浙大食化课件考研.docx

浙大食化课件考研.docx

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浙大食化课件考研

Chapter1Water

2.1ContributionsofWater

Onthisplanet,wateristheonlysubstancethatoccursabundantlyinallthreephysicalstates.Itistheonlycommonliquidandisthemostwidelydistributedpuresolid,beingeverpresentsomewhereintheatmosphereassuspendediceparticles,orontheearth'ssurfaceasvarioustypesofsnowandice.Itisessentialtolife:

asanimportantgovernorofbodytemperature,asasolvent,asacarrierofnutrientsandwasteproducts,asareactantandreactionmedium,asalubricantandplasticizer,asastabilizerofbiopolymerconformation,asalikelyfacililatorofthedynamicbehaviorofmacromolecules,includingtheftcatalytic（enzymatic）properties,andinotherwaysyetunknown,Itistrulyremarkablethatorganiclifeshoulddependsoheavilyonthissmallinorganicmolecule,and,perhapsevenmoreremarkable,thatsofewscientistsareawareofthisfact.

2.2TheWaterMolecule

Water'sunusualpropertiessuggesttheexistenceofstrongattractiveforcesamongwatermolecules,anduncommonstructuresforwaterandice.Thesefeaturesarebestexplainedbyconsideringthenatureoffirstasinglewatermoleculeandhensmallgroupsofmolecules.Toformamoleculeofwater,twohydrogenatomsapproachthetwosp3bondingorbitalsofoxygenandformtwocovalentsigma（σ）bonds（40%partialioniccharacter）,eachofwhichhasadissociationenergyof4.6*102kJ/mol（110kcal/mol）.Thelocalizedmolecularorbitalsremainsymmetricallyorientedabouttheoriginalorbitalaxes,thusretaininganapproximatetetrahedralstructure.AschematicorbitalmodelofawatermoleculeisshowninFigure1.

Figure1SchematicmodelofasingleHOHmolecule:

sp3configuration.

Thebondangleoftheisolatedwatermolecule（vaporstale）is104.5°andthisvalueisneartheperfecttetrahedralangleof109°28’.TheO-Hinternucleardistatanceis0.96ÅandthevanderWaalsradiiforoxygenandhydrogenare,respectively,1.40and1.2Å.

Atthispoint,itisimportanttoemphasizethatthepicturesofarpresentedisoversimplified.PurewatercontainsnotonlyordinaryHOHmoleculesbutalsomanyotherconstituentsintraceamounts.Inadditiontothecommonisotopes16Oand1H,alsopresentare17O,18O,2H（deuterium）and3H（tritium）,givingriseto18isotopicvariantsofmolecularHOH.Wateralsocontainsionicparticlessuchashydrogenions（existingasH30+）.hydroxylions,andtheirisotopicvariants.Waterthereforeconsistsofmorethan33chemicalvariantsofHOH,butthevariantsoccurinonlyminuteamounts.

TheVlikefore1ofanHOHmoleculeandthepolarizednatureoftheO-Hbondresultinanunsymmetricalchargedistributionandavapor-statedipolemomentof1.84Dforpurewater.Polarityofthismagnitudeproducesintermolecularattractiveforces,andwatermoleculesthereforeassociatewithconsiderabletenacity.Water'sunusuallylargeintermolecularattractiveforcecannot,however,befullyaccountedforontilebasisofitslargedipolemoment.Thisisnotsurprising,sincedipolemomentsgivenoindicationofthedegreetowhichchargesareexposedorofthegeometryoftilemolecule,andtheseaspects,ofcourse,haveanimportantbearingontheintensityofmolecularassociation.

Water'slargeintermolecularattractiveforcescanbeexplainedquiteadequatelyintermsofitsabilitytoengageinmultiplehydrogenbondingonathree-dimensionalbasis.Comparedtocovalentbonds（averagebondenergyofabou1335kJ/mol）,hydrogenbondsareweak（typically2-40kJ/mol）andhavegreaterandmorevariablelengths.Thehydrogenbondbetweenoxygenandhydrogenhasadissociationenergyofabout13-25kJ/mol.

Sinceelectrostaticforcesmakeamajorcontributiontotheenergyofthehydrogenbond（perhapsthelargestcontribution）,andsinceanelectrostaticmodelofwaterissimpleandleadstoanessentiallycorrectgeometricpictureofHOHmoleculesastheyareknowntoexistinice,furtherdiscussionofthegeometricalpatternsformedbyassociationofwatermoleculeswillemphasizeelectrostaticeffects.Thissimplifiedapproach,whileentirelysatisfactoryforpresentpurposes,mustbemodifiedifotherbehavioralcharacteristicsofwateraretobeexplainedsatisfactorily.

Thehighlyelectronegativeoxygenofthewatermoleculecanbevisualizedaspartiallydrawingawaythesingleelectronsfromthetwocovalentlybondedhydrogenatoms,therebyleavingeachhydrogenatomwithapartialpositivechargeandaminimalelectronshield;thatis,eachhydrogenatomassumessomecharacteristicsofabareproton.Sincethehydrogen-oxygenbondingorbitalsarelocatedontwooftheaxesofanimaginarytetrahedron（Fig.l）,thesetwoaxescanbethoughtofasrepresentinglinesofpositiveforce（hydrogen-bonddonorsites）.Oxygen'stwolone-pairorbitalscanbepicturedasresidingalongtheremainingtwoaxesoftheimaginarytetrahedron,andthesethenrepresentlinesofnegativeforce（hydrogen-bondacceptorsites）.Byvirtueofthesetourlinesofforce,eachwatermoleculeisabletohydrogen-bondwithamaximumoffourothers.TheresultingtetrahedralarrangementisdepictedinFigure2.

Becauseeachwatermoleculehasanequalnumberofhydrogen-bonddonorandreceptorsites,arrangedtopermitthreedimensionalhydrogenbonding,itisfoundthattheattractiveforcesamongwatermoleculesareunusuallylarge,evenwhencomparedtothoseexistingamongothersmallmoleculesthatalsoengageinhydrogenbonding（e.g.,NH3,HF）.Ammonia,withitstetrahedralarrangementofthreehydrogensandonereceptorsite,andhydrogenfluoride,withitstetrahedralarrangementofonehydrogenandthreereceptorsites,donothaveequalnumbersofdonorandreceptorsitesandthereforecanformonlytwodimensionalhydrogenbondednetworksinvolvinglesshydrogenbondspermoleculethanwater.

Figure2Hydrogenbondingofwatermoleculesinatetrahedralconfiguration.Opencirclesareoxygenatomsandclosedcirclesarehydrogenatoms.Hydrogenbondsarerepresentedbydashedlines.

Conceptualizingtheassociationofafewwatermoleculesbecomesconsiderablymorecomplicatedwhenoneconsidersisotopicvariantsandhydroniumandhydroxylions.Thehydroniumion,becauseofitspositivecharge,wouldbeexpectedtoexhibitagreaterhydrogen-bonddonatingpotentialthannonionizedwater（dashedlinesarehydrogenbonds）.

Thehydroxylion,becauseofitsnegativecharge,wouldbeexpectedtoexhibitagreaterhydrogen-bondacceptorpotentialthanun-ionizedwater（XHrepresentsasoluteorawatermolecule）.

Water'sabilitytoengageinthree-dimensionalhydrogenbondingprovidesalogicalexplanationformanyofitsunusualproperties;itslargevaluesforheatcapacity,meltingpoint,boilingpoint,surfacetension,andenthalpiesofvariousphasetransitionsallarerelatedtotheextraenergyneededtobreakintermolecularhydrogenbonds.

Thepermittivity（dielectricconstant）ofwaterisalsoinfluencedbyhydrogenbonding.Althoughwaterisadipole,thisfactalonedoesnotaccountforthemagnitudeofitspermittivity.Hydrogen-bondedclustersofmoleculesapparentlygiverisetomultimoleculardipoles,whicheffectivelyincreasethepermittivityofwater.

2.3StructureofWater

Tosome,itmayseemstrangetospeakofstructureinaliquidwhenfluidityistheessenceoftheliquidstate.Yetitisanoldandwellacceptedideathatliquidwaterhasstructure,obviouslynotsufficientlyestablishedtoproducelong-rangerigidity,butcertainlyfarmoreorganizedthanthatofmoleculesinthevaporstate,andampletocausetheorientationandmobilityofagivenwatermoleculetobeinfluencedbyitsneighbors.

Evidenceforthisviewiscompelling.Forexample,waterisan“open”liquid,beingonly60%asdenseaswouldbeexpectedonthebasisofclosepackingthatcanprevailinnonstructuredliquids.Partialretentionoftheopen,hydrogenbonded,tetrahedralarrangementoficeeasilyaccountsforwater'slowdensity.Furthermore,theheatoffusionofice,whileunusuallyhigh,issufficienttobreakonlyabout15%ofthehydrogenbondsbelievedtoexistinice.Althoughthisdoesnotnecessarilyrequirethat85%ofthehydrogenbondsexistinginiceberetainedinwater（forexample,morecouldbebroken,butthechangeinenergycouldbemaskedbyasimultaneousincreaseinvanderWaalsinteractions）,resultsofmanystudiessupportthenotionthatmanywater-waterhydrogenbondsdoexist.

Elucidationofthestructureofpurewaterisanextremelycomplexproblem.Manytheorieshavebeensetforth,butallareincomplete,overlysimple,andsubjecttoweaknessesthatarequicklycitedbysupportersofrivaltheories.Thatis,ofcourse,ahealthysituation,whichwilleventuallyresultinallaccuratestructuralpicture（orpictures）ofwater.Inthemeantime,fewstatementscanbemadewithanyassurancethattheywillstandessentiallyunmodifiedinyearstocome.Thus,thissubjectwillbedealtwithonlybriefly.

Threegeneraltypesofmodelshavebeenproposed:

mixture,interstitial,andcontinuum（alsoreferredtoashomogeneousoruniformistmodels）.Mixturemodelsembodytheconceptofintermolecularhydrogenbondsbeingmomentarilyconcentratedinbulkyclustersofwatermoleculesthatexistindynamicequilibriumwithothermoredensespecie-momentarilymeaning~10-11sec.

Continuummodelsinvolvetheideathatintermolecularhydrogenbondsaredistributeduniformlythroughoutthesample,andthatmanyofthebondsexistinginicesimplybecomedistortedratherthanbrokenwheniceismelted.Ithasbeensuggestedthatthispermitsacontinuousnetworkofwatermoleculestoexistthatis,ofcourse,dynamicinnature.

Theinterstitialmodelinvolvestheconceptofwaterretaininganicelikeor