) and basic alkaline earth oxides (CaO) of the soda glasses by boron and

aluminium oxides results in a low thermal coefficient of expansion and high chemical resistance.

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

Uses

It is used in industry for pipelines for corrosive liquids, gauge glasses, superior laboratory apparatus, kitchen

wares, television tubes, chemical plants, electrical insulators.

(iii)Alumino Silicate glass

Raw material

It contains 5% or more of alumina. Addition of alumina makes the glass heat resistant.

Composition

The composition of this glass is

SiO2 (55%), Al2O3(23%), B2O3(7%), MgO(9%), CaO(5%), Na2O + K2O(1%)

Properties

They possess exceptionally high softening temperatures.

Uses

They are used in high-pressure mercury discharge tubes, chemical combustion tubes, certain domestic

equipments. Etc

11. List out the general properties of glass.

b. General properties of glasses

1. It is amorphous.

2. It has no definite melting-point.

3. It is very brittle.

4. It softens on heating.

5. It is affected by alkalis.

6. It can absorb, reflect or transmit light.

7. It is a good electrical insulator.

8. It is not affected by air, water, or acids, or chemical reagents, but soluble in HF, which converts its silica

into SiF4

9. Since it has no crystalline structure, no sliipage between planes can occur, It possesses high compressive

strength.

10. It is light, because it has homogeneous internal structure similar to liquids.

12. Explain the preparation, properties and uses of the following glasses.

(i) Crookes glass (ii) Opal glass (iii) Jena glass

(iv) Optical or Crookes glass

Raw Material


Contain phosphorus and lead silicate together with a small amount of cerium oxide. Cerium oxide is capable

of absorbing UV light (Which is injurious to eyes)

Properties

1. Optical glasses have low melting-points and are relatively soft.

2. Chemical-resistance and durability of optical glasses are lower than those of ordinary glasses

Uses

Optical glasses are used for making lenses

filaments of glass. They are completely alkali free.

The glass filaments are obtained by forcing molten glasses through small

(v)Opal glasses

Raw Material

NaF (or) CaF2 (or) Ca3 (PO4)2 (or) SnO2

Properties

They are translucent white or milky glasses. They are transparent, when they are in liquid, but becomes

opalescent, when they are cooled, because of inclusions

(ii)Boro Silicate glass or pyrex glass or Jena glass.

Raw materials

Silica, boron with a small amount of alumina and some oxides.

Composition

A typical formula for the glass is

SiO2 (80.5%), B2O3(13%), Al2O3(3%), K2O(3%) and Na2O (0.5%)

Properties

1. The substitution of alkali (Na2O) and basic alkaline earth oxides (CaO) of the soda glasses by boron and

aluminium oxides results in a low thermal coefficient of expansion and high chemical resistance.

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

Uses

It is used in industry for pipelines for corrosive liquids, gauge glasses, superior laboratory apparatus, kitchen

wares, television tubes, chemical plants, electrical insulators.

UNIT – V FUELS AND COMBUSTION

PART A

1. Define calorific value.

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

2. What is gross calorific value (GCV)?

Gross calorific value is the total heat generated when a unit quantity of fuel is completely burnt and the products of

combustion are cooled to room temperature.

3. What is Net calorific value (NCV)?

The net heat produced when a unit quantity of fuel is completely burnt and the products of combustion are allowed to

escape.

NCV = GCV – Latent heat of condensation of steam produced.

4. Name the various determinations of proximate analysis.

It involves the determination of percentage of

(i) Moisture content

(ii) Volatile matter

(iii) Ash content

(iv) Fixed carbon in coal.

5. What is the significance of volatile matter in coal? High percentage of volatile matter is undesirable because

i. It reduces the calorific value of coal

ii. It has low ignition temperature.

iii. It burns with a long yellow smokes flame.


6. Distinguish between proximate and ultimate analysis.

7. Why coke is superior as a metallurgical fuel? (JUNE-2013)

(i) Coke is stronger and more porous than coal.

(ii) By coking, much of undesirable sulphur is removed

(iii) Coke does not contain much volatile matter than coal.

(iv) Coke burns with a small flame and without smoke.

8. Define octane number of a petrol? How can it be improved? The octane number is defined as the percentage of iso-octane present in a mixture of iso-octane and n-heptane.

Improving the octane number of a fuel:

(i) The addition antiknock compounds like TEL

(ii) Low octane petrol is blended with high octane compounds like alcohol (straight – run petrol is mixed with

reformed petrol, benzol and alcohol).

9. What is meant by carbonization of coal? When coal is heated strongly in the absence of air, it is converted into a substance of lustrous, dense, porous, coherent

mass called coke. The process of preparing coke from coal is known as carbonization of coal.

10. What are Caking coals and coking coals?

When coals are heated strongly, the mass becomes soft, plastic and fuses to give a coherent mass. Such types of

coals are called Caking Coals. But if the mass so produced is hard, porous and strong then the coals are called

Coking Coals. 11. What is meant by hydrogenation of coal? If coal is heated with hydrogen under high pressure, it is converted into gasoline. This method of preparing liquid fuels

from solid coal is called hydrogenation of coal.

12. What is knocking? (JUN-2013, DEC-2013) Knocking is a kind of explosion occurs in IC engines due to sudden increase of pressure developed by spontaneous

combustion of fuel and air mixture.

13. Explain the chemical structure and knocking. The knocking tendency of fuel hydrocarbons mainly depends on their chemical structures. The knocking tendency

decreases in the following order.

Straight chain paraffins>Branched chain Paraffins>Cycloparaffins>Olefins > Aromatics. Thus olefins of the same

carbon chain length possess better anti knock properties than the corresponding paraffins.

14. Why should leaded petrol not to be used? (i) Lead deposits on the spark plug and on cylinder walls, which is harmful to engine life.

(ii) This creates atmospheric pollution.

15. Define cetane number/ How are diesel oil rated? How are they improved?/ How can the cetane number

of a fuel be improved? The cetane number is defined as "the percentage of hexa decane present in a mixture of hexa decane and α-methyl

naphthalene, which has the same ignition lag as the fuel under test".

Cetane number improvement : The cetane number of a diesel oil can be increased by adding additives called

dopes. Important dopes: Ethyl nitrate, Iso-amyl nitrate. 16. What is the action of TEL in internal combustion engines?

S.No Proximate analysis Ultimate analysis

1

It involves the determination of

physical constituents like moisture,

volatile, ash and fixed carbon

contents in coal

It involves the determination of chemical

constituents like carbon, hydrogen, nitrogen

and sulphur and oxygen contents in coal

2 It gives the approximate composition

of the main constituents of coal

It gives the exact composition of the

elementary constituents of coal.


Tetra ethyl lead is converted into finely divided lead oxide particle in the cylinder and these particles react with

any hydrocarbon peroxide molecules formed, thereby slowing down the chain oxidation reaction and thus

decreasing the chances of early detonation.

17. List out the advantages of gaseous fuels over solid and liquid fuels. (i) Gaseous fuels can flow through pipes and hence can be easily transported to the place of need without any manual

labour.

(ii) It can be lighted at a moment’s notice.

(iii) It burn with high efficiency and a high temperature flame is obtained in no time.

(iv) It does not produce any smoke and ash and it burns freely in the presence of air.

18. What is the drawback of presence of sulphur in the coal? (June 2009)

(i) The harmful gases SO2 and SO3 will create air pollution.

(ii) Sulphur containing coal is not suitable for the manufacture of metallurgical coke.

19. How is water gas superior to producer gas? (Dec 2009)

(i) It’s calorific value is higher than producer gas

(ii) It has very less amount of N2 than producer gas

(iii) It is used for the manufacturing of power alcohol.

20. Distinguish between coal and coke. (June 2010)

S.No Coal Coke

1. It possesses lower strength and

porosity

It possesses higher strength and

porosity

2. It is a natural fuel It is a secondary fuel

21. What are the advantages of CNG? (June2010)

(i) CNG is a much safer fuel, since it ignites at a higher temperature than gasoline and diesel.

(ii) Combustion of CNG leads to lesser Carbon mono oxide emissions than gasoline.

(iii) CNG mixes with air than liquid fuels.

22. Mention any two disadvantages of LPG over other gaseous fuels. (June 2011)

(i) Needs little care for maintenance

(ii) Flexibility and easy control

(iii) Easy to manipulate

(iv) Comparatively, less health hazard, even in case of leakage.

23. Write the characteristics of a good fuel. (July 2010)

(i)It should have a high calorific value

(ii) It should be cheap and readily available

(iii) It should undergo spontaneous combustion.

24. What are the requisites of good coke for metallurgy? (July 2010)

(i)It should have very low moisture, ash and Sulphur content.

(ii) It should be cheap and readily available.

(iii) It should have very high calorific value.

25. Write down the composition of producer gas. (May 2011, June-2013)

Producer gas is mixture of carbon monoxide, hydrogen and nitrogen.

The average composition of producer gas is :

CO = 30%

H2 = 12%

N2 = 55% , CO2 = 3%

26. What do you mean by ignition temperature?

It is defined as the lowest temperature to which the fuel must be heated, so that it starts burning smoothly. In the case of

liquid fuels, the ignition temperature is called flash point.

27. What is bio-diesel?

Vegetable oils comprise of 90-95% triglycerides with small amount of diglycerides, free fatty acids, phospholipids etc. are

known as bio-diesel. The viscosities of vegetable oils are higher and their molecular weights are in the range of 600-900

which are about three times higher than those of the diesel fuels.

28. What do you mean by power alcohol?


When ethyl alcohol is blended with petrol at concentration of 5-10%, it is called power alcohol. In other words absolute

alcohol (100% ethyl alcohol) is also called as power alcohol. Addition of ethyl alcohol to petrol increases its octane

number.

29. What are fuels? Give one example each for primary and secondary solid, liquid and gaseous fuels.

A fuel is a substance, which by combustion gives large amount of heat that can be utilized economically for industrial and

domestic purposes.

S.No Examples Solid fuels Liquid fuels Gaseous fuels

1 Primary

fuel(Natural)

Wood, Peat,

lignite

Petroleum or

crude oil

Natural gas

2 Secondary

fuel(Manufactured)

Semicoke, Coke Gasoline, Diesel Coal gas,

producer gas,

water gas

30. Give the approximate composition of water gas.

It is a mixture of CO and H2 with little amount of non combustible gases such as CO2and N2.It is also known as blue gas as

it burns with non luminous blue flame. Its calorific value is 2800kcals/m3

31.Mention the advantages and disadvantages of power alcohol.

Advantages:

Blending increases the octane number of the fuel.

Blend petrol shows less starting problems as compared to pure petrol.

It shows better anti knock properties.

Disadvantages:

Blended petrol has lower CV than pure petrol.

Alcohol is easily oxidized to organic acids, which can cause corrosion.

32. What are the advantages of biodiesel ?

Pure biodiesel is non toxic.

It is safe to handle because it is biodegradeable.

It has high flash point (148 ºC)

It has high cetane number which improves the ignition quality.

33. Define the term combustion.

Combustion is a process of burning of any combustible elements or compounds in presence of sufficient amount of oxygen

(air) to liberate energy in the form heat and light.

34. What is meant by explosive range of a fuel?

For burning a gaseous fuel,a particular range (minimum and maximum) of concentration of fuel is required in gas air

mixture.The concentration range of gas – air is called the explosive range or limits of inflammability.

35. How is flue gas analyzed? The mixture of gases (like CO2, O2, CO, etc) coming out from the combustion chamber is called flue gases. The

analysis of a flue gas would give an idea about the complete or incomplete combustion process. The analysis of flue

gases is carried out by using orsats apparatus.

36. What is the significance of flue gas analysis? (Nov 2011)

1. Flue gas analysis gives an idea about the complete or incomplete combustion process.

2. If the flue gases contain considerable amount of CO, it indicates that incomplete combustion is

occuring and it also indicates that the short supply of O2.

3. If the flue gases contain considerable amount of O2, it indicates that complete combustion is

occuring and also it indicates that the excess of O2 is supplied.

37. Write the expression for the amount of air required for combustion of 1Kg fuel. (Nov.2010), (May 2011)

For volume: 21 m3 of oxygen is supplied from 100 m3 of air

For amount: 23 kg of oxygen is supplied from 100 kg of air

Net oxygen requirement = Total oxygen required – available oxygen

38. Name the reagents used for absorbing CO2,CO and O2 during flue Gas analysis by Orsat’s apparatus.

Reagents Absorbents

CO2 KOH solution

CO Ammoniacal cuprous chloride solution

O2 Alkaline pyrogallol


39. Write down the Dulong’s formula to determine calorific value. (Nov 2005)

Dulong‟s formula for the theoretical calculation of calorific value is GCV (or) HCV

where C,H,O and S represent the % of the corresponding elements in the fuel 40. What volume of air will be needed to both 42 liters of carbon monoxide measured at the same temp and

pressure?

CO + ½ O2 → CO2

1 vol. 0.5 vol.

1 liter of CO = 0.5 liter of O2

42 liter of CO = 0.5 x 42

= 21 liter of O2

21 liter of O2 = 100 liter of air is needed

21 liter of O2 = (100/21)*21

= 100 lit. of air is required.

41. What is the amount of oxygen required for the complete combustion of 0.096 kg of carbon? (April 2011)

42. Calculate the volume of air (Volume percentage of oxygen present in air equals 21) required for the

complete combustion of two litres of carbon monoxide. (April 1995)

PART - B

1. Explain the proximate analysis of coal. (Dec 2009)

ANALYSIS OF COAL:

In order to assess the quality of coal the following two types of analysis are made.

1.3.1. Proximate Analysis

It involves the determination of percentage of (i) Moisture content (ii) Volatile matter (iii) Ash

content (iv) Fixed carbon in coal.

1. Moisture Content

About 1 gm of powdered coal sample is taken in a crucible, and is heated at 100 − 105°C in an

electric hot-air oven for 1 hour. The loss in weight of the sample is foundout and the % of moisture is


calculated as

% of moisture in coal = loss in weight of the coal X 100 Weight of air – dried coal

2. Volatile matter

After the analysis of moisture content the crucible with residual coal sample is covered with a lid,

and is heated at 950 ± 20°C for 7 minutes in a muffle furnace. The loss in weight of the sample is found out

and the % of volatile matter is calculated as loss in weight of the coal.

% of volatile matter in coal = loss in weight of the coal x 100

Weight of air – dried coal

3. Ash content

After the analysis of volatile matter, the crucible with residual coal sample is heated without lid at

700 ± 50°C for 1⁄2 an hour in a muffle furnace. The loss in weight of the sample is found out and the % of

ash content is calculated as

% of ash content in coal = Weight of ash formed x 100 Weight of air dried coal

4. Fixed carbon

It is determined by subtracting the sum total of moisture, volatile and ash contents from 100.

% of fixed carbon in coal =100 − % of (moisture content + volatile matter + ash)

Significance (or) Importance of Proximate Analysis

S.N Analysis Significances

1 Moisture

content

High percentage of moisture is undesirable because

(i) it reduces the calorific value of coal,

(ii) most of the supplied energy will be wasted for evaporation

(iii) it increases the transport cost.

(iv)Produces smoke

2 Volatile

matter

High percentage of volatile matter is undesirable because


(iv) Produces sooty and smoky flame

(v) Cocking is not possible

(vi) Toxic gases will be evolved

3 Ash

content

High percentage of ash content is undesirable because


(ii) ash causes clinkers, which disturbing the oxygen supply

(iii) forms flying ash which causes air and land pollution

(iv) it increases the transporting, handling and storage costs,

(v) it involves additional cost in ash disposal.


4 Fixed

carbon

High percentage of fixed carbon is desirable because

(i) higher the percentage of fixed carbon in a coal, greater is its calorific value,

(ii) the percentage of fixed carbon helps in designing the furnace

(iii) (iii) classification of coal is based on carbon content.

2. Explain the ultimate analysis of coal. (May 2012)

Ultimate Analysis

It involves the determination of percentage of (i) carbon and hydrogen contents, (ii) nitrogen content

(iii) sulphur content (iv) ash content (v) oxygen content

1. Carbon and Hydrogen contents

A known amount of the coal sample is burnt in a current of O2 in a combustion apparatus. The

carbon and hydrogen, present in the coal sample, are converted into CO2 and H2O respectively according to

the following equations.

C + O2 −−−> CO2 ↑

H2 + 1/2 O2 −−−> H2O ↑

TThhee lliibbeerraatteedd CCOO22 aanndd HH22OO vvaappoouurrss aarree aabbssoorrbbeedd rreessppeeccttiivveellyy iinn KKOOHH aanndd aannhhyyddrroouuss CCaaCCll22 ttuubbeess ooff

kknnoowwnn wweeiigghhttss.. TThhee iinnccrreeaassee iinn wweeiigghhtt ooff KKOOHH ttuubbee iiss dduuee ttoo tthhee ffoorrmmaattiioonn ooff CCOO22 wwhhiillee iinnccrreeaassee iinn wweeiigghhtt

ooff CCaaCCll22 ttuubbee iiss dduuee ttoo tthhee ffoorrmmaattiioonn ooff HH22OO.. FFrroomm tthhee wweeiigghhttss ooff CCOO22 aanndd HH22OO ffoorrmmeedd,, tthhee %% ooff ccaarrbboonn

aanndd hhyyddrrooggeenn pprreesseenntt iinn tthhee ccooaall ccaann bbee ccaallccuullaatteedd aass ffoolllloowwss..

2. Nitrogen content

The determination of nitrogen content is carried out by Kjeldahl‟s method. A known amount of

powdered coal sample is heated with con. H2SO4 in presence of K2SO4 (catalyst) in a long necked flask

(called Kjeldahl‟s flask). Nitrogen in the coal is converted into ammonium sulphate and a clear solution is

obtained.

2N + 3H2 + H2SO4 −−−> (NH4)2 SO4

The clear solution is then heated with excess of NaOH and the liberated ammonia is distilled over

and is absorbed in a known volume of standard N/10 HCl.

(NH4)2 SO4+ 2NaOH −−−> 2NH3 + Na2SO4+ 2H2O

NH3 + HCl −−−> NH4Cl

The volume of unused N/10 HCl is then determined by titrating it against standard N/10 NaOH. Thus

the amount of acid neutralised by liberated ammonia from coal is determined.


3. Sulphur content

known amount of coal sample is burnt completely in a bomb calorimeter. During this process

sulphur is converted into sulphate, which is extracted with water. The extract is then treated with BaCl2

solution so that sulphates are precipitated as BaSO4. The precipitate is filtered, dried and weighed. From the

weight of BaSO4 obtained, the sulphur present in the coal is calculated as follows.

4. Ash content

Determination of ash content is carried out as in proximate analysis

5. Oxygen content

The percentage of oxygen is calculated as follows. % of oxygen in coal = 100 − % of (C + H + N + S + ash)

Significance (or) Importance of Ultimate Analysis

S.N. Analysis Sign

ifica

nces 1 Carbon and

hydroge

n

content

s

(i)Higher the % of carbon and hydrogen, better is the quality of coal and higher is its calorific value.

(ii) The % of carbon is helpful in the classification of coal (iii) Higher % of carbon in coal reduces the size of combustion chamber

required.

2 Nitrogen

content

(i) Nitrogen does not have any calorific value, and its presence in coal is undesirable. (ii) Good quality coal should have very little nitrogen content.

3 Sulph

ur

content

its presence in coal is undesirable because

(i) The combustion products of sulphur, i.e., SO2 and SO3 are toxic

and have corrosion effects on equipments.

(ii) The coal containing sulphur is not suitable for the preparation of

metallurgical coke as it affects the properties of the metal.

3 Ash content High percentage of ash content is undesirable because (i) it reduces the calorific value of coal,

(ii) ash causes clinkers, which disturbing the oxygen supply

(iii) forms flying ash which causes air and land pollution

(iv) it increases the transporting, handling and

storage costs, (v) it involves additional cost in ash

disposal. 4 Oxygen

content

(i)Lower the % of oxygen higher is its calorific value. (ii) As the oxygen content increases its moisture holding capacity

increases, and the calorific value of the fuel is reduced.


3. Describe the Otto – Hoffman of coke manufacture and the recovery of various by product.

(May 2012)

MANUFACTURE OF METALLURGICAL COKE (Otto-Hoffman’s by-product oven)

There are s o many t y p e s of ovens used for the manufacture of metallurgical coke.

But the important one is Otto-Hoffman‟s by product oven.

Objectives & Advantages

(i) increase the thermal efficiency of the carbonisation process and, (ii) recover the valuable by products (like coal gas, ammonia, benzol oil, etc.)

(iii) Heating is done externally by producer gas hence we can save fossil fuel

(iv) The carbonisation time is less.

1. The oven consists of a number of silica chambers. Each chamber is about 10 − 12 m long, 3 − 4 m

height and 0.4 − 0.45 m wide. Each chamber is provided with a charging hole at the top, it is also provided

with a gas off take valve and iron door at each end for discharging coke.

2.Coal is introduced into the silica chamber and the chambers are closed. The chambers are heated to

1200°C by burning the preheated air and the producer gas mixture in the interspaces between the chambers.

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

and 3rd

hot regenerators. Hot flue gases

produced during carbonisation are allowed to pass through 1st

and 4th

regenerators until the temperature has

been raised to 1000°C. While 1st

and 4th

regenerators are heated by hot flue gases, the 2nd

and 3rd

regenerators are used for heating the incoming air and gas mixture.

4.For economical heating, the direction of inlet gases and flue gases are changed frequently. The above system of recycling the flue gases to produce heat energy is known as the regenerative system of heat

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

5.Time taken for complete carbonisation is about 12-20 hours. The yield of coke is about 70%.The

valuable by products like coal gas, tar, ammonia, H2S and benzol, etc. can be recovered from flue gas.

Recovery of by – products

S.N. By - products Recover

ed by 1 Tar By spraying Liquid Ammonia to dissolve tar. NH3 is again recovered by the

heating the solution. 2 Ammonia By spraying water. Here ammonia gets converted to NH4OH. 3 Naphthalene By spraying cooled water, naphthalene gets condensed.

4 Benzene By spraying petroleum, benzene gets condensed to liquid.


5 Hydrogen

Sulphide

The remaining gases are then passed through a purifier packed with moist Fe2O3. Here H2S is retained.

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

4. Describe the manufacture of gasoline by Bergius method. (Dec 2009, Jun 2012Hydrogenation of coal (or)

Synthetic petrol

Coal contains about 4.5% hydrogen compared to about 18% in petroleum. So coal is hydrogen deficient compound. If coal is heated with hydrogen to high temperature under high pressure, it is converted to

gasoline. The preparation of liquid fuels from solid coal is called Hydrogenation of coal (or) synthetic

petrol.

There are two methods available for the hydrogenation of coal

1. Bergius process (or direct method). 2. Fischer-Tropsch process (or indirect method).

1. Bergius process (or) direct method 1. In this process, the finely powdered coal is made into a paste with heavy oil and a catalyst powder

(tin or nickel oleate) is mixed with it.

2. The paste is pumped along with hydrogen gas into the converter, where the paste is heated to

400 − 450°C under a pressure of 200 − 250 atm.

3.During this process hydrogen combines with coal to form saturated higher hydrocarbons, which

undergo further decomposition at higher temperature to yield mixture of lower hydrocarbons.

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

5.The crude oil is then fractionated to yield (i) Gasoline (ii) Middle oil (iii) Heavy oil.

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

recycled for making paste with fresh coal dust.

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

5. How the flue gas analysis is carried out? Explain with neat diagram. (Dec 2009)

FLUE GAS ANALYSIS (ORSAT METHOD)

The mixture of gases (like CO2, O2, CO, etc) coming out from the combustion chamber is called flue gases.

The analysis of a flue gas would give an idea about the complete or incomplete combustion process. The

analysis of flue gases is carried out by using orsat‟s apparatus.

Description of Orsat’s apparatus It consists of a horizontal tube. At one end of this tube, U-tube containing fused CaCl2 is connected

through 3-way stop cock. The other end of this tube is connected with a graduated burette. The burette is


surrounded by a water-jacket to keep the temperature of gas constant. The lower end of the burette is

connected to a water reservoir by means of a rubber tube. The level of water in the burette can be raised or

lowered by raising or lowering the reservoir.

The horizontal tube is also connected with three different absorption bulbs I, II, and III for absorbing

CO2, O2 and CO

. I-Bulb: It contains „potassium hydroxide‟ solution, and it absorbs only CO2. II-Bulb: It contains „alkaline pyrogallol‟ solution, and it absorbs CO2 and O2. III-Bulb: It contains „ammoniacal cuprous chloride solution‟ and it absorbs CO2, O2 and CO.

Precautions:

1. Care must be taken in such a way that, the reagents in the absorption bulb 1, 2 and 3 should be brought to the etched marked level one by one by raising and lowering reservoir bottle.

2. All the air from the reservoir bottle is expelled to atmosphere by lifting the reservoir bottle.

3. It is essential that CO2, O2 and CO are absorbed in that order only.

4. As the CO content in flue gas is very small, it should be measured quite carefully. Working:

The 3-way stop-cock is opened to the atmosphere and the reservoir is raised, till the burette is completely filled with water and air is excluded from the burette. The 3-way stop-cock is now connected to the flue gas supply and the flue gas is sucked into the burette and the volume of flue gas is adjused to 100 cc by raising and lowering the reservoir. Then the 3-way stop cock is closed.

(a) Absorption of CO2: The stopper of the absorption bulb-I, containing KOH solution, is opened and

all the gas is passed into the bulb-I by raising the level of water in the burette. The gas enters into the bulb-I,

where CO2 present in the flue gas is absorbed by KOH. The gas is again sent to the burette. This process is

repeated several times to ensure complete absorption of CO2. The decrease in volume of the flue gas in the

burette indicates the volume of CO2 in 100 cc of the flue gas.

(b) Absorption of O2: Stop-cock of bulb-I is closed and stop cock of bulb-II is opened. The gas is

again sent into the absorption bulb-II, where O2 present in the flue gas is absorbed by alkaline pyrogallol.

The decrease in volume of the flue gas in the burette indicates the volume of O2.

(c) Absorption of CO: Now stop-cock of bulb-II is closed and stop-cock of bulb-III is opened. The

remaining gas is sent into the absorption bulb-III, where CO present in the flue gas is absorbed by

ammoniacal cuprous chloride. The decrease in volume of the flue gas in the burette indicates the volume of

CO. The remaining gas in the burette after the absorption of CO2, O2 & CO is taken as nitrogen. Significance (or) uses of flue gas analysis 1. Flue gas analysis gives an idea about the complete or incomplete combustion process. 2. If the flue gases contain considerable amount of CO, it indicates that incomplete.

3. If the flue gases contain considerable amount of O2, it indicates that complete combustion is curing and also

it indicates that the excess of O2 is supplied.


6. Explain the manufacture and properties of power alcohol.

POWER ALCOHOL

Ethyl alcohol when blended with petrol and used as a fuel for internal combustion of engines, it is called as power alcohol.

A mixture of 90% gasoline and 10% ethyl alcohol is known as gasohol.

Manufacture Ethanol (power alcohol) can be made by fermentation of molasses. The molasses consists of about 50-60% sucrose and invert sugar. Under the conditions of pH 4-5 and temperature 30 C, the enzyme invertase converts

sucorose into glucose and fructose. The whole process takes about 36-38 hours to complete and produces 18-20% of ethyl

alcohol. C12H22O11+H2O−−−> C6H12O6 + C6H12O6 −−−> C2H5OH+CO2

Sucrose Glucose Fructose

Advantage Blending increases the octane number of the fuel and shows better antiknock

properties. Blend petrol shows less starting problems as compared to pure petrol.

Disadvantage Blended petrol has lower CV than petrol. Alcohol is easily oxized to organic acids, which can cause corrosion.

7. Describe the fractional distillation of petroleum in detail. (Jun 2013)

REFINING OF PETROLEUM (or) CRUDE OIL

The process of removing impurities like water, sulphur, dissolved salts like MgCl2 and separating the

crude oil into various fractions having different boiling points is called refining of petroleum. This process

of refining involves the following four steps.

1. Separation of water (Cottrell’s process)

The crude oil is allowed to flow between two highly charged electrodes, where colloidal water droplets combine

to form large drops, which is then separated out from the oil.

2. Removal of sulphur compounds


Constituents Percentage (%)

CO 30

N2 51-56

H2 10-15

CO2+ CH4 rest

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

separated out by filtration.

3. Electrical desalting

This process follows electrolysis, which removes dissolved salts like NaCl, MgCl2 etc from oil.

3. Fractional distillation

The purified crude oil is heated to 400°C and the vapours are passed at the bottom of fractionating column. The

column has a number of horizontal stainless steel trays. On passing the vapours through the trays, they get

condensed and collected according to their boiling points.

When the vapours of the oil go up in the fractionating column, they become cooler and get condensed at

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

lower boiling points condense at higher trays. The gasoline obtained by this fractional distillation is called

straight-run gasoline. Various fractions obtained at different trays are given in table.

8. Explain the synthesis of producer gas. (Dec 2009, Jun 2012, Jun 2013)

PRODUCER GAS

1. It is a mixture of CO & N2 with small amount of H2. 2. Its average composition is as follows.

3.Its calorific value is about 1300 kcal/m3.

4. Manufacture

The reactor used for the manufacture of producer gas is known as gas producer. It consists of a tall

steel vessel inside of which is lined with refractory bricks. It is provided with cup and cone feeder at

the top and a side opening for producer gas exit. At the bottom, it is provided with a inlet pipe for

passing air and steam.

5. When a mixture of air and steam is passed over a red hot coke maintained at about 1100°C in a


reactor, the producer gas is produced.

6. Various Reactions The reactions of producer gas production can be divided into four zones as follows. i.) Ash zone

This is the lowest zone consists mainly of ash. The incoming air and steam mixture is preheated in this zone.

(ii) Combustion (or) Oxidation Zone This is the zone next to ash zone. Here the coke is oxidised to CO and CO2. Both the reactions are

exothermic.Hence, the temperature of the bed reaches around 1,100°C. C + ½ O2 −−−> CO ; exothermic

C + O2 −−−> CO2 ; exothermic (iii) Reduction Zone This is the middle zone. Here both CO2 and steam are reduced.

C + CO2 −−−> 2CO ; endothermic

C + H2O −−−> CO + H2 ; endothermic The above reactions are endothermic. Hence the temperature of the coke bed falls to 1000°C.

(iv) Distillation (or) Drying Zone This is the upper most layer of the coke bed. In this zone (400 − 800°C) the incoming coke is heated by the outgoing gases.

7. Uses 1. It is used as a reducing agent in metallurgical operations. 2. It is also used for heating muffle furnaces, open-hearth furnaces etc.

9. Explain the synthesis of water gas.

WATER GAS

1. It is a mixture of CO and H2 with small amount of N2. 2. The average composition of water gas is as follows.

3. Its calorific value is about 2800 kcal/m3

Constituents Percentage (%)

CO 41

H2 51

N2 4

CO2+ CH4 rest


4. Manufacture The water gas producer consists of a tall steel vessel, lined inside with refractory bricks. It is provided with cup and cone feeder at the top and a side opening for water gas exit. At the bottom it is

provided with two inlet pipes for passing air and steam. 5. When steam and little air is passed alternatively over a red hot coke maintained at about 900

−1000°C in a reactor, water gas is produced. 6. Various Reactions The reactions of water gas production involve the following two steps.

I – Step: In the first stage, steam is passed through the red hot coke, where CO & H2 gases are

produced. The reaction is endothermic. Hence, the temperature of the coke bed falls.

C + H2O −−−> CO + H2 ; endothermic ; ΔH = + ve II – Step: In the second stage, in order to raise the temperature of the coke bed to 1000°C, the steam supply is temporarily cut off and air is blown in. The reaction is exothermic.

C + O2 −−−> CO2 ; exothermic ΔH = − ve

Thus the steam-run and air-blow are repeated alternatively to maintain proper temperature.

7. Uses 1. It is used for the production of H2 and in the synthesis of ammonia. 2. It is used to synthesis gasoline in Fischer-Tropsch process. 3.It is used as an illuminating gas and a fuel.

4. It is also used in the manufacture of power alcohol and carburetted water gas (water gas + oil gas).

10. Explain transesterification and advantages of bio-diesel.

BIO-DIESEL

Chemically, biodiesel is a mixture of fatty acid methyl esters (FAMEs).

M a n u f a c t u r e

It is produced from oils and fats using transesterification.

Transesterification (Alcoholysis) is the reaction of a fat or oil with an alcohol to form esters and glycerol. A catalyst is usually used to improve the reaction rate and yield.

To complete transesterification stoichiometrically, a 3:1 molar ratio of alcohol to triglycerides is needed.

The reaction can be catalyzed by alkalis (NAOH, KOH), acids (H2SO4, HCL), or enzymes (Lipase). But alkali-catalyzed transesterification is much faster than acid or bio-catalyzed reactions.


Among the alcohols (methanol and ethanol) used in the transesterification process, ethanol is most frequently used because it can quickly react with triglycerides in presence of NaOH.

Triglyceride Glycerol Mixture of fatty esters

Advantages:

Pure biodiesel (B100) is nontoxic and environmental friendly diesel fuel. It is safe to handle because it is biodegradable.

It has high flash point (148 ˚C) compared to petroleum diesel fuel (flash point is 52˚C)

Code & Subject:CY6251 & Engg Chemistry- II Dept Name:Chemistry Academic year: 2014-2015

St.Joseph’s College of Engineering/Institute of tech Page No: 1 ISO 9001:2008

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