浙大食化课件考研.docx
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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
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