Acid Base Equilibria

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Chapter 16
Acid Base Equilibria
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Overview

• Definitions Arrhenius Bronsted -- Conjugate
  Pairs Hydronium Ion
• Relative Strengths Strong/Weak acids and
  reactions Strong/Weak bases and reactions
  Kas and Kbs

3

• pH and H2O Ionization
• Calculating Equilibrium Concentrations
• Hydrolysis Acidic and Basic Salts
• Polyprotic Acids
• Molecular Structure/Bonding
• Lewis Acids and Bases

4

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

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

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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
anotherCO32-(aq) H2O(l) HCO3-(aq)
OH -(aq)
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Water Dissociation
2H2O H3O OH -
Hydronium Ion -- because bare protons are unlikely
H transfer
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pH and Water Ionization
2H2O(l) H3O(aq) OH
-(aq)
K H3OOH - K H2O2 H3OOH -
H2O 2
Kw H3OOH - 1.0 x 10 -14
(at 25C)
Reactant Favored
ion-product constant for water
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pH - log H - log H3O
neutral solution H3O OH -
2H2O H3O OH -
1.0 x 10 -14 H3OOH - H3O2
H3O 1.0 x 10 -7 M
pH - log (1.0 x 10 -7) 7.0
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non-neutral solutions
acidic solutions
H gt OH-
pH lt 7.0
basic solutions
H lt OH-
pH gt 7.0
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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. . . .
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What about H3O from water?
2 H2O H3O OH -
initial 0.00100 0
change x x
equil. 0.00100 x x
1 x 10 -14 (0.00100 x)(x) x H3O
and OH- from disso. of water
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Kw H3O OH -
x is very small compared to 0.00100 can be
neglected
0
1 x 10 -14 (0.00100 x)(x)
x 1.00 x 10 -11 M OH -
pOH 11.0
H H3O
H3O (1.0 x 10-11) 1.0 x 10 -14
the conc. of H in a solution of a strong acid is
the conc. of the strong acid
H3O 1.0 x 10 -3 M
pH 3.0
Note pH pOH 14.0
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Bottom Line

• H H3O H5O2 H9O4 hydrated
  hydrogen ions
• neutral solution H3O OH- pH 7
• acidic solution H3O gt OH- pH lt 7
• basic solution H3O lt OH- pH gt 7
• the concentration of H3O in a strong acid
  is the concentration of the acid

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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
strong base, complete rxn, stoichiometric
1.0 x 10 -14 H (0.010 M)
H 1.0 x 10 -12 M
contribution of H from dissociation of H2O is
negligible
pH 12.0
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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
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pH H3O OH- pOH
14 10 -14 1
0
Basic
ammonia
Neutral
human blood
7.0 10 -7 10 -7
7.0
milk
Acidic
vinegar, cola
0 1 10 -14
14
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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

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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
anotherCO32-(aq) H2O(l) HCO3-(aq)
OH -(aq)
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Examples
H
NH4(aq) H2O(l) H3O(aq) NH3(aq)
conjugate acid
conjugate base
acid
base
conjugate pair
H
PO43-(aq) H2O(l) HPO42-(aq) OH -(aq)

conjugate acid
conjugate base
base
acid
conjugate pair
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Some species can act as an acid or base
H
HCO3-(aq) H2O(l) H3O(aq) CO32-(aq)
conjugate acid
conjugate base
acid
base
H
HCO3-(aq) H2O(l) H2CO3(aq) OH -(aq)
conjugate acid
conjugate base
base
acid
HCO3- is an amphiprotic substance
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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

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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- HPO42-H2SO4
HNO3
H2PO4-
H3SO4
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Relative Strengths
HCl H2O H3O Cl -
stronger B
weaker B
stronger A
weaker A
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
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HCN H2O H3O CN -
weaker B
weaker A
stronger A
stronger B
H2O lt CN -
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Which is the weaker acid
H2S CN - HCN HS - HCO3-
SO42- HSO4- CO32- HClO4
H2O H3O ClO4- NH4 H2O
H3O NH3
HCN
HCO3-
H3O
NH4
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The stronger the acid the weaker its conjugate
base
weakest conjugate base
strongest acid
strongest conjugate base
weakest acid
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Given the following, which is the weaker
conjugate
HCl gt CH3CO2H HCN lt H3PO4 H2SO4 gt H2SO3
NH3 gt H2O HSO4- lt CO32- H - gt
NH3
Cl -
CH3CO2-
lt
acids
CN -
H2PO4 -
gt
HSO4 -
HSO3-
lt
NH4
H3O
lt
bases
H2SO4
HCO3 -
gt
H2
NH4
lt
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Predicting Direction of Acid/Base Rxns.
weaker B
stronger B
CH3CO2H CN - HCN CH3CO2 -

stronger A
weaker A
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stronger A
weaker A
HSO4- NH3 NH4 SO42-
stronger B
weaker B
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Strong 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.
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Weak Acids and Bases
HX H2O H3O X -
Ka H3OX - HX
lt 1
B H2O BH OH -
Kb BHOH - B
lt 1
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Weak Acids Can Be

• cations NH4 or Cu(H2O)62
• anions H2PO4- or HCO3-
• neutral CH3CO2H or HCO2H

Weak Bases Can Be

• anions CO32- or CN -
• neutral NH3 or (CH3)3N

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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

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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. gt 100 Ka/b
neglect x when it is added to or subtracted from
the initial conc.
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Answer 1
If this were a strong base then OH- 0.015
M and pOH 1.8 and pH 12.2.
B H2O HB OH-
pH 10.09 and pOH 3. 91 ? OH- 1.2 x
10-4
Kb HBOH- (1.2 x 10 -4)2
B (0.015)
9.6 x 10 -7
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Answer 2
CH3CO2H H2O H3O CH3CO2-
initial 0.20 0 0change
-x x x equil. 0.20 -x x x
Ka 1.8 x 10 -5 H3OCH3CO2-
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
pH 2.7
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What is the percent ionization?
ion H3O x 100
CH3CO2H 0.95
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Polyprotic Acids and Bases
H3PO4 H2O H3O H2PO4-
Ka1 H2PO4- H2O H3O HPO42-
Ka2 HPO42- H2O H3O PO43-
Ka3
H3PO4 3H2O 3H3O PO43--
Ka tot Ka1 Ka2 Ka3
(7.5 x 10 -3)(6.2 x 10 -8)(3.6 x 10 -13) 1.7 x
10 -22
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Weak Bases

• Can be anions such as
• CN-, HSO3-, SO32-, HCO3-, CO32-, 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

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Example
NH3 H2O NH4 OH -
initial 0.20 M 0 0change
-x x x equil. 0.20 -x x x
Kb 1.8 x 10 -5 NH4OH - NH3
Kb 1.8 x 10 -5 (x)(x) x2 3.6 x
10 -6 (0.20 -x) x 1.9 x 10 -3

NH4OH- 1.9 x 10 -3 M NH3 0.20
M
pOH 2.7 pH 12.3
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CO3 -2 H2O OH - HCO3--
Kb1 HCO3 -- H2O OH -
H2CO3 Kb2
CO3 2H2O 2OH --
H2CO32--
Kb tot Kb1 Kb2 (2.1 x 10 -4)(2.4 x 10
-8) 5.0 x 10 -12
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Relationship of an acid Ka and the Kb of its
conjugate base
conjugate pair rxns with water
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
Kb Ka Kw or pKb pKa
pKw
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Find Ka for the conjugate acids of the following
bases Ka Kw/Kb
Base Kb Conj. Acid Ka
NH3 1.8 x 10 -5 C5H5N 1.7 x 10 -9 HS -
1.8 x 10 -7 CO32- 1.8 x 10 -4
NH4
5.6 x 10 -10
C5H5NH
5.9 x 10 -6
5.6 x 10 -8
H2S
5.6 x 10 -11
HCO3 -
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Acid/Base Hydrolysis
Many salts produce solutions that are acidic or
basic
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 -
weak acid
or
B H2O BH OH -
weak base
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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
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Which cations will hydrolyze?
Conjugate acids of weak bases
NH4 Al3 Cu2 etc.
Conjugate bases of weak acids
CO32 - CH3CO2- NH3 F - etc.
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What is the pH of the following salt solutions?
N B A B A N B ?

• NaCl
• NaF
• NH4Cl
• Na2CO3
• Cr(NO3)3
• KNO3
• NaC3H2O4
• NH4CH3CO2

Na -- neutral Cl - -- neutral
Na -- neutral F - -- basic
NH4 -- acidic Cl - -- neutral
Na -- neutral CO32- -- basic
Cr3 -- acidic NO3- -- neutral
K -- neutral NO3 -- neutral
Na -- neutral C3H2O4- -- basic
NH4 -- acidic CH3CO2- -- basic
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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.
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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 Kas Kbs of conjugate
  pairs

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

• Bond Strength
• Polarity or D electronegativity
• Central Atom Charge or Oxidation

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Examples
IVA VA VIA VIIA row 2 CH4 NH3 H2O
HF row 3 SiH4 PH3 H2S HCl
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Acid
Cl ox.
Ka
HClO4 HClO3 HClO2 HClO
7
strong
5
strong
3
1.1 x 10 -2
Increasing acid strength
1
3.0 x 10 -8
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Acid
Ka
HClO HBrO HIO
3 x 10 -8
2.5 x 10 -9
Increasing acid strength
2.3 x 10 -11
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Organic Acids
H
O-
C
C
H
O
Acetate ion resonance
H
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Trifluoroacetic Acid Ka 5.0 x 10 -1
Acetic Acid Ka 1.8 x 10 -5
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Lewis Acids and Bases
Lewis Acid -- accepts a pair of electrons Lewis
Base -- donates a pair of electrons
formation of a coordinate covalent bond
H
F
B
N
H
F
F
H
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NH3 H
H
N
H
H
H
Lewis Acid -- accepts a pair of electrons
NH4
Lewis Base -- donates a pair of electrons