CBSE Class 12 Chemistry - Education Observer
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Transcript of CBSE Class 12 Chemistry - Education Observer
MaterialdownloadedfrommyCBSEguide.com. 1/16
CBSEClass12Chemistry
QuickRevisionNotes
Chapter1
TheSolidState
Solid:Solidisastateofmatterinwhichtheconstitutingparticlesarearrangedveryclosely.
Theconstituentparticlescanbeatoms,moleculesorions.
Propertiesofsolids:
1. Theyhavedefinitemass,volumeandshape.
2. Theyareincompressibleandrigid.
3. Intermoleculardistancesareveryshortandhencetheintermolecularforcesarestrong.
4. Theirconstituentparticleshavefixedposition.sandcanonlyoscillateabouttheirmean
positions.
Classificationofonthebasisofthearrangementofconstituentparticles:
Propertiesofcrystallinesolids:
Theyhaveadefinitegeometricalshape.
Theyhavealongrangeorder.
Theyhaveasharpmeltingpoint.
Theyareanisotropicinnaturei.e.theirphysicalpropertiesshowdifferentvalues
whenmeasuredalongdifferentdirectionsinthesamecrystal.
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Theyhaveadefiniteandcharacteristicheatoffusion.
Theyarecalledtruesolids.
Whencutwithasharpedgedtool,theysplitintotwopiecesandthenewlygenerated
surfacesareplainandsmooth.
Polymorphicformsorpolymorphs:
Thedifferentcrystallineformsofasubstanceareknownaspolymorphicformsor
polymorphs.Forexample:graphiteanddiamond.
Characteristicsofamorphoussolids:
1. Theyhaveanirregularshape.
2. Theyhaveashortrangeorder.
3. Theygraduallysoftenoverarrangeoftemperature.
4. Theyareisotropicinnaturei.e.theirphysicalpropertiesarethesameinalldirections.
5. Whencutwithasharpedgedtool,theycutintotwopieceswithirregularsurfaces.
6. Theydonothavedefiniteheatoffusion.
7. Theyarecalledpseudosolidsorsupercooledliquids.Thisisbecausetheyhavea
tendencytoflow,thoughveryslowly.
Typesofcrystallinesolids:
A.MolecularSolids
ConstituentParticles:Molecules
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Typeof
solid
Constituent
Particles
Bonding/
AttractiveForces
Electrical
conductivity
physical
nature
Melting
pointExamples
Non-polar
solidsMolecules
Dispersionor
LondonforcesInsulator Soft
Very
low
Ar,CCl4,
H2,l2,CO2
Polar
solidsMolecules
Dipole-dipole
interactionsInsulator Soft low
HCl,solid
SO2,solid
NH3
Hydrogen
bondedMolecules
Hydrogen
bondingInsulator Hard low H2O(ice)
B.IonicSolids
ConstituentParticles:Ions
Bonding/AttractiveForces:CoulombicorElectrostatic
ElectricalConductivity:Insulatorsinsolidstatebutconductsinmoltenstateandin
aqueoussolutions
PhysicalNature:Hardbutbrittle
MeltingPoint:High
Examples: ,ZnS,MgO,NaCl
C.MetallicSolids
ConstituentParticles:Positiveionsinaseaofdelocalizedelectrons
Bonding/AttractiveForces:Metallicbonding
ElectricalConductivity:Conductorsinsolidstateaswellasinmoltenstate
PhysicalNature:Hardbutmalleableandductile
MeltingPoint:Fairlyhigh
Examples:Fe,Cu,Ag,Mg
D.CovalentorNetworkSolids
ConstituentParticles:Atoms
Bonding/AttractiveForces:Covalentbonding
ElectricalConductivity:BadConductorsinsolidstateaswellasinmoltenstate
PhysicalNature:Hardbutmalleableandductile
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MeltingPoint:Fairlyhigh
Examples: ,(quartz),SiC,C(diamond),C(graphite)
Crystallattice:Aregularorderedarrangementofconstituentparticlesinthree
dimensionsiscalledcrystallattice.
Latticepointsorlatticesites:thefixedpositionsonwhichtheconstituentparticles
arepresentarecalledlatticepointsorlatticesites.Agroupoflatticepointswhich
whenrepeatedoverandoveragainin3dimensionsgivethecompletecrystallattice.
Unitcell:Itisdefinedasthesmallestrepeatingunitinspacelatticewhichwhen
repeatedoverandoveragaingeneratesthecompletecrystallattice.Thecrystalcan
consistofaninfinitenumberofunitcells.
Parameterswhichcharacterizeaunitcell:
1. Dimensionsoftheunitcellalongthethreeedges,a,bandc:theseedgesmayormaynot
bemutuallyperpendicular.
2. Inclinationoftheedgestoeachother:thisisdenotedbytheanglebetweentheedges , ,
andrespectively. istheanglebetweentheedgesbandc, istheanglebetweentheedgesa
andc,and istheanglebetweenaandb.
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Sevencrystalsystems:
1. Cubic: = = =90°,a=b=c
2. Tetragonal: = = =90°;a=b c
3. Orthorhombic: = = =90°;a b c
4. Monoclinic: = =90°, 90°;a b c
5. Hexagonal: = =90°, =120°;a=b c
6. Rhombohedralortrigonal: = = 90°;a=b=c
7. Triclinic: 90°;a b c
Typesofunitcells:
1. Primitiveorsimpleunitcellshaveconstituentparticlesonlyatitscorners.
2. Centeredunitcellsarethoseunitcellsinwhichoneormoreconstituentparticlesare
presentatpositionsinadditiontothosepresentatthecorners.
Typesofcenteredunitcells:
1. Facecenteredunitcell:Itconsistsofoneconstituentparticlepresentatthecentreofeach
faceinadditiontothosepresentatthecorners.
2. Bodycenteredunitcell:Itconsistsofaoneconstituentparticleispresentatitsbody
centreinadditiontothosepresentatthecorners.
3. Endcenteredunitcell:Itconsistsofoneconstituentparticlepresentatthecentreofany
twooppositefacesinadditiontothosepresentatthecorners.
Numberofparticlesatdifferentlatticepositions:
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Facecentre:ifanatomispresentatthecentreoftheface,itissharedbytwounit
cells.So,onlyhalfoftheatomactuallybelongstotheunitcell.
Bodycentre:ifanatomispresentatthebodycentre,itisnotsharedbyanyotherunit
cell.So,thatoneatomcompletelybelongstothesameunitcell.
Endcentre:fanatomispresentattheedgecentre,itissharedbyfourunitcells.So,
onlyonefourthofanatombelongstotheunitcell.
Numberofatomsindifferentunitcells:
1. Primitiveunitcellhave1atom
2. Facecenteredunitcellhave3atoms
3. Bodycenteredunitcellhave2atoms
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Coordinationnumber:Coordinationnumberisthenumberofnearestneighboursof
aparticle.
Closepackedstructures:
Closepackingintwodimensions:Itisgeneratedbystackingtherowsofclosepacked
spheresintwoways:
i)Squareclosepackingandii)Hexagonalclosepacking.
Closepackinginthreedimensions:Theycanbeobtainedbystackingthetwo
dimensionallayersoneabovetheother.Itcanbeobtainedintwoways:
i)Squareclosepackedlayersandii)Hexagonalclosepackedlayers.
Squareclosepacking:Here,thespheresofthesecondrowwareplacedexactly
abovethoseofthefirstrow.Thiswaythespheresarealignedhorizontallyaswellas
vertically.ThearrangementisAAAtype.Thecoordinationnumberis4.
Hexagonalclosepacking:Here,thesespheresofthesebondrowareplacedabove
thefirstoneinastaggeredmannerinsuchawaythatitsspheresfitinthedepression
ofthefirstrow.ThearrangementisABABtype.Thecoordinationnumberis6.
Threedimensionalclosepackingfromtwodimensionalsquareclosepacked
layers:Here,thespheresoftheupperlayerareplacedexactlyoverthefirstlayer
suchthespheresofthelayersareperfectlyalignedhorizontallyandvertically.Ithasa
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AAAAtypepattern.Thelatticeissimplecubiclattice.
Threedimensionalclosepackingfromtwodimensionalhexagonalclosepacked
layers:Therearetwostepsinvolvedas:
1. Placingthesebondlayeroverthefirstlayer
2. Placingthethirdlayeroverthesecondlayer
Therearetwopossibilities:
Coveringtheoctahedralvoids:Here,octahedralvoidsofthesebondlayermaybe
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coveredbythespheresofthethirdlayer.ItgivesrisetoABCABCABCtypepattern.The
threedimensionalstructureiscalledcubicclosepackedstructureorfacecentered
cubicstructure.Thecoordinationnumberis12.Example:Cu,Ag.
Typesofvoids:
Octahedralvoid-Itisformedatthecentrewhensixspheresarejoinedintheformof
anoctahedron.
Inhexagonalclosepackingorcubicclosepackingarrangement,theoctahedraland
tetrahedralvoidsarepresent.Thenumberofoctahedralvoidspresentinalatticeis
equaltothenumberofclosepackedparticles.Thenumberoftetrahedralvoidsis
twicethenumberofoctahedralvoids.
Forexample:
Ifthenumberofclosepackedparticles=n
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Numberofparticlespresentinoctahedralvoids=n
Then,thenumberofparticlespresentintetrahedralvoids=2n
Packingefficiency:Itisthepercentageoftotalspaceoccupiedbyconstituentparticles
(atoms,moleculesorions).
Packingefficiencyforfacecenteredunitcell=74%
Packingefficiencyforbodycenteredcubicunitcell=68%
Packingefficiencyforsimplecubicunitcell=52.4%
Radiusratioinanoctahedralvoid:Foranatomtooccupyanoctahedralvoid,its
radiusmustbe0.414timestheradiusofthesphere.
Radiusratiofortetrahedralvoid:Foranatomtooccupyatetrahedralvoid,itsradius
mustbe0.225timestheradiusofthesphere.
Densityofaunitcellissameasthedensityofthesubstance.
Relationshipbetweenradiusofconstituentparticle(r)andedgelength(a):
1. Simplecubicunitcell:a=2r
2. Facecenteredunitcell:a=
3. Bodycenteredunitcell:a=
Volumeofaunitcell=(edgelength)3=
1. Simplecubicunitcell:Volume=
2. Facecenteredunitcell:Volume=
3. Bodycenteredunitcell:Volume=
Numberofatomsinaunitcell(z):
1. Simplecubicunitcell:z=1
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2. Facecenteredunitcell:z=4
3. Bodycenteredunitcell:z=2
Densityofunitcell=
Crystaldefectsarebasicallyirregularitiesinthearrangementofconstituentparticles.
Typesofdefects:
1. Pointdefects-Pointdefectsaretheirregularitiesordeviationsfromidealarrangement
aroundapointoranatominacrystallinesubstance.
2. Linedefects-Linedefectsaretheirregularitiesordeviationsfromidealarrangementin
entirerowsoflatticepoints.
Differenttypesofpointdefects:
Differenttypesofstoichiometricdefectsfornon-ionicsolids:
Interstitialdefect:Acrystalissaidtohaveinterstitialdefectwhensomeconstituent
particles(atomsormolecules)occupyaninterstitialsite.Thisdefectresultsin
increaseindensityofthesubstance.
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Differenttypesofstoichiometricdefectsforionicsolids:
Frenkelordislocationdefect:Inthisdefect,thesmallerion(usuallycation)is
dislocatedfromitsnormalsitetoaninterstitialsite.Itcreatesavacancydefectatits
originalsiteandaninterstitialdefectatitsnewlocation.Itdoesnotchangethe
densityofthesolid.Frenkeldefectisshownbyionicsubstanceinwhichthereisa
largeddifferenceinthesizeofions.ItincludesZnS,AgCl,AgBrandAgI.
Differenttypesofnon-stoichiometricdefects:
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Metaldeficiency:Thisdefectarisesbecauseofabsenceofmetalionsfromitslattice
sites.Theelectricalneutralityismaintainedbyanadjacentionhavingahigher
positivecharge.
Reasonsforthecauseofmetalexcessdefect:
a)Anionicvacancies:Acompoundmayhaveanextrametalionifthenegativeionisabsent
fromitslatticesite.Thisemptylatticesiteiscalledahole.Tomaintainelectricalneutrality
thissiteisoccupiedbyanelectron.Theholeoccupiedbyanelectroniscalledf-centreor
Farbenzentercentre.TheF-centreisresponsibleforthecolourofthecompound.
b)Presenceofextracations:Acompoundissaidtohaveextracationsifacationispresent
intheinterstitialsite.Anelectronispresentintheinterstitialsitetomaintaintheelectrical
neutrality.
Classificationofsolidsbasedontheirelectricalconductivities:
a)Conductors:Thesolidswithconductivitiesrangingbetween to are
calledconductors.
b)Insulators:Thesearethesolidswithverylowconductivitiesrangingbetween to
.
c)Semi-conductors:Thesearethesolidswithconductivitiesintheintermediaterangefrom
to .
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b)Incaseofinsulators,theforbiddengapisverylargeandtheelectronsareunabletoexcite
totheconductionband.
c)Incaseofsemiconductors,forbiddengapissmall.Therefore,someelectronsmayjumpto
conductionbandandshowsomeconductivity.Electricalconductivityofsemiconductors
increaseswithriseintemperature,sincemoreelectronscanjumptotheconductionband.
Typesofsemiconductors:
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a)Intrinsicsemiconductors:Thesearethosesemiconductorsinwhichtheforbiddengapis
small.Onlysomeelectronsmayjumptoconductionbandandshowsomeconductivity.They
haveverylowelectricalconductivity.Example:Silicon,germanium.
b)Extrinsicsemiconductors:Whenanappropriateimpurityisaddedtoanintrinsic
semiconductor,itiscalledextrinsicsemiconductors.Theirelectricalconductivityishigh.
Doping:Theprocessofaddinganappropriateamountofsuitableimpurityto
increasetheconductivityofsemiconductorsisknownasdoping.
a)Then-typesemiconductors:Theyareformedwhensiliconisdopedwithelectronrich
impuritylikegroup15elements.Theincreaseinconductivityisduetothenegatively
chargedelectrons.
b)Thep-typesemiconductors:Theyareformedwhensiliconisdopedwithelectron
deficientimpuritylikegroup13elements.Theincreaseinconductivityisduetothe
positivelychargedholes.
Typesofextrinsicsemiconductors:
Diode:Itisacombinationofn-typeandp-typesemiconductorsandisusedasa
rectifier.
Transistors:Theyaremadebysandwichingalayerofonetypeofsemiconductor
betweentwolayersoftheothertypeofsemiconductor.Thenpnandpnptypeof
transistorsareusedtodetectoramplifyradiooraudiosignals.
The12-16compounds:Thesecompoundsareformedbythecombinationofgroup12
andgroup16compounds.Theypossessanaveragevalencyof4.Examples-
ZnS,CdS,CdSeandHgTe.
The13-15compounds:Thesecompoundsareformedbythecombinationofgroup13
andgroup15compounds.Theypossessanaveragevalencyof4.Examples-InSb,AlP
andGaAs.
Everysubstancehassomemagneticpropertiesassociatedwithit.Theoriginofthese
propertiesliesintheelectrons.
Eachelectroninanatombehaveslikeatinymagnet.Itsmagneticmomentoriginates
fromtwotypesofmotions:
(i)itsorbitalmotionaroundthenucleusand
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(ii)itsspinarounditsownaxis.
Classificationofsubstancesbasedontheirmagneticproperties:
1. Paramagneticsubstances:Thesearethosesubstanceswhichareweaklyattractedbythe
magneticfield.Itisduetopresenceofoneormoreunpairedelectrons.
2. Diamagneticsubstances:Diamagneticsubstancesareweaklyrepelledbyamagnetic
field.Diamagnetismisshownbythosesubstancesinwhichalltheelectronsarepaired
andtherearenounpairedelectrons.
3. Ferromagneticsubstances:Thesearethosesubstanceswhichareattractedevery
stronglybyamagneticfield.
4. Antiferromagneticsubstances:Theyhaveequalnumberofparallelandantiparallel
magneticdipolesresultinginazeronetdipolemoment.
5. Ferrimagneticsubstances:Theyhaveunequalnumberofparallelandantiparallel
magneticdipolesresultinginanatdipolemoment.