Chapter 13 Acids and Bases

Chapter 13

Acids and Bases

Arrhenius DefinitionArrhenius Definition Acids produce hydrogen ions in Acids produce hydrogen ions in

aqueous solution.aqueous solution. Bases produce hydroxide ions Bases produce hydroxide ions

when dissolved in water.when dissolved in water. Limited to aqueous solutions.Limited to aqueous solutions. Only one kind of base.Only one kind of base. NHNH33 ammonia could not be an ammonia could not be an

Arrhenius base.Arrhenius base.

Bronsted-Lowry DefinitionsBronsted-Lowry Definitions And acid is an proton (HAnd acid is an proton (H++) donor and a ) donor and a

base is a proton acceptor.base is a proton acceptor. Acids and bases always come in pairs.Acids and bases always come in pairs. HCl is an acid.HCl is an acid. When it dissolves in water it gives its When it dissolves in water it gives its

proton to water.proton to water.

HCl(g) + HHCl(g) + H22O(l) O(l) HH33OO++ + Cl + Cl--

Water is a base making hydronium ionWater is a base making hydronium ion

PairsPairs General equation General equation HA(aq) + HHA(aq) + H22O(l) O(l) H H33OO++(aq) + A(aq) + A--(aq)(aq)

Acid + Base Acid + Base Conjugate acid +Conjugate acid + Conjugate base Conjugate base

This is an equilibrium.This is an equilibrium.

CompetitionCompetition forfor H H++ betweenbetween H H22O and AO and A--

If HIf H22O is a stronger base it takes the HO is a stronger base it takes the H++

Equilibrium moves to right.Equilibrium moves to right.

Acid dissociation constant KAcid dissociation constant Kaa The equilibrium constant for the The equilibrium constant for the general equation.general equation.

HA(aq) + HHA(aq) + H22O(l) O(l) H H33OO++(aq) + A(aq) + A--

(aq)(aq)

KKaa = [H = [H33OO++][A][A--]]

[HA][HA] HH33OO++ is often written H is often written H++ ignoring ignoring

the water in equation (it is implied).the water in equation (it is implied).

Acid dissociation constant KAcid dissociation constant Kaa HA(aq) HA(aq) HH++(aq) + A(aq) + A--(aq)(aq)

KKaa = [H = [H++][A][A--]]

[HA] [HA] We can write the expression for We can write the expression for

any acid.any acid. Strong acids dissociate completely.Strong acids dissociate completely. Equilibrium far to right.Equilibrium far to right. Conjugate base must be weak. Conjugate base must be weak.

Strong acidsStrong acids

KKaa is largeis large

[H[H++] is equal to ] is equal to [HA][HA]

AA-- is a weaker is a weaker base than waterbase than water

Weak acidsWeak acids

KKaa is smallis small

[H[H++] <<< [HA]] <<< [HA] AA-- is a stronger is a stronger

base than waterbase than water

Types of AcidsTypes of Acids Polyprotic Acids- more than 1 Polyprotic Acids- more than 1

acidic hydrogen (diprotic, triprotic).acidic hydrogen (diprotic, triprotic). Oxyacids - Proton is attached to Oxyacids - Proton is attached to

the oxygen of an ion.the oxygen of an ion. Organic acids contain the Carboxyl Organic acids contain the Carboxyl

group -COOH with the H attached group -COOH with the H attached to Oto O

Generally very weak.Generally very weak.

AmphotericAmphoteric Behave as both an acid and a base.Behave as both an acid and a base. Water autoionizesWater autoionizes 2H2H22O(l) O(l) H H33OO++(aq) + OH(aq) + OH--(aq)(aq) KKWW= [H= [H33OO++][OH][OH--]=[H]=[H++][OH][OH--]] At 25ºC KAt 25ºC KWW = 1.0 x10 = 1.0 x10-14-14 In In EVERYEVERY aqueous solution. aqueous solution. Neutral solution [HNeutral solution [H++] = [OH] = [OH--]= 1.0 x10]= 1.0 x10-7-7 Acidic solution [HAcidic solution [H++] > [OH] > [OH--]] Basic solution [HBasic solution [H++] < [OH] < [OH--]]

pHpH pH= -log[HpH= -log[H++]] Used because [HUsed because [H++] is usually very ] is usually very

smallsmall As pH decreases, [HAs pH decreases, [H++] increases ] increases

exponentiallyexponentially Sig figs only the digits after the Sig figs only the digits after the

decimal place of a pH are significantdecimal place of a pH are significant [H[H++] = 1.0 x 10] = 1.0 x 10-8-8 pH= 8.00 2 sig figs pH= 8.00 2 sig figs pOH= -log[OHpOH= -log[OH--]] pKa = -log KpKa = -log K

RelationshipsRelationships KKWW = [H = [H++][OH][OH--]; pH= -log[H]; pH= -log[H++]] [H[H++] = 10] = 10-pH-pH -log K-log KWW = -log[H = -log[H++]+ -log[OH]+ -log[OH--]] pKpKWW = pH + pOH = pH + pOH KKWW = 1.0 x10 = 1.0 x10-14-14

14.00 = pH + pOH14.00 = pH + pOH [H[H++],[OH],[OH--],pH and pOH],pH and pOH

Given any one of these we can Given any one of these we can find the other three.find the other three.

Basic

Acidic Neutral

100

10-

1

10-

3

10-

5

10-

7

10-

9

10-

11

10-

13

10-

14

[H+]

0 1 3 5 7 9 11

13

14

pH

Basic

100

10-

1

10-

3

10-

5

10-

7

10-

9

10-

11

10-

13

10-

14

[OH-]

01357911

13

14

pOH

Calculating pH of SolutionsCalculating pH of Solutions Always write down the major ions Always write down the major ions

in solution.in solution. Remember these are equilibria.Remember these are equilibria. Remember the chemistry.Remember the chemistry. Don’t try to memorize there is no Don’t try to memorize there is no

one way to do this.one way to do this.

Strong AcidsStrong Acids HBr, HI, HCl, HNOHBr, HI, HCl, HNO33, H, H22SOSO44, HClO, HClO44

Completely dissociatedCompletely dissociated [H[H++] = [HA]] = [HA] [OH[OH--] is going to be small because ] is going to be small because

of equilibriumof equilibrium 1010-14-14 = [H = [H++][OH][OH--]] If [HA]< 10If [HA]< 10-7-7 water contributes H water contributes H++

Weak AcidsWeak Acids Ka will be small.Ka will be small. It will be an equilibrium problem It will be an equilibrium problem

from the start.from the start. Determine whether most of the HDetermine whether most of the H++

will come from the acid or the will come from the acid or the water.water.

Compare Ka or KwCompare Ka or Kw Rest is just like equilibrium Rest is just like equilibrium

problemsproblems

ExampleExample Calculate the pH of 2.0 M acetic Calculate the pH of 2.0 M acetic

acid HCacid HC22HH33OO22 with a Ka 1.8 x10 with a Ka 1.8 x10-5-5

Calculate pOH, [OHCalculate pOH, [OH--], [H], [H++]]

A mixture of Weak AcidsA mixture of Weak Acids The process is the same.The process is the same. Determine the major species.Determine the major species. The stronger will predominate.The stronger will predominate. Bigger Ka if concentrations are Bigger Ka if concentrations are

comparablecomparable

Calculate the pH of a mixture 1.0 Calculate the pH of a mixture 1.0 M HF (Ka = 7.2 x 10M HF (Ka = 7.2 x 10-4-4) and 1.0 M ) and 1.0 M HOCHOC66HH55 (Ka = 1.6 x 10 (Ka = 1.6 x 10-10-10))

P705#62

Percent dissociationPercent dissociation = = amount dissociated amount dissociated x 100 x 100

initial concentration initial concentration For a weak acid percent For a weak acid percent

dissociation increases as acid dissociation increases as acid becomes more dilute.becomes more dilute.

As [HA]As [HA]00 decreases [H decreases [H++] decreases ] decreases

but % dissociation increases.but % dissociation increases. Le ChatelierLe Chatelier

Calculate the % dissociation of Calculate the % dissociation of 1.00 M 1.00 M and .00100 M Acetic acidand .00100 M Acetic acid (Ka = 1.8 x 10(Ka = 1.8 x 10-5-5

BasesBases The OHThe OH--

is a strong base. is a strong base. Hydroxides of the alkali metals are Hydroxides of the alkali metals are

strong bases because they dissociate strong bases because they dissociate completely when dissolved.completely when dissolved.

The hydroxides of alkaline earths The hydroxides of alkaline earths Ca(OH)Ca(OH)22 etc. are strong dibasic etc. are strong dibasic

bases, but they don’t dissolve well in bases, but they don’t dissolve well in water.water.

Used as antacids because [OHUsed as antacids because [OH-- ] can’t ] can’t

build up.build up.

Bases without OHBases without OH--

Bases are proton acceptors.Bases are proton acceptors.

NHNH33 + H + H22O O NHNH44++ + OH + OH--

It is the lone pair on nitrogen that It is the lone pair on nitrogen that accepts the proton.accepts the proton.

Many weak bases contain NitrogenMany weak bases contain Nitrogen BB(aq) (aq) + H+ H22O(l)O(l) BH BH++(aq) (aq) + OH+ OH- -

(aq)(aq)

KKbb = [ = [BHBH++][][OHOH- - ]] [ [BB] ]

Strength of BasesStrength of Bases

N:

Hydroxides are strong.Hydroxides are strong. Others are weak.Others are weak. Smaller Smaller KKbb weaker base. weaker base.

Calculate the pH of a solution of Calculate the pH of a solution of 4.0 M pyridine4.0 M pyridine (Kb = 1.7 x 10(Kb = 1.7 x 10-9-9))

Calculate the pH of a solution of Calculate the pH of a solution of 0.20 M pyridine0.20 M pyridine (Kb = 1.7 x 10(Kb = 1.7 x 10-9-9))

P706#86

Polyprotic acidsPolyprotic acids Always dissociate stepwise.Always dissociate stepwise. The first HThe first H++ comes of much easier comes of much easier

than the second.than the second. Ka for the first step is much bigger Ka for the first step is much bigger

than Ka for the second.than Ka for the second. Denoted KaDenoted Ka11, Ka, Ka22, Ka, Ka33

Polyprotic acidPolyprotic acid HH22COCO33 HH++ + HCO + HCO33

--

KaKa11= 4.3 x 10= 4.3 x 10-7-7

HCOHCO33-- HH++ + CO + CO33

-2-2

KaKa22= 4.3 x = 4.3 x

1010-10-10

Base in first step is acid in second.Base in first step is acid in second. In calculations we can normally In calculations we can normally

ignore the second dissociation.ignore the second dissociation.

Calculate the ConcentrationCalculate the Concentration Of all the ions in a solution of 0.20 Of all the ions in a solution of 0.20

M Arsenic acid HM Arsenic acid H33AsOAsO44

KaKa11 = 5.0 x 10 = 5.0 x 10-3-3

KaKa22 = 8.0 x 10 = 8.0 x 10-8-8

KaKa33 = 6.0 x 10 = 6.0 x 10-10-10

P707#96

Sulfuric acid is specialSulfuric acid is special In first step it is a strong acid.In first step it is a strong acid. KaKa22 = 1.2 x 10 = 1.2 x 10-2-2

Calculate the concentrations in a Calculate the concentrations in a 1.0 M solution of H1.0 M solution of H22SOSO44

P685#14.16

Salts as acids an basesSalts as acids an bases Salts are ionic compounds.Salts are ionic compounds. Salts of the cation of strong bases Salts of the cation of strong bases

and the anion of strong acids are and the anion of strong acids are neutral.neutral.

for example NaCl, KNOfor example NaCl, KNO33

There is no equilibrium for strong There is no equilibrium for strong acids and bases.acids and bases.

We ignore the reverse reaction.We ignore the reverse reaction.

Basic SaltsBasic Salts If the anion of a salt is the conjugate If the anion of a salt is the conjugate

base of a weak acid = basic solution.base of a weak acid = basic solution. In an aqueous solution of NaFIn an aqueous solution of NaF The major species are NaThe major species are Na++, F, F--, and H, and H22OO FF- - + H+ H22O O HF + OHHF + OH--

KKbb =[HF][OH=[HF][OH--]]

[F[F- - ]]

but Ka = [Hbut Ka = [H33OO++][F][F--]]

[HF] [HF]

Basic SaltsBasic Salts KKaa x K x Kb b = [HF][OH= [HF][OH--]] x [Hx [H33OO

++]]

[F[F--]] [F[F- - ] ] [HF][HF]


++]]

[F[F--]] [F[F- - ] ] [HF][HF]


++]]

[F[F--]] [F[F- - ] ] [HF][HF]

Basic SaltsBasic Salts KKaa x K x Kb b = [HF][OH= [HF][OH--]] x [Hx [H++][F][F--]]

[F[F- - ] ] [HF][HF]

KKaa x K x Kb b =[OH=[OH--] [H] [H++]]

Basic SaltsBasic Salts KKaa x K x Kb b = [HF][OH= [HF][OH--]] x [Hx [H++][F][F--]]

[F[F- - ] ] [HF][HF]

KKaa x K x Kb b =[OH=[OH--] [H] [H++]]

KKaa x K x Kb b = K= KWW

KKaa tells us K tells us Kbb The anion of a weak acid is a semi-The anion of a weak acid is a semi-

strong base.strong base. Calculate the pH of a solution of 0.050M Calculate the pH of a solution of 0.050M

NaCN. Ka of HCN is 6.2 x 10NaCN. Ka of HCN is 6.2 x 10-10-10 The CNThe CN-- ion competes with OH ion competes with OH-- for the for the

HH++

P707#105b

Acidic saltsAcidic salts A salt with the cation of a weak base A salt with the cation of a weak base

and the anion of a strong acid will be and the anion of a strong acid will be basic.basic.

The same development as bases leads The same development as bases leads to to

KKaa x K x Kb b = K= KWW

Calculate the pH of a solution of 0.40 M Calculate the pH of a solution of 0.40 M NHNH44Cl (the KCl (the Kbb of NH of NH33 1.8 x 10 1.8 x 10-5-5).).

Other acidic salts are those of highly Other acidic salts are those of highly charged metal ions.charged metal ions.

Calculate the pH of a solution of Calculate the pH of a solution of 0.10 M NH0.10 M NH44Cl (the KCl (the Kbb of NH of NH33 1.8 x 1.8 x

1010-5-5).).

p691

Anion of weak acid, cation of Anion of weak acid, cation of weak baseweak base

KKaa > K > Kb b acidicacidic

KKaa < K < Kb b basicbasic

KKaa = K = Kb b NeutralNeutral

Structure and Acid base Structure and Acid base PropertiesProperties

Any molecule with an H in it is a potential Any molecule with an H in it is a potential acid.acid.

The stronger the X-H bond the less acidic The stronger the X-H bond the less acidic (compare bond dissociation energies).(compare bond dissociation energies).

The more polar the X-H bond the The more polar the X-H bond the stronger the acid (use stronger the acid (use electronegativities).electronegativities).

The more polar H-O-X bond -stronger The more polar H-O-X bond -stronger acid.acid.

Strength of oxyacidsStrength of oxyacids The more oxygen hooked to the The more oxygen hooked to the

central atom, the more acidic the central atom, the more acidic the hydrogen.hydrogen.

HClOHClO44 > HClO > HClO33 > HClO > HClO22 > HClO > HClO

Remember that the H is attached Remember that the H is attached to an oxygen atom.to an oxygen atom.

The oxygens are electronegativeThe oxygens are electronegative Pull electrons away from hydrogenPull electrons away from hydrogen

Strength of oxyacidsStrength of oxyacids

Electron Density

Cl O H


Electron Density

Cl O HO


Cl O H

O

O

Electron Density


Cl O H

O

O

O

Electron Density

Hydrated metalsHydrated metals Highly charged Highly charged

metal ions pull metal ions pull the electrons of the electrons of surrounding surrounding water water molecules molecules toward them.toward them.

Make it easier Make it easier for Hfor H++ to come to come off.off.

Al+3 OH

H

Acid-Base Properties of OxidesAcid-Base Properties of Oxides Non-metal oxides dissolved in Non-metal oxides dissolved in

water can make acids.water can make acids. SOSO33 (g) + H (g) + H22O(l) O(l) H H22SOSO44(aq)(aq)

Ionic oxides dissolve in water to Ionic oxides dissolve in water to produce bases.produce bases.

CaO(s) + HCaO(s) + H22O(l) O(l) Ca(OH) Ca(OH)22(aq)(aq)

Lewis Acids and BasesLewis Acids and Bases Most general definition.Most general definition. Acids are electron pair acceptors.Acids are electron pair acceptors. Bases are electron pair donors.Bases are electron pair donors.

B FF

F

:NH

H

H

Lewis Acids and BasesLewis Acids and Bases Boron triflouride wants more Boron triflouride wants more

electrons.electrons.

B FF

F

:NH

H

H

Lewis Acids and BasesLewis Acids and Bases Boron triflouride wants more Boron triflouride wants more

electrons.electrons. BFBF33 is Lewis acid NH is Lewis acid NH33 is a Lewis is a Lewis

Base. Base.

BF

F

F

N

H

H

H

Lewis Acids and BasesLewis Acids and Bases

Al+3 ( )H

HO

Al ( )6

H

HO

+ 6

+3

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Chapter 13 Acids and Bases