Acidsand

Bases

Several concepts of acid-base theory:

The Arrhenius concept

The Bronsted-Lowry concept

The Lewis concept

Arrhenius Acids & BasesAccording to the Arrhenius concept of acids and bases

An acid is a substance that, when dissolved in water, increases the concentration of Hydrogen ion, H+(aq)

The H+(aq) ion, called the Hydrogen ion is actually chemically bonded to water, that is, H3O+, called the Hydronium ion

The Hydronium Ion (H3O+) actually exists as a hydrogen

bonded H9O4+ cluster.

)(3)(2)( aqlaq OHOHH

According to the Arrhenius concept of acids and bases

A base is a substance that when dissolved in water increases the concentration of the hydroxide ion, OH-(aq)

HBr(aq) H+(aq) + Br-(aq)Acid + H2O Anion + H3O+

HA + H2O A– + H3O+

HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2

−(aq)

a. Metal HydroxidesNaOH(s) Na+

(aq) + OH–(aq)

Mg(OH)2(s) Mg2+(aq) + 2OH–

(aq)

b. Molecular bases: NH3(aq)+H2O NH4

+(aq)+ OH-

(aq)

Arrhenius acid-base neutralization Acid and base combine to produce water and salt.

Ex. HCl(aq) + NaOH(aq) H2O + NaCl(aq)

H3O+(aq) + Cl–(aq) + Na+(aq) + OH–(aq)

2H2O + Cl–(aq) + Na+(aq)

However, many reactions resemble this without forming H3O+ or OH– in solution!!!

5

Gas Phase Acid-Base• Not covered by Arrhenius definition

Ex. NH3(g) + HCl(g) NH4Cl(s)

Brønsted-Lowry Definition• Acid = proton donor• Base = proton acceptor

Allows for gas phase acid-base reactions

HCl + H2O H3O+ + Cl–

HCl = acid : Donates H+ Water = base : Accepts H+

7

Bronsted-Lowry Acids & Bases

Amphoteric Species: can act as either as acid or base

WaterWater can react as an acid by donating a proton to a base

Water can react as a base by accepting a proton from an acid

+ -3 2 4N H (a q ) + H O (l) N H (a q ) + O H (a q )

H+

+2 3H F (a q ) + H O (l) (a q ) + H O (a q )F

H+

Learning Check

• Write a reaction that shows that HCO3– is a

Brønsted acid when reacted with OH–

• HCO3–(aq) + OH–(aq)

• Write a reaction that shows that HCO3– is a

Brønsted base when reacted with H3O+(aq)

• HCO3–(aq) + H3O+(aq)

9

H2CO3(aq) + H2O(ℓ)

H2O(ℓ) + CO32–(aq)

Lone pair binds H+

Lone pair binds H+

Conjugate Acid-Base

Pair

H

H

Learning Check

conjugate base

conjugate acid

HCl

NH3

HC2H3O2

CN–

HF

12

Identify the Conjugate Partner for Each

Cl–

NH4+

C2H3O2–

HCNF–

)()(3)(2)( aqaqlaq AOHOHHA

][

][][ 3

HA

AOHKa

The concentration of water is almost constant.

Ka very large Ka very small

solutes

ElectrolytesNon-

electrolytesSolute undergoes Dissociation

strong weakcomplete

dissociationpartial dissociation

NaCl / H2OHCl / H2O

HAc / H2O

Sugar / H2O

)()()(2

aqaqOH

s ClNaNaCl )()()(2

aqaqOH

l AcHHAc

)()(3)(2)( aqaqlaq AOHOHHA

Relative Strengths of

Acid and Bases

As acid strength decreases, base

strength increases; the weaker the acid,

the stronger its conjugate base.

As base strength decreases, acid

strength increases; the weaker the base,

the stronger its conjugate acid.

Position of Acid-Base Equilibrium

• Acetic acid (HC2H3O2) is weak acid– Ionizes only slightly in water

HC2H3O2(aq) + H2O(ℓ) H3O+(aq) + C2H3O2–(aq)

weaker acid weaker base stronger acid stronger base

• Hydronium ion– Better H+ donor than acetic acid – Stronger acid

• Acetate ion – Better H+ acceptor than water – Stronger base

• Position of equilibrium favors weaker acid and base19

Calculate [H+] or [OH-] as required for each of the following solutions at 25oC, and state whether the solution is neutral, acidic, or basic

MH

HOHH

OHHKw

713

132

13

100.3100.1][

100.1][][][

100.1][][

As T is increased, the product concentration increases,Reaction shifts to the right,

Forward reaction proceeds faster,Forward reaction is endothermic.

A common way to express acidity and basicity is with pH

pH = - log [H3O+] or [H3O+]=10-pH

Acidity and Basicity

The # of sig figs in the concentration

= the number of decimal places in pH

In an acidic solution,

[H3O+]= 1.25 x 10-4 M at 25 oC

pH =-log (1.25 x 10-4)= -(-3.903) = 3.903

UA GenChem

Your Turn!What is the pH of Black Coffee, [H3O+]= 1.0 x 10-5 M?

What is the H3O+ concentration in sea water if pH=8.30?

What is the OH¯ concentration in acid rain if pH=5.25?

[H3O+], [OH-] and pH

General conclusion @25ºC Basic solution pH > 7 Neutral pH = 7

Acidic solution pH < 7

Other pX ScalesIn general pX = -log X

and so pOH = - log [OH-]

Which solution is more basic, one that has a pH of 5.5 or one with a pOH of 8.5?

If Kw = [H3O+] [OH-] = 1 x 10-14

Taking the log of both sides

-log (10-14) = - log [H3O+] + (-log

[OH-])

pKw = 14 = pH + pOH

The pH of a sample of human blood was measured to be 7.41 at 25oC. Calculate pOH, [H+], and [OH-] for the sample.

33 NOHHNO

OHHNOsolution HHH23][][][

0.1 M 0.1 M< 10-7 M

3][][ HNOsolution HH

acidstrongHNOsolution HHHpH ]log[]log[]log[3

pH =-log (0.1)= -(-1) = 1

ClHHCl

OHHClsolution HHH2][][][

1x10-10 M 1x10-10 M

= 10-10 M

OHsolution HH2][][

OHsolution HHpH2]log[]log[

pH ≈-log (1x10-7)= -(-7) ≈7

≈ 10-7 M

slightly smaller

Calculate the pH of 1.0 M HF solution.

OHHFsolution HHH2][][][

FHHFco

chang

e

ceq

molxx 242 107.2102.700.1

%7.2%1001

107.2%100

2

oc

x < 5%

For any weak acid: HA → H+ + A-

check

If < 5%

pH =-log(x)=-log (2.7x10-2)= 1.57

autoionization of water neglected

Calculate the pH of a solution that contains 1.00 M HCN (Ka=6.2×10-10) and 5.00 M HNO2 (Ka= 4.0×10-4). Also calculate the concentration of cyanide ion (CN) in this solution at equilibrium.

co

chang

e

ceq

Mx

x

Kcx ao

2

4

105.4

100.45

35.1)105.4log(105.4][ 22 pHMxH

co

chang

e

ceq

][

][][

HCN

CNHKa

MCN

CN 8

2

10 104.1][00.1

][105.4102.6

FHHF

Calculate the pH of 1.0 M HF solution.

co1 0 0

chang

e

-x x x

ceq1-x x x

amount dissociated of

HF = x%100%

oc

x

co1.0 M 0.1

x 2.7x10-

2

0.0081

a 2.7% 8.1%

AcHHAc

Calculate the percent dissociation of 1.0 M and 0.10 M

HAc solution.

coco 0 0

chang

e

-x x x

ceqco-x x x

amount dissociated of

HAc = x

%100% oc

x

co1.0 M 0.1

x 4.2x10-

3

1.3x10-

3

a 0.42% 1.3%

Ka(Hac)=1.8x10-5

ao

oo

a Kcxc

x

xc

xK

22

In a 0.100 M aqueous solution, lactic acid is 3.7% dissociated. Calculate the value of Ka for this acid.

AHHAco

1 0 0

chang

e

-x x x

ceq1-x x x

%100% oc

x

xco %100

%

0037.0%100

%7.31.0 x

4

22

104.10037.01.0

0037.0

xc

xK

o

a

OHNaNaOH aq )(

OHNaOHsolution OHOHOH2][][][

0.05 M 0.05 M< 10-7 M

NaOHsolution OHOH ][][

basestrongNaOHsolution OHOHOHpOH ]log[]log[]log[

pOH =-log (0.05)= -(-1.3) = 1.3

Calculate the pH of 5.0x10-2 M NaOH solution.

pH =14-pOH=14-1.3 = 12.7

NH3(aq) + H2O NH4+

(aq) + OH-(aq)

Weak Bases•Partial Ionization•Small Kb

Bases

Calculate the pH of 15.0 M NH3 solution (Kb=1.8x10-5).

NH3(aq) + H2O NH4+

(aq) + OH-(aq)

co15 0 0

change -x x x

ceq15-x x x

2522

106.1108.11515

xx

x

xK b

8.1)102.1log(log]log[ 2 xOHpOH solution

pH =14-pOH=14-1.8 = 12.2

Calculate the pH of 1.0 M methylamine solution (Kb=4.38x10-4)

Check the 5% rule!

triprotic acid

Calculate the pH of a 5.0 M H3PO4 solution and the equilibrium concentrations of the species H3PO4, H2PO4

-, HPO42- and PO4

3-.

4243 POHHPOH

co5 0 0

chang

e

ceq13

22

1 109.1105.755

xx

x

xK a

72.0)109.1log(log]log[ 1 xHpH

][109.1][ 1

42

HMxPOH

2

442 HPOHPOHco 0.19 0.19 0

change -y +y +y

ceq 0.19-y 0.19+y y

yy

yyK a

)19.0(

)19.0(2

2

2

4

8 ][102.6 aKHPOy

3

4

2

4 POHHPOco 6.2x10-8 0.19 0

change -z +z +z

ceq 6.2x10-8-z 0.19+z z

][106.119.0

102.6108.4

108.4)102.6(

)19.0(

3

4

198

13

13

83

POMz

z

zzK a

442 HSOHSOH1 M 1 M

2

44 SOHHSOco 1 1 0

change -x +x +x

ceq 1-x 1+x x

1 M

2

2 102.1)1(

)1(

x

xxK a

2102.1)1(

)1( x

0)012.1log()1log(]log[ xHpH

442 HSOHSOH0.01 M 0.01 M

2

44 SOHHSOco 0.01 0.01 0

change -x +x +x

ceq 0.01-x 0.01+x x

0.01 M

2

2 102.1)01.0(

)01.0(

x

xxK a

2102.1)01.0(

)01.0( x

84.1)0145.0log()0045.001.0log(]log[ HpH

Calculate the pH of a 0.01 M H2SO4.

Approximation?

%120%10001.0

012.0%100

oc

x

Solve quadratic equation: Mx 3105.4

saltscation anion

acidic

1+ and 2+cations

)(2

)(2

)()(

,

,,

aqaq

aqaq

CaMg

KNa

conjugate acid of weak bases

NH4+

3324 NHOHOHNH

No effect on pH

3+ cationsacidic

)(3

)(3 , aqaq FeAl

OHOHOHAlOHOHAl 3

2

522

3

62 )()()(

As Size of Cation, AciditySmaller, more concentrated charge: Means pull of e

density from O—H bond: acidity of H+

saltscation anion

basic

conjugate base of strong acid

Cl-, Br-, I- , NO3-

OHHAcOHAc 2

No effect on pH

conjugate base of weak acidAc- , PO4

3-

][

][][

Ac

OHHAcKb

OHHAcOHAc 2

][

][][

Ac

OHHAcKb

OHAcOHHAc 32

][

][][ 3

HAc

AcOHKa

][

][][

][

][][ 3

Ac

OHHAc

HAc

AcOHKK ba

][][ 3

OHOHKK ba wK

Acid dissociation

constantx

constant for conjugate

base

Water ionization constant

=

OHHFOHF 2

co 0.3 0 0

change -x +x +x

ceq 0.3-x +x +xx

x

F

OHHFKb

3.0][

][][ 2

11

4

14

104.1102.7

100.1

a

wb K

KK

611 100.2104.130.0 x

69.5)100.2log(log]log[ 6 xOHpOH solution

pH =14-pOH=14-5.69 = 8.31

neutralanionKcationKif

basicanionKcationKif

acidicanionKcationKif

ba

ba

ba

)()(

)()(

)()(

10106.5 aK10106.5 bK

neutral

10106.5 aK5106.1 bK

basic

5104.1 aK13

422

103.8)(

SOHK

KK

a

wb

acidic

Binary Acids = HnX

X = Cl, Br, P, As, S, Se, etc.

1. Acid strength from left to right

within same period (across row)– Acid strength as electronegativity of X

Ex. HCl is stronger acid than H2S which is

stronger acid than PH3

– or PH3 < H2S < HCl

Trends in Binary Acid Strength

Binary Acids = HnX

X = Cl, Br, P, As, S, Se, etc.

•X: group VIIA, from bottom to top :• I, Br, Cl, F

•Electronegativity inccreases•Bond polarity higher

•Bond stronger (also because size smaller)

•More difficult to break the bond •Dissociation less, acid weaker

Trends in Oxoacid Strength

Oxoacids (HnYOm)

– Acids of H, O, and one other element– HClO, HIO4, H2SO3, H2SO4, etc.

1. Acids with same number of O’s and differing Ya. Acid strength from bottom to top within

group• HIO4 < HBrO4 < HClO4

b. Acid strength from left to right within period• H3PO4 < H2SO4 < HClO4

• as electronegativity of central atom 55

why

OClHHOCl

not ClHOHOCl

or ClHOHOCl

Positive ClPositive O

NaHOHONa Electronegativity of atom attached to O

Trends in Oxoacid Strength

Oxoacids (HnYOm)2. For same Y

– Acid strength as number of O’s • H2SO3 < H2SO4

• More oxygens, remove more electron density from central atom, weakening O—H bond and make H more acidic

57

Your Turn!Which of the following is the strongest organic acid?

OHC

O

CH

H

H

OHC

O

CF

H

H

OHC

O

CBr

H

H

OHC

O

CI

H

H

OHC

O

CCl

H

H

A B C

D E

oxidesOxides of nonmetals Oxides of metals

Strong covalent bond NM-O

acidicAcid anhydride

basicbase anhydride

Ionic bond M-O