Acid-Base Equilibrium

Chapter 18Chapter 18

Acids and BasesArrheniusArrhenius

acids:acids: generate Hgenerate H++ in water in waterbases:bases: generate OHgenerate OH-- in water in water

BrBrønsted-Lowryønsted-Lowryacids:acids: HH++ donors donorsbases:bases: HH++ acceptors acceptors

HCl + HHCl + H22O O Cl Cl + H + H33OO++

acidacid base base

H3O+ = H+(aq) = proton in water

Conjugate acid-base pairsConjugate base:Conjugate base:

remains after Hremains after H++ is lost is lostacid: HClacid: HCl conj. base:conj. base: ClCl--

Conjugate acid:Conjugate acid:remains after Hremains after H++ is gained is gainedbase: NHbase: NH33 conj. acid:conj. acid: NHNH44

++

Base Acid+Acid Base+

Conjugate Pair

Conjugate Pair

Strong and Weak Acids

Strong:Strong: 100% dissociation100% dissociationgood Hgood H++ donor donorequilibrium lies far to right (HNOequilibrium lies far to right (HNO33))

generates weak base (NOgenerates weak base (NO33--))

Weak:Weak: <100% dissociation<100% dissociationnot-as-good Hnot-as-good H++ donor donorequilibrium lies far to left (CHequilibrium lies far to left (CH33COOH)COOH)

generates strong base (CHgenerates strong base (CH33COOCOO--))

Very strong

Strong

Weak

Very weak

Relative Acid Strength

Relative Conj. Base Strength

Very weak

Weak

Strong

Very strong

Acid Dissociation Constant

O H A O H HA 3-

2

[HA]]][A[H

[HA]]][AO[H K

--3

a

Ka I

ncre

ases

Strength vs. Ka

Cat o25101]][OHO[H K 14-3w

AcidAcid Base Base Conj. Acid Conj. Base Conj. Acid Conj. Base

Water is amphoteric

Proton Transfer in Water

O HH

O HH

O HH

O HH

O HH

O HH

O HH

O HH

+-

O HH

OH -

H O HH

O HH

[H3O+] and [OH-]

[H[H33OO++]] 1x10 1x1000 to 1x10 to 1x10-14-14 in waterin water

[OH[OH--]] 1x10 1x10-14-14 to 1x10 to 1x1000 in waterin water

]O[H

K ][OH3

w-

14-3w 101]][OHO[H K

][OH

K ]O[H -w

3

Finding [H3O+] and [OH-]

pH Scale

]log[H- ]Olog[H- pH 3

14-3w 101]][OHO[H K

pH 0 to 14 in water

]log[OH- ]log[OH- pOH pOH 14 to 0 in water

pKw = pH + pOH = 14

H+H+

H+

OH-OH-

OH-

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14Acidic Neutral Basic

pH and pOH

As [HAs [H33OO++] rises, [OH] rises, [OH--] falls] falls

As pH falls, pOH risesAs pH falls, pOH rises

Indicators

Polyprotic Acids

1.1. HH33POPO44 HH22POPO44-- + H + H++

KKa1a1 = 7.2 x 10= 7.2 x 10-3-3

2.2. HH22POPO44-- HPOHPO44

2-2- + H+ H++

KKa2a2 = 6.3 x 10= 6.3 x 10-8-8

3.3. HPOHPO442-2- POPO44

3-3- + H+ H++

KKa3a3 = 4.2 x 10= 4.2 x 10-13-13

Ka1 > Ka2 > Ka3

Strength of Bases

Strong:Strong:100% dissociation100% dissociationOHOH-- supplied to solution supplied to solutionNaOHNaOH(s)(s) Na Na++

(aq)(aq) + OH + OH--(aq)(aq)

Weak:Weak:<100% dissociation<100% dissociationOHOH-- by reaction with water by reaction with waterCHCH33NHNH2(aq)2(aq) + H + H22OO(l) (l) CHCH33NHNH2(aq)2(aq) + OH + OH--

(aq)(aq)

Base Dissociation Constant

OH BH O H B 2-

][B][BH][OH K -

-

b

Kw, Ka and Kb

14-3w 101]][OHO[H K

14baw 101K KK

KK K

b

wa

KK K

a

wb

Structure and Acid-Base Behavior

Binary hydrides:Binary hydrides:

1.1. Bond polarity (high Bond polarity (high acid strength) acid strength)

2.2. Bond strength (low Bond strength (low acid strength) acid strength)

Structure and Acid-Base Behavior

Binary oxides:Binary oxides:

1.1. Acidic oxides (acid anhydrides)Acidic oxides (acid anhydrides)OO—X bond:—X bond: strong and polarstrong and polarSOSO22, SO, SO33, NO, NO22, P, P44OO1010, …, …

2.2. Basic oxides (basic anhydrides)Basic oxides (basic anhydrides)OO—X bond:—X bond: ionicionicKK22O, CaO, …O, CaO, …

Structure and Acid-Base Behavior

Oxoacids (HOxoacids (HaaXXbbOOcc):):

1.1. Same number of O on different XSame number of O on different XEN of XEN of X

2.2. Different number of O on same XDifferent number of O on same Xmore Omore O

Relative Strengths of Oxoacids

Cl O H

Electron density

Cl O H

Electron density

O

Cl O H

Electron density

O

O

Cl O H

Electron density

O

OO

1 O

2 O

3 O

4 O

Increasing Acid Strength

Lewis Acids and Bases

Lewis Acid:Lewis Acid: electron pair acceptorelectron pair acceptorLewis Base:Lewis Base: electron pair donorelectron pair donor

Al3+ + 6H

OH

Al OH

H 6

3+

Acid Base