Solutions--University-of-Massachusetts-Lowell解决的方案

SolutionsAhomogeneousmixtureoftwoormoresubstances.TheSolutionProcessWewillfocusonsolidorliquidsolutesdissolvedinaliquidsolvent.Sinceallparticlesareincontactwitheachother,thesolute-soluteandsolvent-solventforcesofattractionaredisrupted,andnew,solute-solventforcesofattractionarecreated.TheSolutionProcessThedisruptionofsolute-soluteandsolvent-solventforcesofattractionrequiresenergy,andisendothermic.Theinteractionofsolventandsoluteusuallyreleasesenergy.Thesumoftheenergyofallthreestepsiscalledtheenthalpyofsolution,ΔHosoln.Notethatsolutionsmayformwhetherthenetprocessisendothermicorexothermic.TheSolutionProcessTheSolutionProcessInadditiontotheenthalpyofsolution,wemustalsoconsidertheentropyofmixing.Entropyisameasureofrandomnessordisorder.Anincreaseinentropymakesaprocessmorelikelytooccur.Sincemixingpuresubstancesincreasesentropy,thisfactormakesprocessesthatareslightlyendothermicfavorable.EntropyofMixingTheSolutionProcessThegeneralruleonsolutionformationis:Likedissolveslike.Polarandioniccompoundsdissolveinpolarsolvents.Non-polarcompoundsdissolveinnon-polarsolvents.LikeDissolvesLikeVitaminAconsistsalmostentirelyofcarbonandhydrogen,andisnon-polar.Asaresult,vitaminAisfat-soluble,andcanbestoredinthebody.LikeDissolvesLikeVitaminCcontainspolarC-OandO-Hbonds.Itiswatersoluble,andmustbeconsumedoften,asitisexcretedeasily.O-HbondsC-ObondLikeDissolvesLikeIonicCompoundsLikeDissolvesLikeTheSolutionProcessDisrupt-ionofsoluteDisrupt-ionofsolventSolute/Solventinteract-ionIonicAqueousSolutionsWhenanioniccompoundisdissolvedinwater,theenergyrequiredtoseparatetheionsofthesoluteisequalto–(latticeenergy),or-ΔHlattice.Theenergyreleasedasthegaseousionsdissolveinwateriscalledthehydrationenergy,ΔHhydration.ThenetenergychangeisΔHsoln.HeatofHydrationFactorsAffectingSolubilityMolecularStructurePressure(forgaseoussolutes)TemperaturePressureEffectsGasesdissolvedinaliquidsoluteobeyHenry’sLaw:C=kPwhereCistheconcentration,kisaconstantspecifictosoluteandsolvent,andPisthepressureofthegasabovethesolutionPressureEffectsGasesdissolvedinaliquidsoluteobeyHenry’sLaw:C=kPTheamountofagasdissolvedinasolutionisdirectlyproportionaltothepressureofthegasabovethesolution.Henry’sLawPressureEffectsTemperatureEffectsForgasesdissolvedinliquids,thesolubilitydecreasesastemperatureincreases.Thatis,gasesdissolvebetterincoldliquidsthaninwarmerliquids.TemperatureEffectsForsolidsolutesdissolvedinwater,theeffectoftemperatureonsolubilityisdifficulttopredict,althoughmanysolidsdissolvemoreastemperatureincreases.SolutionConcentrationMasspercent=(massofsolute)(100%)(massofsolution)Molefraction(XA)=(molesofA)total#ofmolesMolality(m)=molesofsolutekgofsolventSolutionConcentrationAlthoughmolarity(M)isusedforstoichiometrycalculations,therearemanyotherwaystoexpresstheconcentrationofasolution.Molaritywillvaryslightlywithchangesintemperatureasthevolumeexpandsorcontracts.Unitssuchasmasspercent,molefraction,ormolalityremainconstantastemperaturechanges.VeryDiluteSolutionsTheconcentrationofverydilutesolutionsareexpressedinpartspermillion(ppm)orpartsperbillion(ppb).ppm=[(masssolute)x106]÷(masssoln)ppb=[(masssolute)x109]÷(masssoln)TheColligativePropertiesThecolligativepropertiesarepropertiesthatdependupontheconcentrationofparticles(moleculesorions)dissolvedinavolatilesolvent,andnotonthenatureoftheparticles.Theyinclude:vaporpressurefreezingpointboilingpointosmoticpressureTheColligativePropertiesRelativelysimplemathematicalrelationshipscanbeusedtopredictthechangesinvaporpressure,freezingandboilingpoint,etc.Thepropertiescanbepredictedfordilutesolutions(0.1M)ofnon-volatilesolute(usuallysolids)dissolvedinavolatilesolvent(usuallyaliquid).VaporPressureTheadditionofanon-volatilesolutetoavolatilesolventlowersthevaporpressureofthesolvent.VaporPressureThedecreaseinvaporpressurecanbeunderstoodbylookingattheevaporationprocess.Weneedtocomparetheenthalpychange(ΔHvap)andentropychangeofevaporation.VaporPressureThevaporpressureofthepuresolventorthesolutionistheresultofsolventmoleculesescapingtheliquidsurfaceandbecominggaseous.Sincethesoluteisnon-volatile,itdoesnotevaporate.Sinceonlysolventmoleculesevaporate,theenthalpychangeforpuresolventorthesolutionisthesame.VaporPressureThedecreaseinvaporpressureofthesolutionistheresultofchangesinentropy.Thevaporineithercontainerisdisordered,duetotherandommotionofgaseoussolvent.VaporPressureTheliquidphasesdifferinentropy.Thepuresolventisrelativelyorderedsinceallofthemoleculesarethesame(solvent).VaporPressureTheliquidphaseofthesolutionismuchmorerandom,sinceitisamixture.VaporPressureUponevaporation,thepuresolventundergoesagreaterincreaseinentropythanthesolution.VaporPressureSystemstendtomaximizeentropy.Thepuresolventevaporatesmorereadily,becauseitundergoesagreaterincreaseinentropy.BoilingPointElevationVaporPressureLoweringThechangeinvaporpressurecanbecalculatedasfollows:∆vp=-XsolutePsolventwhereXisthemolefractionofsoluteparticlesPosolventisthevaporpressureofthepuresolventoVaporPressureLowering∆vp=-XsolutePosolventThesignisnegativebecausethevaporpressuredecreases.VaporPress