FLT• Iwillbeableto:– Explaintherelativestrengthsofacidsandbasesbasedonmolecularstructure,
interparticleforces,andsolutionequilibrium– IdentifycompoundsasBrønsted-Lowryacids,bases,and/orconjugateacid-
basepairs,usingproton-transferreactionstojustifytheidentification– Generateoruseaparticulaterepresentationofanacid(strongorweakor
polyprotic)andastrongbasetoexplainthespeciesthatwillhavealargeversussmallconcentrationsatequilibrium
– ReasonaboutthedistinctionbetweenstrongandweakacidsolutionswithsimilarvaluesofpH,includingthepercentionizationoftheacids,theconcentrationsneededtoachievethesamepH,andtheamountofbaseneededtoreachtheequivalencepointinatitration
– Reasonthatneutrality,basedonthedependenceofKWontemperature,requires[H+]=[OH-]asopposedtorequiringpH=7,includingespeciallytheapplicationstobiologicalsystems
– Identifyagivensolutionasbeingthesolutionofamonoproticweakacidorbase(includingsaltsinwhichoneisaweakacidorbase),calculatethepHandconcentrationofallspeciesinthesolution,and/orinfertherelativestrengthsoftheweakacidsorbasesfromgivenequilibriumconcentrations
bycompletingCh.14NotesA
Ch.14:AcidsandBases
Recall
8
Electrolytes
9
Electrolytes and Nonelectrolytes • Electrolytes=Dissolveintoions&conductelectricityinaq.soln.
Electrolytes and Nonelectrolytes • Nonelectrolytes=Donotconductelectricityinaq.soln.
• Mostaremolecular/covalent(noions)
Electrolytes and Nonelectrolytes • Notallelectrolytesconducttothesamedegree
• Thereareweakelectrolytes,andstrongelectrolytes
• Dependson:thedegreeofionization• Weakelectrolytesonlypartiallyionize.
Electrolytes and Nonelectrolytes • Weakelectrolytes=onlypartiallyionizeWeakelectrolyteshaveonlyafractionofthesolutethatexistsasions(about1%)– Weakacidsandbases;ammonia,aceticacid
Acids&Bases
Acids&Bases• PropertiesofAcids:
– Sourtaste(ex/vinegar,citrus)– Changethecolorofindicators
• Turnbluelitmuspaperred
– Electrolytes– Candissolvemanymetals– Reactwithcertainmetalstoproducehydrogengas
2HCl+MgàMgCl2+H2– ReactwithcarbonatestoproduceCO2gas
2HCl+CaCO3àCaCl2+H2O+CO2– Reactwithbases(neutralizationreactions)
Tro,Chemistry:AMolecularApproach 16
CommonAcidsChemical Name Formula Uses Strength
Nitric Acid HNO3 explosive, fertilizer, dye, glue Strong
Sulfuric Acid H2SO4 explosive, fertilizer, dye, glue,
batteries Strong
Hydrochloric Acid HCl metal cleaning, food prep, ore refining, stomach acid Strong
Phosphoric Acid H3PO4 fertilizer, plastics & rubber,
food preservation Moderate
Acetic Acid HC2H3O2 plastics & rubber, food preservation, Vinegar Weak
Hydrofluoric Acid HF metal cleaning, glass etching Weak
Carbonic Acid H2CO3 soda water Weak
Boric Acid H3BO3 eye wash Weak
Tro,Chemistry:AMolecularApproach 17
StructureofAcids• oxyacidshaveacidhydrogensattachedtoanoxygenatom– H2SO4,HNO3
Tro,Chemistry:AMolecularApproach 18
StructureofAcids• carboxylicacidshaveCOOHgroup– HC2H3O2,H3C6H5O7
• onlythefirstHintheformulaisacidic– theHisontheCOOH
Acids&Bases• PropertiesofBases:
– Bittertaste– Slipperyfeel– Changethecolorofindicators
• Turnredlitmuspaperblue
– Areelectrolytes– Canneutralizeacids
Acids&Bases• Alkaloidsarenaturallyoccurringorganicbases,suchasthosefoundinplants,thatoftenhaveasignificantphysiologicalaffectonhumans
Tro,Chemistry:AMolecularApproach 21
CommonBasesChemical
Name Formula Common Name Uses Strength
sodium hydroxide NaOH lye,
caustic soda soap, plastic,
petrol refining Strong
potassium hydroxide KOH caustic potash soap, cotton,
electroplating Strong
calcium hydroxide Ca(OH)2 slaked lime cement Strong
sodium bicarbonate NaHCO3 baking soda cooking, antacid Weak
magnesium hydroxide Mg(OH)2
milk of magnesia antacid Weak
ammonium hydroxide
NH4OH, {NH3(aq)}
ammonia water
detergent, fertilizer,
explosives, fibers Weak
Acids&Bases• Canwerecognizethestructureofbases?
• MostionicbasescontainOH-ions– NaOH,Ca(OH)2
• SomecontainCO32-ions
– CaCO3,NaHCO3• MolecularbasescontainstructuresthatreactwithH+– Mostlyaminegroups
DefinitionsofAcidsandBases
DefinitionsofAcids&Bases1. Arrhenius2. Brønsted-Lowry3. Lewis“Thedifferencesbetweenthevariousacid-baseconceptsarenotconcernedwithwhichisright,
butwhichismostconvenienttouseinaparticularsituation.”
-JamesHuheey
TheArrheniusDefinition
TheArrheniusDefinition• SvanteArrhenius(1880s)• Acid
– AsubstancethatproducesH+ionsinaqueoussolution
• Base– AsubstancethatproducesOH-ionsinaqueoussolution
ArrheniusTheory
HClionizesinwater,producingH+andCl–ions
NaOHdissociatesinwater,producingNa+andOH–ions
TheArrheniusDefinition• Note:• TheH+ionsarehighlyreactiveinaqueoussolution,andtheybondtowatertoformH3O+
• Consequently,H+(aq)andH3O+
(aq)aresometimesusedinterchangeablywhendiscussingaqueoussolutions
Brønsted-LowryDefinition
TheBrønsted-LowryDefinition• 1923• Morewidelyapplicabledefinition• Looksatthetransferofprotons(H+)inanacid-basereaction
• Acid=protondonor• Base=protonacceptor• Inthisdefinition,acidsandbasesalwaysoccurtogether
TheBrønsted-LowryDefinition• Wecanlabelouracid-basereactionsusingproton-transfer
• Ex1:
HCl(aq)+H2O(l)àH3O+(aq)+Cl-(aq)
• Ex2:
NH3(aq)+H2O(l)ßàNH4+(aq)+OH-
(aq)
Donated(Lost)H+
Donated(Lost)H+
Accepted(gained)H+
Accepted(gained)H+
Acid
Acid
Base
Base
TheBrønsted-LowryDefinition• AmphotericSubstances
– Substancesthatcanactasacidsorbases– Example:H2O
TheBrønsted-LowryDefinition• ConjugateAcid-BasePairs
– Twosubstancesrelatedtoeachotherbythetransferofaproton
• ConjugateAcid– Speciesformedafterabasehasacceptedaproton– Functionsastheacidinthebackreaction
• ConjugateBase– Speciesformedafteranaciddonatesaproton– Functionsasthebaseinthebackreaction
HCl(aq)+H2O(l)àH3O+(aq)+Cl-(aq)
TryThis:• IdentifyandLabelallspecies:
HCO3-(aq)+H2O(l)ßàH2CO3(aq)+OH-
(aq)
H2SO4(aq)+H2O(l)ßàHSO4-(aq)+H3O+
(aq)
AcidStrength&theAcidIonizationConstant(Ka)
AcidStrengthandKa• Recall:– Astrongacidisastrongelectrolyte,andaweakacidisaweakelectrolyte
• Inotherwords,thestrengthofanaciddependsontheequilibrium
HA(aq)+H2O(l)ßàH3O+(aq)+A-
(aq)• Iftheequilibriumliesfartotheright,theacidisstrongbecauseitcompletelyionizes.Iftheequilibriumliestotheleft,theacidisweakbecauseonlyasmall%oftheacidmoleculesionize
ArrowConventions• chemistscommonlyusetwokindsofarrowsinreactionstoindicatethedegreeofcompletionofthereactions
• asinglearrowindicatesallthereactantmoleculesareconvertedtoproductmoleculesattheend
• adoublearrowindicatesthereactionstopswhenonlysomeofthereactantmoleculeshavebeenconvertedintoproducts
AcidStrengthandKa• StrongacidsaremorewillingtodonateH• StrongacidsdonatepracticallyalloftheirH’s
– 100%ionizedinwater• [H3O+]=[strongacid]• Examples:• HCl• HBr• HI• HNO3• HClO4
• H2SO4
HCl→H++Cl-
HCl+H2O→H3O++Cl-
AcidStrengthandKa• WeakacidsdonatesmallfractionofH’s• Lessthan1%ionizedinwater• [H3O+]<<[acid]
• Examples:• HF• HC2H3O2(acetic)• HCHO2(formic)• H2SO3(sulfurous)• H2CO3(carbonic)• H3PO4(phosphoric)
HF⇔H++F-HF+H2O⇔H3O++F-
Tro,Chemistry:AMolecularApproach 40
Acids Conjugate Bases HClO4 ClO4
-1 H2SO4 HSO4
-1 HI I-1
HBr Br-1 HCl Cl-1
HNO3 NO3-1
H3O+1 H2O HSO4
-1 SO4-2
H2SO3 HSO3-1
H3PO4 H2PO4-1
HNO2 NO2-1
HF F-1 HC2H3O2 C2H3O2
-1 H2CO3 HCO3
-1 H2S HS-1
NH4+1 NH3
HCN CN-1 HCO3
-1 CO3-2
HS-1 S-2 H2O OH-1
CH3-C(O)-CH3 CH3-C(O)-CH2-1
NH3 NH2-1
CH4 CH3-1
OH-1 O-2
Increasin
gAcidity
IncreasingBasicity
AcidStrengthandKa• Thedegreetowhichanacidisstrongorweakdependsontheattractionbetweentheanionoftheacid(conjugatebase)andthehydrogenion,relativetotheattractionsoftheseionstowater
HA(aq)+H2O(l)ßàH3O+(aq)+A-
(aq)• IftheattractionsbetweenH+andA-isweak,thentherxnfavorstheforwarddirection,andtheacidisstrong
• Iftheattractionb/tH+andA-isstrong,thentherxnfavorsthebackdirection,andtheacidisweak
AcidStrengthandKa• Generallyspeaking,thestrongertheacid,theweakertheconjugatebaseandviceversa
• Iftheforwardreactionhasahightendencytooccur,thenthereversereactionhasalowtendencytooccur
GeneralTrendsinAcidity• ThestrongeranacidisatdonatingH,theweakertheconjugatebaseisatacceptingH
• higheroxidationnumber=strongeroxyacid– H2SO4>H2SO3;HNO3>HNO2
• cationstrongeracidthanneutralmolecule;neutralstrongeracidthananion– H3O+1>H2O>OH-1;NH4
+1>NH3>NH2-1
– basetrendopposite
AcidStrengthandKa• Awordofwarning…• Donotconfusetheterms“strong”and/or“weak”with“concentrated”and“diluted”.
TheAcidIonizationConstant(Ka)
TheAcidIonizationConstant(Ka)• Ka=Acidionizationconstant=theequilibriumconstantfortheionizationreactionofaweakacid
• GivenHA(aq)+H2O(l)ßàH3O+
(aq)+A-(aq)
or
HA(aq)ßàH+(aq)+A-
(aq)• Then
Ka= =
[H3O+][A-]
[HA]
[H+][A-]
[HA]
TheAcidIonizationConstant(Ka)• Forweakacids,Kashouldberelativelysmall