intermolecular forces of attraction - cobequid educational ：分子间的吸引力——cobequid教育

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THEHIGHESTBOILINGPOINTCCL4DOES,BECAUSEITISABIGGERMOLECULEWITHMOREELECTRONSTHEINCREASEINTHEDISPERSIONFORCESMORETHANCOMPENSATESFORTHELOSSOFDIPOLEDIPOLEINTERACTIONSTHEBOILINGPOINTSARECHCL3612CCCL4768CINTERMOLECULARBONDINGHYDROGENBONDSTHEEVIDENCEFORHYDROGENBONDINGMANYELEMENTSFORMCOMPOUNDSWITHHYDROGENREFERREDTOAS“HYDRIDES“IFYOUPLOTTHEBOILINGPOINTSOFTHEHYDRIDESOFTHEGROUP4ELEMENTS,YOUFINDTHATTHEBOILINGPOINTSINCREASEASYOUGODOWNTHEGROUPTHEINCREASEINBOILINGPOINTHAPPENSBECAUSETHEMOLECULESAREGETTINGLARGERWITHMOREELECTRONS,ANDSOVANDERWAALSDISPERSIONFORCESBECOMEGREATERIFYOUREPEATTHISEXERCISEWITHTHEHYDRIDESOFELEMENTSINGROUPS5,6AND7,SOMETHINGODDHAPPENSALTHOUGHFORTHEMOSTPARTTHETRENDISEXACTLYTHESAMEASINGROUP4FOREXACTLYTHESAMEREASONS,THEBOILINGPOINTOFTHEHYDRIDEOFTHEFIRSTELEMENTINEACHGROUPISABNORMALLYHIGHINTHECASESOFNH3,H2OANDHFTHEREMUSTBESOMEADDITIONALINTERMOLECULARFORCESOFATTRACTION,REQUIRINGSIGNIFICANTLYMOREHEATENERGYTOBREAKTHESERELATIVELYPOWERFULINTERMOLECULARFORCESAREDESCRIBEDASHYDROGENBONDSTHEORIGINOFHYDROGENBONDINGTHEMOLECULESWHICHHAVETHISEXTRABONDINGARENOTETHESOLIDLINEREPRESENTSABONDINTHEPLANEOFTHESCREENORPAPERDOTTEDBONDSAREGOINGBACKINTOTHESCREENORPAPERAWAYFROMYOU,ANDWEDGESHAPEDONESARECOMINGOUTTOWARDSYOUNOTICETHATINEACHOFTHESEMOLECULESTHEHYDROGENISATTACHEDDIRECTLYTOONEOFTHEMOSTELECTRONEGATIVEELEMENTS,CAUSINGTHEHYDROGENTOACQUIREASIGNIFICANTAMOUNTOFPOSITIVECHARGEEACHOFTHEELEMENTSTOWHICHTHEHYDROGENISATTACHEDISNOTONLYSIGNIFICANTLYNEGATIVE,BUTALSOHASATLEASTONE“ACTIVE“LONEPAIRLONEPAIRSATTHE2LEVELHAVETHEELECTRONSCONTAINEDINARELATIVELYSMALLVOLUMEOFSPACEWHICHTHEREFOREHASAHIGHDENSITYOFNEGATIVECHARGELONEPAIRSATHIGHERLEVELSAREMOREDIFFUSEANDNOTSOATTRACTIVETOPOSITIVETHINGSCONSIDERTWOWATERMOLECULESCOMINGCLOSETOGETHERTHEHYDROGENISSOSTRONGLYATTRACTEDTOTHELONEPAIRTHATITISALMOSTASIFYOUWEREBEGINNINGTOFORMACOORDINATEDATIVECOVALENTBONDITDOESNTGOTHATFAR,BUTTHEATTRACTIONISSIGNIFICANTLYSTRONGERTHANANORDINARYDIPOLEDIPOLEINTERACTIONHYDROGENBONDSHAVEABOUTATENTHOFTHESTRENGTHOFANAVERAGECOVALENTBOND,ANDAREBEINGCONSTANTLYBROKENANDREFORMEDINLIQUIDWATERIFYOULIKENTHECOVALENTBONDBETWEENTHEOXYGENANDHYDROGENTOASTABLEMARRIAGE,THEHYDROGENBONDHAS“JUSTGOODFRIENDS“STATUSONTHESAMESCALE,VANDERWAALSATTRACTIONSREPRESENTMEREPASSINGACQUAINTANCESWATERASA“PERFECT“EXAMPLEOFHYDROGENBONDINGNOTICETHATEACHWATERMOLECULECANPOTENTIALLYFORMFOURHYDROGENBONDSWITHSURROUNDINGWATERMOLECULESTHEREAREEXACTLYTHERIGHTNUMBERSOFHYDROGENSANDLONEPAIRSSOTHATEVERYONEOFTHEMCANBEINVOLVEDINHYDROGENBONDINGTHISISWHYTHEBOILINGPOINTOFWATERISHIGHERTHANTHATOFAMMONIAORHYDROGENFLUORIDEINTHECASEOFAMMONIA,THEAMOUNTOFHYDROGENBONDINGISLIMITEDBYTHEFACTTHATEACHNITROGENONLYHASONELONEPAIRINAGROUPOFAMMONIAMOLECULES,THEREARENTENOUGHLONEPAIRSTOGOAROUNDTOSATISFYALLTHEHYDROGENSINHYDROGENFLUORIDE,THEPROBLEMISASHORTAGEOFHYDROGENSINWATER,THEREISEXACTLYTHERIGHTNUMBEROFEACHWATERCOULDBECONSIDEREDASTHE“PERFECT“HYDROGENBONDEDSYSTEMMORECOMPLEXEXAMPLESOFHYDROGENBONDINGTHEHYDRATIONOFNEGATIVEIONSWHENANIONICSUBSTANCEDISSOLVESINWATER,WATERMOLECULESCLUSTERAROUNDTHESEPARATEDIONSTHISPROCESSISCALLEDHYDRATIONWATERFREQUENTLYATTACHESTOPOSITIVEIONSBYCOORDINATEDATIVECOVALENTBONDSITBONDSTONEGATIVEIONSUSINGHYDROGENBONDSTHEDIAGRAMSHOWSTHEPOTENTIALHYDROGENBONDSFORMEDTOACHLORIDEION,CLALTHOUGHTHELONEPAIRSINTHECHLORIDEIONAREATTHE3LEVELANDWOULDNTNORMALLYBEACTIVEENOUGHTOFORMHYDROGENBONDS,INTHISCASETHEYAREMADEMOREATTRACTIVEBYTHEFULLNEGATIVECHARGEONTHECHLORINEHOWEVERCOMPLICATEDTHENEGATIVEION,THEREWILLALWAYSBELONEPAIRSTHATTHEHYDROGENATOMSFROMTHEWATERMOLECULESCANHYDROGENBONDTOHYDROGENBONDINGINALCOHOLSANALCOHOLISANORGANICMOLECULECONTAININGANOHGROUPANYMOLECULEWHICHHASAHYDROGENATOMATTACHEDDIRECTLYTOANOXYGENORANITROGENISCAPABLEOFHYDROGENBONDINGSUCHMOLECULESWILLALWAYSHAVEHIGHERBOILINGPOINTSTHANSIMILARLYSIZEDMOLECULESWHICHDONTHAVEANOHORANNHGROUPTHEHYDROGENBONDINGMAKESTHEMOLECULES“STICKIER“,ANDMOREHEATISNECESSARYTOSEPARATETHEMETHANOL,CH3CH2OH,ANDMETHOXYMETHANE,CH3OCH3,BOTHHAVETHESAMEMOLECULARFORMULA,C2H6OTHEYHAVETHESAMENUMBEROFELECTRONS,ANDASIMILARLENGTHTOTHEMOLECULETHEVANDERWAALSATTRACTIONSBOTHDISPERSIONFORCESANDDIPOLEDIPOLEATTRACTIONSINEACHWILLBEMUCHTHESAMEHOWEVER,ETHANOLHASAHYDROGENATOMATTACHEDDIRECTLYTOANOXYGENANDTHATOXYGENSTILLHASEXACTLYTHESAMETWOLONEPAIRSASINAWATERMOLECULEHYDROGENBONDINGCANOCCURBETWEENETHANOLMOLECULES,ALTHOUGHNOTASEFFECTIVELYASINW