CHEM 112:
General Chemistry I Lecture
University of Illinois
at Chicago
UIC
General Chemistry 1
Lecture 37
Instructor: Dr. Chad Landrie
Lecture CRN: 18644
Time/Day: M,W,F; 2:00-2:50 pm
April 14, 2010
i>Clicker Question
Codeine (CodN) is an opiate analgesic. It is a Bronsted base that contains
an amine functional group and has a pKb of 5.95. Codeine is typically
administered as its conjugate acid (codeine hydrochloride, CodNHCl).
Determine the pH of a 0.025 M solution of codeine hydrochloride.
CodNH+ Cl–
conjugate acid
pKa = 14.00 – 5.95 = 8.05
CodN
base
pKb = 5.95
K a = 10 –8.05
K a = 8.913 × 10 –9
CodNH+(aq) + H2O(l) ⇌ CodN(aq) + H3O+(aq)
I
C
E
Ka =
0.025
–x
0.025–x
0
+x
0
+x
x
A.
4.75
B.
4.83
pH = –log [H3O+ ]
C.
4.92
pH = –log (1.685 × 10 −5 )
pH = 4.83
D.
4.64
E.
4.55
x
+
[CodN][H3O ]
[CodNH+ ]
x2
8.913 × 10 =
0.025 − x
x = 1.493 × 10 −5
−9
University of
Illinois at Chicago
UIC
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 2
Lecture: April 14
i>Clicker Question
A 0.100 M solution of hydrochloric acid (HCl) contains 100.%
dissociated HCl. Determine the percent dissociated HF in a
0.100 M solution of HF if the pKa of this acid is 3.200.
pK a = 3.20
HF is a much weaker acid
than HCl. Why?
K a = 10 −3.20
K a = 6.310 × 10 –4
HF(aq) + H2O(l) ⇌ H3O+(aq) + F–(aq)
I
0.100
C –x
E 0.100–x
0
+x
x
0
+x
x
[F – ][H3O+ ]
Ka =
[HF]
x2
6.310 × 10 =
0.100 − x
2
x + (6.310 × 10 −4 )x − 6.310 × 10 −5 = 0
−4
x = 7.634 × 10 −3
University of
Illinois at Chicago
UIC
% HAc dissociated =
[HF]dissoc
× 100%
[HF]init
A.
B.
C.
D.
E.
6.31%
25.%
3.20%
5.25%
7.63%
(7.634 × 10 –3 )
% HAc dissociated =
× 100% = 7.63%
(0.100)
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 3
Lecture: April 14
Structural Effects on the strengths of Acids
•
•
•
1. Electronegativity:
Dominant effect for atoms in the same period (row).
More electronegative conjugate base = more stable conjugate base
= Ka lies further to right
Alternate reasoning: H of conjugate acid (HA) becomes more
positive with increasing electronegativity of A–.
H–A ⇌ H+ + A–
more electronegative
= stabilizes negative charge
= more stable (lower NRG)
= larger Ka
University of
Illinois at Chicago
UIC
[H+ ][A – ]
Ka =
[HA]
A system at equilibrium lies furthest
toward the side that is the most stable
(lowest in energy). This is a major topic
in CHEM 114.
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 24
4
Lecture: April 14
Structural Effects on the strengths of Acids
Increasing electronegativity of atom in bold.
H
Acid
C
H
N
H
H
H
O
H
H
pKa = 60
pKa = 36
H
F
H
pKa = 16
H
pKa = 3
Increasing acid strength.
Conjugate
base
C
H
N
H
H
H
O
H
F
H
Increasing stability (lower NRG) of conjugate base.
University of
Illinois at Chicago
UIC
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 24
5
Lecture: April 14
Inductive Effect
Inductive effect: polarization of a bond by an electron
withdrawing atom (electronegative) through two or more bonds.
University of
Illinois at Chicago
UIC
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 6
Lecture: April 14
Structural Effects on Acid Strength
•
•
•
2. Bond Strength:
Dominant effect for atoms in the same group (column).
Bond strength for H–A decreases as A moves down a column.
Size of A increases as it moves down a column; bigger A = less
overlap between orbitals of H–A = weaker bond.
4
3
2
1
2
3
University of
Illinois at Chicago
UIC
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 7
Lecture: April 14
Structural Effects on Acid Strength
•
•
•
2. Bond Strength:
Dominant effect for atoms in the same group (column).
Bond strength for H–A decreases as A moves down a column.
Size of A increases as it moves down a column; bigger A = less
overlap between orbitals of H–A = weaker bond.
•
Covalent bonds are formed when two
electrons are shared between two
atoms.
•
Electrons are shared when the orbitals
(electron cloud) that contain the
electron for each atom overlap.
•
The closer in size the orbitals are, the
better (stronger) the overlap. Better
orbital overlap = stronger bond.
University of
Illinois at Chicago
UIC
2
3
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 8
Lecture: April 14
Structural Effects on Acid Strength
Similar Size = Better Overlap = Strong Bond
The shared electrons spend a larger percentage
of the time in between the two atoms.
University of
Illinois at Chicago
UIC
Dissimilar Size = Poorer Overlap = Weaker Bond
The shared electrons spend a smaller percentage
of the time in between the two atoms.
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 9
Lecture: April 14
Structural Effects on Acid Strength
•
•
•
2. Bond Strength:
Dominant effect for atoms in the same group (column).
Bond strength for H–A decreases as A moves down a column.
Size of A increases as it moves down a column; bigger A = less
overlap between orbitals of H–A = weaker bond.
HF
HCl
HBr
HI
pKa = 3
pKa = –4
pKa = –6
pKa = –10
University of
Illinois at Chicago
UIC
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 10
Lecture: April 14
Structural Effects on Acid Strength
University of
Illinois at Chicago
UIC
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 11
Lecture: April 14
Structural Effects on Acid Strength
•
•
3. Resonance (Electron Delocalization):
Delocalization of electrons in the conjugate base = increased stability
(lower NRG) of conjugate base
More stable (lower energy) conjugate base = larger Ka = smaller pKa =
stronger acid.
H2SO4 ⇌ H+ + HSO4–
[H+ ][HSO 4– ]
Ka =
= 1,000
[H2SO 4 ]
H2SO3 ⇌ H+ + HSO3–
[H+ ][HSO 3– ]
Ka =
= 0.0150
[H2SO 3 ]
For oxyacids, the greater the number of oxygen
atoms, the more resonance structures can be drawn.
University of
Illinois at Chicago
UIC
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 12
Lecture: April 14
Review: Drawing Lewis Structures
Drawing Lewis Structures
(Abbreviated Procedure)
1. Central atom(s), then others
2. Count total valence electrons
3. Draw single bonds
4. Distribute remaining electrons
5. Form multiple bonds for octet
or expanded valence shells
6. Determine formal charge.
Total VE
31+1=32
Bonds VE
10
Lone-pairs VE
22
Remaining
0
University of
Illinois at Chicago
UIC
Special Cases: Octet Expansion
Non-metals in periods 3 and higher
can exceed an octet.
O
–1
O
S
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
HSO41–
–1
+2
O
Example
–1
O
H
hydrogen
sulfate
Slide 13
Lecture: April 14
Review: Drawing Lewis Structures
Drawing Lewis Structures
(Abbreviated Procedure)
1. Central atom(s), then others
2. Count total valence electrons
3. Draw single bonds
4. Distribute remaining electrons
5. Form multiple bonds for octet
or expanded valence shells
6. Determine formal charge.
Total VE
31+1=32
Bonds VE
14
Lone-pairs VE
18
Remaining
0
University of
Illinois at Chicago
UIC
Special Cases: Octet Expansion
Non-metals in periods 3 and higher
can exceed an octet.
Example
HSO41–
O
–1
O
S
O
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
O
H
hydrogen
sulfate
Slide 14
Lecture: April 14
Review: Drawing Lewis Structures
Drawing Lewis Structures
(Abbreviated Procedure)
1. Central atom(s), then others
2. Count total valence electrons
3. Draw single bonds
4. Distribute remaining electrons
5. Form multiple bonds for octet
or expanded valence shells
6. Determine formal charge.
Total VE
8
Lone-pairs VE
18
Remaining
0
UIC
Non-metals in periods 3 and higher
can exceed an octet.
O
–1
O
S
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Example
HSO31–
–1
+1
25+1=26
Bonds VE
University of
Illinois at Chicago
Special Cases: Octet Expansion
O
H
hydrogen sulfite
Slide 15
Lecture: April 14
Review: Drawing Lewis Structures
Drawing Lewis Structures
(Abbreviated Procedure)
1. Central atom(s), then others
2. Count total valence electrons
3. Draw single bonds
4. Distribute remaining electrons
5. Form multiple bonds for octet
or expanded valence shells
6. Determine formal charge.
Total VE
10
Lone-pairs VE
16
Remaining
0
UIC
Non-metals in periods 3 and higher
can exceed an octet.
Example
HSO41–
O
25+1=26
Bonds VE
University of
Illinois at Chicago
Special Cases: Octet Expansion
–1
O
S
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
O
H
hydrogen sulfite
Slide 16
Lecture: April 14
Review: Drawing Resonance Structures
Resonance structures are formed by “pushing” electrons
into different allowed arrangements.
O
O
O
S
OH
O
University of
Illinois at Chicago
OH
O
O
O
O
S
O
O
O
S
S
OH
UIC
O
S
OH
O
OH
more resonance structures =
more charge delocalization =
more stable conjugate base =
equilibrium lies further to the
right (larger Ka) =
stronger acid
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 17
Lecture: April 14
Trends in Acid Strength of Oxoacids
Acid
pKa
H2SO4
–3
H2SO3
1.81
Acid
pKa
HClO4
–10
HClO3
–1
HClO2
1.96
HClO
7.50
University of
Illinois at Chicago
UIC
•
more oxygen atoms bonded to
central atom =
•
•
•
•
•
more resonance structures =
more delocalization of charge =
more stable conjugate base =
larger Ka =
stronger acid
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
Slide 18
Lecture: April 14
i>Clicker Question
Rank the following bases in order of
increasing pKb.
BrO4–(aq) + H2O(l) ⇌ HBrO4(aq) + OH–(aq)
HBrO4(aq) + H2O(l) ⇌ BrO4–(aq) + H3O+(aq)
A
B
C
D
BrO4–
0.10 M
BrO–
0.25 M
BrO2–
0.10 M
BrO3–
0.015 M
The stronger the acid, the weaker the conjugate base.
The weaker the acid, the stronger the conjugate base.
University of
Illinois at Chicago
UIC
A.
B.
C.
D.
E.
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
B < C < D <A
A<D<C<B
D < C <A < B
B <A < C < D
D <A < C < B
Slide 19
Lecture: April 14
i>Clicker Question
Rank the following oxoacids in order of increasing pKa.
The ionizable proton in each acid is highlighted in blue.
A
B
O
H
S
OH
H3C
OH
H
H3C
more electronegative groups/atoms, more
stable conjugate base
more stable conjugate base, more acidic acid
UIC
O
F
C
more Os, more resonance, more stable
conjugate base
University of
Illinois at Chicago
D
O
O
C
H
C
OH
A.
B.
C.
D.
E.
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
O
S
OH
C
F
F
B < C < D <A
A<D<C<B
D < C <A < B
B <A < C < D
D <A < C < B
Slide 20
Lecture: April 14
i>Clicker Question
Rank the following acids in order of
increasing pKa.
A
B
C
D
H2O
0.10 M
H2Se
0.25 M
H2Te
0.10 M
H2S
0.015 M
size of atom’s orbital increases down a
group
bigger orbital = poorer overlap = weaker
bond
weaker bond = stronger acid
University of
Illinois at Chicago
UIC
A.
B.
C.
D.
E.
© 2010, Dr. Chad L. Landrie
CHEM 112: General Chemistry 1, Spring 2010
B < C < D <A
C < B < D <A
D < C <A < B
A<B<C<D
D <A < C < B
Slide 21
Lecture: April 14
CHEM 112:
General Chemistry I Lecture
University of Illinois
at Chicago
UIC
Next Lecture...
Sections: 18.7-18.8