Chapter 16

Overview:•Definitions Arrhenius Bronsted -- Conjugate Pairs Hydronium Ion

•Relative Strengths Strong/Weak acids and reactions Strong/Weak bases and reactions Ka’s and Kb’s

•pH and H2O Ionization

•Calculating Equilibrium Concentrations

•Hydrolysis Acidic and Basic Salts

•Polyprotic Acids

•Molecular Structure/Bonding

•Lewis Acids and Bases

Acids•sour•corrosive•reddens blue vegetable colors•react with bases

Bases•bitter•soapy•restores vegetable colors reddened by acids•react with acids

Arrhenius Acid:substance that releases or produces H+

HCl(aq) H+(aq) + Cl -

(aq) Arrhenius Base:substance that releases or produces OH - NaOH(aq) Na+

(aq) + OH -(aq) Bronsted Acid:

substance that donates a H+ to anotherHNO3(aq) + H2O(l) H3O+

(aq) + NO3-(aq)

Bronsted Base:substance that accepts a H+ from anotherCO3

2-(aq) + H2O(l) HCO3

-(aq) + OH -

(aq)

Water Dissociation:Water Dissociation:

2H2O H3O+ + OH -

Hydronium Ion -- because bare protons are unlikely

H+ transfer

HO

H

HO

H

HO

H

HO

H

++

-

- +

pH and Water Ionization:pH and Water Ionization:

2H2O(l) H3O+(aq) +

OH -(aq)

Kw = [H3O+][OH -] = 1.0 x 10 -14

Reactant Favored

ion-product constant for water

(at 25C)

K = [H3O+][OH -] = K [H2O]2 = [H3O+][OH -] [H2O] 2

pH = - log [H+] = - log [H3O+]pH = - log [H+] = - log [H3O+]

neutral solution: [H3O+] = [OH -]

2H2O H3O+ + OH - 1.0 x 10 -14 = [H3O+][OH -] =

[H3O+]2

[H3O+] = 1.0 x 10 -

7 M

pH = - log (1.0 x 10 -7) = 7.0 pH = - log (1.0 x 10 -7) = 7.0

non-neutral solutions:

[H+] > [OH-]

acidic solutions:

pH < 7.0pH < 7.0

basic solutions:[H+] < [OH-]pH > 7.0pH > 7.0

pH of Strong Acid and Base Solutions: Calculate the pH of a solution containing 0.00100 M of a strong acid such as HCl.

HCl(aq) H+(aq) + OH-

(aq) 0.00100 0.00100 0.00100

pH = - log (0.00100) = 3.00

but. . . .

What about H3O+ from water?

2 H2O H3O+ + OH -

initial 0.00100 0

change +x +xequil. 0.00100 + x x

1 x 10 -14 = (0.00100 + x)(x)

x = H3O+ and OH- from disso. of water

[H3O+] (1.0 x 10-11) = 1.0 x 10 -14 [H3O+] = 1.0 x 10 -3 M

pH = 3.0

1 x 10 -14 = (0.00100 + x)(x)x = 1.00 x 10 -11 M =

[OH -]

Kw = [H3O+] [OH -] 0

x is very small compared to 0.00100 & can be neglected

[H+] [H3O+]

the conc. of H+ in a solution of a strong acid is the conc. of the strong acid

pOH = 11.0

Note: pH + pOH = 14.0

Bottom Line:Bottom Line:

[H+] = [H3O+] = [H5O2+] = [H9O4

+] hydrated

hydrogen ions neutral solution [H3O+] = [OH-] pH = 7

acidic solution [H3O+] > [OH-] pH < 7

basic solution [H3O+] < [OH-] pH > 7

the concentration of [H3O+] in a strong acid

is the concentration of the acid

Calculate the pH of a solution containing 0.010 M KOH.

KOH K+(aq) +

OH-(aq) 0.010 M 0.010 M

0.010 M

1.0 x 10 -14 = [H+] (0.010 M)[H+] = 1.0 x 10 -12 M

pH = 12.0

strong base, complete rxn, stoichiometric

contribution of [H+] from dissociation of H2O is negligible

Calculate the [H+] and [OH -] in a solution that has a pH = 8.60

- log [H+] = pH

- log [H+] = 8.60

log [H+] = -8.60

[H+] = anti log (-8.60)

[H+] = 1.8 x 10 -4 M

[OH -] = 5.6 x 10 -11 M

Basic

Neutral

Acidic

pH [H3O+] [OH-] pOH

7.0 10 -7 10 -7 7.0

14 10 -14 1 0

0 1 10 -14 14

vinegar, cola

milk

ammonia

human blood

Measuring pH:

pH meter -- electrodes measure pH– most precise method

acid-base indicators– less precise but good when a pH meter is

not available– substances which are differently colored at

different pH values– litmus, phenolphthalein, thymol blue

Arrhenius Acid:substance that releases or produces H+

HCl(aq) H+(aq) + Cl -

(aq) Arrhenius Base:substance that releases or produces OH - NaOH(aq) Na+

(aq) + OH -(aq) Bronsted Acid:

substance that donates a H+ to anotherHNO3(aq) + H2O(l) H3O+

(aq) + NO3-(aq)

Bronsted Base:substance that accepts a H+ from anotherCO3

2-(aq) + H2O(l) HCO3

-(aq) + OH -

(aq)

Examples:

NH4+

(aq) + H2O(l) H3O+(aq) +

NH3(aq) acid

base

conjugate acid

conjugate base

PO43-

(aq) + H2O(l) HPO42-

(aq) + OH -

(aq)

H+

base

acid

conjugate acid

conjugate base

H+

conjugate pair

conjugate pair

HCO3-(aq) + H2O(l) H3O+

(aq) + CO3

2-(aq)

HCO3-(aq) + H2O(l) H2CO3(aq) + OH

-(aq)

Some species can act as an acid or base:

acid

base

conjugate acid

conjugate base

base

acid

conjugate acid

conjugate base

HCO3- is an amphiprotic

substance

H+

H+

You Must Know:You Must Know:

what an acid and a base is and how to identify both know definitions and properties

the reaction of an acid and a base with water

how to identify acid, base, conjugate acid and conjugate base

what the hydronium ion is and the ionization reaction of water

Problems: HX + H2O H3O+ + X - A B CA

CB

•What is the conjugate base of:

H2SNH4

+

NH3 H2OOH -

HS -

NH3 NH2

- OH -

O2-

•What is the conjugate acid of:NO3

- HPO4

2-

H2SO4

HNO3 H2PO4

- H3SO4

+

Relative Strengths:Relative Strengths:

HCl + H2O H3O+ + Cl -

equilibrium is a competition between the bases H2O and Cl

- -- the equilibrium will lie toward the direction of the weaker acid and base

in this case, H2O is a stronger base than Cl - as it competes much more effectively for the H+

stronger B

weaker Bstronger A

weaker A

HCN + H2O H3O+ + CN -

H2O < CN -

weaker Bweaker A stronger A

stronger B

Which is the weaker acid:Which is the weaker acid:

H2S + CN - HCN + HS -

HCO3- + SO4

2- HSO4- +

CO32-

HClO4 + H2O H3O+ + ClO4

-

NH4+ + H2O H3O+ +

NH3

HCN

HCO3-

H3O+

NH4+

The stronger the acid the weaker its conjugate base:

strongest acid

weakest acid strongest conjugate base

weakest conjugate base

Given the following, which is the weaker conjugate: Given the following, which is the weaker conjugate:

HCl > CH3CO2H

HCN < H3PO4

H2SO4 > H2SO3

NH3 > H2O

HSO4- < CO3

2-

H - > NH3

acids

bases

Cl - CH3CO2-<

CN - H2PO4 -

>

HSO4 -

HSO3-

<

NH4+ H3O+<

H2SO4 HCO3 -

>

H

2 NH4

+ <

Predicting Direction of Acid/Base Rxns.Predicting Direction of Acid/Base Rxns.

CH3CO2H + CN - HCN + CH3CO2 - stronger

Aweaker A

weaker B

stronger B

HSO4- + NH3 NH4

+ + SO4

2-

stronger A

weaker A

stronger B

weaker B

Strong Acids and BasesStrong Acids and Bases•Strong Acids HCl HNO3

HClO4

HClO3 H2SO4 HBr HI

•Strong Bases Grp I hydroxides Grp II hydroxides (except Be)

HX + H2O H3O+ + X -

MOH M+ + OH -

essentially complete rxns.

Weak Acids and Bases:Weak Acids and Bases:

HX + H2O H3O+ + X -

B + H2O BH+ + OH -

Ka = [H3O+][X -] [HX]

Kb = [BH+][OH -] [B]

< 1< 1

< 1< 1

Weak Acids Can Be:Weak Acids Can Be:

•cations NH4+ or [Cu(H2O)6]2+

•anions H2PO4- or HCO3

-

•neutral CH3CO2H or HCO2H

Weak Bases Can Be:Weak Bases Can Be:

•anions CO32- or CN -

•neutral NH3 or (CH3)3N

You Must:You Must:Be able to determine direction of rxn based on acid or base relative strengths

Know the strong acids and strong bases

Be able to recognize weak acids and bases

Know water ionization rxn, constant and expression

Know what pH is and how to calculate it

Know how to calculate equil. conc., pH, pOH for weak acids and bases

Problem 1: A 0.015 M solution of an unknown base has a pH of 10.09. Is it a strong or weak base? What is the Kb, if it is weak?

Problem 2: What are the equil. conc. of H3O+, acetate ion, acetic acid in a 0.20 M aqueous solution of acetic acid, CH3CO2H?

Use the approximation whenever possible: When the initial weak acid/base conc. > 100 a/b neglect x when it is added to or subtracted from the initial conc.

If this were a strong base then [OH-] = 0.015 M and pOH = 1.8 and pH = 12.2.

pH = 10.09 and pOH = 3. 91 [OH-] = 1.2 x 10-4

Kb = [HB+][OH-] = (1.2 x 10 -4)2

[B] (0.015)

B + H2O HB+ + OH-

= 9.6 x 10 -7

Answer 1:Answer 1:

CH3CO2H + H2O H3O+ + CH3CO2-

initial 0.20 0 0change -x +x +xequil. 0.20 -x x x

Ka = 1.8 x 10 -5 = [H3O+][CH3CO2-]

[CH3CO2H]

Ka = 1.8 x 10 -5 = (x)(x) x2 = 3.6 x 10 -6 (0.20 -x) x = 1.9 x 10 -3

[CH3CO2-]=[H3O+] = 1.9 x 10 -

3 M [CH3CO2H] = 0.20 M

[CH3CO2-]=[H3O+] = 1.9 x 10 -

3 M [CH3CO2H] = 0.20 M

pH = 2.7pH = 2.7

Answer 2:Answer 2:

What is the percent ionization?

% ion = [H3O+] x 100 [CH3CO2H]

= 0.95 %

Polyprotic Acids and Bases:Polyprotic Acids and Bases:

H3PO4 + H2O H3O+ + H2PO4-

Ka1

H2PO4- + H2O H3O+ + HPO4

2- Ka2

HPO42- + H2O H3O+ + PO4

3- Ka3

H3PO4 + 3H2O 3H3O+ + PO4

3--

(7.5 x 10 -3)(6.2 x 10 -8)(3.6 x 10 -13) = 1.7 x 10 -22

Ka tot = Ka1 Ka2 Ka3Ka tot = Ka1 Ka2 Ka3

Weak Bases:Weak Bases: Can be anions such as:

– CN-, HSO3-, SO3

2-, HCO3-, CO3

2-, etc.

Can be N-containing compounds, such as:– NH3, (CH3)NH2, (CH3)2NH, (CH3)3N, etc.

React with water

– B- + H2O HB + OH- or

– B + H2O HB+ + OH-

Have base dissociation constants, Kb

NH3 + H2O NH4+ + OH -

initial 0.20 M 0 0change -x +x +xequil. 0.20 -x x x

Kb = 1.8 x 10 -5 = [NH4+][OH -]

[NH3]Kb = 1.8 x 10 -5 = (x)(x) x2 = 3.6 x 10 -6 (0.20 -x) x = 1.9 x 10 -3

[NH4+]=[OH-] = 1.9 x 10 -3 M

[NH3] = 0.20 M[NH4

+]=[OH-] = 1.9 x 10 -3 M [NH3] = 0.20 M

pOH = 2.7pH = 12.3

pOH = 2.7pH = 12.3

Example:Example:

CO3 -2 + H2O OH - + HCO3--

Kb1

HCO3 -- + H2O OH - + H2CO3 Kb2

CO3 + 2H2O 2OH -- + H2CO3

2--

Kb tot = Kb1 Kb2

(2.1 x 10 -4)(2.4 x 10 -8) = 5.0 x 10 -12

Relationship of an acid Ka and

the Kb of its conjugate base:

Relationship of an acid Ka and

the Kb of its conjugate base:

NH3 + H2O NH4+ + OH - Kb

= 1.8 x 10 -5 NH4

+ + H2O NH3 + H3O+ Ka = 5.6 x 10 -10 2H2O H3O+ + OH - Kw = 1 x 10 -14

For Conjugate Acid/Base Pairs:For Conjugate Acid/Base Pairs:Kb Ka = Kw or pKb pKa = pKw

conjugate pair rxns with water

Find Ka for the conjugate acids of the following bases: Ka = Kw/Kb

Find Ka for the conjugate acids of the following bases: Ka = Kw/Kb

NH3 1.8 x 10 -5

C5H5N 1.7 x 10 -9

HS - 1.8 x 10 -7

CO32- 1.8 x 10 -4

Base Kb Conj. Acid Ka

NH4+ 5.6 x 10 -10

C5H5NH + 5.9 x 10 -6

H2S 5.6 x 10 -8

5.6 x 10 -11 HCO3 -

Acid/Base HydrolysisAcid/Base HydrolysisMany salts produce solutions that are acidic or basic

Why?Why?

because either (or both) the cation or anion of the salt acted as a weak acid or a weak base

HX + H2O H3O+ + X - or

B + H2O BH+ + OH -

weak acid

weak base

Which cations will not hydrolyze?Cations of strong bases -- Grp I and II (except Be) Li+ Na+ K+ Rb+ Cs+ Mg2+ Ca2+

Sr2+ Ba2+

Anions of strong acids --

ClO4- ClO3

- SO42- NO3

- Cl - Br - I -

All other cations and anions will hydrolyze

Which cations will hydrolyze?

Conjugate acids of weak bases:

NH4+ Al3+ Cu2+ etc.

Conjugate bases of weak acids:

CO32 - CH3CO2

- NH3 F - etc.

What is the pH of the following salt solutions?What is the pH of the following salt solutions?

•NaCl

•NaF

•NH4Cl

•Na2CO3

•Cr(NO3)3

•KNO3

•NaC3H2O4

•NH4CH3CO2

Na+ -- neutral Cl - -- neutralNa+ -- neutral F - -- basicNH4

+ -- acidic Cl - -- neutralNa+ -- neutral CO3

2- -- basicCr3+ -- acidic NO3

- -- neutral

K+ -- neutral NO3+ -- neutral

Na+ -- neutral C3H2O4- --

basicNH4

+ -- acidic CH3CO2- --

basic

N

B

A

B

A

N

B

?

Is NH4CH3CO2 acidic or basic?

NH4+ + H2O H3O+ +

NH3

Ka = 5.6 x 10 -10

CH3CO2- + H2O CH3CO2H + OH

-

Kb = 5.6 x 10 -

10 weak acid & weak base strength equal -- neutral soln.

You Must:You Must:Know what hydrolysis is

Know which cations hydrolyze or produce acidic solutions

Know which anions hydrolyze or produce basic solutions

Be able to estimate and calculate the pH of a salt solution

Know how to determine Ka’s & Kb’s of conjugate pairs

Strength of Acids:Can be increased by anything that facilitates the loss of a Proton (H+)

•Bond Strength

•Polarity or electronegativity

•Central Atom Charge or Oxidation #

Examples:Examples:

IVA VA VIAVIIA

row 2 CH4 NH3 H2O HF

row 3 SiH4 PH3 H2S HCl

Increasing acid strengthIncreasing acid strength

Incre

asin

g a

cid

stre

ng

thIn

cre

asin

g a

cid

stre

ng

th

HClO4

HClO3

HClO2

HClO

Incre

asin

g a

cid

str

en

gth

Acid

Cl ox. #

+7

+5

+3

+1

Ka

strong

strong

1.1 x 10 -2 3.0 x 10 -8

HClO

HBrO

HIO In

cre

asin

g a

cid

str

en

gth

Acid

Ka

3 x 10 -8

2.5 x 10 -9 2.3 x 10 -11

Organic Acids:Organic Acids:

C C O-

O

H

H

H

C C O

O-

H

H

H

Acetate ion resonance

C C OH

O

H

H

H

Trifluoroacetic Acid Ka = 5.0 x 10 -1

C C OH

O

F

F

F

Acetic AcidKa = 1.8 x 10 -5

Lewis Acids and Bases:Lewis Acids and Bases:

Lewis Acid -- accepts a pair of electrons

Lewis Base -- donates a pair of electrons

formation of a coordinate covalent bond

NH

HH

B

FF

F

NH

HH

H+

NH4+

NH3 + H+

Lewis Base -- donates a pair of electrons

Lewis Acid -- accepts a pair of electrons