定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件

QuantitativeAnalysis

定量分析

QuantitativeAnalysis

定量分析CHAPTER10

EFFECTOFELECTROLYTESONCHEMICALEQUILIBRIA

CHAPTER10

EFFECTOFELECTROLY

Thepositionofmostsolutionequilibriadependsontheelectrolyteconcentrationofthemedium,evenwhentheaddedelectrolytecontainsnoionincommonwiththoseinvolvedintheequilibrium. H3AsO4+3I-+2H+ H3AsO3+I3-+H2O Ifanelectrolyte,suchasbariumnitrate,potassiumsulfate,orsodiumperchlorate,isaddedtothissolution,thecolorofthetriiodideionbecomeslessintense.ThisdecreaseincolorintensityindicatesthattheconcentrationofI3-hasdecreasedandthattheequilibriumhasbeenshiftedtotheleftbytheaddedelectrolyte. Thepositionofmostsolutio定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件

HowDoIonicChargesAffectEquilibria? Themagnitudeoftheelectrolyteeffectishighlydependentonthechargesoftheparticipantsinanequilibrium.Whenonlyneutralspeciesareinvolved,thepositionofequilibriumisessentiallyindependentofelectrolyteconcentration.Withionicparticipants,themagnitudeoftheelectrolyteeffectincreaseswithcharge.Ina0.02Msolutionofpotassiumnitrate,thesolubilityofbariumsulfateislargerthanitisinpurewaterbyafactorof2.Thesolubilityofbariumiodatebyafactorofonly1.25andthatofsilverchlorideby1.2 HowDoIonicChargesAffectE定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件 WhatIstheEffectofIonicStrengthonEquilibria? Theeffectofaddedelectrolyteonequilibriaisindependentofthechemicalnatureoftheelectrolytebutdependsonapropertyofthesolutioncalledtheionicstrength.Thisquantityisdefinedas ionicstrength=

=1/2([A]ZA2+[B]ZB2+[C]ZC2+…) Where[A],[B],[C],…representthemolarspeciesconcentrationsofionsA,B,C,…andZA,ZB,ZC,…aretheirioniccharges. Forsolutionswithionicstrengthsof0.1Morless,theelectrolyteeffectisindependentofthekindofionsanddependentonlyontheionicstrength.Thisindependencewithrespecttoelectrolytespeciesdisappearsathighionicstrengths. WhatIstheEffectofIonicS TheSaltEffect Theelectrolyteeffectresultsfromtheelectrostaticattractiveandrepulsiveforcesthatexistbetweentheionsofanelectrolyteandtheionsinvolvedinanequilibrium.Theseforcescauseeachionfromthedissociatedreactanttobesurroundedbyasheathofsolutionthatcontainsaslightexcessofelectrolyteionsofoppositecharge.Whenabariumsulfateprecipitateisequilibratedwithasodiumchloridesolution,eachdissolvedbariumionissurroundedbyanionicatmospherecarriesasmallnetnegativecharge. TheSaltEffect…continued…

Eachsulfateionissurroundedbyanionicatmospherethattendstobeslightlypositive.Thesechargedlayersmakethebariumionssomewhatlesspositiveandthesulfateionssomewhatlessnegativethantheywouldbeintheabsenceofelectrolyte.Theconsequenceofthiseffectisadecreaseinoverallattractionbetweenbariumandsulfateionsandanincreaseinsolubility,whichbecomesgreaterasthenumberofelectrolyteionsinthesolutionbecomeslarger.…continued… ACTIVITYCOEFFICIENTS

Chemistsusethetermactivity,a,toaccountfortheeffectsofelectrolytesonchemicalequilibria.Theactivity,oreffectiveconcentration,ofspeciesXdependsontheionicstrengthofthemediumandisdefinedas aX=x[X] where,aXistheactivityofX,[X]isitsmolarconcentration,andxisadimensionlessquantitycalledtheactivitycoefficient. ACTIVITYCOEFFICIENTS…continued… TheactivitycoefficientandthustheactivityofXvarywithionicstrengthsuchthatsubstitutionofaXfor[X]inanyequilibrium-constantexpressionfreesthenumericalvalueoftheconstantfromdependenceontheionicstrength.IfXmYnisaprecipitate,thethermodynamicsolubilityproductexpressionisdefinedbytheequation Ksp=aXm.aYn Ksp=Xm.Yn.[X]m[Y]n=Xm.Yn.K`sp HereK`spistheconcentrationsolubilityproductconstantandKspisthethermodynamicequilibriumconstant.…continued… PropertiesofActivityCoefficients

1.Theactivitycoefficientofaspeciesisameasureoftheeffectivenesswithwhichthatspeciesinfluencesanequilibriuminwhichitisaparticipant.Inverydilutesolution,wheretheionicstrengthisminimal,thiseffectivenessbecomesconstantandtheactivitycoefficientbecomesunity.Undersuchcircumstances,theactivityandthemolarconcentrationareidentical(asarethermodynamicandconcentrationequilibriumconstants).Astheionicstrengthincreases,however,anionlosessomeofitseffectivenessanditsactivitycoefficientdecreases.Atmoderateionicstrengths,x<1;asthesolutionapproachesinfinitedilution,however,x1andthusax[X]andK`sp

Ksp PropertiesofActivityCoeffi定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件…continued…

2.Insolutionsthatarenottooconcentrated,theactivitycoefficientforagivenspeciesisindependentofthenatureoftheelectrolyteanddependentonlyontheionicstrength. 3.Foragivenionicstrength,theactivitycoefficientofaniondepartsfartherfromunityasthechargecarriedbythespeciesincreases.Theactivitycoefficientofanunchargedmoleculeisapproximatelyunity,regardlessofionicstrength. 4.Atanygivenionicstrength,theactivitycoefficientsofionsofthesamechargeareapproximatelyequal.Thesmallvariationsthatdoexistcanbecorrelatedwiththeeffectivediameterofthehydratedions. 5.Theactivitycoefficientofagiveniondescribesitseffectivebehaviorinallequilibriainwhichitparticipates.…continued…

TheDebye-HuckelEquation

P.DebyeandE.Huckelderiveatheoreticalexpressionthatpermitsthecalculationofactivitycoefficientsofionfromtheirchargeandtheiraveragesize.Thisequationtakestheform

where,x=activitycoefficientofthespeciesX

Zx=chargeonthespeciesX =ionicstrengthofthesolution x=effectivediameterofthehydratedionXin nanometer(10-9m)

2 TheDebye-HuckelEquation2…continued…

Theconstants0.51and3.3areapplicabletoaqueoussolutionat25oC;Themagnitudeofxappearstobeapproximately0.3nmformostsinglychargedions;then,thedenominatoroftheDebye-Huckelequationsimplifiestoapproximately1+.Forionswithhighercharge,xmaybeaslargeas1.0nm.…continued… 定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件 OmittingActivityCoefficientsin EquilibriumCalculations Weshallordinarilyneglectactivitycoefficientsandsimplyusemolarconcentrationsinapplicationoftheequilibriumlaw.Thisrecoursesimplifiesthecalculationsandgreatlydecreasestheamountofdataneeded.Formostpurposes,theerrorintroducedbytheassumptionofunityfortheactivitycoefficientisnotlargeenoughtoleadtofalseconclusions.Significantdiscrepanciesoccurwhentheionicstrengthislarge(0.01orlarger)orwhentheionsinvolvedhavemultiplecharges.Withdilutesolutions(ionicstrength<0.01)ornonelectrolytesorofsinglychargedion,theuseofconcentrationsinamass-lawcalculationoftenprovidedreasonablyaccurateresults. OmittingActivityCoefficient定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件

QuantitativeAnalysis

定量分析

QuantitativeAnalysis

定量分析CHAPTER10

EFFECTOFELECTROLYTESONCHEMICALEQUILIBRIA

CHAPTER10

EFFECTOFELECTROLY

Thepositionofmostsolutionequilibriadependsontheelectrolyteconcentrationofthemedium,evenwhentheaddedelectrolytecontainsnoionincommonwiththoseinvolvedintheequilibrium. H3AsO4+3I-+2H+ H3AsO3+I3-+H2O Ifanelectrolyte,suchasbariumnitrate,potassiumsulfate,orsodiumperchlorate,isaddedtothissolution,thecolorofthetriiodideionbecomeslessintense.ThisdecreaseincolorintensityindicatesthattheconcentrationofI3-hasdecreasedandthattheequilibriumhasbeenshiftedtotheleftbytheaddedelectrolyte. Thepositionofmostsolutio定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件

HowDoIonicChargesAffectEquilibria? Themagnitudeoftheelectrolyteeffectishighlydependentonthechargesoftheparticipantsinanequilibrium.Whenonlyneutralspeciesareinvolved,thepositionofequilibriumisessentiallyindependentofelectrolyteconcentration.Withionicparticipants,themagnitudeoftheelectrolyteeffectincreaseswithcharge.Ina0.02Msolutionofpotassiumnitrate,thesolubilityofbariumsulfateislargerthanitisinpurewaterbyafactorof2.Thesolubilityofbariumiodatebyafactorofonly1.25andthatofsilverchlorideby1.2 HowDoIonicChargesAffectE定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件 WhatIstheEffectofIonicStrengthonEquilibria? Theeffectofaddedelectrolyteonequilibriaisindependentofthechemicalnatureoftheelectrolytebutdependsonapropertyofthesolutioncalledtheionicstrength.Thisquantityisdefinedas ionicstrength=

=1/2([A]ZA2+[B]ZB2+[C]ZC2+…) Where[A],[B],[C],…representthemolarspeciesconcentrationsofionsA,B,C,…andZA,ZB,ZC,…aretheirioniccharges. Forsolutionswithionicstrengthsof0.1Morless,theelectrolyteeffectisindependentofthekindofionsanddependentonlyontheionicstrength.Thisindependencewithrespecttoelectrolytespeciesdisappearsathighionicstrengths. WhatIstheEffectofIonicS TheSaltEffect Theelectrolyteeffectresultsfromtheelectrostaticattractiveandrepulsiveforcesthatexistbetweentheionsofanelectrolyteandtheionsinvolvedinanequilibrium.Theseforcescauseeachionfromthedissociatedreactanttobesurroundedbyasheathofsolutionthatcontainsaslightexcessofelectrolyteionsofoppositecharge.Whenabariumsulfateprecipitateisequilibratedwithasodiumchloridesolution,eachdissolvedbariumionissurroundedbyanionicatmospherecarriesasmallnetnegativecharge. TheSaltEffect…continued…

Eachsulfateionissurroundedbyanionicatmospherethattendstobeslightlypositive.Thesechargedlayersmakethebariumionssomewhatlesspositiveandthesulfateionssomewhatlessnegativethantheywouldbeintheabsenceofelectrolyte.Theconsequenceofthiseffectisadecreaseinoverallattractionbetweenbariumandsulfateionsandanincreaseinsolubility,whichbecomesgreaterasthenumberofelectrolyteionsinthesolutionbecomeslarger.…continued… ACTIVITYCOEFFICIENTS

Chemistsusethetermactivity,a,toaccountfortheeffectsofelectrolytesonchemicalequilibria.Theactivity,oreffectiveconcentration,ofspeciesXdependsontheionicstrengthofthemediumandisdefinedas aX=x[X] where,aXistheactivityofX,[X]isitsmolarconcentration,andxisadimensionlessquantitycalledtheactivitycoefficient. ACTIVITYCOEFFICIENTS…continued… TheactivitycoefficientandthustheactivityofXvarywithionicstrengthsuchthatsubstitutionofaXfor[X]inanyequilibrium-constantexpressionfreesthenumericalvalueoftheconstantfromdependenceontheionicstrength.IfXmYnisaprecipitate,thethermodynamicsolubilityproductexpressionisdefinedbytheequation Ksp=aXm.aYn Ksp=Xm.Yn.[X]m[Y]n=Xm.Yn.K`sp HereK`spistheconcentrationsolubilityproductconstantandKspisthethermodynamicequilibriumconstant.…continued… PropertiesofActivityCoefficients

1.Theactivitycoefficientofaspeciesisameasureoftheeffectivenesswithwhichthatspeciesinfluencesanequilibriuminwhichitisaparticipant.Inverydilutesolution,wheretheionicstrengthisminimal,thiseffectivenessbecomesconstantandtheactivitycoefficientbecomesunity.Undersuchcircumstances,theactivityandthemolarconcentrationareidentical(asarethermodynamicandconcentrationequilibriumconstants).Astheionicstrengthincreases,however,anionlosessomeofitseffectivenessanditsactivitycoefficientdecreases.Atmoderateionicstrengths,x<1;asthesolutionapproachesinfinitedilution,however,x1andthusax[X]andK`sp

Ksp PropertiesofActivityCoeffi定量分析10电解质对化学平衡的影响EFFECT-OF-ELECTROLYTES-ON-CHEMICAL-EQUILIBRIA课件…continued…

2.Insolutionsthatarenottooconcentrated,theactivitycoefficientforagivenspeciesisindependentofthenatureoftheelectrolyteanddependentonlyontheionicstrength. 3.Foragivenionicstrength,theactivitycoefficientofaniondepartsfartherfromunityasthechargecarriedbythespeciesincreases.Theactivitycoefficientofanunchargedmoleculeisapproximatelyunity,regardlessofionicstrength. 4.Atanygivenionicstrength,theactivitycoefficientsofionsofthesamechargeareapproximatelyequal.Thesmallvariationsthatdoexistcanbecorrelatedwiththeeffectivediameterofthehydratedions. 5.Theactivitycoefficientofagiveniondescribesitseffectivebehaviorinallequilibriainwhichitparticipates.…continued…

TheDebye-HuckelEquation

P.DebyeandE.Huckelderiveatheoreticalexpressionthatpermitsthecalculationofactivitycoefficientsofionfromtheirchargeand