class10 chemistryg12 notes and homework
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GRADE12CHEMISTRY
OLYMPIADSCHOOL
WEDNESDAY2:304:30
FRIDAY4:456.45
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,
and
pH
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DifferentcombinationsofstrongArrheniusacidsandbasesreactwiththesameexothermicresult.Measurementsalwaysshowthereleaseof56kJofenergypermoleofwaterformed.Thismakessense,becausethenetionic
equationisthesameregardlessofthespecificneutralizationreactionthatoccurs.
TheArrheniustheoryhaslimitations,however.Forexample,H+(aq),abareproton,doesnotexistinwater.Thepositivechargeonaprotonisattractedtotheregionofnegativechargeonthelonepairofelectronsonawater
moleculesoxygenatom.Thecombinationisahydratedprotoncalleday ron um on, + aq + aq + + aq
Thehydronium ionformshydrogenbondswithotherwatermolecules.Thus,abetterformulafortheionthatispresentinacidicsolutionsis
H H2O n +,
w ere
n
is
usua y
4
or
5.
For convenience,
owever,c emists
usuallyuseasinglehydronium ionwhenwritingequations.
TheArrheniustheoryalsohaslimitationsforexplainingcertainreactions.Forexample,aqueoussolutionso ammoniaarebasic.Theyreactwithacidsinneutralizationreactions,eventhoughammoniadoesnotcontainthe
hydroxideion.Manyaqueoussolutionsofsaltswithnohydroxideionsareas c, oo. omereac ons a ep acew ou any qu so ven . orexample,ammoniumchloridecanbeformedbythereactionbetweenammoniaandhydrogenchloride,whicharebothgases:
g + g s
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TheBrnstedLowryTheory
ThelimitationsoftheArrheniustheoryofacidsandbasesareovercomebyamore eneraltheor calledtheBr nstedLowr theor .Thistheor was
proposedindependently,in1923,byJohannesBrnsted,aDanishchemist,andThomasLowry,anEnglishchemist.Itrecognizesanacidbasereactionasachemicalequilibrium,havingbothaforwardreactionandareversereactionthatinvolvethetransferofaproton.TheBrnstedLowrytheorydefinesacidsandbasesasfollows:
*Anacidisasubstancefromwhichaprotoncanberemoved.(SomechemistsdescribeBrnstedLowryacidsasprotondonors.)
*Abaseisasubstancethatcanacceptaproton.(SomechemistsdescribeBrnstedLowr basesas rotonacce tors.
Notethatthewordprotonreferstothenucleusofahydrogenatom anH+ionthathasbeenremovedfromtheacidmolecule.Itdoesnotrefertoa
rotonremovedfromthenucleusofanotheratom suchasox enorsulfur,thatmaybepresentintheacidmolecule.Asmentionedpreviously,H+ionsshareelectronswithanyspecies(ionormolecule)thathasalone
pairofelectrons.Inaqueoussolution,theprotonbondswithawatermoleculetoformthehydronium ion.UnliketheArrheniustheory,however,theBrnstedLowrytheoryisnotrestrictedtoaqueoussolutions.Forexample,thelonepairofelectronsonanammoniamoleculecanbondwith
+,an qu ammon acanac asa ase.
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ConjugateAcidBasePairs
proceedsinbothdirections.Aceticacidisweak,soonlyafewionsdissociate.Thepositionofequilibriumliestotheleft,andthereversereaction is favoured. In the reverse reaction the h dronium ion ives u aprotontotheacetateion.Thus,theseionsareanacidandabase,respectively.Theacidontheleft(CH3COOH)andthebaseontheright(CH3COO)differbyoneproton.Theyarecalledaconjugateacidbasepair.Similarly,H2OandH3O+areaconjugateacidbasepair.
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Aqueousammoniaisaweakbase,sorelativelyfew
y rox e
ons
orm.
epos on
o
equ r um
es
o
theleft.Intheforwardreaction,thewatermolecule.
canactasaprotondonor(anacid)inonereactionand
beamphoteric.(Wateractsasanacidinthepresence,
strongeracid.
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StrongAcidsandBases
Whenastrongacidorbasedissolvesinwater,almosteveryacidorbasemoleculedissociates.Whiletherearemany
acidsandbases,mostareweak.Thus,thenumberofstrongacidsandstrongbasesisfairlysmall.
StrongAcids
Binaryaci st at avet egenera ormu aHX aq ,w ereX=Cl,Br,andI(butnotF):forexample,hydrochloricacid,HCl,
andhydrobromic acid,HBr (HCl andHBr arehydrohalic
halogenelements).Oxoacids (acidscontainingoxygenatoms)inwhichthenumberofoxygenatomsexceeds,bytwoormore,thenumberofprotonsthatcanbedissociated:orexamp e,n tr cac , ,su ur c ac , ,
perchloric acid,HClO4,andchloric acid,HClO3
Thebinaryacidsofnonmetalsexhibitperiodictrendsin.
trend:theelectronegativity oftheatomthatisbondedtohydrogen,andthestrengthofthebond.
, , .Theyhaveonlyasinglehydrogenatomthatdissociatesinwater.Someacidshavemorethanonehydrogenatomthatdissociates.Theseacidsarecalledpolyprotic acids.
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StrongbasesareconfinedtotheoxidesandhydroxidesfromGroups1(IA)and2(IIA).
rong asesAlloxidesandhydroxidesofthealkalimetals:forexample,
, , ,
Alkalineearth(Group2(IIA))metaloxidesandhydroxides
belowberyllium:forexample,calciumhydroxide,Ca(OH)2,andbariumhydroxide,Ba(OH)2
Thestrongbasicoxideshavemetalatomswithlowe ec ronega v y. us, e on ooxygen s on can srelativelyeasilybrokenbytheattractionofpolarwatermolecules.Theoxideionalwaysreactswithwatermoleculestoproducehydroxideions.O2(aq)+H2O() 2OH(aq)
Magnesiumoxideandmagnesiumhydroxidearenotveryso u e. eyares rong ases, owever, ecause esma amountthatdoesdissolvedissociatesalmostcompletelyintoions.Ber lliumoxideisaweakbase. Itistheexce tionin
Group2(IIA).)Itisarelativelysmallatom,sothebondtooxygenisstrongandnoteasilybrokenbywatermolecules.
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CalculationsThatInvolveStrongAcidsand
Bases enas rongac ssoc a escomp e e y n o ons
inwater,theconcentrationofH3O+(aq)isequaltotheconcentrationofthestrongacid.Similarly,when
astron basedissociatescom letel inwater,the
concentrationofOH(aq)isequaltothe.
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TheEquilibriumofWeakAcidsandBases
solutionproducesionsthatinteractwithwater.ThepHoftheaqueoussolutionisdeterminedbythepositionofequilibriuminreac ons e ween e ons a arepresen nso u onan ewatermolecules.Purewatercontainsafewions,producedbythe
dissociationofwatermolecules:2H2O() H3O+(aq)+OH(aq)
At25C,onlyabouttwowatermoleculesinonebilliondissociate,.
neutralwater,at25C,theconcentrationofhydronium ionsisthesameastheconcentrationofhydroxideions:1.0 107mol/L.
eseconcentrat onsmust et esame ecauset e ssoc at onofwaterproducesequalnumbersofhydronium andhydroxide
ions.Becausethisisanequilibriumreaction,andbecausethepositionofequilibriumofallreactionschangeswithtemperature,[H3O+]isnot1.0 107mol/Latothertemperatures.Thesameistrueof OH .
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TheIonProductConstantforWater
Theequilibriumconstant,Kc,forthedissociationofwaterisgivenbythefollowingexpression.
Sofewionsformthattheconcentrationofwaterisessentiallyconstant.TheproductKc[H2O]2 isequaltotheproductoftheconcentrationsofhydronium ions
andhydroxideions.TheequilibriumvalueoftheconcentrationionproductH3O+ OH at25 Cisca e t e onpro uctconstant orwater.Itisgivent e
symbolKw.
Kc[H2O]2=[H3O+][OH]=1.0 107mol/L 1.0 107mol/L=1.0 1014=Kw
eun tsarecommon y roppe ,as not erequ r umexpress onsyou ave
encountered.TheconcentrationofH3O+inthesolutionofastrongacidisequaltotheconcentrationofthedissolvedacid,unlessthesolutionisverydilute.Consider
+ . .dissociateinwater,formingahydronium ionconcentrationthatequals0.1mol/L.Theincreased[H3O+]pushesthedissociationreactionbetweenwatermoleculesto
theleft,inaccordancewithLeChteliers principle.Consequently,theconcen ra ono y ron um ons a resu s rom e ssoc a ono wa er sevenlessthan1 107mol/L.This[H3O+]isnegligiblecomparedwiththe0.1mol/Lconcentrationofthehydrochloricacid.Unlessthesolutionisverydilute(about1
[H3O+]ofastrongacid.
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Similarly,theconcentrationofhydroxideionscanbe
thesolutionisastrongbase,youcanignorethedissociationofwatermoleculeswhendetermining[OH],unlessthe
. known,youcanusetheionproductconstantforwater,Kw,to
determinetheconcentrationoftheotherion.Althou hthevalueofKw forwateris1.0 1014at25Conly,youcanusethisvalueunlessanothervalueisgivenforadifferent
.
Inanacidicsolution,[H3O+]isgreaterthan1.0 107mol/L
and[OH]islessthan1.0 107mol/L.Inaneutralsolution,both[H3O+]and[OH]areequalto1.0 107mol/L.Inabasicsolution,[H3O+]islessthan1.0 107mol/Land[OH]
. .
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pHandpOH Youcandescribetheacidit ofana ueoussolution uantitativel b statin
theconcentrationofthehydronium ionsthatarepresent.[H3O+]isoftenaverysmallnumber.ThepHscalewasdevisedasaconvenientwayto
represen ac y an , yex ens on, as c y . esca e s ogar m c,
basedon10.Thinkoftheletterpasamathematicaloperationrepresenting
log.ThepHofasolutionistheexponentialpowerofhydrogen(orhydronium)ions,inmolesperlitre.Itcanthereforebeexpressedasfollows:pH= log[H3O+]
T epractica rangeo t epHsca eis rom0to14.Aso utiono astrong
acidthatismoreconcentratedthan1.0mol/LwouldgiveanegativepH.
Since oucandetermine H3O+ ofsuchsolutionsdirectl fromthe
concentrationoftheacid,thepHscaleoffersnoadvantage.Similarly,the
pHofastrongbasethatismoreconcentratedthan1.0mol/Lisgreaterthan
. o e a p sa mens on essquan y. no erwor s, asnoun s.
YoucancalculatethepOH (thepowerofhydroxideions)ofasolutionfrom
=
Kw=[H3O+][OH]=1.0 1014at25C pH+pOH =14
Th A id Di i i C
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TheAcidDissociationConstant
Manycommonfoods(suchascitrusfruits),pharmaceuticals,andsomevitaminsareweakacids.Whenaweakaciddissolvesinwater itdoesnot
completelydissociate.Theconcentrationofthehydronium ions,andtheconcentrationoftheconjugatebaseoftheacidthatisformedinsolution,dependontheinitialconcentrationoftheacidandtheamountofacidthatdissociates.Youcanrepresentanyweakmonoprotic acidwiththegeneralformulaHA.Theequilibriumofaweakmonoprotic acidinaqueoussolution
canbeexpressedasfollows:HA(aq)+H2O(aq) H3O+(aq)+A(aq)
TheequilibriumexpressionforthisreactionisKc =[H3O+][A]/[HA][H2O]
, .
Multiplyingbothsidesoftheequilibriumexpressionby[H2O]givestheproductoftwoconstantsontheleftside.Thisnewconstantiscalledtheaciddissociationconstant Ka.Kc[H2O]=Ka=[H3O+][A]/[HA]
YoucandeterminethevalueofKaforaparticularacidbymeasuringthepH.
WeakacidshaveKavaluesthatarebetween1andabout1 1016.VeryweakacidshaveKavaluesthatarelessthan1 1016.Thesmallerthe
, .
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The Base Dissociation Constant
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TheBaseDissociationConstant
Manycompoundsthatarepresentinplantsareweakbases.
a e ne
nco ee
an
p per ne n
ac
pepper
are
wo
examples.Aweakbase,representedbyB,reactswithwatertoformane uilibriumsolutionofions.
B(aq)+H2O() HB+(aq)+OH(aq)
Theequilibriumexpressionforthisgeneralreactionisgivenasfollows:
Kc =[HB+][OH]/[B][H2O]
Theconcentrationofwaterisalmostconstantindilutesolutions.Multiplyingbothsidesoftheequilibrium
theleftside.Thenewconstantiscalledthebasedissociationconstant,Kb.
Kc[H2O]=[HB+][OH]/[B]=Kb
NitrogencontainingcompoundsareBrnstedLowrybases,
bondwithH+fromwater.
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AcidsandTheirConjugateBases
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j g
Thereisanimportantrelationshipbetweenthedissociationconstantforanacid Ka andthedissociationconstant oritscon u atebase Kb.Consider
aceticacidanditsdissociationinwater.CH3COOH(aq)+H2O(aq) H3O+(aq)+CH3COO(aq)
Theacetateionistheconjugatebaseofaceticacid.Asolublesaltofthe
conjugatebase,
such
as
sodium
acetate,
forms
acetate
ions
in
solution.
The
.
CH3COO(aq)+H2O(aq) CH3COOH(aq)+OH(aq)
Kbisgivenbytheexpression:Kb=[CH3COOH][OH]/[CH3COO]
TheproductKaKb givesaninterestingresult.
KaKb =[H3O+][CH3COO]/[CH3COOH] [CH3COOH][OH]/[CH3COO]
= =
Thus,foranacidanditsconjugatebase(orabaseanditsconjugateacid),KaKb =Kw.Oneinterpretationoftheresultisthestrongeranacid,the
. ,strongacidgivesupaprotonfromeachmolecule.Therefore,itsconjugatebasedoesnotbondwiththeproton.Insummary,then,thestrengthofan
acidisalwaysaweakbase,and,conversely,theconjugateofastrongbase
isalwaysaweakacid.
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