AcidBase Theory

1
Acid/Base Theory

• Lecture 1
• 1/10/03

2
The Chemistry of HCO3-

• HCO3-(aq) Ca2(aq)? CaCO3 (s) H (aq)
• HCO3-(aq) H(aq) ? H2CO3 (aq) ? H2O (l)
  CO2 (g)

Acid behavior Proton donor
Base behavior Proton acceptor
3
Bronsted-Lowry A/B Theory

• Proton transfer and the equilibrium that is
  established represents this theory
• Acids proton donors
• Bases proton acceptors

a proton refers to a H
4
Proton Transfer Reaction
Conjugate Base
Acid

• Deprotonation
• HCl (aq) H2O (l) ? H3O (aq) Cl-
  (aq)

Strong Acid all acid molecules deprotonate
Conjugate Base
Acid
HCO3- (aq) H2O (l) ? H3O (aq) CO32- (aq)
Weak Acid some of the acid molecules deprotonate
5
List of Strong Acids
This table is found in fundamental section J.
6
Weak Acid acetic acid
7
Proton Transfer Reaction
Conjugate Acid
Base

• Protonation
• O2- (aq) H2O ? 2 OH- (aq)

Strong Base all base molecules protonate
Conjugate Acid
Base
NH3 (aq) H2O (l) ? NH4 (aq) OH-
(aq)
Weak Base some of the base molecules protonate
8
List of Strong Bases
This table is found in fundamental section J.
9
Acid/Conjugate Base Pairs

• HSO3- (aq) NH4 (aq) ? NH3 (aq) H2SO3 (aq)
• What are the acid/conjugate base (base/conjugate
  acid) pairs?

Conjugate base
Acid
Conjugate acid
Base
10
Lewis Acid Theory
Metal oxides form O2-, therefore are basic

• Acid electron pair acceptor
• Base electron pair donor
• O2- (aq) H2O ? 2 OH- (aq)

BaO CaO Na2O
Lewis Base
..
H
..
OH-

..
O 2-
..
O H
..
..
OH-
H
..
Lewis Acid
11
Lewis Acid Theory
Nonmetal oxides accept an electron pair,
therefore are acids

• C O H

O
H


CO2 P4O10 SO2
..

H
O
/
O
\
..
C O
\
//
H
O
12
Amphoteric Oxides

• Substances which can behave both as an
  acid or a base

13
Autoprotolysis

• The transfer of a proton from one water molecule
  to a second water molecule
• H2O (l) H2O (l) ? H3O (aq) OH- (aq)

Water is amphiprotic the ability to act as both
a proton donor and as a proton acceptor
14
Kw
H2O (l) H2O (l) ? H3O (aq) OH- (aq)

• DGro SnDGfo (products)
• SnDGfo (reactants)
• ln Kw -DGro
• RT
• Kw 1.0 x 10-14 at 298 K

• Kw (aH3O) (aOH-)
• (aH2O)2
• aH3O H3O
• aOH- OH-
• aH2O 1
• Kw H3O OH-

Kw 1.0 x 10-14 at 298 K
In pure H2O at 25 oC, H3O and OH- 1 x 10-7
M
15
Example Problem

• Calculate the molarity of OH- in the following
  solutions.
• 0.20 M H3O b) 1.0 x 10-4 M H3O
• c) 3.1 x 10-2 M H3O

16
pH Scale
pOH
13

12
11
Acid

• pH - log H3O

10
9
8
Neutral
7
6
pOH - log OH-
5

4
Base
3
2
1
0
17
Example Problem

• Calculate the pH pOH on the following
  solutions.
• 0.20 M H3O b) 1.0 x 10-4 M H3O
• c) 3.1 x 10-2 M H3O

18
Relating Kw with pH and pOH

• Kw H3O OH-
• take the log of both sides of the equation
• log Kw (- log H3O) (- log OH-)
• pKw pH pOH
• since Kw 1.0 x 10-14
• pKw 14
• 14 pH pOH

19
Solvent Leveling Strong Acids

• HCl (aq) H2O (l) ? H3O (aq) Cl- (aq)

?
Very few HCl molecules remain
Because the proton-donating power of the HCl is
greater than the proton-donating power of the
conjugate acid, H3O
20
(No Transcript)
21
Solvent Leveling Weak Acids

• CH3COOH (aq) H2O (l) ? H3O (aq) CH3COO-
  (aq)

Most CH3COOH molecules remain
Because the proton-donating power of the CH3COOH
is less than the proton-donating power of the
conjugate acid, H3O
22
Solvent Leveling
These statements are true if the solvent is water.

• Strong Acid the proton-donating power of the
  acid (HA) is greater than that of the conjugate
  acid (H3O because in water)
• Weak Acid the proton-donating power of the acid
  (HA) is less than that of the conjugate acid
  (H3O because in water)
• Strong Base the proton-accepting power of the
  base is greater than that of the conjugate base
  (OH- because in water)
• Weak Base the proton-accepting power of the
  base is less than that of the conjugate base
  (OH- because in water)

23
Solvent Leveling

• If the solvent is not water, then being a strong
  or weak acid, or a strong or weak base depends on
  the strength of the conjugate acid or base formed
  in the solvent.
• CH3COOH (aq) H2O (l) ? H3O (aq) CH3COO-
  (aq)

24
Example pH and pOH problem

• Calculate the pH and pOH for the following
  solutions
• 0.725 M HCl
• 10.9 mg of KOH dissolved in 10.0 mL of water
• 5.0 mL of 3.5 x 10-4 M HClO4 after diluting to
  25.0 mL

25
Example pH and pOH problem

• Calculate the pH and pOH of the following
  solution (also calculate the percentage
  deprotonation)
• 0.29 M lactic acid CH3CH(OH)COOH

26
Example pH and pOH problem

• Calculate the pH and pOH of the following
  solution (also calculate the percentage
  protonation) The pKa of the
• 0.045 M quinine conjugate acid of
  quinine 8.52

27
Example Problem

• The percentage deprotonation of cacodylic acid,
  (CH3)2AsOOH, is 0.77. If the initial
  concentration of cacodylic acid was 0.0110 M,
  what is the pH and Ka of the cacodylic acid
  solution?

28
Example pH and pOH problem

• Calculate the pH and pOH of the following
  solution (also calculate the percentage
  protonation) The pKa of the
• 0.045 M quinine conjugate acid of
  quinine 8.52

29
Molecular Structures of Acids

• HA H2O ? H3O A-
• Strength of HA depends on the ease with which the
  H-A bond is broken
• The easier it is to break the H-A bond, the
  stronger the HA acid

30
Factors Affecting Strength of H-A Bond

• Bond Polarity
• Depends on the electronegativity difference
  between H and A
• As the electronegativity of A increases, the
  polarity of H-A increases, therefore the larger
  the electronegativity, the more acidic

• Atomic Radii
• Depends on the size of A
• As the atomic radii of A increases, the bond
  strength of H-A decreases, and the molecule
  becomes more acidic

31
Example Problem

• Which of the following substances is the
  strongest acid?
• H2S, PH3, HCl
• H2O, H2S, H2Se

32
Oxoacids
The greater the electronegativity of the central
atom, the stronger the acid.
33
Example Problem

• Which of the following is the strongest acid?
• HClO2, HBrO2, HIO2

34
Oxoacids
35
Oxoacids

• The greater the number of oxygen atoms on the
  central atom (which also means the central atom
  has a larger, more positive oxidation number),
  the stronger the acid.
• Which is the stronger acid?
• H2SO3, H2SO4

36
Oxoacids
The greater the electronegativities of the atoms
attached to the carboxyl group, the stronger the
acid.

• Organic acids are oxoacids.

Compare the acidity of an organic acid with that
of an alcohol.
H3C
H3C
\
..
\
C O
H - C - H
..
/
/
CH3CH2OH
CH3COOH
O
O
\
\
H
H
37
Example Problem

• Which of the following is the strongest acid?
• CH3COOH, CH2ClCOOH, CH2FCOOH

38
Salts

• Salt ionic compound produced during a
  neutralization reaction
• HCl NaOH ? HOH NaCl
• Since this is a neutralization reaction, what
  will be the resulting pH?

39
A/B Properties of Salts

• Why is NaHCO3 used to clean up an acid spill?

NaHCO3 (s) ? Na (aq) HCO3- (aq)
HCO3- (aq) H2O (l) ? OH- (aq)
H2CO3 (aq)
HCO3- (aq) HCl (aq) ? Cl- (aq)
H2CO3 (aq) Acid-base neutralization
40
A/B Properties of Salts

• If a salt contains an ion that is a conjugate
  base from a weak acid (formed during an acid/base
  reaction), the salt solution will be slightly
  basic.

Kw Ka Kb
HC2H3O2(aq) H2O (l) ? H3O(aq)
C2H3O2- (aq)
NaC2H3O2(aq) ? Na(aq)
C2H3O2- (aq)
C2H3O2-(aq) H2O (l) ? OH-(aq)
HC2H3O2 (aq)
41
A/B Properties of Salts

• If a salt contains an ion that is a conjugate
  acid from a weak base (formed during an acid/base
  reaction), the salt solution will be slightly
  acidic.

Kw Ka Kb
NH3(aq) H2O (l) ? OH-(aq)
NH4(aq)
NH4NO3(aq) ? NH4(aq)
NO3- (aq)
NH4(aq) H2O (l) ? NH3(aq)
H3O (aq)
42
A/B Properties of Salts

• Salt Content
• Cation Anion A/B Property

NaC2H3O2
NH4NO3
NaCl
CH3NH3F
From SB From WA basic
From WB From SA acidic
From SB From SA neutral
Write the acid or base reaction that each of
these salts would undergo. (The proton transfer
equilibria in H2O)
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If the cation is from a weak base, the salt will
be acidic
44
If the anion is from a weak acid, the salt will
be basic
45
Concept Map

• H2O, acidic salt, basic salt, amphiprotic salt,
  pH, CaCO3, FeCl2, KNO3
• Draw a map that includes all of these concepts.
  You will try to link the maps together by drawing
  lines between concepts and describing why they
  are related.
• Summarize the map by stating how these terms (or
  some of them) could help you to solve a problem.

46
Calculate the pH of a Salt Solution

• Calculate the pH of 0.10 M NH4NO3.

47
Very Dilute Acids/Bases and Polyprotics

• Lecture 6
• 1/24/03

48
Very Dilute Acids (or Bases)

• We will consider only very dilute strong acids
  and bases
• Therefore, section 10.16 regarding very dilute
  weak acids will not be covered (or tested on).

What is a very dilute strong acid?
49
Very Dilute Strong Acid (Base)

• The pH of such solutions is significantly
  affected by the autoprotolysis of water.

This occurs when an acid or base has a
concentration of less than 1 x 10-6 M
50
Calculating the pH of a Very Dilute Strong Acid
Charge Balance
1.5 x 10-9 M HNO3
Material Balance
Kw Expression
What species exist in a solution of a strong acid
like HNO3? (solvent is H2O)
51
Calculating the pH of a Very Dilute Strong Base
Charge Balance
7.3 x 10-11 M KOH
Material Balance
Kw Expression
What species exist in a solution of a strong base
like KOH? (solvent is H2O)
52
Polyprotic Acid

• Can donate more than one proton during proton
  transfer equilibria
• H2S H2O ? H3O HS-
• HS- H2O ? H3O S2-

Ka H3OHS- H2S
Ka expression for the first step in the
equilibria ? Ka1
Ka H3OS2- HS-
Ka expression for the second step in the
equilibria ? Ka2
Which is greater? Ka1 or Ka2
53
Calculate the pH for a Polyprotic

• What is the pH of a 0.0037 M solution of H2CO3?

54
Calculate the Concentrations for all Species in a
Polyprotic Equilibria

• Calculate the concentrations of all the species
  in a 0.20 M H2S solution

55
Salts of Polyprotic Acids

• The conjugate base of a polyprotic acid is
  amphiprotic.
• H2S H2O ? H3O HS-

KHS
Is this salt acidic or basic?
What is the pH of a solution containing this salt?
56
pH of Salt of polyprotic

• Calculate the pH of a 0.05 M Na2S solution.
• H2S H2O ? H3O HS- Ka1 1.3 x
  10-7
• HS- H2O ? H3O S2- Ka2 7.1 x
  10-15

pH ½ (pKa1 pKa2)
57
Chapter 10

• Bronsted-Lowry Theory
• Proton Transfer Equations
• For weak and strong acids and bases
• Deprotonation and Protonation
• Lewis Theory
• Oxides
• Autoprotolysis

• Kw
• pH, pOH, pKa, pKb
• Calculate pH of strong and weak acid and base
  solutions
• Relationship between Ka and Kb of conjugate pairs
• Strength of acids
• Molecular structure and various trends

58
Chapter 10

• Strength of Oxoacids
• trends
• Identify salts as acidic, basic, or neutral
• Calculate pH
• Very dilute strong acids and bases
• Polyprotic acids
• Proton transfer equation