Engineering Chemistry -Engineering-VTU-July- 2016-Solution - Grad Plus

# Engineering Chemistry -Engineering-VTU-July- 2016-Solution

## Engineering Chemistry

TIME: 3HOURS                                                                                                                                                    MAXIMUM MARKS-80

Note-Answer FIVE full questions, choosing one full question from each module.

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Q1.a) Derive Nernst equation for single electrode potential.    [5M]

ANS:-The Nernst’s equation is used for finding single electrode potential and EMF of a given cell.

Taking into consideration a redox reaction.

Mn++ne⇔M

The change in free energy for the above reaction is as follows:

ΔG=-nFE  ……………………………… (a)

Under standard conditions, when the concentration is unity, the standard free energy change is as follows:

ΔG0=-nFE0  ………………………………..(b)

The equilibrium constant K is associated with the change in free energy change by the VantHoff equation,

ΔG=ΔG0+RT ln K ……………………….(c)

Where K=[M]/[Mn+] put the value ok K in equation (c)

ΔG=ΔG0+RT ln [M]/[Mn+]

ΔG=ΔG0+RT ln [M]-RT ln[Mn+]……………(d)

Put the values of ‘G’ and ‘G0‘ from equations (b) and (c) in equation (d).

-nFE=-nFE0 +RT ln[M] -RT ln[Mn+]

Divide both sides by -nF

\frac{-nFE}{-nF}=\frac{-nFE^0}{-nF}+\frac{RT\;ln\lbrack M\rbrack}{-nF}-\frac{RT\;ln\mathit\;\lbrack\mathrm M^{\mathrm n+}\rbrack}{-nF}

Under standard conditions,M=1 hence above equation becomes,

E=E^0+\frac{RT\mathit\;ln\;\lbrack M^{n+}\rbrack}{nF}

Put R=8.314 JK-1 mol-1

F=96500 C mol-1 ,T=250C

Now, covert ln to log base 10 the above equation becomes,

And the equation becomes,

E=E^0+\frac{0.0591}n\log_{10}\;\;\lbrack M^{n+}\rbrack\;

b)What is electrolyte concentration cell? The emf of the cell Cu|CuSo4(0.001M)||CuSo4(XM)|Cu is 0.0595 V at 250C.Find the value of X .   [5M]

ANS:-Concentration cells:

a)In these cells the two electrodes of the same material are dipped in the two different solutions containing the same electrolyte of different concentrations.

b)Electrodes are connected through the salt bridge as shown in the figure. It helps to minimize the junction potential and the cell works without transfer of ions from the electrodes.

c)Consider that concentration cell containing Zn2+ions. The cell has two Zn electrodes of different concentrations of ZnSO4 solution. The electrodes of the cell can be represented as,

Zn(S)|ZnSO4(C1M) and  Zn |ZnSO4(C2M)

The cell can be represented as,

Zn(S)|ZnSO4(C1M) || ZnSO4(C2M) |Zn(S)

This indicates that C2>C1

d)Reactions occurs at electrodes are,

At oxidation electrode : Zn⇔Zn2+(c1)+2e

At Reduction electrode:Zn2+(c2)+2e⇔Zn

The Overall cell reaction:Zn2+(c2)⇔Zn2+(c1)

Solution:-Given emf=0.0595V ,C1=0.001M ,C2=XM ,n=2

$\begin{array}{l}E_{cell}=\frac{0.0591}n\log\frac{C_2}{C_1}\\\\0.0595=\frac{0.0591}2\;\log\frac X{0.001}\end{array}$

X=0.103 M.

c)Explain the following battery characteristics: [6M]

i)Cell potential

ii)Capacity

iii)Shelf life

ANS:-

i)Cell potential-A good cell is one that offers large and almost constant EMF. It also depends on the change in the free energy of the reaction.

ΔG=-nF Ecell

More will the change in free energy, higher will be the EMF.

According to Nernst Equation,

E_{cell}=E_{cell}^0-\frac{2.303RT}{nF}\;\log\frac{\lbrack Products\rbrack}{\lbrack Reac\tan ts\rbrack}

where,

E0cell=E0R-E0L=E0cathode – E0anode

The larger the potential difference between the electrodes higher is E0cell.

ii)Capacity:-The capacity of a battery is the amount of electric current that can be obtained from a  battery. It is charge in ampere-hours(Ah). It depends on the size of the battery and discharge condition. The capacity of a battery is to be determined by the Faraday relation.

C=WnF/M

where,

C=Capacity of battery (Ah)

W=Weight of the active material

n=Number of electrons

M=Molar mass.

iii)Shelf life:-The duration of storage under specified conditions at the end of which the cell still has the ability to deliver specified performance. A good battery should possess a long shelf life.

Q2.a)Define reference electrode. Discuss the construction and working of the calomel electrode.    [5M]

ANS:-Reference Electrode

a)The electrode of standard potential, with reference to the potential of another electrode is called reference electrode.

b)The most commonly used reference electrode is the Hydrogen electrode. The potential of all other electrodes are measured with reference to the hydrogen electrode. Therefore, it is called as reference electrode.

c)Other more Reference electrodes available which are standardized using a standard electrode and then used as reference electrodes for measuring the potential difference of other electrodes. So, these electrodes are called as secondary reference electrodes.

Calomel Electrode.

Calomel Electrode is a secondary reference electrode. Calomel is also known as mercurous chloride and it is soluble in water.

Construction:

i)It consists of a thin layer of pure mercury at the lowest portion of the container. And on that paste of Hg+Hg2Cl2  is placed.

ii)The remaining part of the cell is filled with a solution of a normal or saturated solution of KCl.

iii)A wire made of platinum is dipped into the layer of mercury is used for making electrical contact. The side-tube is used for making electrical contact with the help of a salt bridge.

Working:

i)The electrode is represented as,

Pt,Hg(l) ,Hg2Cl2(S)|KCl (xM) saturated with Hg2Cl2

ii)The electrode is of reversible type, if the reduction is occurring at this electrode, the reaction is as follows,

Hg2Cl2(S)⇔Hg2++2Cl

Hg2+2  +2e⇔2Hg

Hg2Cl2(S)+2e⇔2Hg+2Cl

Nernst Equation for the calomel electrode is,

$E_{Hg/Hg_2Cl_2}=E_{Hg/Hg_2cl_2}^0-\frac{2.303RT}{nF}\;\log\lbrack Cl^-\rbrack^2$

The potential of this electrode depends on the concentration of KCl. Calomel electrode with saturated KCl and 1M KCl have value of 0.2412 V and 0.28V.

b)Describe the construction and working of the Ni-MH battery. Mention its applications. [5M]

ANS:-Nickel-metal hydride battery:

Usually these batteries are alklaine and rechargeable.

Construction:-

i)The anode used is made up of metal hydride like MH and MH2. Active hydrogen storage alloy like LaNi5 are put on a very thin, highly porous nickel sheet.

ii)Cathode,is made of NiO(OH) is also put on thin and highly porous nickel sheet.

iii)Electrolyte used is aqueous potassium hydroxide.

iv)A thin layer of polypropylene is poured into the separator.

v)All the components are in glass container.

Working:-

Cell Representation:

MH2 |KOH(5M)| Ni(OH)2,NiO(OH)

Reactions:

At anode: MH2+2OH→M+2H2O+2e

At cathode: NiO(OH)+H2O+e→Ni(OH)2+OHX 2

Net Reaction: MH2+2NiO(OH)→M+2Ni(OH)2

EMF of the battery is 1.3V.

1)High capacity

2)Zero maintenance required

3)Long life

4)If there charge there is long shelf life.

5)Less environmental problem.

6)Recharging capability is rapid.

Applications:

There are various use of Nickel-metal hydride batteries. These batteries are used in computers, cellular phones, and other portable and consumer electronics applications where high specific energy is required.

c)What is fuel cell? Distinguish between conventional cell and fuel cell. [6M]

ANS:-Fuel Cell:-Electrochemical cells are usually fuel cells. It consists of two electrodes and an electrolyte which help to convert the chemical energy of the chemical reaction between fuel and oxidant which converts directly into electrical energy.

The ordinary combustion process of fuel is

Fuel+Oxygen→ Combustion products+Heat

The prcess of fuel cell is ,

Fuel+ Oxygen→Oxidation products+Electricity

The process is the same as the electricity generator set, which helps to convert the chemical energy of diesel and air into electricity.

Q3.a)Define corrosion. Explain the electrochemical theory of corrosion by taking iron as an example. [6M]

ANS:- It is defined as the natural process that causes the transformation of pure metals to undesirable substances when they react with substances like water or air. This reaction causes damage and disintegration of the metal starting from the portion of the metal exposed to the environment and spreading to the entire bulk of the metal.

Mechanism of  Wet electrochemical corrosion:

Wet corrosion happens when an electrochemical attack on metal in an aqueous solution or in an aqueous environment.

i)Electrons are released at anode and those electrons are absorbed at the cathode.

ii)Oxidation is the process where corrosion occurs at anode region.

iii)At the cathode region reduction of O2 occurs in the presence of water to hydroxyl(OH) ions.

iv)Mechanism involves following reaction as follows:

Reaction at anode region:- Corrosion occurs at the anode and the metal undergoes corrosion in which oxidation with the release of electrons.

Fe→Fe2++2e

Reaction at cathode region:-The electron released from the anode are absorbed at the cathode.The flow of electron towards cathode are responsible for the reaction at the cathode. It depends on the nature of the environment.

Reduction occurs in following ways:-

1)In the absence of oxygen in an acidic medium hydrogen ions combine with electrons and form hydrogen gas.

2H++2e→H2(g)

2)In acidic medium in presence of oxygen reaction occurs :

4H++O2+4e→4OH

3)In an alkaline medium dissolved oxygen to combine with electrons and form hydroxyl ions and hydrogen gas is released with the formation of OH ions.

2H2O+O2+4e→4OH

2H2O+2e→H2+2OH

The Fe metal ions formed at the anode with hydroxyl ions form Fe(OH)2, hydrated ferric oxide i.e. rust is formed when oxidized.

2Fe++4OH→2Fe(OH)2

2Fe(OH)2+1/2 O2+(x-2)H20 →Fe2O3.xH2O (Rust)

Black rust is formed in presence of limited oxygen.

3Fe(OH)2+1/2O2→Fe3O4.3H20  (Black Rust)

b)Explain the following factor affecting corrosion.  [5M]

i)Ratio of anodic to cathodic areas

ii)Nature of corrosion product.

ii)Temperature.

ANS:-i)Ratio of anodic to cathodic areas-

i)The rate of corrosion depends on the size of anodic and cathodic areas.

ii)When the ratio of anodic to cathodic  region is smaller then this leads to faster and intensive anodic reaction and due to this the overall corrosion is effective.

iii)When the ratio of anodic to cathodic  region is larger due to this thr corrosion at anodic region to occur very slowely,

ii)Nature of corrosion product.

i)When there is moist atmosphere all metals get covered with a thin surface flim of metal oxide as the product of corrosion.

ii)If the product of corrosion is insoluable ,adherent stable,non-porous and non-volatile then it is acting as prtective flim which help to prevent further corrosion.This layer acts as a barrier between the metal surface and corrosive environment.

iii)If the product of corrosion is soluable ,non-adherent unstable,porous and conducting then the corrosion process continuous after the formation of corrosion product.

c)Describe electroplating of nickel using Watt’s bath. Mention its applications.  [5M]

OR

Q4.a)Explain differential aeration corrosion with one example.   [5M]

ANS:- Differential aeration corrosion:

• When a metal is exposed to differential air or oxygen concentration it causes Differential Aeration Corrosion.
• The part of metal exposed to a higher concentration of oxygen is acting as cathode and part to metal exposed to lower concentraion of oxygen is acting as anode and due to this corrosion happens.

Example:-

1)In an aerated solution of Sodium chloride pure iron plate is immersed partially as shown in the figure.

2)Oxygen is required at cathode reaction the higher concentration of oxygen always acts as cathode and part of metal immersed in NaCl is an anodic region.

Reactions are as follows:-

At anode:- Fe2+→Fe+2e

At cathode:-1/2 O2+H2O+2e→2OH

b)What is metal finishing? Mention the technological importance of Metal Finishing. [6M]

ANS:-Metal finishing is the process of changing the surface of an object, for the purpose of improving its appearance. Metal finishing is related to electroplating, which is the production of a thin surface coating of the metal upon another by electrodeposition.

Technological importance of Metal Finishing.

1)Electrical conductivity, thermal conductivity, Good thermal conductivity, and optical reflectivity.

2)It also improved corrosion resistance, heat resistance, surface hardness, and solder ability.

3)It also helps in manufacturing electrical and electronic components such as PCB, capacitors, etc

4)It also helps in imparting hardness.

5)It also help in building up of material.

6)It also enhances the form and shapes of components and also increases the bonding of adhesive or organic coatings.

7)The processes in industry are platting operating which is divided into electroplating and there is electrochemical conversion,metallic coating and chemical coating in which includes electroplating and electroless plating and surface treatments.

c)Define electroless platting. Distinguish between electroplating and electroless plating.   [5M]

ANS:-In this process of plating deposition is done without any current applied. It is a chemical reaction and is autocatalytic.

Q5.a)Explain the determination of calorific value of a solid fuel using bomb calorimeter. [6M]

ANS:-Principle of Bomb Calorimeter:

Construction:-

i)As shown in the figure, the Bomb calorimeter consists of a cylindrical stainless steel bomb where fuel combustion is carried out.

ii)Bomb has a gas-tight lid that is screwed to the body. The lid has two stainless steel electrodes and an oxygen inlet valve.

iii)There is a small ring connected to one of the electrodes. Nickel or stainless steel crucible is supported to the ring in which fuel of known mass/volume is taken.

iv)In a copper calorimeter containing a known mass of water the entire bomb pot is placed.

v)To prevent heat losses due to radiation copper calorimeter is surrounded by an air-jacket and water jacket.

vi)It also contains an electric stirrer and Beckmann’s thermometer.

Working:-

• This crucible is placed over the ring, in this, a known mass of fuel is taken.
• A wire which is fine and made up of magnesium is stretched across the electrode, touching the fuel sample.
• The lid of the pot is screwed tightly and the bomb pot is filled with 25 atmospheric pressure of oxygen.
• In the copper calorimeter containing a known mass of water, the bomb pot is placed.
• The stirrer is working and water temperature at the initial stage is noted.
• The electrodes in the calorimeter are connected to 6V battery. The circuit is completed. When the battery is connected it provides ignition to the magnesium fuse wire and it burns and also provides ignition to fuel.
• The sample of fuel taken burns and heat is liberated.
• Due to the combustion of fuel heat is liberated which is absorbed by the water in the calorimeter. For uniform distribution of heat stirring of water is continued and due to temperature of water rises. And the maximum temperature is recorded.

Calculation:-

Let,

x=Mass of fuel taken in the crucible, whose calorific value is to be determined.

W=Mass of water in the copper calorimeter

w=Water equivalent of the calorimeter, stirrer, thermometer, bomb, etc in grams.

t1=Initial temperature of water in the calorimeter.

t2=Final temperature of water in the calorimeter.

L=Gross/higher calorific value of the fuel in cal/gm.

The heat liberated by  fuel =x.L

Heat absorbed by water and apparatus =M×S×(t2-t1)

Heat absorbed by water and apparatus =(W+w)×1×(t2-t1)

Heat liberated by fuel=Heat absorbed by water, apparatus etc.

x.L=(W+w)(t2-t1)

\begin{array}{l}GCV\;of\;fuel\;(L)=\frac{S(W+w)(t_2-t_1)}x\;cal/gm\;\\or\\GCV\;(L)=\frac{4.187(W+w)(t_2-t_1)}x\;J/gm\end{array}

If fuel contains x% hydrogen , NCV of the fuel is calculated as ,

2 atoms of hydrogen produce 1 molecule of water.

2gm of hydrogen produce 18gm of water

x gm of hydrogen produce 9gm of water.

x%  hydrogen=\frac{9\times\;x}{100}\;gm\;of\;water

=0.09 x x gm of water

NCV=GCV-Latent heat of steam formed

NCV=GCV-0.09 X  x  X  Latent heat of steam

Latent heat of steam=2454 KJkg-1

b)What is the reforming of petroleum? Give any three reactions involved in reforming.   [5M]

ANS:-Reforming is a process designed to increase the volume of gasoline that can be produced from a barrel of crude oil. By controlling the temperature and flow rate of the reformer, refinery operators can increase the octane rating of the reformate, but that also has the effect of producing less reformate.

There are four major types of reactions that occur during reforming processes: (1) dehydrogenation of naphthenes to aromatics, (2) dehydrocyclization of paraffin to aromatics, (3) isomerization, and (4) hydrocracking.

c)What is photovoltic cell?Explain the construction and working of a photovoltaic cell.  [5M]

ANS:-These cells used to convert solar energy to electrical energy it is based on the principle of the photoelectric effect.

Construction:

i)Photovoltaic cells are made up of thin semiconductor silicon alloy which are light-sensitive p-n junction photo diode as shown in figure.

ii)There is the formation of the p-n junction by joining p-type with n-type and the region of separation is called junction.

iii)p-type region contains holes which is connected to the positive terminal of the battery.

iv)n-type region contains an excess of electrons which is connected to the negative terminal of the battery.

v)Positive charge of battery repelled holes while the negative charge of battery repelled electrons.

vi)Diffusion of electrons across the junction is created by the imbalance of charge  and potential is developed  and current flow through the diode,

Working:

i)Solar radiation contains photons that enter n-type semiconductor breaks barrier potential and moves to p-type semiconductor electrons are knocks by photons in p-type and there is the formation of electron pair.

ii)Electrons are travelling from n-type to p-type semiconductors, due to this electric circuit is completed. Hence it leads to the formation of electricity which is shown by the glowing of bulb attached on it.

OR

Q6.a)0.75 g of coal sample (carbon -90%, hydrogen -6%, and ash 4% ) was subjected to combustion in a bomb calorimeter. The mass of water taken in the calorimeter was 3500g and the water equivalent of the calorimeter was 750g. The rise in temperature was found to be 3.20C. Calculate the gross and net calorific values of a sample (Specific heat of water=4.187 kJ/ kg / oC; Latent heat of steam =2454 kJ/kg.)  [5M]

ANS:-Weight of coal=0.75g=0.75×10-3 kg

Mass of water taken =W=3500g=3500×10-3 kg

Water equivalent of calorimeter=w=750g=750×10-3 kg

Rise in temperature=T2-T1=3.20

Specific heat of water=S=40187kJ/kg/0C

Latent heat of steam=2454 kJ/kg

%H=6

\begin{array}{l}GCV=\frac{(W+w)\times(T_2-T_1)\times S}x\\\\\;\;\;\;\;\;\;\;\;=\frac{(3500+750\times10^{-3})\times3.2\times4.187}{0.75\times10^{-3}}\\\\\;\;\;G=75924.26\;kJ/kg\end{array}

b)Explain the modules, panels, and arrays of PV cells.     [6M]

c)Explain the production of solar grade silicon by the Union-Carbide process. [5M]

ANS:-Solar grade silicon by Union-Carbide Process:-

There are the following steps in the production of solar-grade silicon by the Union-Carbide process.

Step I:- In an electric arc furnace SiO2 is treated with carbon at 1500-20000 C and silicon is produced.

SiO2+2C→Si+2CO↑

Step II:-Bypassing oxygen purification is done over the molten silicon obtained which helps to remove metallic impurities as metal oxides. And the obtained silicon is called metallurgical grade silicon. This silicon mixed with HCl to obtain trichlorosilane.

Si+HCl→HSiCl3+H2

Step III:-Trichlorosilane is passed through a fixed bed column containing Quaternary ammonium ion  exchange resin acting as a catalyst to convert it into hydride.

6HSiCl3→3H2SiCl2+3SiCl4

3H2SiCl2→SiH4+2HSiCl3

Step IV:-By distillation and trichlorosilane is recycled to the exchange resin to obtain Silane. In a reactor containing heated silicon seed rods to yield polysilicon in that pure silane undergoes pyrolysis.

SiH4→2H2+Si

Q7.a)Explain the type of polymerization with an example.  [4M]

ANS:-POLYMERIZATION: Polymerization is the process of connecting these monomers together and creating large macromolecules of different sizes and shapes. A polymer is a long molecule consisting of many identical or similar building blocks linked by covalent bonds – like how a train consists of a chain of cars. Most large molecules, or macromolecules, are polymers. The repeating units that serve as the building blocks of a polymer are small molecules called monomers.

The two major types of polymerization are addition polymerization and condensation polymerization.

Polymerization that occurs through the coupling of monomers using their multiple bonds is called addition polymerization. The simplest example involves the formation of polyethylene from ethylene molecules. In this reaction, the double bond in each ethylene molecule opens up, and two of the electrons originally in this bond are used to form new carbon-carbon single bonds with two other ethylene molecules.

2)Condensation Polymerization

The chemical mechanism that cells use to make and break polymers are basically the same in all cases. Monomers are connected by a reaction in which two molecules are covalently bonded to each other through the loss of a water molecule; this is called a condensation polymerization because the lost molecule is water. When a bond forms between two monomers, each monomer contributes part of the water molecule that is lost; one molecule provides a hydroxyl group, while the other provides hydrogen. To make a polymer, this reaction is repeated as monomers are added to the chain one by one.

b)What is glass transition temperature? Discuss any two factors affecting the glass transition temperature. [6M]

ANS:-Glass Transition Temperature (Tg) is the point at which a material alters state – going from a glass-like rigid solid to a more flexible, rubbery compound.  Tg is normally measured on a Differential Scanning Calorimeter (DSC)  equipment.  The DSC automatically plots a chart from which you can calculate the approximate Tg from. The glass transition takes place over a range on the graph so doesn’t automatically come out with an exact figure, but a skilled eye can interpret a good graph and may be able to make a qualified estimate if a specific figure is required.

Factors affecting glass transition temperature are-

1) Chain Stiffness-Stiffening groups: in the polymer chain reduce the flexibility of the chain and raise the value of Tg

Tg

.

poly(ethylene terephthalate) Tg=69oC

Tg=69oC

2) Intermolecular Forces-

Stronger intermolecular forces lead to a higher Tg. PVC has stronger intermolecular forces than polypropylene because of the dipole-dipole forces from the C-Cl bond.

Atactic Polypropylene Tg=−20oC

Tg=−20oC

c)Explain the synthesis and applications of the following: [6M]

i) Plexiglass

ii)Polycarbonate

OR

Q8.a) In a polymer sample, 20% of molecules have a molecular mass of 15000g/mol. 45% of molecules have a molecular mass of 25000g/mol remaining molecules have a molecular mass of 27000 g/mol. Calculate the number average weight average molecular mass of the polymer.  [6M]

ANS:-

b)Explain the synthesis, properties, and applications of silicone rubber.  [5M]

ANS: Synthesis of Silicon Rubber:- Overall, silicone rubbers synthesis majorly involves three steps i.e. preparation of chlorosilanes followed by hydrolysis and then polymerization yielding silicone elastomers.

Today, silicones are obtained commercially from chlorosilanes prepared following the direct process of Rochow. The reaction giving chlorosilanes takes place in a fluidised bed of silicon metal powder in which flows a stream of methylchloride, usually at temperatures of 250 to 350°C and at pressures of 1 to 5 bars. A copper-based catalyst is used.

A mixture of different silanes is obtained containing mainly the dimethyldichlorosilane, Me2SiCl2.
(Source: Dow Corning)

The dimethyldichlorosilane is separated via distillation and used as the monomer to produce Polydimethylsiloxanes by the hydrolysis of dimethydichlorosilane in the presence of excess water.

(Source: Dow Corning)

This heterogeneous and exothermic reaction gives formally a disilanol “Me2Si(OH)2” [2] which readily condenses, with HCl acting as a catalyst, to give a mixture of linear [3] or cyclic [4] oligomers by inter- or intramolecular condensation.

The linear and cyclic oligomers obtained by hydrolysis of the dimethyldichlorosilane have too short a chain for most applications. They must be condensed (linears) or polymerized (cyclics) and crosslinking to obtain elastomers.

PROPEERTIES:-

The strong Si-O chemical structure and high bond energy give Si Elastomers their unique performance properties. Several benefits of silicone rubbers include:

• Wide service temperature range – excellent thermal and thermoxidative resistance (-Si-O-Si- binding energy is higher than C-C bonds)
• Excellent resistance to attack by oxygen, ozone and sunlight
• Excellent non-stick and non-adhesive properties
• Low toxicity
• Good excellent insulation properties
• Low chemical reactivity
• High bio-compatibility
• Excellent mechanical properties

APPLICATIONS:-

Silicone rubber is a perfect choice of material in applications requiring:

1. For applications exceeding working temperature above 100°C
2. Non-toxic food contact applications
3. High electrical resistance
4. High steam sterilization resistance especially in seals & gaskets used in medical devices
5. High durability, color matching options, etc.

c)Explain the mechanism of conduction in polyaniline.  [5M]

ANS:-

Q9.a)Explain the scale and sludge formation in the boiler. [5M]

ANS:- In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentrations reach saturation point, they are thrown out of water in the form of precipitates on the inner walls of the boiler. If the precipitation takes place in the form of loose and slimy precipitate, it is called sludge. On the other hand, if the precipitated matter forms a hard, adhering crust/coating on the inner walls of the boiler, it is called scale.

Sludge is a soft, loose and slimy precipitate formed within the comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends. Sludge’s are formed by substances which have greater solubility in hot water than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc.

DISADVANTAGES OF SLUDGE FORMATION :

1. Sludges are poor conductor of heat, so they tend to waste a portion of heat generated.
2. If sludges are formed along with scales, then former gets entrapped in the latter and both get deposited as scales.
3. Excessive sludge formation disturbs the working of the boiler. It settles in the regions of poor water circulation such as pipe connection, plug opening, gauge-glass connection, thereby causing even choking of the pipes.

PREVENTION OF SLUDGE FORMATION :

(1) By using well softened water

(2) By frequently ‘blow-down operation’, i.e., drawing off a portion of the concentrated water. Scales are hard deposits, which stick very firmly to the inner surfaces of the boiler. Scales are difficult to remove, even with the help of hammer and chisel. Scales are the main source of troubles. Formation of scales may be due to;

(1) Decomposition of calcium bicarbonate

Ca(HCO3)2 → CaCO ↓ + H2O + CO2 ↑

Scale

However, scale composed chiefly of calcium carbonate is soft and is the main cause of scale formation in low-pressure boilers. But in high-pressure boilers, CaCO3 is soluble.

CaCO3  +  H2O   →  Ca(OH2)2  (soluble) + CO2 ↑

Disadvantages of scale formation :

(1) Wastage of fuel : Scales have a low thermal conductivity, so the rate of heat transfer from boiler to inside water is greatly decreased. In order to provide a steady supply of heat to water, excessive or over heating is carried out  and this causes increase in fuel consumption.

(2) Lowering of boiler safety : Due to scale formation, over-heating of boiler is to be done in order to maintain a constant supply of steam. The over-heating of the boiler tube makes the boiler material softer and weaker and this causes distortion of boiler tube and makes the boiler unsafe to bear the pressure of the steam, especially in high-pressure boilers.

(3) Decrease in efficiency : Scales may sometimes deposit in the valves and condensers of the boiler and choke them partially. Tills results in decrease in efficiency of  boiler.

(4) Danger of explosion :  When thick scales crack, due to uneven expansion, the water comes suddenly in contact with over-heated iron plates. This causes  formation of a large amount of steam suddenly. So sudden high-pressure is developed, which may even cause explosion of the boiler.

Removal of scales : (i) With the help of scraper or piece of wood or wire brush, if they are loosely adhering.
|(ii) By giving thermal shocks (i.e., heating the boiler and then suddenly cooling with cold water), if they are brittle.
(iii) By dissolving them by adding them chemicals, if they are adherent and hard. Thus, calcium carbonate scales can be dissolved by using 5-10% HCl. Calcium sulphate scales can be dissolved by adding EDTA.
(iv) By frequent blow-down operation, if the scales are loosely adhering.

PREVENTION OF SCALES FORMATION :

(1)  External treatment includes efficient ‘softening of water’ (i.e.  removing  hardness producing constituents of water).

(2)  Internal treatment : In this process (also called sequestration), an ion is prohibited to exhibit its original character by ‘complexing’ or converting it into other more soluble salt by adding appropriate reagent. An internal treatment is accomplished by adding a proper chemical to boiler water either : (a) to precipitate the scale forming impurities in the form of sludges, which can be removed by blow-down operation, or (b) to convert them into compounds, which will stay in dissolved form in water  and thus do not cause any harm.

Internal treatments methods are, generally, followed by ‘blow-down operation’, so that an accumulated sludge is removed. Important internal conditioning/treatment methods are;

(i) Colloidal conditioning : In low-pressure boilers, scale formation can be avoided by adding organic substances like kerosene, tannin, agar-agar (a gel), etc., which get coated over the forming precipitates, thereby yielding non-sticky and loose deposits, which can easily be removed by pre-determined blow-down operations.

(ii) Phosphate conditioning : In high-pressure boilers, scale formation can be avoided by adding sodium phosphate, which reacts with hardness of water forming non-adherent and easily removable, soft sludge of calcium and magnesium phosphates, which can be removed by blow – down operation, e.g.,

3CaCl2  +  2Na3PO4    →   Ca3(PO4)2  +  6 NaCl

The main phosphates employed are : (a) NaH2PO4, sodium dihydrogen phosphate (acidic); (b) Na2HPO4, disodium hydrogen phosphate (weakly alkaline); (c) Na3PO4, trisodium  phosphate (alkaline).

(iii) Carbonate conditioning : In low-pressure boilers, scale-formation can be avoided by adding sodium carbonate to boiler water, when CaSO4 is converted into calcium carbonate in equilibrium.

CaSO4 +Na2CO→  CaCO3 + Na2SO4

Consequently, deposition of CaSO4 as scale does not take place and calcium is precipitated as loose sludge of CaCO3, which can be removed by blow-down operation.

(iv) Calgon conditioning involves in adding calgon [sodium hexameta phosphate (NaPO3)6 to boiler water. It prevents the scale and sludge formation by forming soluble complex compound with CaSO4.

Na2[Na4(PO3)6]     →  2  Na+  +   [Na4P6O18]2-

2 CaSO4  +  [Na4P6O18]2 −   →  [Ca2P6O18]2 − +  2 Na2SO4

(v) Treatment with sodium aluminates (NaAlO2) : Sodium aluminates gets hydrolyzed yielding  NaOH and a gelatinous precipitate of  aluminium hydroxide.

NaAlO2  +  2H2O     →  NaOH    +    Al(OH)3

The sodium hydroxide, so-formed, precipitates some of the magnesium as Mg(OH),

MgCl2 +  2NaOH   →   Mg(OH)2  +  2 NaCI

The flocculent precipitate of Mg(OH)2 plus Al(OH)3, produced inside the boiler, entraps finely suspended and colloidal impurities, including oil drops and silica. The loose precipitate can be removed by pre-determined blow-down operation.

(vi) Electrical conditioning: Sealed glass bulbs, containing mercury connected to a battery, are set rotating in the boiler. When water boils, mercury bulbs emit electrical discharges, which prevents scale forming particles to adhere together to form scale.

(vii) Radioactive conditioning: Tablets containing radioactive salts are placed inside the boiler water at a few points. The energy radiations emitted by these salts prevent scale formation.

(viii) Complex metric method involves addition of 1.5 % alkaline (pH = 8.5) solution of  EDTA to feed-water. The EDTA binds to the scale-forming cations to form stable and soluble complex. As a result, the sludge and scale formation in boiler is prevented. Moreover, this treatment : (1) prevents the deposition of iron oxides in the boiler, (2) reduces the carryover of oxides with steam, and (3) protects the boiler units from corrosion by wet steam (steam containing liquid water).

b)Define COD. Discuss of experiment determination COD of wastewater. [6M]

ANS:-Chemical Oxygen Demand (COD) analysis is a measurement of the oxygen-depletion capacity of a water sample contaminated with organic waste matter. Specifically, it measures the equivalent amount of oxygen required to chemically oxidize organic compounds in water.

Determination of COD

i)To promote oxidation of straight-chain aliphatic compounds, aromatic compounds, and pyridine Silver sulphate is used.

ii)Take 25 ml volume of waste water and it is refluxed with a known potasium dichromate and dilute sulphuric acid in presence of silver sulphate.

iii)The sample containing the organic matter is oxidized to ammonia, carbon dioxide, and water. There is titration of unreacted K2Cr2O7aganist FeSO4(NH4)2 SO4 6H20 it is Mohr’s salt. The amount of K2Cr2O7used to calculate the oxygen required by the waste water for degradation of pollution.

iv)The COD of the sample can be determined as follows:-

$COD=\frac{(V_1-V_2)\times N\times8\times1000}Vmg/l$

c)Write a note on fullerenes. [5M]

ANS:-Fullerenes are allotropes of carbon. They have a large number of carbon atoms which are hollow spherical clusters. The most common fullerene is of C60 molecules is known as Buckminister fullerene.

Structure of fullerenes.

• Fullerene is a zero-dimensional allotrope of carbon.
• Carbon atoms are arranged in a series of 5 membered and 6 membered rings to form a structure like a football.C60 is a spherical cluster of carbon atoms and the diameter of the ball is about 0.7nm.
• The structure of C60 is highly symmetric.60 carbon atoms are used and they are arranged in a closed net with 20hexagonal faces and 12 pentagonal faces in such a way that no two pentagons are adjacent to each other. Pentagonal rings are more strained than hexagonal rings.
• There is delocalization of Π electrons of the double bond. Delocalization of electrons does not take place over the whole molecule.
• Members of fullerene, such as C24, C70, C76, C78, C80  and more have been isolated under certain conditions.
• Fullerenes show reactions such as electrophilic addition reaction, Diel’s -alder reaction, reactions with alkali metals, etc. because of the presence of Double bonds.

Properties:

• Fullerene is tough and has high impact resistance.
• It is a black powdery material. It dissolves in benzene forming a deep magenta-colored solution.
• It acts with nucleophiles and alkali metals. Its conductivity increases several times when reacted with alkali metals.
• Fullerenes show electrophilic addition reactions and give Diels-alder reactions with anthracene, Furan, etc.

Q10.a) Explain desalination of seawater by ion-selective electrodialysis process.  [5M]

ANS:-

b)Explain the synthesis of two materials of Sol-Gel process. Mention its advantages. [6M]

ANS:-Sol-gel process :

• In the fields of material science and ceramic engineering, the Sol-gel process is a wet-chemical technique also called as a chemical solution technique which is used widely.
• For the fabrication of material, such methods are primarily used for a chemical solution which acts as the precursor for an integrated network of either discrete particles or network polymers.
• The reaction involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides MOR can be described as follows:

MOR+H2O→MOH+ROH (Hydrolysis)

MOH+ROM→M-O-M+ROH (Condensation)

The following process is of Sol-Gel Synthesis:-

Step1:-There is the formation of different stable solutions of the alkoxide or solvated metal precursor (Sol)

Step2:-By polycondensation or poly-esterification reaction there is the formation of an oxide or alcohol bridged network that causes a tremendous increase in the viscosity of the solution.

Step3:-During ploy-condensation reaction aging of the gel until the gel transforms into a solid mass, there is a contraction of the gel network and expansion of solvent from a gel pores.

Step4:-When water and other volatile liquids are removed from the gel network there is drying of gel. Due to this, there is fundamental changes in the process of the structure of gel.

Step5:-During Dehydration the surface-bound M-OH groups are removed which hepls to stabilized the gel.This is achieved by heating at a temperature upto 8000C.

Step6:-There is decomposition and densification of the gels at high temperature and the pores present grl network are collapsed and the remaining organic species are volatilized.

c)Write a note on carbon nanotubes. [5M]

ANS:-Carbon Nanotubes(CNTs):

Structure:

• Carbon Nanotubes are the novel allotropes of carbon. The diameter of CNT is of the order of few nanometers.CNT’s can be imagined as cylinders formed by rolling graphene sheets and capping by hemisphere structure similar to fullerene hemisphere.

There are two main types of carbon nanotubes

1)Single-Walled Nanotubes (SWNT):- Structure of SWNT can be wrapping a single atom thick layer of graphite into a cylinder. The bonding in carbon nanotubes is SP2, with each atom joined to three neighbors as in the case of graphite.

2)Multi-Walled Nanotubes (MWNT):-Collection of concentric SWNTs with different diameters considered as Multi-Walled Nanotubes. It is multiple rolled layers or concentric tubes of graphene. The length, diameter, and hence properties of MWNTs differ from SWNTs.

In general, CNTs are stiff than steel. They have high tensile strength and high resistance to damage from physical force.

Applications:-

1)Hydrogen storage material

2)It is used in Sensors.

3)It is also used as Composite material.

4)Field emitting transistors (FETs)

Properties:-

1)Mechanical:- The carbon nanotubes are highly elastic and it is measure with the help of Young’s Modulus. They have large strains than steel. Without breaking they can be bent.

2)Electrical:-The electrical properties of carbon nanotubes depends upon their diameter. As the diameter of the CNT is increases the energy gap decreases.

3)Thermal:-Thermal conductivity of carbon nanotubes is more than twice that of a diamond. Carbon nanotubes are good conductors of heat.

4)Physical:-CNTs have a very high strength-to-weight ratio.

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