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
24
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