acids and bases. an introduction

7/30/2019 Acids and Bases. an Introduction

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AcidsandBases

AnIntroduction

DavidAKatzPimaCommunityCollege,

Tucson,AZ,USA

BasedonaPowerPointb JohnD.Bookstaver St.CharlesCommunit Colle e St.PetersMOforChemistry,TheCentralScience,10thedition,byTheodoreL.Brown;H.EugeneLeMay,

Jr.;

and

Bruce

E.

Bursten


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1. Sourtaste

(examples:vinegar,citricacid,lemonjuice)2. Turnslitmusfrombluetored(alsoaffectsother

indicators)

3. ReactswithmetalstoliberateH2

4. Reactswithbaselosingitsacidproperties

5. Reactswithsaltstogiveanewacidandanewsalt

6. Conductselectricity(electrolyte)


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1. Bittertaste(examples:bakingsoda,soap)

2. Turnslitmusfromredtoblue(alsoaffectsotherindicators bestknownisphenolphthalein)

3. Soapyfeel

4. Reactswithacidlosingitsbasicproperties

5. Conductselectricity(electrolyte)


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Acid: Substancethatproducesh dro enionsinwatersolution.

HCl(aq) H+

(aq) + Cl

(aq)

hydroxideionsinwatersolution.

NaOH Na+ +OH

Anacidneutralizesabase+

aq aq


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hydrogenionsproduced.DefinitionincludesacidssuchasHCl,HBr,HI,

HNO3,H2SO4,HSO4,HC2H3O2,etc.

Basestrengthdependsonamountofy roxi eionspro uce .

DefinitionincludesbasessuchasNaOH,KOH,

2,

2,

3,

.


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Thesolventislimitedtowater,butwaterisnotinvolvedintheacidbasereaction

AsolutionofCO2inwaterisacidic,butH2CO3 existsata

concentrationless

than

1%

ofthe

CO2

3 , 4 exist (Note:theformulaNH4OHwasinventedby

ArrheniustoexplainthebasicnatureofNH3 solutions)

ThereactionofHClandNH3 resultsinanacidicsolution


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Thesolvent water canionizeto roduceH+ andOH

ionsH2O(l) H

+(aq) + OH

(aq)

Acid: Substancethatincreasestheconcentrationof.

HCl(aq) H+

(aq) + Cl

(aq)

Astrongacidis100%dissociated

Aweakacidisonlyslightlydissociated


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Base: Substancethatincreasestheconcentrationof

hydroxideionsinwatersolution.NaOH(a ) Na

+(a ) +OH

(a )

Astrongbaseis100%dissociated

Aweakbaseisonlyslightlydissociated

HydrolysisreactionscanchangeH+ andOH

CO2 +H2O H+ + HCO3

CO2isanacid

+


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Solventsotherthanwatercanbeused:

Thesolventcanionizetoproducepositiveandnegative

ions

2NH3 NH4+ + NH2

c : u s ance a ncreases econcen ra ono epositiveioninsolution.

HCl +NH3 NH4+ + Cl

Base: Substancethatincreasestheconcentrationofthenegativeioninwatersolution.

NaOH +NH3 Na+ +NH2

+H2O(l)


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Neutralizationproducesthesolvent:

, 3

NH4+ + NH2

2NH3


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JohannesNicolaus Brnsted (18791947)andThomasMartinLowry(18741936)

independentlyproposedtheprotondefinitionofacidsandbasesin1923

QuotingBrnsted:

"...acidsandbasesaresubstancesthatareca able of s littin off or takin u h dro enions,respectively.

or

Anacidbasereactionconsistsofthetransferof

aproton(orhydrogenion)fromanacidtoabase


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Thatis,anacidisasubstancefromwhicha+ .

ABaseisaprotonacceptorThatis,abaseisasubstancethathasanon

bondedelectronpairthatcanbondwitha

proton(H+)fromanacid.

Nosolventisspecified


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proton,thenitis amphiproticSomeexam lesare

HCO3 ,HSO4

,andH2O

asanacid: HCO3 + H2O H3O

+ + CO32

asabase: HCO3 + H3O+ H2CO3 + H2ONotethattheproton(H+)iscombinedwith

water orm ng 3


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HCl+H2O H3O

+ +Cl

Lookingattheforwardreaction:HClisanacid,becauseithasaprotonavailabletobe

transferred.

H2Oisabase,

since

itbonds

with

the

proton

that

the

acid

.

Lookingatthereversereaction:

H O+ is an acid because it can ive a roton to the Cl

Cl isabase,sinceitcanbondwiththeprotonfromH3O+

NotethateachpairHClandCl,and,H2OandH3O+ differby

oneproton.Thesepairsarecalledconjugateacidbasepairs


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So,inthereaction:

HCl+H2O H3O+ +Cl

Acid Base

HClisanacid,andCl istheconjugatebase ofHCl

2

andH3O

+2 , 3 2


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Th Br n L wr Th r

WhenanAcidDissolvesinWater:

WateractsasaBrnsted

Lowrybaseandabstractsa+

Acid1 Base2

.

Asaresult,theconjugate

base oftheacidanda

hydroniumion areformed.

Base1 Acid2


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ConjugateAcids

and

Bases:

FromtheLatinwordconjugare,meaningtojointo ether.

Reactionsbetweenacidsandbasesalwaysyieldtheircon u atebasesandacids.

Anotherexampleis:


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StrongAcids

HNO3, HCl, H2SO4 and HClO4 are the mostcommon y nown s rong ac s.


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StrongAcids

submicroscopicartists view


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WeakAcidsWeakAcids

Weakacidsaremuchlessthan100%ionizedinwater

Oneofthebestknownweakacidsisaceticacid, CH3CO2H


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WeakAcidsWeakAcids


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StrongBases

AStrongBaseis100%dissociatedinwater.100%dissociatedinwater.

NaOHNaOH((aqaq)) NaNa

++

((aqaq)) +

OH+

OH

((aqaq))

OthercommonstrongbasesincludeKOHandCa(OH)

CaO (lime)+H2O Ca(OH)2 (slakedlime)

CaOCaO


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AWeakbaseislessthan100%ionizedinwater

Oneofthebestknownweakbasesisammonia

NH3(aq) +H2O(l) NH4+

(aq) +OH(aq)


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ThesevenstrongacidsareHCl,HBr,HI,HNO3,

2 4, 3, 4.

, ,existtotallyasionsinaqueoussolution(i.e.,they

are100%ionicinsolution)

Forthemonoprotic strongacids,the

concentrat ono H3+

sequa tot econcentrationoftheacid:

3


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Strongbasesarethesolublehydroxides,whichare

t ea a meta an eav era a neeart meta

hydroxides(Ca2+,Sr2+,andBa2+).

Strongbasesdissociatecompletelyinaqueous. ., .

oramono y roxy s rong ase, econcen ra onofOH isequaltotheconcentrationofthebase:

= ase


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Acetic acid, HC2H3O2,isaweakacid

HC H O +H O H O+ +C H O '

AcetateisastrongerbasethanH2O,sothemolecular

.

Onlyabout1%oftheaceticacidisionic.


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PolyproticAcids

Havemorethanoneionizable H

, 2 4

Thedissociationofapolyprotic acidiswrittento

H2SO

4(aq)+ H

2O

(l)HSO

4

(aq)+ H

3O+

(aq)

HSO4(aq) + H2O(l) SO4

2(aq) + H3O

+(aq)

,

theequationarepresentinsolution.


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PolyproticAcids


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P l r i A i


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Waterisamphoteric.

Inpurewater,afewmoleculesactasbasesandafewactasacids.

H2O(l) +

H2O(l) H3O

+

(aq)+

OH

(aq)'

Thisprocessisreferredtoasautoionization.


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AutoionizationofWater

TheproductoftheconcentrationsofH3O+ andOH is

g ven yt eequat on:

Kw = [H3O+][OH]= 1.00x1014 at25oC

+ 3

so [H3O+]= [OH] =1.00x107 M


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H

Acommonwaytoexpressacidityand

basicityiswithpH

Hisdefinedasthene ativelo arithm tothebase

10)ofthehydrogenion(hydronium)concentration

+ 3

Inaneutralsolution,

[H3O+] = [OH]=1.00x107 at25oC

pH = log(1.00x107)

= (7) = 7


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H

, ,

pH= log(1.0 107)=7.00+ 3 ,

itspHis7.


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H

TheconceptofpHwasfirstintroducedbyDanishchemistSrenPederLauritzSrensen(18681939),theheadofthe

CarlsbergLaboratorysChemicalDepartment,in1909

Dr.SrensendevelopedthepHscaleduringhispioneeringresearchintoproteins,aminoacidsandenzymes thebasis

oftodaysproteinchemistryinapapertitledEnzyme

StudiesII.TheMeasurementandMeaningofHydrogenIon

ConcentrationinEnzymaticProcesses:The value of the hydrogen ion concentrationwill accordingly be expressed by thehydrogen ion based on the normality factorof the solution used, and this factor will

.

Since in the following section I usually referto this, I will explain here that I use the name"hydrogen ion exponent" and the

designation PH for the numerical value of the

exponents of this power.Srensen and the Carlsberg Chemistry Department


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H

Today,werefertopHasmeaningthepowerofhydrogen.

The Hscale rovidesasim leanduniversalmeasurementoftheamountofhydrogenionsinasolution,whichaffectsitsacidityand

howitreactschemically.

Eachvalueof HmeanstheH+ concentrationchan esb afactorof10

AstheH+ concentrationdecreases,theOH concentrationincreases

pH 1 pH 7 pH 14strong weak neutral weak strong

The pH scale according to the late Dr. Hubert Alyea, Princeton University


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pHvalues

common


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TheLevelingEffect

ThepHscalerangesfrom0to14

CananacidorbasehaveapHlowerthan0orhigherthan14?

Whenastrongacid,suchasHCl,isdissolvedinwater,itreac s o orm 3 n e o ow ngreac on:

+ aq aq aq

AnyacidthatisstrongerthanH3O+ reactswithH2Oto

orm3

; ere ore,noac can es ronger an3

inH2O.


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TheLevelingEffect

Inthesameway,abasewhichisstrongerthantheconjugatebaseofwater,suchassodiumoxide,willreact

withwatertogivetheweakerbasewhichisthehydroxide

ion,OH: +

2 (s) 2 (aq) (aq)Note: Theactualbasehereistheoxideionsincethesodiumionisanextremelyweakacidorbase.

Allbaseswhicharestrongerthanthehydroxideionwill.

Althoughoxideion,amideion,ethoxideion,andmethoxideionareallstrongerbasesthanhydroxideion,

theirstrengthinaqueoussolutionisleveledtothatofhydroxideion.

2

OH;therefore,nobasecanbestrongerthanOH inH2O.


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TheLevelingEffect

Canthestrengthsofstrongacidsorbasescanbedifferentiated?

Thelevelingeffectoperatesinanyprotonicsolvent. Ifasolventotherthanwaterisused,differencesbetween

.

Inliquidammonia,forexample,allacidsarelevelledto

thestrengthoftheammoniumion,NH4+

,andallbasesareleveledtothestrengthoftheamideion,NH2

.

2NH3 NH4+ + NH2

Manyoftheacidswhichareweakinwateractasstrongacidsinliquidammoniabecausetheyarestrongerthanammoniumion.

Nota o t e asesw ic arestronginwaterarea sostronginliquidammonia.


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TheLevelingEffect

Glacialaceticacidisanotherprotonicsolventinwhichthelevelingeffecttakesplace.

HC2H3O2 H+

+C2H3O2

Glacialaceticacidcanbeusedtoshowthath dro enchlorideisaweakeracidthanisperchloricacid,since

hydrogenchloridebehavesasaweakacidinglacial

.

Methanolisalsoaprotonicsolventinwhichsomeoftheacidswhicharestrong(completelydissociated)in

waterarefoundpartiallyinmolecularform


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rev ouswesaw:

H O+ OH =K =1.0 1014

Ifwetakethenegativelogof[H3

O+]and[OH]

log[H3O+]+ log[OH]= logKw=14.00

or, no erwor s,H+ OH= K =14.00


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M rin H Forasimpleacidorbasedeterminationweusean

n cator nownas tmuspaper Litmusisawatersolublemixtureofdifferentdyesextractedfromlichens,especially

Roccellatinctoria.

ename muscomes rom e ng s emose o can nav anor g n;a n

toOldNorselitmosi,dyer'sherbs:litr,color,dye+mosi,bog,moss)andMiddle

Englishlykemose(fromMiddleDutchlijkmoes,variantoflcmoes:lken,todrip+

moes,moss).

Red litmus paper turnsblue above ~ H = 8

Blue litmus paper turnsred below ~pH = 5


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M rin H

usingamixtureofdyestoindicatepHvaluesovera

widerangeor,formore

accuratemeasurements,

ereares or range

papers.


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MeasuringpH

Indicatorsaredyesormixturesofdyesthatcanbeaddedto

solutionsofacidsorbasestodeterminepH.

Somecommonindicatorsandtheircolorchangesareshownin

thechartbelow:


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M rin HFormoreaccurate

measurements,apH

meterisused.

Usingspecialglass

electrodes,themeter

measures evo ageo

thesolutionasa

functionoftheactivityof

thehydrogenionsnearthetipoftheelectrode.


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ThepHMeter

FirstpHmeterwasconstructedin1934byArnoldBeckman(19002004).

AglasspHelectrodethathadapotentialdependenton

activityofH+ ionshadbeenconstructedin1906byFritzHaberandZygmuntKlemensiewicz,buttherewere

tec n ca cu t es uetoa arge nterna res stanceo

glasselectrodes.Toobtainreliableresultsonewas

forcedtouseverysensitivegalvanoscope expensive

.

ToovercometheproblemArnoldBeckmanproposedtousesimplehighgainamplifiermadeusingtwovacuum

.

withcheapmiliamperometers.

Beckmans first pH meter, 1934 (middle photo)

The Model G pH meter, the first successful marketed pHmeter, 1936 (bottom photo)

acids and bases. an introduction

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