West Bengal Institute of Technology (WBIT) 2009 B.Tech Computer Science and Engineering Engineering chemistry - Question Paper

,    SN² follows 2nd order kinetics ie they are bimolecular.

SN¹ onechanism is generally followed by testiary halides, eg. alkaline hydrolyisis Of testiary butyl bromide.

KOH K++OH-

(aq) (Nu)    

CH,    CH3    CH3

First step : CH-C-Br    CHi-C+Pr>CHj-C-OH

| r/d step    |    |

ch3    ch3    ch3    .

tert. butyl bromide    tert. butyl bromide

SN² mechanism is generally followed by primary halidex. eg. alkaline hydrolysis of ethyl bromide and is a one step onechanism.

|-j H

CM-Br-[HO^s~    Br^S'l HO-C-H-Br⁰

I    L    I J    l

.    (11    Br    CH    

SN¹ mechanism are favoured by weak nucleophiles whereas SN² mechanism are favaured by strong mucleophiles in the above example OH^-.

SN¹ mechanism are favoured by polar solvents and SN² mechanism are favoured by polar aprolic solvents.

Both, back side and front side attack of nudeophile takes place in SN¹ reaction. In case of optical active substrate, configuration dominates because of back side attack till the formation of canbocation.

The nucleophile ex-clusively attacks from back side of substrate in SN² reaction. If the substrate in optically active inversion of config takes place.

The electronic factories influence the reaction rates largely in SN¹ mechanism. For the same halogen, the reactisity sequence is tert. > sec. > primary ? CH₃x.

Stenic factors influence the rate if reaction in SN² mechanism. The electronic factors do not influence the rate. Thereactivity sequence is CH₃> Primary > Sec.> tert.

Here transition state (T.S.) being more polar man than that of stanting material. This is because, new change is developed and concentrated in the Transition state I (T.S.I) as compared to the stanting material. So in solvents of high polarity, T.S. gets greater stability that of the starting material. As a result, the energy of activation value is decreased and consequently the reaction rate is increased. Moreover, the reaction, rate increases with increasing the polarity of solvent, because the stability if carbocation intermediate increases with increasing the polarity of solvent. Here usually polar profit solvent is used to separate leaving group from carbocation (R⁺) is a solvation through inteamoleculair H-bonding between X^- and H-atom of the solvent molecule. Some example of polar solvent. HjO, RoH, R COOH, R CON₂ etc.    '

Ans. (b) Hesss law : Same as Q. 10(c) 2007.    .

Hesss law follows 1st law of thermodynamics : From 1st law of thermodynamics we know, AE = q-v\ - q-PAV

AE is a state function and its value depend, only on the initial and final states of the system but not on the path of the transformation. However, the values of a and w depend on the path by which the change is carried out

At constant \olume, Av=0, so w = 0 .\ AE-q

Since AE is a definite quantity, therefore at constant volume, q is also a definite quantity. At constant pressure the work done w becomes a definite quentity. Thus at constant prossure, q = AE + PAv. becomes a definite quantity. So at constant volume or pressure, the heat evolved or absorbed in the reaction must therefore, Hens law is only special case of 1st law of thermodynamics.    '

Ans. (c) Proof : The efficiency of a heat engine is always less than one using cannot cycle :

When q = 0, ie no hert is rejected to the sink.

W = q-q,

T-Ti    , q, Tt    a. T,    ^Ti

* q-qrq-T¹ or, l-~:il- = i-L or, = or, q = qi

¹ T    q T    q T    T

Therefore W_max =qx^{T T(} xTx =r(T ~ Ti) = 0 ie no work can obtained.

T I) T 1|

Hence to have work, heat must be supplied to the system from the source and more over

some heat must be rejected to the sink by the system ie heat flow fromcource to sink. So

when system do some work, some amount of heat (qj) must be rejected to the sink.

. . q - q j must be < q    

w< q    r| (efficiency of a heat enjine) < 1

5l<l

^a ' ' ' ' .

¹ ie 100% conversion is not feasible. Thus only a fractional quantity of supplied heat can be

converted to work and this fraction is,5-.

8. (a) What is fingerprint region ? Why is methanol a good solvent for UV but not for ER. determination ? What solvents are generally used for IR technique ? Which groups are detected if absorption data are 2841(w), 2755(w), 1686(s), 1605, 1460(m) cm^-1 cm^-1 ?

(b) What is synthetic metal ? What is electronic polymer ? Write notes on conducting polymers and their importance.    I '    9+6

Ans. (a) Finger print Region : -The region 1300 to 625 cm^-1 known as fringer print region. It is here that the pattern'of peaks varies from compound to compound. These are some substancees containing the same functional group show similar absorption above 1500 cm^-1. However absorption positions differ in fingerprint region. This region is very useful in identifying an unknown compound by comparing its IR spectrum will a set of standard

spectrum recorded under indentical conditions.

' . . . ' ' '

UV spectra are recorded by dissolving the compounds in suitable solvents. A good solvent should be transparent over the desired^- range of coavelengths. Usually solvent, which do not contain conjugated system are most suitable for running the UV spectrum. Methanol is a good solvent for UV because it is obtained as very high grade purity and it is free from UV absgrbing substances.    . ' '    -

In case if IR spectra, method is not used as solvent because it is not transparent in the IR range and it has strong IR absorption band. It not only dissolve oni the cells made of rock salt but would also give overlapping bands in certain cases.

The most convenient way to obtain IR spectra of solid substance is to use them in solution form. Effective solvent are those which have poor absorption of their own. No one solvent is transparent in the entire region and the solvent lias to be selected keeping in view the region where the compound under study is expected to absorb. The most commonly used solvents in IR spectra scopy are CC1₄, CHC1₃ and .CS₂.

Cm"¹

2841 (W)    Aliphatic C-H, Aldehyde C-H.

2755 (W)    Aldehyde C-H

1686 (S)    Amide, > C = 0

1605    _f Aromatic

1460 (m)    Ester (aromatic) C = 0, Amine C = ;N

Ans. (b) Synthetic Metal : Polymer that possess the characteristic properties of metals , such as electrical conductivity while retainting the mechanical properties processibility, etc commonly associated-with conventional polymers are called intvinsically conducting polymers (1CP) or synthelic metal. Example of syathetic metal is the crystalline poly sulphubmitride polymer (SN)_n. It has an appreciable conductivity at room temperature.

Electrinic Polymer : Semiconducting polymers whose conductivity can be increased by several times are generally known as electronic polymers. The conductivity of electronic polymers can be increased by doping.

Conducting polymers and their importance : In general polymers are poor conductors of electricity while metals are good conductors. There are some organic polymers such as poly aniline, poly pyoole, poly this phene, etc. They are good conductors like metals. These polymer compound are wholly composed of elements like carbon, hydrogen and occasiowally nitrogen, oxygen or sulphur. More over they have the mechanical properties of polymers, such as flexibility. Such type of polymers are called conducting by a zieglat-Natta process is a

conditctong polymer because the conjugated double bonds in polvacetylene make it possible to conduct electicity down its back bone.

Zi*lar-N.ua__{cH = CH[CH = CH]nCH =} CH_

HC = CH-Aeetvlene

Polyacctylene

There are some non conducting polymers can be made conducting by doping will an electron dnar or electron aceptor. Thus non conducting polymers are a physical mixture of non conducting; polymers and doping materials. Both n-type and p-type dopants can be used to transform an electronic polymer from an insulator to conductor.

Most of the characteristic shown by inorganic sensiconductors are also shown by semiconducting polymers and the high performance devise's suph as photovoltic cells, photodelectors. 4ightemining diodes, etc made from these polymers have been found to perform to the same levels or even better man the devices made from inorganic semiconductors conducting polymer are such lighter than normal metals and can be'used form making light weight batteries. One & their greater flexibility, the electronic devices such as transistors, made from them can be bended like plastics.    '

9. (;i) What are rectifiers? Where are they used?

< b) Why ZnO is white but changes colour on heating ?

(c)    W hat are photovoltaic cells?

(d)    What is the effect of temperature on the conductivity of p-type and n-type semiconductors?    4+3+4-M

Aiis. (a) Same as. Q. 8(b) 2002.

Ans. (b) Same as Q. 3(c) 2002.

Ans. (c) Photovoltaic Cells : Photovoltaic Cells in a device that directly converts sunlight to electicity.

The most commonly used photovoltaic cells are of the bassier type like iron-selenium

cells or Cu - CuO, cells. In the iron selenium cell, a selenium layer is placed on iron disc.

Now an extreml; thin transparent layer of gold or silver is deposited on the selenium to act as

a front electrode. A contact ring on the silver    Light _

layer acts as one electrode and the iron base as    ///// rh^Rin8|

-    Transparent r-----------------------

the other.    _r.. ¹ *

Film

When light falls on the semiconductor. ieBassicr selenium, it ejects electrons which travel fromlaj'er selenium to the front silver electrode through the bassier layer. The flow of electrons in the opposite direction is not permitted by the bassier

because it act as a rectifier. The e.m.f generated internally between silver electrode and selenium is directly proportional to the intensity of incident light radiations.

It inquires no extern al battery for its own operations, ie it is self generating.

The internal emf and hence current generated by it are large enough to be measured on a pointer galvanometer Hence, such cells aie used in devices like portable expouse meters, direct reading illumination meters and low resistance relays for on/off operations and other monitoring operations in industry.

Ans. (d) Effect of temperature on the Conductivity :

Conductivity of n-type and p-tvpe semiconductors increases with increase in temperature. In case ot conductors the free clectom concentration and hole depends temperature.

n - type Semiconductors : For loop n-type semiconductors close to absolute zero, there

are no free carriers in the conduction band, as was in the case of-----semiconductors. Hcnce

the conductivity is negligible. With increase in temperature, the conductivity increases bin

alter a particular temperature it become constant and then again increases with increase in temperature.,

P - type semiconductors : Also for p-type semiconductors, the conductivity increases with increase in temperature, reacting a maximum value and then it becomes constant as in case of n-type semiconductors. However, the increase in conductivity with increase in temperature is mainly due to the increase in number of electrons front the values band of the semiconductors to the accptor band if the doping agent. When all the acceptor alom get filled up will the electrons from the value band, conductivity becomes constant. Now above this

temperature, ihe rise in conductivity of the semi conductor is mainly due to the increase in___

- conductivity.

Effect of temp erature on conductivity of type terms conductor.

10. (a) Phosphorous forms both PF₃ and PF_S but nitrogen forms only NF₃. Exnlain.

(b)    Arrange the following in increasing order of bond lengths : 0₂, Oj^-.

(c)    Using Kohlrauschs law how do we determine the equivalent conductance of acetic acid at infinite dilution?

(d)    Carboxylic acid is stronger acid than phenol.* Explain.

(e)    What is calorific value of fuel ? Distinguish between gross and net calorific

values.

(0 What arc the constituents of coal as determined by proximate analysis?

Am. (a) The ground state electronic_: configuration of phospHorous is-

3s 3p    3d

P : d in? If.

Here phosphorous has vacant 3d orbital. In excited state phosphorous may promote an electron from 3s to vacant ed-orbital. It PF₅ phosphorous having gained five electrons from fluorine alons and through sp³d hybridization phosopherous formed PFj. It can also form PF₃

by Sp³ d hybridization phosphorous having gained three electrons from---atoms in PF₃

molecule.

The ground state electronic of nitrogen is

2s 2p    '

n : rrr

it

^gS        .    .    3

S,ince nitrogen does not have any accant d-orbital, it can only from NF₃ by Sp hybridization.

Ans. (b) O2 => 16 e

2py n2pz *2py *2pz
2Px		B.O. =

IS ^CTIS

>2Px

B.O. = = 1-5

greater the Bond lower the bond length.

So the in creasing order of bond length is o| <0₂ <03' Ans. (c) Equivalent Conductance of acltic acid at infinite dilution using kohlreushs

law : Equivalent conductance of eveak electrolytes such as CH₃COOH, can not found out by extra polution of the curve obtained when exponmentally determined values of equivalent conductance of the electrolyte are plotted against Vconc However kohlraushs law helps to find out A_a for CH3COOH (weak electrolyte) by knowing a ₀ values for certain strong electrolytes as CHjCOONa, Hel and Nacl.

Ao(CH₃COOH)-%_jC00.+>.⁰h.

Adding and subtracting limiting ionic conductances of each of Na* and cl to the R.H.S. w e get.

Ao (CH3COOH) = A₀(CHjCOONa) + A₀(Hcl)-A₀(NacI)

Hence by knowing A₀ for strong electrolytes ie CHCOONa, Hct and Nacl, A₀ for CHCOOH can be found out.

Ans. (d) The stability of conjugate bases is in the order

R = alkyl group.

The delocalisation structure ofcarboxylate anion is as follows :

9

R- C ur* R- C

and the phenside ion is

Phenols are considerably weaker acid than carbonytic acid. This is due to the fact that

delocalisation of the (-) ve charge in the carbonylate anion-----structures of indentical energy

content and of the centres of highly electronegative oxygen atom ; where as in the phenoside anion, the structures be higher energy content than the one in which it is on oxygen atom. The relative stabilization of anion w.r. to the undissociated molecule is these likely to be less effective with aohenol than with a carbolytk acid, leading to the lower relative acidity of the former.

Ans. (e) Carbolitic value of fuel: Calorific value of a fuel may be defined as the amount of heat produced by completely burning of unit man or volume of the fuel. Higher the calorific value better the fuel will be. S.I. unit: J kg * and C.G.S unit: caj g *

Distinguish lactocen gross and net calorific value : Same as Q. 9(a) 2003.

Ans. (0 Constituents of coal as determined by pnzimate analyes. : This analysis provides data for a first general assessment of the quality and type of coal. The properties tested include.

(i)    The determination of noister content.

(ii)    Volatile Carbonaceons Matter (VCM)

(iii)    Ash content    .

(iv)    Fixed carbon

(a)    Biodegradable polymers.

(b)    Mass spectroscopy and its applications.

(c)    Straight run gasoline and-jet fuel.

(d)    Calomel electrode.

(e)    LDPE and HDPE

(0 Clausius - Clevperon equation.    .

OR    '

Gibbs - Helmholtz equation.    .    

Ans. (a) Biodegradable Polymers (out of syllabus) : The polymers that can be broken down rapidly by enzymen catalysed seactions are called bio degradable polymers.

The enzymes are produced by micro-organisms. The carbon carbon bonds of chain growth polymers are insert to enzyme satalysed seactions so they are now biodegradable unless bonds that can be broken by enzymes are inserted into polymer. When the polymer is burried waste, micro-organisms present in the. soil can degrade the polymer. One method of making a polymer biodegradable insolves inserting hydrolyzable ester* group into the polymer. For example, if the acctal (shown below) is added to an alkene undergoing free radical polymerzation, ester groups will be inserted into the polymer. ,

.    /HKcn,    vjjo'cii.'vni

CHy-CII-Cll. C    lu    I,

² 1 '    ¹¹    I " N>-CII^{( 11}

11    -    R    -    - , '

' ' ' O / '    ' ' O

- II .    pu_ _ rjjp . . II - . ,

CH.-CH-CH,-C~O-CH.CH.CHXIICHXH <    CH, -CH~CH₂-C -Q-CH₂CH_:CH,

I . \ I    |

, pi A weak line r    . R

The ester link being a weak link is susceptible to enzyme catalysed hydrolysis. Among the most common biodegradable polymers are poly glycolic acid (PGA), poly lactic acid (PLA) and poly hydroxy butyrate (PHB). AH are'polyester and are therefore suceplible towards enzyme catalysed hydrolysis of there ester link. Co-polymers of PGA and PLA hare found wide variety of.uses. For example, a 90/10 co-poly glycolis acid with poly laptic acid used to make absorbable sutures (thread used for goining the edges of wound by stiching). Dextron was the first biodegradable sature. She Stares are entirely degraded arid absorbed by the body within 90 days aftpr surgery. Poly hydroxy butyrate which can be used for making films for packaging as well as molded items degrades within fore weaks in land fills. However at present this polymer has limited use due its high cost which is about four times as compared to polypropylene.

Ans. (b) Same as Q.l2(c) 2002.

Applications : (i) Identification of substances : Man spectrum is highly characteristic of a compound. No two compounds can have exactly similar man spectra.

(ii)    Determination of molecular man and formula.

(iii)    Determination of molecular structure.

Ans. (c) Same as Q,8(c) 2005.

Ans. (d) Calomel Electrode : Calomel electrodes is an example of secondary reference electrode. A calomel electrode consists of mercury in contact with a paste of spasingle soluble salt of mcrcurous chloride in mercury and a solution of soluble chloride ions such as Kcl known concentration. It is represented as :

Cl (aq)/Hg₂Cl_:(&)/Hg(/)

I'he half cell reaction is

Hg₂CU<fc) _T 2Hg(/) + 2 Cl" (aq) .

Which is the resultant of the following reactions taking place :

noiJulos 0>f p W*.
aboimia tomoleO

Hg CMs) H;²(aq) + 2CI-(aq)

HgJ^:(aq) + 2e" 2Hg (1) '

I he calomel electrode is, therefore, reversible w.r. to Cl ions (anions).

In case a saturated solution of KCI is used, the electrode is called saturated calomel electrode. This electrode is commonly "used in laboratory. A dip type saturated calomel electrode is now available for use.

The electrode potential of electrode on H-scale, represented as

KCI(aq)/Hg₃CI₂(s)/Hg depends upon the concentration of KCI solution contained in it and on the temperature.

Hg CI₂(S) + 2e 2Hg (/) + 2Cr(aq)

The tenth molar electrode (0.1M KCI) has the lowest temperature Co-efficient and is preferred for accurate work. However, saturated calomel electrode is tiie most convenient due to the case of replacing the solution and is, thus frequently employed.

Ans. (e) LDPE : It is manufactured by heating ethylene to 350 K to 570 K under a pressure of 1000 - 2000 atm. and in presence of traces of oxygen (0 03 - 01y.)

350k-570k,02

n CH, - CH₂ ]ooo-2000 aim ^(CH2 ^CH2> polyethylene

This polymersiation reaction occurs through a free radical mechanism initiated by oxygen. Poly there produced by this process has a molecular mass around 20,000 and has a branched chain structure. Since branched chain molecule do not pack well, this type of polythene has low density (0-92 gm cm^-3) and low melting point (384 K). That is why polythene produced bv this method is called low density polythene.

Propcrtis : It is transparent, has moderate tensile strength and high toughness. It is chens cally insert, slightly flexible and poor conductor of electricity.

Uses : It is used (2) as packing material (in the form of this film, bags, etc. ; (ii) for insulating sires and cables, etc.

HDPE : It is manufactured by heating ethylene at about 333 - 343 K under a pressure 96

- 7 atm. in presence of- a catalyst such as trieltyl-aluminium and titeanium tetrachloride (zeigiar-Natta catelyet).

333 - 343K,6 ~ 7atm    "

n CH₂ - CH_{2 Seigler}__{Natta Catalyst} (CH₂ - CHj) Poly ethylene.

Properties : Polythene produced by this process mainly consists of lineear chains. Therefore, the moleculer pack well and hence this type of poly thene has density (0-97 gm cm ³), higher melting point (313 - 423K) and is inert but harder, tongher and has greater tensile strength than LDPE.

Uses : It is used (i) In the manufacture of container (buckets, tubes, etc.)

(ii)    for making pipes, bottles, toys, etc.

(iii)    As insulators, anticorrosise and packaging material.

Ans. (f) Same as Q. 10(a) 2003.

OR. Same as Q.3(d) 2005.

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