Intersection 15: The End

1
Intersection 15 The End

• 12/12/06

2
The Final Exam Questions

1. Electron configurations, atomic structure,
   periodic trends, atomic orbitals.
2. Quantitative calculations.
3. Lewis VSEPR Structures and polarity.
4. Thermochemistry
5. Thermochemistry
6. Thermochemistry
7. Acid/Base chemistry
8. Acid/Base chemistry
9. Oxidation number
10. Redox reactions
11. Redox reactions
12. Electrochemistry

3
Research at the University of MichiganA unique
opportunity in your life
For Help Finding Research Labs

• Women in Science and Engineering (WISE)
• http//www.wise.umich.edu/
• Undergraduate Research Opportunities Program
• http//www.lsa.umich.edu/urop/
• Your future class professors
• Dr. Gottfried and Dr. Banaszak Holl
• Send specific request with information about
  yourself and interests (CV). Use contacts.
  Blanket emails not great approach.

4
Outline

• Vitamin C note
• Electrochemistry
• Nernst
• Ampere
• Electrolytic cells
• Environmental
• Hybrid cars

5
Vitamin C

• Which fruit keeps best over a long period of
  time?
• Fruits known to sailors
• Orange, lemon 1400
• Limes 1638
• Shaddock (grapefruit) 1700
• Used lemon or lime juice preserved in brandy

6
M
Nernst
Picture from www.corrosion-doctors.org/
Biographies/images/
7
Is potential always the same?
M
Standard conditions 1 atm, 25oC, 1 M What will
influence the potential of a cell?
8
Mathematical Relationships Nernst
M
The Nernst Equation   Eo standard potential
of the cell R Universal gas constant 8.3145
J/molK T temperature in Kelvin n number of
electrons transferred F Faradays constant
96,483.4 C/mol Q reaction quotient
(concentration of anode divided by the
concentration of the cathode)
E Eo - RT ln Q nF
Cu2 Zn(s) ? Zn2 Cu(s) Q
9
Applying the Nernst Equation
M
Cu2 Zn(s) ? Zn2 Cu(s)
This cell is operating at 25oC with 1.00x10-5M
Zn2 and 0.100M Cu2? Predict if the voltage
will be higher or lower than the standard
potential
E Eo - RT ln Q nF
10
Cu2 Zn(s) ? Zn2 Cu(s)
M
E Eo - RT ln Q nF
Zn2 2e- -gt Zn -0.76 V Cu2 2e- -gt
Cu 0.34 V
Eo standard potential of the cell R Universal
gas constant 8.3145 J/molK T temperature in
Kelvin n number of electrons transferred F
Faradays constant 96,483.4 C/mol Q reaction
quotient (concentration of anode divided by the
concentration of the cathode)
25oC 273 K
n
1.00x10-5M Zn2 and 0.100M Cu2
Q Zn2/Cu2
11
Were your predictions correct?
M
12
A
Ampere
Picture from musee-ampere.univ-lyon1.fr/
13
The Units of Electrochemistry
A

• Coulomb
• 1 coulomb equals 2.998 x 109 electrostatic units
  (eu)
• eu is amount of charge needed to repel an
  identical charge 1 cm away with unit force
• Charge on one electron is -1.602 x 10-19 coulomb

Problem An aluminum ion has a 3 charge.
What is this value in coulombs? magnitude
of charge is same at that of e-, opposite
sign 3 x 1.602 x10-19 4.806 x 10-19
coulomb Key Point electrons or ions charges
can be measured in coloumbs
14
The Units of Electrochemistry
A

• Ampere
• Amount of current flowing when 1 coulomb passes
  a given point in 1 second
• Units of Amperes are Coulombs per second
• Current (I) x time (C/s x s) gives an amount of
  charge.

Problem How much current is flowing in a wire in
which 5.0 x 1016 electrons are flowing per
second? The charge transferred each second
(5.0 x 1016 electrons/sec) x (1.602 x 10-19
coulomb/electron) 8.0 x 10-3 coulombs/sec
amps
15
The Units of Electrochemistry
A

• Ampere
• Amount of current flowing when 1 coulomb passes
  a given point in 1 second
• Units of Amperes are Coulombs per second
• Current (I) x time (C/s x s) gives an amount of
  charge.
• We can express electron or ION flow in amps!

Problem If 1 mol Al3 ions passes a given point
in one hour, what is the current flow?
1 mol Al 3 ions 6.022 x 1023 Al 3 ions 4.806 x 10-19 coulomb 1 hour
Hour 1 mol Al 3 ions 1 Al3 ion 3600 sec
80 C/s 80 A
16

• From Moore, Stanitski, and Jurs Chemistry
  The Molecular Science 2nd Edition/

17
Batteries
A

• Primary Cells
• non-reversible, non-rechargeable electrochemical
  cell
• "dry" cell alkaline cell 1.5 v/cell
• mercury cell 1.34 v/cell
• fuel cell 1.23v/cell
• Secondary Cells
• reversible, rechargeable electrochemical cell
• Lead-acid (automobile battery) 2 v/cell
• NiCad 1.25 v/cell
• Lithium batteries

18
Flash Light Batteries
A

• Primary Cells
• "Dry" Cell
• Zn(s) 2 MnO2(s) 2 NH4 ?
• Zn2(aq) 2 MnO(OH)(s)
  2 NH3
• Alkaline Cell
• Zn(s) 2 MnO2(s) ? ZnO(s) Mn2O3(s)

19
Leclanche Dry Cell
A
20
Mercury Battery
A

• Primary Cells

Zn(s) HgO(s) ? ZnO(aq) Hg(l)
21
Fuel Cells
A

• anode
• H2(g) 2 OH-(aq) ? 2 H2O(l)
  2 e-
• cathode
• O2(g) 2 H2O(l) 4 e- ? 4 OH-(aq)

Picture from http//www.bpa.gov/Energy/N/project
s/fuel_cell/education/fuelcellcar/
22
Lead-Acid (Automobile Battery)
A
Secondary Cell

• Pb(s) PbO2(s) 2 H2SO4 2 PbSO4(s) 2
  H2O

2 v/cell thus 12 volt battery 6-2 volt cells
23
Nickel-Cadmium (Ni-Cad)
A

• Secondary Cell
• Cd(s) 2 Ni(OH)3(s) ? Cd(OH)2(s) Ni(OH)2(s)

24
Lithium Batteries
M

• Lithium batters (developed in 1970 used in
  watches, pacemakers, etc.)
• The 4-volt lithium battery, which has up to 33
  percent higher energy density and 60 less weight
  than a nickel-metal hydride battery of the same
  size, has made possible the miniaturization of
  the current generation of electronic devices

25
Lithium chemistry
M

• What is the ½ reaction involving lithium?
• Is this a reduction or oxidation reaction?
• Does it take place at the anode or cathode?

26
The other ½ of the battery
M

• Co3 e- ? Co2 (1.92 V)
• What is the standard potential of the cell?
• Li e- ? Li (-3.045 V)
• A lithium battery has a potential listed at 4V.
  Does this differ from the number you calculated?
  Why?

27
Alternative Anodes
M

• Which elements would you predict to have
  oxidation potentials similar to lithium?
• (no peeking at the table of standard reduction
  potentials!)
• Is what way is lithium preferable to these
  elements?

28
Alternative Cathodes
M

• Co3 e- ? Co2
• Disadvantage Use of cobalt oxide can lead to
  thermal runaway. Increased temperature and
  pressure cause case of battery to break and fumes
  are released. Lithium is exposed to oxygen and
  hydrogen in the air.
• What are the alternatives and why arent they
  being used in conjunction with lithium to build a
  battery with higher potential?

29
Table of Standard Reduction Potentials
M
Eo Eo
F2 2e- --gt 2F- 2.87 Fe3 3e- ---gt Fe -0.04
Co3 e- --gt Co2 1.80 Pb2 2e- ---gt Pb -0.13
Cl2 2e- ---gt 2Cl- 1.36 Ni2 2e- ---gt Ni -0.25
O2 4H 4e- -gt 2H2O 1.23 Co2 2e- ---gt Co -0.29
Hg2 2e- ---gt Hg 0.85 Cr3 e- ---gt Cr2 -0.40
Ag e- ---gt Ag 0.80 Fe2 2e- ---gt Fe -0.41
I2 2e- ---gt 2I- 0.54 Zn2 2e- ---gt Zn -0.76
Cu e- ---gt Cu 0.52 Mn2 2e- ---gt Mn -1.18
2H 2e- ---gt H2 0.00 Al3 3e- ---gt Al -1.66
30
Other commercially viable batteries
M

• Besides cobalt, two other reduction reactions
  used in lithium batteries are shown below. What
  is the potential of these batteries?
• Fe2 2e- ? Fe (-0.41 V)
• ½ I2 e- ? I- (0.54 V)

31
Other cathode reactions being explored
M

• Al and Mg are also being explored as materials
  for the cathode. What are the advantages and
  disadvantages of these materials?

32
A
33
Corrosion A Case of Environmental
Electrochemistry
A

• Facts about formation of rust
• Iron does not rust in dry air moisture must be
  present
• Iron does not rust in air-free water O2 must be
  present
• The loss of iron and the deposition of rust often
  occur at different places on the same object
• Iron rusts more under acidic conditions (low pH)
• Iron rusts more quickly in contact with ionic
  solutions
• Iron rusts more quickly in contact with a less
  active metal (such as Cu) and more slowly in
  contact with a more reactive metal (such as Zn)

34
Balance the Redox Reaction

• Fe(s) O2 (g) ? Fe2(aq) H2O(l) in acid

35
Corrosion A Case of Environmental
Electrochemistry
A

• O2(g) 4 H(aq) 4 e- gt 2 H2O(l)
  Eo 1.23 V
• Fe(s) gt Fe2(aq)
  2 e- Eo 0.44 V
• 2 Fe(s) O2(g) 4 H(aq) gt 2 H2O(l)
  Fe2(aq)
• 
  Eo 1.67 V

2Fe2(aq) ½O2(g) (2n)H2O(l) ? Fe2O3.nH2O(s)
4H(aq)
36
Corrosion A Case of Environmental
Electrochemistry
A

• Sacrificial cathode

37
A

• During the reconstruction of the Statue of
  Liberty, Teflon spacers were placed between the
  iron skeleton and the copper plates that cover
  the statue. What purpose do these spacers serve?

Cu2 2e- ? Cu 0.153 V Fe2 2e- ?
Fe -0.44 V
38
Hybrid Cars
M
39
Electrochemistry and the Honda Civic
M
The battery in the 2004 Honda Civic is rated for
6.0 Ampere-hours. If the electric motor draws 42
amps to accelerate from 0 to 60 mph, and the time
required is 15 seconds. The Honda Civic battery
is a nickel metal hydride battery. The
half-reaction for generating the electrical
current is MH OH- ? M H2O e- How
much metal (assume its nickel) is produce on
accelerating from zero to 60?
42 C 15 sec 1 mole e- 1 mol Ni 58.69 g Ni 0.38 g Ni
sec 96, 487 C 1 mole e- 1 mol Ni
40
Hybrid Cars Can We PowerUsing Sunlight and Fuel
Cells?
M
41
Some Needed Data
M

• The Toyota Prius can generate 100 Amps
• Lets assume our commute requires on average at
  20 max current for 1 hour.
• A 1 m2 solar cell array can generate 3 amps in
  full summer sunlight

• Questions
• If the electrical power was generated with a fuel
  cell, how much hydrogen would be consumed? How
  much volume would be required at 1 atm pressure
  to hold the hydrogen?
• Assume you can place 10 m2 of solar cells on your
  car. How long would it take to generate the
  needed hydrogen assuming full summer sunlight?

42
1. Hydrogen Consumption
M
At the anode (where oxidation occurs) 2 H2 (g)
4 H2O (l) ? 4 H3O (aq) 4 e- At the cathode
(where reduction occurs) O2 (g) 2 H2O (l)
4e- ? 4 OH- (aq) Overall reaction 2 H2(g) O2
? 2 H2O (l)
Average 20 of maximum power 20 amps 20 amps x
3600 secs 72,000 coulombs
72,000 C 1 Faraday 1 mol electrons 2 mol H2 0.37 mol H2
96,487 C 1 Faraday 4 mol electrons
V nRT/P (0.37 x 0.08206 x 300) / 1 9 Liters
43
2. Hydrogen Generation
M
A 1 x 1 m2 solar panel generates a peak current
of 3 amps. Our car with 10 m2 of solar panels
could generate 30 amps.
Electrolysis of water generates hydrogen with the
following half-reaction At the cathode (where
reduction occurs) 4 H3O (aq) 4 e- ? 2 H2 (g)
4 H2O (l)
0.37 mol H2 4 mol e- 1 Faraday 96, 487 C sec 1 hour 0.66 h
2 mol H2 1 mol e- 1 Faraday 30 C 3600 sec
44
Issues Arising
M

• Can generate hydrogen from water quickly enough
  to supply average demand by fuel cell.
• Important since solar cells cannot supply peak
  current flow used in Prius of 100 amps.
• Where to store hydrogen??
• As water is good. High density storage.
• Molarity??
• 110 M
• As gas is bad, requires too much space ( 9 L)
• Need good storage materials!! (Active research in
  Yaghi group)

45
(No Transcript)
46
Evaluations

• Please fill out both an evaluation for Dr.
  Banaszak Holl and Dr. Gottfried and put them in
  the correct envelopes!
• Constructive comments very helpful!

Thanks for a great semester!