Ch 14. Acids and Bases

Chem 30A

Acids and Bases

Acids and Bases

•  Acids–  Sourtaste– Dissolvemanymetals–  Turnlitmuspaperred.–  Egs.Ace9cacid(vinegar),

citricacid(lemons)

•  Bases–  Bi>ertaste,slipperyfeel–  Turnlitmuspaperblue.–  Egs.Drano,ammonia,caffeine

Arrhenius Definition of Acids and Bases

BasedonH+orOH-Produc9oninWater•  Acid:SubstancethatproducesH+ions(protons)inaqueoussolu9ons§  HCl(aq)àH+(aq)+Cl–(aq)

§  HNO3(aq)àH+(aq)+NO3–(aq)

•  Base:SubstancethatproducesOH-(hydroxide)ionsaqueoussolu9ons§  NaOH(aq)àNa+(aq)+OH–(aq)

•  Defnislimited–restrictedtoreac9onsinwateronly

Bronsted-Lowry Definition of Acids and Bases

BasedonProtonTransfer:•  Acids:Protondonors HClàH++Cl-

•  Bases:Protonacceptors NH3+H+àNH4

+

Bronsted-Lowry Acid

•  Anacidmusthaveanacidicprotonthatcanbedonated.

acidicproton

Bronsted-Lowry Base

•  Abasemusthavealonepairtoacceptaproton.

•  Abasecanbeneutralornega9velycharged.

Number of Acidic Protons on a Molecule

•  Anacidmoleculecanbemonopro9c,dipro9c,ortripro9c(basedonnumberofacidicprotons).

Water Acts as Both Acid and Base (Amphoteric)

•  Watercanactasabase,accep9ngaprotonfromanacidàFormshydroniumionH3O+.

•  Watercanactasanacid,dona9ngaprotontoabaseàFormshydroxideionOH-

The Proton in Water

•  AH+ioninwaterisnotisolated!AH+ioninwatera>ractsthenega9vepoleofwatermoleculesostronglythatitformsacovalentbondtowater.

HCl(aq)+H2O(l)àH3O+(aq)+Cl–(aq) hydroniumion(hydratedproton)

*Alsowri>enas:HCl(aq)àH+(aq)+Cl–(aq)

Bronsted-Lowry Acid and Base Work Together

•  Anacidandbasemustalwaysworktogethertotransferaproton!

Inaqueoussolu9onsofacidsorbases,wateractsasthebaseoracid“partner”:

§  HCl+H2OàCl-+H3O+(HClàCl-+H+)

acid base§  NH3+H2OàNH4

++OH-(NH3àNH4++OH-)

baseacid

Acid-Base Reactions Don’t Have to Involve Water

•  ABronsted-Lowryacid-basereac9ondoesnotalwaysoccurinwater.

ExProbs

List of Common Acids and Bases

StrongAcids HCl hydrochloricacidHBr hydrobromicacidHI hydroiodicacidHNO3 nitricacid H2SO4 sulfuricacid HClO4 perchloricacid

StrongBases:MetalHydroxidesofGroup1Aca9onsLiOH,NaOH,KOH,etc.andHeavierGroup2Aca9ons:Ca(OH)2,Sr(OH)2,Ba(OH)2

CommonWeakAcids HC2H3O2orCH3COOH ace9cacidH2CO3carbonicacid

CommonWeakBaseNH3ammonia

Conjugate Acid-Base Pairs

Conjugate Acid-Base Pair

•  NH3andNH4+areaconjugateacid-basepair.

•  H2OandOH–areaconjugateacid-basepair.

Conjugateacid-basepair:twosubstanceswhoseformulasdifferbyonlyahydrogenion

Conjugate Acid-Base Pair

•  H2SO4andHSO4–areaconjugateacid-base

pair.•  H2OandH3O+areaconjugateacid-basepair.

Conjugate Acid-Base Pair

Example Problems

Whichofthefollowingrepresentconjugateacid-basepairs?a.  HF,F-b.  NH4

+,NH3

c.  HCl,H2Od.  HClO4,ClO4

-

e.  HCl,ClO-

f.  H2PO4-,HPO4

2-

g.  HNO3,NO3-

✔

✔

✔

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✔ExProbs

Reactions of Acids and Bases

Acid-Base Reaction: Neutralization

Neutraliza9on:areac9oninwhichanacidandbasereactquan9ta9velywitheachother

Acid-Base Reaction: Acid + Strong Base

Whenanacidandastrongbase(metalhydroxide)react,theyformwaterandasalt(ioniccompound).

•  HCl(aq)+NaOH(aq)àH2O(l)+NaCl(aq)

Netioniceqn:H+(aq)+OH-(aq)àH2O(l)

Acid-Base Reaction: Gas-Forming

Whenanacidandabicarbonate(HCO3–)or

carbonate(CO32–)react,waterandgasareformed.

•  HCl(aq)+KHCO3(aq)àKCl(aq)+[H2CO3(aq)]à

KCl+H2O(l)+CO2(g)H+(aq)+HCO3

–(aq)à[H2CO3(aq)]àH2O(l)+CO2(g)

•  2HCl(aq)+K2CO3(aq)à2KCl(aq)+[H2CO3(aq)]à 2KCl(aq)+H2O(l)+CO2(g)

2H+(aq)+CO32–(aq)à[H2CO3(aq)]àH2O(l)+CO2(g)

Reaction of Acids with Metals (Redox Rxn)

•  Acidsdissolvemanymetals:Acidsoxidizemetals,causingmetalstogointosolu9on.

•  2HCl(aq)+Mg(s)àH2(g)+MgCl2(aq)acid metalH2gassalt

•  H2SO4(aq)+Zn(s)àH2(g)+ZnSO4(aq)acid metalH2gassalt

Titration

•  Titra9on:Determina9onoftheconcentra9onofasolu9onwithanunknownconcentra9on(analyte)bycombiningitwithastandardsolu9onofknownconcentra9on(9trant).

Acid-Base Titration •  Equivalencepoint:thepointwhereenough9tranthasbeenaddedtoreactexactlywiththeanalytepresent

•  Indicatorforacid-base9tra9on:Compoundwhosecolorisdifferentinacidthaninbase

•  Endpoint:thepointatwhichtheindicator’ssignalistriggered(Theendpointmayormaynotcomeexactlyatequivalencepoint).

moleH+=moleOH–

Acid-Base Titration

(titrant)

(analyte)

CH3COOH(aq)+NaOH(aq)àH2O(l)+Na+(aq)+CH3COO-(aq)

Acid-Base Titration

•  Acid-base9tra9onproblemsarestoichiometryproblems(solu9onstoichiometry).

ExProbs

Acid Strength

Acid Strength

•  Strongacid:Completelyionized(dissociated).HCl(aq) H+(aq)+Cl-(aq)

•  Weakacid:Mostoftheacidmoleculesremainintact.

CH3COOH(aq) H+(aq)+CH3COO-(aq) equilibriumarrow

Acid Strength

Strong Acid Solutions

•  Strongacidsionizecompletely: HCl à H+ + Cl-

So:0.01MHClà 0.01MH+ + 0.01MCl-

0.5MHClà 0.5MH+ + 0.5MCl-

•  Weakacidsdonotionizecompletely:

Acids as Electrolytes

Electrolyte:asolu9onoffreeions,conductselectricity

WeakacidWeakelectrolyte

StrongacidStrongelectrolyte

List of Common Acids and Bases

StrongAcids HCl hydrochloricacidHBr hydrobromicacidHI hydroiodicacidHNO3 nitricacid H2SO4 sulfuricacid HClO4 perchloricacid

StrongBases:MetalHydroxidesofGroup1Aca9onsLiOH,NaOH,KOH,etc.andHeavierGroup2Aca9ons:Ca(OH)2,Sr(OH)2,Ba(OH)2

CommonWeakAcids HC2H3O2orCH3COOH ace9cacidH2CO3carbonicacid

CommonWeakBaseNH3ammonia

Some Weak Acids and Weak Bases

ExProbs

Ion-Product Constant for Water

Water as an Acid and Base

•  Waterisamphoteric:canreactaseitheranacidorbase

•  Waterundergoesauto-ioniza9on:H2O(l)+H2O(l)àH3O++OH-

•  Forpurewater,25°C:[H3O+]=[OH–]=1.0×10–7M

•  Ion-productconstantforwater(Kw)Kw=[H3O+][OH–]=1.0×10–14at25°C *Trueforpurewaterandallaqueoussolu9ons!

Relationship between [H+] and [OH-]

Wecanrelate[H+]and[OH-]throughKw: Kw=[H+][OH–]=1.0x10-14

forwaterandallaqueoussolu9ons

Definitions: Neutral, Acidic, Basic

•  Neutralsolu9on:[H+]=[OH–]•  Acidicsolu9on:[H+]>[OH–]•  Basicsolu9on: [H+]<[OH–]

Ineachcase,however,Kw=[H+][OH–]=1.0×10–14 forwaterandallaqueoussolu9ons

ExProbs

pH and pOH

The pH Scale

pH=–log[H+]•  Acompactwaytorepresentsolu9onacidity.eg.pH=-log(1.0x10-7M)=7

Logarithmic Function

y=logx à10y=xpH=-log[H+]-pH=log[H+]à 10-pH=[H+]

•  As[H+]increases,pHdecreases.

pH Range

pH7 [H+]=[OH-] NeutralpH<7 [H+]>[OH-] AcidicpH>7 [H+]<[OH-] Basic

•  LowerthepH,moreacidicthesolu9on.•  HigherthepH,morebasicthesolu9on.

pH Range

10-pH=[H+]Every9mepHdropsby1,thereis10xincreasein[H+].

Calculating pH and [H+]

•  TogetpHfrom[H+]:pH=-log[H+]

•  Toget[H+]frompH:pH=-log[H+]-pH=log[H+]Inverselog(-pH)=inverselog(log[H+])Inverselog(-pH)=[H+]OR10-pH=[H+]

ExProbs

pOH

pH=-log[H+]

pOH=–log[OH-]

Calculating pOH and [OH-]

•  TogetpOHfrom[OH-]:pH=-log[OH-]

•  Toget[OH-]frompOH:

Inverselog(-pOH)=[OH-]OR10-pOH=[OH-]

ExProbs

Relationship Between pH and pOH

1.0x10-14=[H+][OH–]

log1.0x10-14=log[H+]+log[OH–] -14.00=–log[H+]–log[OH–]

14.00=log[H+]+log[OH–]

14.00=pH+pOH

ExProbs

Equations for pH Calculation Problems

1.  pH=-log[H+] (analogousforpOH)2.  Invlog[-pH]=[H+](analogousfor[OH–])3.  1.0×10–14=[H+][OH–]4.  14.00=pH+pOH

Exprobs

Determining pH in Laboratory

Buffers

Buffers

•  Buffer:asolu9onthatcontainsbothanacidandabase,thusresistspHchange

•  Bufferscontainsignificantamountsofbothaweakacidanditsconjugatebase.

•  Theweakacidneutralizesaddedbase. Theconjugatebaseneutralizesaddedacid.

•  Eg.ofbuffer:CH3COOHandCH3COO-

(CH3COO-comesfromNaCH3COO)

Water vs. Buffer