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CBSEQuestionPaper2018
Class12Chemistry
Timeallowed:3hours
MaximumMarks:70
GeneralInstructions:
i. Allquestionsarecompulsory.
ii. Questionsnumber1to5areveryshortanswerquestionsandcarry1markeach.
iii. Questionsnumber6to10areshortanswerquestionsandcarry2markseach.
iv. Questionsnumber11to22arealsoshortanswerquestionsandcarry3markseach.
v. Questionnumber23isavaluebasedquestionandcarries4marks.
vi. Questionsnumber24to26arelonganswerquestionsandcarry5markseach.
vii. Uselogtables,ifnecessary.Useofcalculatorsisnotallowed.
1. AnalysisshowsthatFeOhasanon-stoichiometriccompositionwithformulaFe0·95O.
Givereason.
Ans.Showsmetaldeficiencydefect/ItisamixtureofFe2+andFe3+/SomeFe2+ionsare
replacedbyFe3+/Someoftheferrousionsgetoxidisedtoferricions.
2. CO(g)andH2(g)reacttogivedifferentproductsinthepresenceofdifferent
catalysts.Whichabilityofthecatalystisshownbythesereactions?
Ans.Selectivityofacatalyst
3. WritethecoordinationnumberandoxidationstateofPlatinuminthecomplex
[Pt(en)2Cl2].
Ans.CoordinationNumber=6,OxidationState=+2
4. Outofchlorobenzeneandbenzylchloride,whichonegetseasilyhydrolysedby
aqueousNaOHandwhy?
Ans.Benzylchloride;Duetoresonance,stablebenzylcarbocationisformed
5. WritetheIUPACnameofthefollowing:
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Ans.3,3-Dimethylpentan-2-ol
6. Calculatethefreezingpointofasolutioncontaining60gofglucose(Molarmass=
180gmol-1)in250gofwater.
(Kfofwater=1·86Kkgmol-1)
Ans.
=2.48K
2.48=273.15-Tf
Tf=270.67K/270.52K/-2.48oC
7. Forthereaction
therateofformationofNO2(g)is .Calculatetherateof
disappearanceofN2O5(g).
Ans.
RateofdisappearanceofN2O5
(Deducthalfmarkifunitiswrongornotwritten)
8. AmongthehydridesofGroup-15elements,whichhavethe
a. lowestboilingpoint?
b. maximumbasiccharacter?
c. highestbondangle?
d. maximumreducingcharacter?
Ans.
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a. PH3
b. NH3
c. NH3
d. BiH3
9. Howdoyouconvertthefollowing?
a. EthanaltoPropanone
b. ToluenetoBenzoicacid
OR
Accountforthefollowing:
a. AromaticcarboxylicacidsdonotundergoFriedel-Craftsreaction.
b. pKavalueof4-nitrobenzoicacidislowerthanthatofbenzoicacid.
Ans.
a.
b.
OR
a. becausethecarboxylgroupisdeactivatingandthecatalystaluminiumchloride
(Lewisacid)getsbondedtothecarboxylgroup
b. Nitrogroupisanelectronwithdrawinggroup(-Ieffect)soitstabilisesthecarboxylate
anionandstrengthenstheacid/DuetothepresenceofanelectronwithdrawingNitro
group(-Ieffect).
10. Completeandbalancethefollowingchemicalequations:
a.
b.
Ans.
a.
b.
11. Givereasonsforthefollowing:
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a. Measurementofosmoticpressuremethodispreferredforthedeterminationof
molarmassesofmacromoleculessuchasproteinsandpolymers.
b. Aquaticanimalsaremorecomfortableincoldwaterthaninwarmwater.
c. Elevationofboilingpointof1MKClsolutionisnearlydoublethanthatof1M
sugarsolution.
Ans.
a. Ascomparedtoothercolligativeproperties,itsmagnitudeislargeevenforverydilute
solutions/macromoleculesaregenerallynotstableathighertemperaturesand
polymershavepoorsolubility/pressuremeasurementisaroundtheroom
temperatureandthemolarityofthesolutionisusedinsteadofmolality.
b. Becauseoxygenismoresolubleincoldwateroratlowtemperature.
c. DuetodissociationofKCl/KCl(aq) K++Cl-,iisnearlyequalto2
12. Anelement‘X’(At.mass=40gmol-1)havingf.c.c.structure,hasunitcelledge
lengthof400pm.Calculatethedensityof‘X’andthenumberofunitcellsin4gof
‘X’. .
Ans.
=4.15g/cm3
Noofunitcells=totalnoofatoms/4
13. Afirstorderreactionis50%completedin40minutesat300Kandin20minutesat
320K.Calculatetheactivationenergyofthereaction.(Given:log2=0·3010,log4=
0·6021,R=8·314JK-1mol-1).
Ans.k2=0.693/20,
k1=0.693/40
Ea=27663.8J/molor27.66kJ/mol
14. Whathappenswhen
a. afreshlypreparedprecipitateofFe(OH)3isshakenwithasmallamountofFeCl3
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solution?
b. persistentdialysisofacolloidalsolutioniscarriedout?
c. anemulsioniscentrifuged?
Ans.
a. Peptisationoccurs/ColloidalsolutionofFe(OH)3isformed
b. Coagulationoccurs
c. Demulsificationorbreaksintoconstituentliquids
15. WritethechemicalreactionsinvolvedintheprocessofextractionofGold.Explain
theroleofdiluteNaCNandZninthisprocess.
Ans.4Au(S)+8CN-(aq)+2H2O(aq)+O2(g) 4[Au(CN)2]-(aq)+4OH-(aq)
2[Au(CN)2]-(aq)+Zn(s) 2Au(S)+[Zn(CN)4]
2-(aq)
NaCNleachesgold/NaCNactsasaleacingagent/complexingagentZnactsasreducing
agent/Zndisplacesgold.
16. Givereasons:
a. EovalueforMn3+/Mn2+coupleismuchmorepositivethanthatforFe3+/Fe2+.
b. Ironhashigherenthalpyofatomizationthanthatofcopper.
c. Sc3+iscolourlessinaqueoussolutionwhereasTi3+iscoloured.
Ans.
a. ThecomparativelyhighvalueforMnshowsthatMn2+(d5)isparticularlystable/
MuchlargerthirdionisationenergyofMn(wheretherequiredchangeisfromd5to
d4)
b. Duetohighernumberofunpairedelectrons.
c. Absenceofunpairedd-electroninSc3+whereasinTi3+thereisoneunpairedelectron
orTi3+showsd-dtransition.
17. a. Identifythechiralmoleculeinthefollowingpair:
b. Writethestructureoftheproductwhenchlorobenzeneistreatedwithmethyl
chlorideinthepresenceofsodiummetalanddryether.
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c. Writethestructureofthealkeneformedbydehydrohalogenationof1-bromo-1-
methylcyclohexanewithalcoholicKOH.
Ans.
a. (i)/
b.
c.
18. (A),(B)and(C)arethreenon-cyclicfunctionalisomersofacarbonylcompoundwith
molecularformulaC4H8O.Isomers(A)and(C)givepositiveTollens’testwhereas
isomer(B)doesnotgiveTollens’testbutgivespositiveIodoformtest.Isomers(A)
and(B)onreductionwithZn(Hg)/conc.HClgivethesameproduct(D).
a. Writethestructuresof(A),(B),(C)and(D).
b. Outof(A),(B)and(C)isomers,whichoneisleastreactivetowardsadditionof
HCN?
Ans.
a. A=CH3CH2CH2CHO
B=CH3COCH2CH3
C=(CH3)2CHCHO
D=CH3CH2CH2CH3
b. B
19. Writethestructuresofthemainproductsinthefollowingreactions:
i.
ii.
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iii.
Ans.
i.
ii. C6H5CH(OH)CH3
iii. C2H5I+C6H5OH(Nosplittingofmarks)
20. a. Whyisbithionaladdedtosoap?
b. Whatistinctureofiodine?Writeitsoneuse.
c. Amongthefollowing,whichoneactsasafoodpreservative?
Aspartame,Aspirin,SodiumBenzoate,Paracetamol
Ans.
a. Toimpartantisepticproperties
b. 2-3%solutionofiodineinalcohol–watermixture/iodinedissolvedinalcohol,used
asanantiseptic/appliedonwounds.
c. Sodiumbenzoate/Aspartame
21. Definethefollowingwithanexampleofeach:(a)Polysaccharides(b)Denatured
protein(c)Essentialaminoacids
OR
a. WritetheproductwhenD-glucosereactswithconc.HNO3.
b. Aminoacidsshowamphotericbehaviour.Why?
c. Writeonedifferencebetween and -pleatedstructuresofproteins.
Ans.
a. Carbohydratesthatgivelargenumberofmonosaccharideunitsonhydrolysis/large
numberofmonosaccharidesunitsjoinedtogetherbyglycosidiclinkageStarch/
glycogen/cellulose(oranyother)
b. Proteinsthatlosetheirbiologicalactivity/proteinsinwhichsecondaryandtertiary
structuresaredestroyedCurdlingofmilk(oranyother)
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c. Aminoacidswhichcannotbesynthesisedinthebody.Valine/Leucine(oranyother)
OR
a. Saccharicacid/COOH-(CHOH)4-COOH
b. Duetothepresenceofcarboxylandaminogroupinthesamemolecule/dueto
formationofzwitterionordipolarion.
c. α-helixhasintramolecularhydrogenbondingwhileβpleatedhasintermolecular
hydrogenbonding/α-helixresultsduetoregularcoilingofpolypeptidechainswhile
inβpleatedallpolypeptidechainsarestretchedandarrangedsidebyside.
22. a. Writetheformulaofthefollowingcoordinationcompound:Iron(III)
hexacyanoferrate(II)
b. Whattypeofisomerismisexhibitedbythecomplex[Co(NH3)5Cl]SO4?
c. Writethehybridisationandnumberofunpairedelectronsinthecomplex
[CoF6]3-.(AtomicNo.ofCo=27)
Ans.
a. Fe4[Fe(CN)6]3
b. Ionisationisomerism
c. sp3d2,4
23. Shyamwenttoagroceryshoptopurchasesomefooditems.Theshopkeeperpacked
alltheitemsinpolythenebagsandgavethemtoShyam.ButShyamrefusedto
acceptthepolythenebagsandaskedtheshopkeepertopacktheitemsinpaper
bags.Heinformedtheshopkeeperabouttheheavypenaltyimposedbythe
governmentforusingpolythenebags.Theshopkeeperpromisedthathewoulduse
paperbagsinfutureinplaceofpolythenebags.
Answerthefollowing:
a. Writethevalues(atleasttwo)shownbyShyam.
b. Writeonestructuraldifferencebetweenlow-densitypolytheneandhigh-density
polythene.
c. WhydidShyamrefusetoaccepttheitemsinpolythenebags?
d. Whatisabiodegradablepolymer?Giveanexample.
Ans.
a. Concernedaboutenvironment,caring,sociallyalert,lawabidingcitizen(oranyother
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2values)
b. Lowdensitypolytheneishighlybranchedwhilehighdensitypolytheneislinear.
c. Asitisnon-biodegradable.
d. Whichcanbedegradedbymicroorganisms,egPHBV(oranyothercorrectexample)
24. a. Givereasons:
i. H3PO3undergoesdisproportionationreactionbutH3PO4doesnot.
ii. WhenCl2reactswithexcessofF2,ClF3isformedandnotFCl3.
iii. DioxygenisagaswhileSulphurisasolidatroomtemperature.
b. Drawthestructuresofthefollowing:
i. XeF4
ii. HClO3
OR
a. Whenconcentratedsulphuricacidwasaddedtoanunknownsaltpresentina
testtubeabrowngas(A)wasevolved.Thisgasintensifiedwhencopperturnings
wereaddedtothistesttube.Oncooling,thegas(A)changedintoacolourless
solid(B).
i. Identify(A)and(B).
ii. Writethestructuresof(A)and(B).
iii. Whydoesgas(A)changetosolidoncooling?
b. Arrangethefollowinginthedecreasingorderoftheirreducingcharacter:
HF,HCl,HBr,HI
c. Completethefollowingreaction
Ans.
a. i. In+3oxidationstateofphosphorustendstodisproportionatetohigherandlower
oxidationstates/OxidationstateofPinH3PO3is+3soitundergoes
disproportionationbutinH3PO4itis+5whichisthehighestoxidationstate,soit
cannot.
ii. Fcannotshowpositiveoxidationstateasithashighestelectronegativity/Because
Fluorinecannotexpanditscovalency/AsFluorineisasmallsizedatom,itcannot
packthreelargesizedClatomsaroundit.
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iii. Oxygenhasmultiplebondingwhereassulphurshowscatenation/Dueto
bondinginoxygenwhereassulphurdoesnot/Oxygenisdiatomicthereforeheld
byweakintermolecularforcewhilesulphurispolyatomicheldbystrong
intermolecularforces.
b. i.
ii.
OR
a. i. A=NO2,B=N2O4
ii. ,
iii. BecauseNO2dimerisestoN2O4/NO2isanoddelectronspecies.
b. HI>HBr>HCl>HF
c.
25. a. Writethecellreactionandcalculatethee.m.f.ofthefollowingcellat298K:
b. Givereasons:
i. OnthebasisofEovalues,O2gasshouldbeliberatedatanodebutitisCl2gas
whichisliberatedintheelectrolysisofaqueousNaCl.
ii. ConductivityofCH3COOHdecreasesondilution.
OR
a. Forthereaction
2AgCl(s)+H2(g)(1atm) 2Ag(s)+2H+(0.1M)+2Cl-(0.1M),
at25oC
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Calculatethee.m.f.ofthecell.
[log10-n=-n]
b. Definefuelcellandwriteitstwoadvantages.
Ans.
a. (Equationmustbebalanced)
=0.14-0.0295log10=0.11V/0.1105V
b. i. Duetooverpotential/OvervoltageofO2
ii. Thenumberofionsperunitvolumedecreases
OR
a.
Eo=0.226V
E=Eo-0.059/2log([H+)2[Cl-]2/[H2])
=0.226-0.059/2log[(0.1)2 (0.1)2]/1
=0.226-0.059/2log10-4
=0.226+0.118=0.344V
b. Cellsthatconverttheenergyofcombustionoffuels(likehydrogen,methane,
methanol,etc.)directlyintoelectricalenergyarecalledfuelcells.Advantages:High
efficiency,nonpolluting(oranyothersuitableadvantag)
26. a. Writethereactionsinvolvedinthefollowing:
i. Hofmannbromamidedegradationreaction
ii. Diazotisation
iii. Gabrielphthalimidesynthesis
b. Givereasons:
i. (CH3)2NHismorebasicthan(CH3)3Ninanaqueoussolution.
ii. Aromaticdiazoniumsaltsaremorestablethanaliphaticdiazoniumsalts.
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OR
a. Writethestructuresofthemainproductsofthefollowingreactions:
i.
ii.
iii.
b. GiveasimplechemicaltesttodistinguishbetweenAnilineandN,N-
dimethylaniline.
c. ArrangethefollowingintheincreasingorderoftheirpKbvalues:
C6H5NH2,C2H5NH2,C6H5NHCH3
Ans.
a. i. (i)Ar/R-CONH2+Br2+4NaOH Ar/R-NH2+2NaBr+Na2CO3+2H2O
ii. (or
anyothercorrectequation)
iii.
b. i. Becauseofthecombinedfactorsofinductiveeffectandsolvationorhydration
effect
ii. Duetoresonancestabilisationorstructuralrepresentation/resonatingstructures.
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OR
a. i. C6H5NHCOCH3
ii. C6H5SO2N(CH3)2
iii. C6H6
b. AddchloroforminthepresenceofKOHandheat,Anilinegivesaoffensivesmell
whileN,Ndimethylanilinedoesnot.(oranyothercorrecttest)
c. C2H5NH2<C6H5NHCH3<C6H5NH2
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