topic 3 2009 chemical bonding prelim soln

ChemicalBonding-SuggestedSolutions

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Topic3:ChemicalBonding

ACJC/P3/Q2a1.

I Ix Ixx xx

xxLinear

AJC/P2/Q1b(i)2. Al2O3 has a giant ionic lattice structure with strong electrostatic forces of attraction

betweenoppositelychargedAl3+andO2-ions.SO2andSO3havesimplemolecularstructureswithweakvanderWaalsforcesbetweenthemolecules.

ACJC/P2/Q2d(i)3. Br2hasasmallerelectroncloudsizethanI2.

electroncloudofBr2islesspolarisable weakervanderWaalsforcesbetweenthemolecules lessenergyisrequiredtoovercometheinteractionbetweenBr2molecules,leadingtoa

lowerboilingpoint.

AJC/P3/Q5b,c4(a) MoreenergyisneededtoovercomethestronghydrogenbondsbetweenHFmolecules,

as compared to the weaker permanent dipole permanent dipole interactions betweenHClmolecules,henceHFhasahigherboilingpointthanHCl.HIhasahigherboilingpointcomparedtoHCl,asithasalargerelectroncloud,whichismore polarisable, leading to stronger and more extensive van der Waals forces thatrequiremoreenergytoovercome.

(b) BrF3has2lonepairsand3bondpairsaroundthecentralBratom,whichleadstoaT-shapegeometry.InBrF2+,thereare2lonepairsand2bondpairsaroundthecentralBratomwhichleadstoabent(V-shape)geometry.


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SbF5 has 5 bond pairs around the central Sb atom, which leads to a trigonalbipyramidalgeometry. InSbF6-, thereare6bondpairsaround thecentralSbatom,whichleadstoanoctahedralgeometry.

DHS/P2/Q2c5.

DHS/P3/Q4b,c6(a) SiO2 has a giant molecular structure with extensive covalent bonding in a giant

three-dimensionalstructure. PbO2hasagiant ionicstructure. In thesolidstate, the ionscanonlyvibrateabout

fixedpositions.(b)A largeamountof energy is required toovercome thestrongelectrostatic forcesof

attractionbetweenthecationsandseaofdelocalisedelectrons.

HCI/P3/Q1a(ii)7.

ShapewithrespecttoeachP:Trigonalpyramidal;Bondangle:107o

HCI/P3/Q1d(i)8. BothCl2and I2havesimplemolecularstructures,andexistasdiatomicmoleculeswith

weakvanderWaalsforcesexistingbetweenmolecules.


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I2hasabiggerelectroncloudthanCl2andhence,strongerandmoreextensivevanderWaalsforcesbetweenthemolecules.ThereforeI2islessvolatilethanCl2.

HCI/P3/Q3a

9(a)

Na Mg Al Si P Si Cl Ar

M.p./oC

HighmeltingpointfromNatoAlastheyexistasgiantmetallicstructureswithstrongmetallicbondsof increasingstrengthsdue tosmallercationicradiusand increasednumberofdelocalizedelectrons.

Very high melting point for Si as it exists as a giant covalent structure with anextensive network of strong covalent bonds. These strong bonds require a lot ofenergytobreakbeforemeltingcanoccur.

Low melting point from P to Ar as they exist as simple molecular structures,consisting of discrete molecules with weak van der Waals forces between themolecules.MeltingpointdecreasesfromS8>P4>Cl2>Arbecausethesizeoftheelectroncloudsdecreases fromS8>P4>Cl2>Ar,such thatweakvanderWaalsforcesareweaker.

IJC/P2/Q1b10. No.AlthoughionicbondsbetweenK+andCl- ionsarebrokenin theboilingofKCl, it is

weak van der Waals forces between tetrachloromethane molecules that are brokenduringboiling,notcovalentbondswithintetrachloromethanemolecules.

S

Melting

point /oC


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IJC/P2/Q2a(ii)11. SiO2 exists as a giant covalent structure with an extensive network of strong covalent

bonds. These strong bonds require a lot of energy to break before melting can occur,henceSiO2hasaveryhighmeltingpoint.

IJC/P2/Q2b(i)12.

JJC/P2/Q1a(i)13.

Bent/angular/v-shapetrigonalplanar

MI/P3/Q4b

14. Shapeisbent,hencebondangle=104.5o

MJC/P3/Q2c(I,ii)15(i)Iodine has a large enough atomic size to accommodate the number of O atoms

surroundingitinIO53-andIO65-.(ii)

SOO

OSOO

x

xx xx

xx

xxxx

xx

xx

xx

x

x

xx

xx xx

xxx

O

OO

IOO

O

x x

x x

xx

xx

xx

xx

x

5-

HO

OH


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N Nxxxxxxxx xx N Nxxxxxxxx xx O

NJC/P3/Q1a,b(i)16 (a)Electronegativity is defined as the ability of an atom to attract bonding pair of

electronstoitselfinacovalentbond.

Theoxygenatomismoreelectronegativethanthenitrogenatom.

OxygenhasagreaternuclearchargethanNitrogenatom. Theyhavesimilarshielding/screeningeffect. EffectivenuclearchargeofOatomisgreaterthanthatofNatom; itwillattract the

bondingelectronsmorestrongly.

(b)(i)

ShapeofN2Oislinear.Bondangleis180o.

NJC/P3/Q4a17.

NYJC/P3/Q1c18. MgO>NaF>H2O>CH3NH2>CO2 CO2havesimplemolecularstructureconsistingofCO2moleculesheldtogetherbyvander

Waalsforces. CH3NH2 and H2O have simple molecular structures consisting of CH3NH2 and H2O

moleculesheldtogetherbystrongerhydrogenbonds. H2Ohas2lonepairhydrogenunitswhileCH3NH2has1thusH2Ohashigherboilingpoint. NaFandMgOhaveionicstructuresconsistingofoppositelychargedionsheldtogetherby

strong electrostatic forces. MgO has higher boiling point because it has higher ionicchargesthanNaF.

CaIx

x

x

xx

O2+ O

xxO

o

-

2


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NYJC/P3/Q5a19.

N

ClClCl

BCl

ClCl

3bp1lpTrigonalpyramidal

3bp0lpTrigonalplannar

x xx

xx

x

x x

NCl3andBCl3havesimplemolecularstructurebutNCl3hasanetoveralldipolemomentthusthemoleculesareheldtogetherbyvanderWaalsforcesduetopermanentdipoles.BCl3 has no overall dipole moment thus the molecules are held together by van derWaalsforcesduetoinduceddipolesandthereforerequirelessenergytobreakthebonds.

PJC/P2/Q1b20.NaClgiantioniclatticestructurewithstrongelectrostaticforcesofattractionbetween

oppositelychargedNa+andCl-ionsAlCl3simplecovalent/molecularstructureweaktemporaryinduceddipole-inducedipoleattractionbetweenmolecules.PCl5simplecovalent/molecularstructurepermanentdipole-permanentdipoleattractionsbetweenmolecules.AlCl3existasdimers,withstrongerinstantaneousdipole-induceddipole(duetogreaterno.ofelectrons)thanpermanentdipole-permanentdipoleattractionsofPCl5.Hencem.p.ofAlCl3ishigherthanPCl5.

PJC/P2/Q2b(ii)21.


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PJC/P3/Q2b22.

SAJC/P2/Q2b23(ii) Carbon Shape Bondangle C1 Trigonalplanar 120

C2 tetrahedral 109

SAJC/P2/Q3a24(a) (i) In water, ethanoic acid molecules form intermolecular hydrogen bonds with water

moleculesandexistasCH3CO2H.HenceitsMris60.

In a non-aqueous solvent, hexane, the id-id interactions between ethanoic acid andhexane is weaker so ethanoic acid molecules dimerise by forming strongerintermolecularhydrogenbondswithethanoicacidmolecules.HenceitsMristwicethatofCH3CO2Handbecomes120.

SAJC/P3/Q1b

25. CompoundAisheldbyhydrogenbonding.

Compound B is held by weak induced dipole-induced dipole interactions (id-id)/weakVDW.Lessenergyisneededtoovercometheweakerid-idinteractionsinB.

+ -

+ -

CH3CO

OH

CCH3

:O

HOHydrogenbond


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Although Compound C does not have H-bonding, having more electrons causesstrongerid-id(orVDW,butnotpd-pd)whichcompensatesfortheabsenceofH-bonding.HencemoreenergyisrequiredtoovercometheintermolecularforcesofattractioninC.

SRJC/P3/Q2a(i)26. MgO

hasagiantioniclatticestructure largeamountofenergy isneededtoovercomethestrongelectrostaticforces

ofattractionbetweenoppositelychargedions. hasahighboilingpoint

P4O6 hasasimplemolecularstructure smallamountofenergyisneededtoovercometheweakintermolecularvander

Waalsforcesofattraction hasalowboilingpoint

TJC/P2/Q1b27(i)

(ii) POCl3andPCl5botharesimplediscretemoleculeswithweakvanderWaalsforcesofattraction between the molecules (or POCl3 has permanent-dipole-permanent-dipoleinteractions while PCl5 has induced-dipole-induced-dipole interactions betweenmolecules).

PCl5hasamuchlargerMrthanPOCl3,andthereforealargerelectroncloudthatismoreeasilydistorted,givingrisetolargerpartialcharges,resultinginstrongervanderWaalsforces.

P

Cl

O

Cl Cl

POCl3Tetrahedral

PCl5Trigonalbipyramidal

PClCl

Cl

Cl

Cl


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TJC/P2/Q1a28. Phosphorus, sulphur and chlorine all have simple molecular structure with weak

instantaneous dipole-induced dipole forces between the molecules. Sulfur exists as S8molecules,whilephosphorusexistsasP4moleculesandchlorineexistsasCl2molecules.Hence, sulphur has the strongest intermolecular instantaneous dipole-induced dipoleforces as its molecules have the biggest electron cloud which makes them morepolarisable.

TJC/P3/Q1a29. AlF3hasagiantioniclatticestructurewithstrongelectrostaticforcesofattractionbetween

oppositelychargedAl3+andF-ions.AlCl3hasasimplemolecularstructurewithweakvanderWaalsforcesbetweenitsmolecules.MoreenergyisneededtoovercomethestrongionicbondsinAlF3thantheweakintermolecularforcesinAlCl3henceAlF3hasamuchhighermeltingpoint.

TPJC/P2/Q1b(iii)30.

TPJC/P3/Q2d31.

VJC/P2/Q3a(i),c(i)32(a)(i) NaClisanioniccompoundwithgiantionicstructureheldbystrongelectrostatic

forcesofattractionbetweentheoppositelychargedNa+andCl-ions. Silicon tetrachloride has a simple molecular structure. The molecules are non-

polarandheldbyweakinduceddipole-induceddipoleattractionswhichcanbeeasilybrokenduringmelting.

Since ionicbondsaremuchstronger than id-id,NaClmeltsatamuchhighertemperature.


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(c)(i)

VJC/P2/Q3d33. Formulaeofions:[ClO2]+and[ClO4]-

VJC/P3/Q3b34(i)Themolecular formulaesuggest that thestructureofchlorosulfonicacid isderived from

thatofsulfuricacid,with1OHgroupbeingreplacedbyCl.

Hence the extent of hydrogen bonding is less between chlorosulfonic acid moleculeswhichexplainsitslowerboilingpoint.

(ii)

SO

OO

H

O

109O

105O

OS

O

O

OH

VJC/P3/Q3e35(i)AsMgislesselectronegativethanS,theelectronegativitydifferencebetweenMgandF

islargercomparedtothatbetweenSandF. HenceMgFisionicwhereasSF2iscovalent.

(ii) MgF2hasamuchhighermeltingpointthatSF2.

Al Al

Cl Cl

Cl

Cl

Cl Cl


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ORMgF2conductselectricityinthemoltenstatebutSF2doesnotconductelectricityinanystate.

(iii) To form MgF4, Mg2+ has to lose two more electrons from the penultimate shell whichresultsinaprohibitivelyhighamountofenergyrequired.ThiscannotbecompensatedbytheenergyevolvedfromtheformationoftheioniclatticeofMgF4.

YJC/P2/Q1b(ii)36. HydrogenbondingexistsbetweenHFmolecules

YJC/P2/Q2a37.


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YJC/P3/Q5a

38.

topic 3 2009 chemical bonding prelim soln

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