AP 化学 BONDING.docx
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AP 化学 BONDING.docx
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AP化学BONDING
TheAdvancedPlacementExaminationinChemistry
PartII-FreeResponseQuestions&Answers
1970to2009
Bonding&MolecularStructure
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1973D
Discussbrieflytherelationshipbetweenthedipolemomentofamoleculeandthepolarcharacterofthebondswithinit.Withthisasthebasis,accountforthedifferencebetweenthedipolemomentsofCH2F2andCF4.
Answer:
Inordertohaveadipolemoment(i.e.,tobeapolarmolecule)amoleculemusthavepolarbondsandmusthaveamoleculargeometrywhichisnotsymmetrical(i.e.,oneinwhichthevectorsumofthebonddipoles0).
InCH2F2theC-FandC-Hbondsarepolarandthemoleculeisnotsymmetrical;therefore,themoleculesispolarandwouldshowadipolemoment.
InCF4theC-Fbondsarepolar,butthemoleculeissymmetrical;therefore,themoleculeisnon-polarandwouldnotshowadipolemoment.
1974D
ThepossiblestructuresforthecompounddinitrogenoxideareNNOandNON.Byexperimentationithasbeenfoundthatthemoleculeofdinitrogenoxidehasanon-zerodipolemomentandthationsofmass44,30,28,16,and14areobtainedinthemassspectrometer.Whichofthestructuresissupportedbythesedata?
Showhowthedataareconsistentwiththisstructure.
Answer:
ThecorrectstructureisNNO.N-N=O;N-Obonddi-pole;non-linearstructure,non-symmetrical;moleculardipolemoment.
SpectralData(massofmolecularfragments):
44=NNO28=NN14=N30=NO16=O
Afragmentof28couldn’tbemadeifthestructurewasNON.
1974D
Theboilingpointsofthefollowingcompoundsincreaseintheorderinwhichtheyarelistedbelow:
CH4 Discussthetheoreticalconsiderationsinvolvedandusethemtoaccountforthisorder. Answer: CH4-weakLondondispersion(vanderWaals)forces H2S-Londonforces+dipole-dipoleinteractions NH3-London+dipole+hydrogenbonding 1975D SupposethatamoleculehastheformulaAB3.Sketchandnametwodifferentshapesthatthismoleculemayhave.Foreachofthetwoshapes,giveanexampleofaknownmoleculethathasthatshape.Foroneofthemoleculesyouhavenamed,interprettheshapeinthecontextofamodernbondingtheory. Answer: Example: trigonalplanar,BF3;trigonalpyramid,NH3 ForBF3,theboronatomissurroundedbythreepairsofelectrons,thearrangementthatwillminimizetherepulsionsisaflat(planar)arrangementwiththeelectronpairsfurthestapartat120ºangles.OR TheNH3moleculehasfourpairsofelectrons: threebondingpairsandonenon-bondingpair.Thebestarrangementforfourelectronpairsisatetrahedralstructure(109.5º)withthelone(non-bonding)electronpairattheapexrequiringmorespacethanthebondingpairs,compressingthebondingpairstoanangleof107º.Themolecularstructureisalwaysbasedonthepositionsoftheatoms,thereforeitisatrigonalpyramidratherthanatetrahedron. 1976D NF3andPF5arestablemolecules.Writetheelectron-dotformulasforthesemolecules.Onthebasisofstructuralandbondingconsiderations,accountforthefactthatNF3andPF5arestablemoleculesbutNF5doesnotexist. Answer: Describethesp3bondingforNF3andthesp3dforPF5. NonexistenceofNF5becauseofnolowenergydorbitalforN. 1978D Statepreciselywhatismeantbyeachofthefollowingfourterms.Thendistinguishclearlybetweeneachofthetwotermsinpart(a)andbetweeneachofthetwotermsinpart(b),usingchemicalequationsorexampleswherehelpful. (a)Bondpolarityandmolecularpolarity(dipolemoment) (b)ForametalM,ionizationenergyandelectrodepotential. Answer: (a)Bondpolarity-resultingfromunequalsharingofelectronsbetweenbondingatoms;orfrombondingofatomswithdifferentelectronegativities. Molecularpolarity-resultoftheseparationofthecentersofpositiveandnegativechargesinanentiremolecule(Thedipolemomentisameasure).;ortheresultofthenon-zerovectorsumofbonddipolesandlone-pairelectrons. Distinction(normallyincludedwithinthedefinitions). (b)Ionizationenergy-energyrequiredtoremoveanelectronfromanatom[ifatomisdescribedasgaseousorisolated-1additionalpoint] Electrodepotential-relatedtoenergyassociatedwithoxidationorreductionorassociatedwithatendencytogainorloseelectrons. Aquantitymeasuredrelativetothehydrogenelectrodeorrelatedtotheenergychangesinanelectrochemicalcell. Distinction(normallyincludedwithinthedefinitions). 1979D DrawLewisstructuresforCO2,H2,SO3andSO32-andpredicttheshapeofeachspecies. Answer: 1979D Butane,chloroethane,acetone,and1-propanolallhaveapproximatelythesamemolecularweights.Dataontheirboilingpointsandsolubilitiesinwaterarelistedinthetablebelow. Compound Formula BoilingPt.(ºC) Solubilityinwater Butane CH3CH2CH2CH3 0 insoluble Chloroethane CH3CH2Cl 12 insoluble Acetone CH3C CH3 56 completelymiscible 1-Propanol CH3CH2CH2OH 97 completelymiscible Onthebasisofdipolemoments(molecularpolarities)and/orhydrogenbonding,explaininaqualitativewaythedifferencesinthe (a)boilingpointsofbutaneandchloroethane. (b)watersolubilitiesofchloroethaneandacetone. (c)watersolubilitiesofbutaneand1-propanol. (d)boilingpointsofacetoneand1-propanol. Answer: (a)Butaneisnonpolar;chloroethaneispolar.Intermolecularforcesofattractioninliquidchloroethanearelargerduetodipole-dipoleattraction;thusahigherboilingpointforchloroethane. (b)Bothchloroethaneandacetonearepolar.However,acetoneformshydrogenbondstowatermuchmoreeffectivelythanchloroethanedoes,resultingingreatersolubilityofacetoneinwater. (c)Butaneisnon-polarandcannotformhydrogenbonds;1-propanolispolarandcanformhydrogenbonds.1-propanolcaninteractwithwaterbybothdipole-dipoleforcesandhydrogenbonds.Butanecaninteractwithwaterbyneithermeans.Thus,1-propanolismuchmoresoluble. (d)AcetonemoleculesareattractedtoeachotherbyvanderWaalsattractionanddipole-dipoleattraction.1-propanolmoleculesshowthesetwotypesofattraction.However,1-propanolcanalsoundergohydrogenbonding.Thisdistinguishingfeatureresultsinthehigherboilingpointof1-propanol. 1982D (a)DrawtheLewiselectron-dotstructuresforCO32-,CO2,andCO,includingresonancestructureswhereappropriate. (b)WhichofthethreespecieshastheshortestC-Obondlength? Explainthereasonforyouranswer. (c)Predictthemolecularshapesforthethreespecies.Explainhowyouarrivedatyourpredictions. Answer: (a) (b)COhastheshortestbondbecausethereisatriplebond.ORbecausethereisthegreatestnumberofelectronsbetweenCandOinCO. (c)CO32-trigonalplanar(planarandtriangular).Cbondingissp2hybrid-or-Chasthreebondingpairsandnolonepair. CO2linear.Cbondingissphybrid-or-Chastwobondingpairsandnolonepairs-or-CO2isnonpolarandmustbelinear. COlinear.Twoatomsdetermineastraightline. 1982D Thevaluesofthefirstthreeionizationenergies(I1,I2,I3)formagnesiumandargonareasfollows: I1 I2 I3 (kJ/mol) Mg 735 1443 7730 Ar 1525 2665 3945 (a)GivetheelectronicconfigurationsofMgandAr. (b)Intermsoftheseconfigurations,explainwhythevaluesofthefirstandsecondionizationenergiesofMgaresignificantlylowerthanthevaluesforAr,whereasthethirdionizationenergyofMgismuchlargerthanthethirdionizationenergyofAr. (c)IfasampleofArinonecontainerandasampleofMginanothercontainerareeachheatedandchlorineispassedintoeachcontainer,whatcompounds,ifany,willbeformed? Explainintermsoftheelectronicconfigurationsgiveninpart(a). (d)ElementQhasthefollowingfirstthreeionizationenergies: I1 I2 I3 (kJ/mol) Q 496 4568 6920 WhatistheformulaforthemostlikelycompoundofelementQwithchlorine? Explainthechoiceofformulaonthebasisoftheionizationenergies. Answer: (a)Mg: 1s22s22p63s1 Ar: 1s22s22p63s23p6 (b)ValenceelectronsforMgandArareinthesameprincipalenergylevel,butAratomissmallerandhasagreaternuclearcharge.Thus,ionizationenergiesforMgarelessthanthoseforAr.RemovalofthirdelectronfromMgatomisfromn=2levelandelectronsinthislevelexperiencestrongnuclearattraction. (c)OnlyMgCl2forms.Mgatomsreadilylose2valenceelectronseach.Ionizationenergyforthirdelectronveryhigh.ElectronaffinityforArislow,andionizationenergiesforAratomsarehigh. (d)FormulaisQCl.Very
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