Aqueous Equilibria: Acids and Bases

1
Chapter 14

• Aqueous Equilibria Acids and Bases

2
AcidBase Concepts 01
Arrhenius Acid A substance which dissociates to
form hydrogen ions (H) in solution.
HA(aq) ? H(aq) A(aq) Arrhenius Base A
substance that dissociates in, or reacts with
water to form hydroxide ions (OH).
MOH(aq) ? M(aq) OH(aq)
3
AcidBase Concepts 02

• BrønstedLowry Acid Substance that can donate H
• BrønstedLowry Base Substance that can accept H
• Chemical species whose formulas differ only by
  one proton are said to be conjugate acidbase
  pairs.

4
Strong vs. Weak acids 03
5
Hydrated Protons and Hydronium Ions
Due to high reactivity of the hydrogen ion, it is
actually hydrated by one or more water molecules.
H(H2O)n1
For our purposes, H1 is equivalent to H3O1.
6
AcidBase Concepts
7
Lewis AcidBase Concepts
8
AcidBase Concepts 05

• A Lewis Acid is an electron-pair acceptor. These
  are generally cations and neutral molecules with
  vacant valence orbitals, such as Al3, Cu2, H,
  BF3.
• A Lewis Base is an electron-pair donor. These are
  generally anions and neutral molecules with
  available pairs of electrons, such as H2O, NH3,
  O2.
• The bond formed is called a coordinate bond.

9
AcidBase Concepts 06
-

10
Lewis Acids and Bases
Lewis Acid An electron-pair acceptor.
Lewis Base An electron-pair donor.
11
Lewis Acids and Bases
Lewis Acid An electron-pair acceptor.
Lewis Base An electron-pair donor.
12
AcidBase Concepts 07

• Write balanced equations for the dissociation of
  each of the following BrønstedLowry acids.
• (a) H2SO4 (b) HSO4 (c) H3O
• Identify the Lewis acid and Lewis base in each of
  the following reactions
• (a) SnCl4(s) 2 Cl(aq) æ SnCl62(aq)
• (b) Hg2(aq) 4 CN(aq) æ Hg(CN)42(aq)
• (c) Co3(aq) 6 NH3(aq) æ Co(NH3)63(aq)

13
Dissociation of Water 01

• Water can act as an acid or as a base. H2O(l) æ
  H(aq) OH(aq)
• Kc HOH
• This is called the autoionization of water.
  H2O(l) H2O(l) æ H3O(aq) OH(aq)

14
Dissociation of Water 02

• This equilibrium gives us the ion product
  constant for water.
• Kw Kc HOH 1.0 x 1014
• If we know either H or OH then we can
  determine the other quantity.

15
Dissociation of Water 03

• The concentration of OH ions in a certain
  household ammonia cleaning solution is 0.0025 M.
  Calculate the concentration of H ions.
• Calculate the concentration of OH ions in a HCl
  solution whose hydrogen ion concentration is 1.3
  M.

16
pH A Measure of Acidity 01

• The pH of a solution is the negative logarithm of
  the hydrogen ion concentration (in mol/L).
• pH log H, H 10-pH
• pH pOH 14
• Acidic solutions H gt 1.0 x 107 M, pH lt
  7.00Basic solutions H lt 1.0 x 107 M, pH
  gt 7.00Neutral solutions H 1.0 x 107 M,
  pH 7.00

17
pH A Measure of Acidity 02

• Nitric acid (HNO3) is used in the production of
  fertilizer, dyes, drugs, and explosives.
  Calculate the pH of a HNO3 solution having a
  hydrogen ion concentration of 0.76 M.
• The pH of a certain orange juice is 3.33.
  Calculate the H ion concentration.
• The OH ion concentration of a blood sample is
  2.5 x 107 M. What is the pH of the blood?

18
pH A Measure of Acidity 04
Color of Tea Polyphenols, Thearubigins
Color of Red Cabbage Anthocyanin 
19
pH A Measure of Acidity 04
20
Strength of Acids and Bases 03
ACID CONJ. BASE
ACID CONJ. BASE

• HClO4
• HI
• HBr
• HCl
• H2SO4
• HNO3
• H3O
• HSO4

HSO4 HF HNO2 HCOOH NH4 HCN H2O NH3
ClO4 I Br Cl HSO4 NO3 H2O SO42
SO42 F NO2 HCOO NH3 CN OH NH2
Increasing Acid Strength
Increasing Acid Strength
21
Strength of Acids and Bases 04

• Stronger acid stronger base ?
• weaker acid weaker base
• Predict the direction of the following
• HNO2(aq) CN(aq) æ HCN(aq) NO2(aq)
• HF(aq) NH3(aq) æ F(aq) NH4(aq)

22
Acid Ionization Constants 01

• Acid Ionization Constant the equilibrium
  constant for the ionization of an acid. HA(aq)
  H2O(l) æ H3O(aq) A(aq)
• Or simply HA(aq) æ H(aq) A(aq)

23
Conjugate Base Ionization Const
Ka ?
Kw

• Ka ? Kb Kw

24
Acid Ionization Constants 02
ACID Ka
CONJ. BASE Kb
7.1 x 10 4 4.5 x 10 4 3.0 x 10 4 1.7 x 10
4 8.0 x 10 5 6.5 x 10 5 1.8 x 10 5 4.9 x 10
10 1.3 x 10 10
HF HNO2 C9H8O4 (aspirin) HCO2H (formic) C6H8O6
(ascorbic) C6H5CO2H (benzoic) CH3CO2H
(acetic) HCN C6H5OH (phenol)
F NO2 C9H7O4 HCO2 C6H7O6 C6H5CO2
CH3CO2 CN C6H5O
1.4 x 10 11 2.2 x 10 11 3.3 x 10 11 5.9 x 10
11 1.3 x 10 10 1.5 x 10 10 5.6 x 10 10 2.0 x
10 5 7.7 x 10 5
25
Strength of Acids and Bases 03

• (a) Arrange the three acids in order of
  increasing value of Ka.
• (b) Which acid, if any, is a strong acid?
• (c) Which solution has the highest pH, and which
  has the lowest?

K
(42/2) 8 12/5 0.2 Very Large
26
Acid Ionization Constants Determine the pH of
0.50 M HA solution at 25C. Ka 7.1 x 104 05

• Initial Change Equilibrium Table.

-


H

A
æ
HA
(aq)
(aq)
(aq)
Initial

(
M
)

0.50
0.00
0.00
Change
(M)

x

x

x
Equilib
0.50

x
x
x
(M)
27
What is the pH of a 0.50 M Citric acid solution
(at 250C)?
Initial (M)
0.50
0.00
0.00
Change (M)
-x
x
x
Equilibrium (M)
0.50 - x
x
x
100Ka lt Co ? 100 x 7.1 x 10-4 0.071 lt 0.5
0.50 x ? 0.50
x2 3.55 x 10-4
x 0.019 M
H A- 0.019 M
pH -log H 1.72
HA 0.50 x 0.48 M
28
Acid Ionization Constants 06

• pH of a Weak Acid (Contd)
• Substitute equilibrium concentrations into
  equilibrium expression.
• If 100Ka lt Co then (C0 x) approximates to
  (C0).
• The equation can now be solved for x and pH.
• If 100Ka is not significantly smaller than Co
  the quadratic equation must be used to solve for
  x and pH.

29
Acid Ionization Constants 07

• The Quadratic Equation
• The expression must first be rearranged to
• The values are substituted into the quadratic and
  solved for a positive solution to x and pH.

30
Acid Ionization Constants 09

• Percent Dissociation A measure of the strength
  of an acid.
• Stronger acids have higher percent dissociation.
• Percent dissociation of a weak acid decreases as
  its concentration increases.

31
Percent dissociation of a weak acid decreases as
its concentration increases

• Concentration Dependence

32
Weak Bases Base Ionization Constants 01

• Base Ionization Constant
• The equilibrium constant for the ionization of a
  base.
• The ionization of weak bases is treated in the
  same way as the ionization of weak acids.
  B(aq) H2O(l) æ BH(aq) OH(aq)
• Calculations follow the same procedure as used
  for a weak acid but OH is calculated, not H.

33
Base Ionization Constants 02
BASE Kb
CONJ. ACID Ka
5.6 x 10 4 4.4 x 10 4 4.1 x 10 4 1.8 x 10
5 1.7 x 10 9 3.8 x 10 10 1.5 x 10 14
C2H5NH2 (ethylamine) CH3NH2 (methylamine) C8H10N4O
2 (caffeine) NH3 (ammonia) C5H5N
(pyridine) C6H5NH2 (aniline) NH2CONH2 (urea)
C2H5NH3 CH3NH3 C8H11N4O2 NH4 C5H6N C6H5NH3 N
H2CONH3
1.8 x 10 11 2.3 x 10 11 2.4 x 10 11 5.6 x 10
10 5.9 x 10 6 2.6 x 10 5 0.67
Note that the positive charge sits on the
nitrogen.
(caffeine)
34
Base Ionization Constants 03

• Product of Ka and Kb multiplying out the
  expressions for Ka and Kb equals Kw.
• Ka ? Kb Kw

35
pH of Basic Solutions

• What is the pH of a 0.15 M solution of NH3?

NH3, M NH4, M OH-, M
Initially 0.15 0 0
At Equilibrium 0.15 - x x x
36
pH of Basic Solutions
100 x Kb lt C0 ? 1.8 ? 10-3lt 0.15 0.15 x 0.15

• (1.8 ? 10-5) (0.15) x2
• 2.7 ? 10-6 x2
• 1.6 ? 10-3 x2

37
pH of Basic Solutions

• Therefore,
• X OH- 1.6 ? 10-3 M
• pOH -log (1.6 ? 10-3)
• pOH 2.80
• pH 14.00 - 2.80
• pH 11.20

38
Acid-Base Properties of Salts

• Calculate the pH of a 0.10 M solution of Na2CO3.
• Na H2O ---gt neutral
• CO32- H2O æ HCO3- OH-
• base acid acid base
• Kb 2.1 x 10-4

39
Acid-Base Properties of Salts

• Calculate the pH of a 0.10 M solution of Na2CO3.
• Na H2O ---gt neutral
• CO32- H2O æ HCO3- OH-
• base acid acid base
• Kb 2.1 x 10-4
• Step 1. Set up ICE table
• CO32- æ HCO3- OH- initial 0.1
  0 0
• change -x x x
• equilib 0.10 - x x x

40
Acid-Base Properties of Salts

• Calculate the pH of a 0.10 M solution of Na2CO3.
• Na H2O ---gt neutral
• CO32- H2O e HCO3- OH-
• base acid acid base
• Kb 2.1 x 10-4

Step 2. Solve the equilibrium expression
Assume 0.10 - x 0.10, because 100Kb lt Co x
HCO3- OH- 0.0046 M
41
Acid-Base Properties of Salts

• Calculate the pH of a 0.10 M solution of Na2CO3.
• Na H2O ---gt neutral
• CO32- H2O e HCO3- OH-
• base acid acid base
• Kb 2.1 x 10-4

Step 3. Calculate the pH OH- 0.0046 M pOH
- log OH- 2.34 pH pOH 14, so pH
11.66, and the solution is Basic.
42
Diprotic Polyprotic Acids 01
H2SO4 H3PO4

• Diprotic and polyprotic acids yield more than one
  hydrogen ion per molecule.
• One proton is lost at a time. Conjugate base of
  first step is acid of second step.
• Ionization constants decrease as protons are
  removed.

43
Diprotic Polyprotic Acids 02
ACID Ka
CONJ. BASE Kb
Very Large 1.3 x 10 2 6.5 x 10 2 6.1 x 10
5 1.3 x 10 2 6.3 x 10 8 4.2 x 10 7 4.8 x 10
11 9.5 x 10 8 1 x 10 19 7.5 x 10 3 6.2 x 10
8 4.8 x 10 13
H2SO4 HSO4 C2H2O4 C2HO4 H2SO3 HSO3 H2CO3 HCO3
H2S HS H3PO4 H2PO4 HPO42
HSO4 SO4 2 C2HO4 C2O42 HSO3 SO3
2 HCO3 CO3 2 HS S 2 H2PO4 HPO42 PO43
Very Small 7.7 x 10 13 1.5 x 10 13 1.6 x 10
10 7.7 x 10 13 1.6 x 10 7 2.4 x 10 8 2.1 x 10
4 1.1 x 10 7 1 x 10 5 1.3 x 10 12 1.6 x 10
7 2.1 x 10 2
44
Molecular Structure and Acid Strength 01

• The strength of an acid depends on its tendency
  to ionize.
• For general acids of the type HX
• The stronger the bond, the weaker the acid.
• The more polar the bond, the stronger the acid.
• For the hydrohalic acids, bond strength plays the
  key role giving HF lt HCl lt HBr lt HI

567 kJ/mol for HF
299 kJ/mol for HI
45
Molecular Structure and Acid Strength 02

• The electrostatic potential maps show all the
  hydrohalic acids are polar. The variation in
  polarity is less significant than the bond
  strength which decreases from 567 kJ/mol for HF
  to 299 kJ/mol for HI.

46
(pm)
47
Molecular Structure and Acid Strength 03

• For binary acids in the same group, HA bond
  strength decreases with increasing size of A, so
  acidity increases.
• For binary acids in the same row, HA polarity
  increases with increasing electronegativity of A,
  so acidity increases.

48
Molecular Structure and Acid Strength 04

• For oxoacids bond polarity is more important. If
  we consider the main element (Y) YOH
• If Y is an electronegative element, the YO bond
  will pull more electrons, the OH bond will be
  more polar and the acid will be stronger.

49
Molecular Structure and Acid Strength 05

• For oxoacids with different central atoms that
  are from the same group of the periodic table and
  that have the same oxidation number, acid
  strength increases with increasing
  electronegativity.

50
Polar Covalent Bonds 02
Pauling Electronegativities
Detailed List of Electronegativity
http//environmentalchemistry.com/yogi/periodic/el
ectronegativity.html
51
Molecular Structure and Acid Strength 07

• Oxoacids of Chlorine

52
Molecular Structure and Acid Strength 08

• Predict the relative strengths of the following
  groups of oxoacids
• a) HClO, HBrO, and HIO.
• b) HNO3 and HNO2.
• c) H3PO3 and H3PO4.

53
Acid-Base Properties of Salts
54
Strong bases

• Strong bases
• The following metals make strong hydroxy base
• Alkali metal cations of group 1A
• Alkaline earth metal cations of group 2A
• except for Be

55
AcidBase Properties of Salts 01

• Salts that produce neutral solutions are those
  formed from strong acids and strong bases.
• Salts that produce basic solutions are those
  formed from weak acids and strong bases.
• Salts that produce acidic solutions are those
  formed from strong acids and weak bases.

56
Salts That Contain Cation from a Weak Base and
anion from a Weak Base
The pH of an ammonium carbonate solution,
(NH4)2CO3, depends on the relative acid strength
of the cation and the relative base strength of
the anion.
Is it acidic or basic?
57
Acid-Base Properties of Salts
Salts That Contain Acidic Cations and Basic Anions
(NH4)2CO3
Ka
Kb

• Three possibilities
• Ka gt Kb The solution will contain an excess of
  H3O1 ions , Acidic solution, (pH lt 7).
• Ka lt Kb The solution will contain an excess of
  OH1- ions, Basic solutions, (pH gt 7).
• Ka Kb The solution will contain approximately
  equal concentrations of H3O1 and OH1- ions (pH
  7).

58
Salts That Contain Cation from a Weak Acid and
anion from a Weak Base
(NH4)2CO3
Ka
Kb
5.6 x 10-10
Ka for NH41

1.8 x 10-4
Kb for CO32-

Basic, Ka lt Kb
59
Acid-Base Properties of Salts
60
Hydrated Cation of Al3
61
AcidBase Properties of Salts 03

• Metal Ion Hydrolysis

62
AcidBase Properties of Salts 04

• Calculate the pH of a 0.020 M Al(NO3)3 solution
• Ka 1.4 x 10-5.
• Predict whether the following solutions will be
  acidic, basic, or nearly neutral
• (a) NH4I (b) CaCl2 (c) KCN (d) Fe(NO3)3