BaseSolutions

16

BaseSolutions•  Strongbasesarestrongelectrolytes•  Somecommonstrongbases:

LiOH NaOHSr(OH)2 Ca(OH)2KOH Ba(OH)2

•  Notethatmoststrongbasesaregroup1Aand2Ametalhydroxides

BaseSolutions•  Strongbasesarestrongelectrolytes•  Writeasthebasedissociatingintoions•  Ex/NaOH

NaOH(aq)àNa+(aq)+OH-(aq)

BaseSolutions•  However,bewarned–basescompletelydissociateinonestep

•  Thisisincontrasttopolyproticacids,thatdissociateinsteps

NaOH(aq)àNa+(aq)+OH-(aq)

Ba(OH)2(aq)àBa2+(aq)+2OH-(aq)

•  Thismeansthat[OH-]forastrongbaseisequalto[strongbase]xcoefficientinbalancedequation

BaseSolutions•  Forweakbases,wewritethedissociationwithwater

B+(aq)+H2O(l)ßàBH+(aq)+OH-

(aq)

•  Kb=baseionizationconstant=describesthedegreeofionizationofaweakbase

KaxKb=Kw•  ThesmallertheKb,theweakerthebase

21

NH3(aq)+H2O(l)ßàNH4+(aq)+OH-

(aq)

22

BaseSolutions•  Justlikeasinacids,wecanneglectthe[OH-]contributionfromtheautoionizationofwater,andsimplylookatthebase

•  Knowingthat,canwecalculatethe[OH-]orpHofbasicsolutions:

•  Thingstoremember:pH+pOH=14.00[H3O+][OH-]=Kw

KaxKb=Kw

CalculationswithStrongBases

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CalculationswithStrongBases•  Determinethe[OH-]andpHof0.225MKOH

KOH(aq)àK+(aq)+OH-(aq)

•  Andthereisa1:1stoichiometricratiobetweenthebaseandthehydroxide

•  [OH-]=[KOH]=0.225M•  SopOH=-log(0.225M)=0.648•  pH=13.352•  Trythis:Findthe[OH-]andpHof0.0015MSr(OH)2

CalculationswithWeakBases

26

CalculationswithWeakBases•  Findthe[OH-]andpHofa0.100MNH3(theKbat25°Cis1.76x10-5

•  Steps:– Writethebalanceddissociationequation– Createanicetable(whichnumberisbeingapproximated?)

– SetupyourKbexpression(alwayssetupbeforeplugginginvalues)

– Pluginvaluesandsolveforx(verifyifyou’retryingtouseanapproximation)

– UsextofindconcentrationandthenpH– Tryit!

IonsandSalts

28

IonsandSalts•  Doionshaveacid/baseproperties?•  Asyoumayhavenoticed,theysometimesdo!•  Generallyspeakng…– Anionstendtoformeitherbasicorneutralsolutions– Cationstendtoformeitheracidicorneutralsolutions

IonsandSalts•  Wecanthinkofananion(A-)astheconjugatebaseofsomeacid(HA)

•  Ananionthatistheconjugatebaseofaweakacidisaweakbase(onthecompletescaleofstrong/weak).

F−(aq)+H2O(l)⇔HF(aq)+OH−(aq)– Thestrongertheacid,theweakertheconjugatebase

•  AnanionthatistheconjugatebaseofastrongacidispH-neutral

Cl−(aq)+H2O(l)←HCl(aq)+OH−(aq)

Tro,Chemistry:AMolecularApproach 31

UsetheTabletoDetermineiftheGivenAnionIsBasicorNeutral

a)  NO3−

theconjugatebaseofastrongacid,thereforeneutral

b)  NO2−

theconjugatebaseofaweakacid,thereforebasic

DeterminingthepHofasolutionContaininganAnionActingasaBase

AnionActingasaBase•  FindthepHofa0.100MNaCHO2solution.ThesaltcompletelydissociatesintoNa+(aq),andtheNa+ionhasnoacidorbaseproperties.TheKaforHCHO2at25°Cis1.8x10-4.– Writeequation– PrepareICETable+plug-in– UseKaxKb=KwtofindtheKbofyourconjugatebase– Calculateandcheck!

CationsasWeakAcids

CationsasWeakAcids•  Cationscansometimesactasweakacids① Thecounterionofahydroxidebase(ex/Na+is

thecounterionofNaOH)ispHneutral② Theconjugateofaweakbaseisusuallyweakly

acidic(NH4+istheconjugateofNH3)

③ Small,highlychargedmetals(Al3+,Fe3+)formweaklyacidicsolutions

Al3+(aq)+6H2O(aq)àAl(H2O)63+(aq)Al(H2O)63+(aq)+H2O(aq)ßàAl(H2O)52+(aq)+H3O+

(aq)

CationsasWeakAcids•  Example:ClassifyeachcationasaweakacidorpH-neutral

a.  C5H5NH+b.  Ca2+c.  Cr3+Answers:a)  Weakacid(conjugate)b)  pH-neutral(counterionofCa(OH)2)c)  Weakacid(small,highlychargedmetal)

ClassifyingSaltSolutionsasAcidic,Basic,orNeutral

ClassifyingSaltSolutions•  Sincesaltscontainbothacationandananion,willtheyformneutral,acidic,orbasicsolutions?

•  Wehavetolookatthespecificionsinvolved.① Saltsinwhichneithercationnortheanionactas

anacidorabase② Saltsinwhichthecationdoesnotactasanacid,

buttheanionactsasabase③ Saltsinwhichthecationactsasanacid,butthe

aniondoesnotactasabase④ Saltsinwhichthecationactsasanacidandthe

anionactsasabase

ClassifyingSaltSolutions① Saltsinwhichneithercationnortheanionactas

anacidorabase•  ThesewillformpH-neutralsolutions•  Example:NaCl•  Na+isthecounterionofastrongbase•  Cl-istheconjugatebaseofastrongacid

ClassifyingSaltSolutions② Saltsinwhichthecationdoesnotactasanacid,

buttheanionactsasabase•  Thesewillformbasicsolutions•  Example:NaF•  Thecationisthecounterionofastrongbase•  Theanionistheconjugatebaseofaweakacid

ClassifyingSaltSolutions③ Saltsinwhichthecationactsasanacid,butthe

aniondoesnotactasabase•  Theseformacidicsolutions•  Example:NH4BrorFeCl3•  ThecationistheconjugateofaweakbaseORisasmall,highlychargedmetalion

•  Theanionisaconjugatebaseofastrongacid

ClassifyingSaltSolutions④ Saltsinwhichthecationactsasanacidandthe

anionactsasabase•  Iftheybothhaveacidic/basicproperties,thenweneedtocomparetheKaandKbvalues

•  Example:FeF3•  IfKa>Kb,thesolutionisacidic•  IfKb>Ka,thesolutionisbasic

TryThis•  Determineifthesolutionformedbyeachsaltisacidic,basic,orneutral:

a)  SrCl2b)  AlBr3c)  CH3NH3NO3d)  NaCHO2e)  NH4F•  TheKaofNH4

+is5.68x10-10andtheKbofF-is2.9x10-11

PolyproticAcids

PolyproticAcids•  Polyproticacidsionizeinsuccessivesteps,eachwithitsownKaH2SO3(aq)ßàH+

(aq)+HSO3-(aq)Ka1=1.6x10-2

HSO3-(aq)ßàH+

(aq)+SO32-(aq)Ka2=6.4x10-8

•  sincepolyproticacidsionizeinsteps,eachHhasaseparateKa

•  Ka1>Ka2>Ka3

PolyproticAcids•  Ka1>Ka2>Ka3•  BecauseKa1issomuchlargerthansuccessivevalues,andbecausethefirstionizationstepwillproducethegreatestconcentrationofH3O+(thereforeinhibitingformationofadditionalH3O+),wetreatmostpolyproticacidsolutionsasifthefirststepweretheonlyonethatcontributesto[H3O+]– exceptH2SO4⇒use[H2SO4]asthe[H3O+]forthesecondionization

47

48

[H2C6H6O6] [H3O+] [HC6H6O6-]

I 0.0100 ≈0.00 0.0100 C E

Example:FindthepHof0.100MH2C6H6O6solution@25°C(Ka1=8.0x10-5)

Write the reactions for the acid with water Construct an ICE table for the reaction

Make sure to confirm if you approximate for one of your steps

H2C6H6O6(aq)+H2O(l)⇔H3O++HC6H6O6-(aq)

49

[HSO4 -] [SO4

2 -] [H3O+] I 0.0100 ≈0.00 0.0100 C E

NowTry:FindthepHof0.0100MH2SO4(aq)

Write the reactions for the acid with water Construct an ICE table for the reaction

Enter the initial concentrations – assuming the [HSO4

−] and [H3O+] is ≈ [H2SO4]

HSO4-(aq)+H2O(l)⇔SO4

2-(aq)+H3O+

(aq)Ka2=0.012

H2SO4(aq)+H2O(l)⇔HSO4(aq)-+H3O+(aq)STRONG

50

Note:youmustaddinthecontributionof[H3O+]fromthefirststepsinceit’sstrong.

represent the change in the concentrations in terms of x

sum the columns to find the equilibrium concentrations in terms of x substitute into the equilibrium constant expression

( )( )( )x

xxK−+

==+

0100.00100.0

]HSO[]OH][[SO

-4

3-24

a

[HSO4 -] [SO4

2 -] [H3O+] initial 0.0100 0 0.0100

change −x +x +x equilibrium 0.0100 −x x 0.0100 −x

AcidStrengthandMolecularStructure

StrengthsofBinaryAcids•  themoreδ+H-Xδ-polarizedthebond,themoreacidicthebond

•  thestrongertheH-Xbond,theweakertheacid

•  binaryacidstrengthincreasestotherightacrossaperiod–  H-C<H-N<H-O<H-F

•  binaryacidstrengthincreasesdownthecolumn–  H-F<H-Cl<H-Br<H-I

StrengthsofOxyacids,H-O-Y•  themoreelectronegativetheYatom,thestrongertheacid– helpsweakenstheH-Obond

•  themoreoxygensattachedtoY,thestrongertheacid–  furtherweakensandpolarizestheH-Obond

LewisAcid-BaseTheory

LewisAcid-BaseTheory•  Basedonthesharingofelectronpairs•  Electrondonor=LewisBase=nucleophile– musthavealonepairofelectrons

•  Electronacceptor=LewisAcid=electrophile– electrondeficient

•  WhenLewisBasegiveselectronsfromlonepairtoLewisAcid,acovalentbondformsbetweenthemoleculesNucleophile:+Electrophile⇔Nucleophile:Electrophile

•  productcalledanadduct•  otheracid-basereactionsalsoLewis

DrawingLewisAcid-BaseReactions

OHH C H

⊕

+OH-1⇔

OHH C H

OH

OHH C H

⊕

+OH-1⇔

Electrophile Nucleophile

••

••••

Practice-CompletetheFollowingLewisAcid-BaseReaction.Labelthe

NucleophileandElectrophileanddrawanarrowtoshowelectrondirection

•  BF3+HF⇔

SampleQuestions•  WhatarethemajorspeciesinanaqueoussolutionofHF?– TheKavalueforHFis7.2×10–4

a.  H2O,H+,F–,andOH–

b.  H2O,H+,andF–

c.  H2O,HF,andOH–

d.  H2O,HF,H+,andF–

e.  H2OandHF

SampleQuestions•  Considertwoseparatesolutionsofequalconcentration– Thefirstsolutioncontainssodiumhydroxide,andthesecondsolutioncontainsbariumhydroxide

– Whichofthefollowingstatementsistrue?a.  ThesodiumhydroxidesolutionhasthelowerpHb.  ThebariumhydroxidesolutionhasthelowerpHc.  ThepHvaluesofbothsolutionsareequal

SampleQuestions•  Theconjugateacidandconjugatebaseofbicarbonateion(HCO3

–)are,respectively,a.  H3O+andOH–b.  H3O+andCO3

2–c.  H2CO3andOH–

d.  H2CO3andCO32–

e.  CO32–andOH–

SampleQuestions•  Ifthesolidsodiumcyanide(NaCN)isdissolvedinpurewater,willtheresultingsolutionbeacidic,basic,orneutral?a.  Acidicb.  Basicc.  Neutral

SampleQuestions•  Ifthesolidammoniumfluoride(NH4F)isdissolvedinpurewater,willtheresultingsolutionbeacidic,basic,orneutral?– Note:ForNH3,Kb=1.8×10–5;forHF,Ka=7.2×10–4

a.  Acidicb.  Basicc.  Neutral