Outline 92001

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Outline 4/22/2005

• Announcements
• Today Chapter 17 - Electrochemistry
• redox reactions (read section 5.5 in textbook)

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Definitions

• A chemical reaction in which there is an electron
  transfer is a redox rxn.

redox reduction gain of electrons oxidation
loss of electrons
Reduction and oxidation always occur together. If
an electron is gained by one compound, it is lost
by another.
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Redox Mnemonics
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More definitions

• Reducing agent
• causes the reduction of another species
• is itself oxidized

• Oxidizing agent
• causes the oxidation of another species
• is itself reduced

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Assigning Oxidation Numbers

• treat ions separately
• ox. state of atoms in pure, neutral elements is
  zero
• hydrogen has an ox. num. of 1 (or -1
  when bound to metals)
• start with the most electronegative atom (use the
  octet rule)
• the sum of all ox. num. net charge

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Assigning Oxidation Numbers
Example SiCl4 (l) 2 Mg (s) ? Si (s) 2 MgCl2
(s) SiCl4 Cl -1 each Si 4 Mg 0 Si
0 MgCl2 Cl -1 each Mg 2
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Assigning Oxidation Numbers
Another Example 2 H2O2 (aq) ? 2 H2O (l) O2
(g) H2O2 H2O O2
H 1 each O -1 each H 1 each O -2 O 0
each
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How to balance redox rxns

• Identify charge states is it a redox rxn?
• Separate into half reactions
• Balance each half-reaction for mass
• Balance for O by adding H2O
• Balance for H by adding H (then OH- if basic)
• Balance each half-reaction for charge e-
• Equate charge (multiply by LCM)
• Add half-reactions together

Worksheet
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Worksheet

• Example 1 Acidic conditions

8 H MnO4- ? Mn2 4 H2O
5 e- 8 H MnO4- ? Mn2 4 H2O
MnO4- ? Mn2
MnO4- ? Mn2 4 H2O
Br- ? Br2
2 Br- ? Br2
2 Br- ? Br2 2 e-
Now, combine half-reactions
16 H 2 MnO4- 10 Br- ? 2 Mn2 8 H2O 5 Br2
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Worksheet

• Example 2 Basic conditions

MnO4- ? MnO42-
e-
? 2
H2O SO32- ? SO42- 2 H
SO32- ? SO42-
2 OH- H2O SO32- ? SO42- 2 H2O
2 OH- H2O SO32- ? SO42- 2 H2O 2 e-
H2O SO32- ? SO42-
Now, combine half-reactions
2 OH- 2 MnO4- SO32- ? 2 MnO42- SO42- H2O
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Do 3 on Worksheet

• Answer to 3
• 14 H Cr2O72- 6 I- ? 2 Cr3 7 H2O 3 I2
• 4 Do this one on your own
• Answer
• 2 OH- 3 ClO- 2 Cr(OH)4-?3 Cl- 2 CrO42- 5 H2O

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Outline 4/25/2005

• Announcements
• Today Chapter 17
• electrochemical cells

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Gummy Bear Sacrifice demo

• lots of energy is released in the redox reaction
  of carbohydrates

2 NaClO3 (s) ? 2 NaCl (s) 3 O2 (g) C12H22O11
(s) 3 O2 (g) ? 9 C (s) 3 CO2 (g) H2O (g)
5635 kJ ox. num of Cl before and after ox. num
of O before and after ox. num of C before and
after ox. num of O before and after
5 to -1
-2 to 0
probably -4 to 0 and 4
0 to -2
http//www.woodrow.org/teachers/chemistry/institut
es/1988/gummybear.htm
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this reaction will eventually stop because of
charge build-up
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So, why does it happen?

• Zn wants to be an ion more than Cu
• In other words, it takes less energy to ionize Zn
  than Cu
• Since Cu is already an ion, the metals exchange
  electrons
• the difference in energy becomes the potential on
  the cell
• this is summarized as reduction potentials (Eo )

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Who invented the first battery?
Volta apparently tested his new battery on his
skin, his tongue, his eyes, and various other
parts of his male anatomy read from science
historian Bern Dibner
Alessandro Volta in 1800
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Components of a Battery

• two half-cells
• one half-cell undergoes oxidation
• one half-cell undergoes reduction
• two electrodes
• a wire connecting the electrodes
• a salt bridge or a porous membrane
• demo

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Outline 4/27/2005

• Announcements
• Today Chapter 17
• electrochemical cells

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How to find Eo or? o of a battery?
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Definitions

• galvanic (voltaic) cell ? a cell that is
  spontaneous
• electrolytic cell ? a cell that is not
  spontaneous
• electrolysis ? the process of driving a
  non-spontaneous redox reaction
• electrolysis can be
• charging a battery
• plating jewelry or other metals
• reducing metals (ie., making sodium metal)
• etc.

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How to find Eo ?

• For example, what happens when Cu metal is put
  into a Zn2 solution?

• Look up std reduction potentials
• Cu2 2e? ? Cu 0.342 V
• Zn2 2e???? ? Zn -0.762 V

Does this result in a spontaneous reaction?
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Finding Eo and spontaneity

• Cu Zn2 ? Cu2 Zn - 1.104 V

Eorxnlt 0 , means non-spontaneous!!
If we reverse the entire cell, however(that
means Zn metal in a soln of Cu2)
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Finding Eo and spontaneity

• Cu2 Zn ? Cu Zn2 1.104 V
• Erxngt0 , means spontaneous!!

• Galvanic cell (a cell that is spontaneous)
• Electrodes Anode
• Cathode

Oxidation Reduction

• Zn ? Zn2 2e? _at_anode
• Cu2 2e???? Cu _at_cathode

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Passive vs. Active electrodes
Passive electrodes

• are not chemically altered by the redox reaction
• serve only to conduct current

Active electrodes

• are chemically altered by the redox reaction

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The Nernst Equation

• E E o ? (0.0257/n) lnQ
• or E E o ? (0.05916/n) log Q
• (T 298K is always assumed)

• What good is this?
• You can calculate E at non-standard conditions
  (voltages of some batteries change with
  concentration)

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Example

• What is the voltage produced by the following
  cell ? ? E o (V)
• MnO4? 8H 5e? ? Mn2 4H2O 1.51
• O2(g) 4H 4e? ? ?2 H2O 1.23
• at pH7.00, pO2 0.20 atm,
• MnO4? Mn2 0.10 M

• First balance redox equation for cell ...
• Set up expression for Q
• Solve in Nernst equation

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Set up redox equation for cell

• Which equation gets reversed?

MnO4? 8H 5e? ? Mn2 4H2O 1.51
2 H2O???O2 4H 4e? ?1.23

• What is n?

• n20

• What is E o?

• 0.28V

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What is Q?

• Products/reactants

What is E? ?

• E E o ? (0.05916/n) log Q
• 0.28 - 0.24
• 0.04 V

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The Life of a Battery

• ??

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Outline 4/29/2005

• Announcements
• Today Chapter 17
• electrochemical cells

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Battery Life/Electrolysis calcs(sections 17.6
17.7)

• Current ( I ) charge/time
• 1 Ampere 1 Coulomb / second

• Faradays constant ( F )
• 96485 C / mol e
• think about unit analysis when doing these
  problems

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Steps to solve 1) find the number of moles of
PbO2 2) use the 1/2 reaction to find moles of
e- 3) convert mol e- to time using F and I F
(C/mol) I (C/s) Amp
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• 1) 250 g PbO2 1 mol/ 239.2 g
• 1.045 mol PbO2

2) PbO2 ? PbSO4 (4 ? 2) 2e? ? 1.045 mol
PbO2 2 mol e? / mol PbO2
2.090 mol e?

• 3) (2.090 mol e?)(96485 C/mol)

? time 3361 sec 9.34 hours
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An Electrolysis Example
How many grams of silver would be
electrodeposited in 2.00 minutes from a
100. mL solution of 0.250M Ag with an applied
current of 100. mA?
Steps to solve 1) find the number of moles of e-
2) use the 1/2 reaction to convert moles of e-
to moles of Ag 3) convert mol Ag to grams of Ag
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• 1) 2 min

60 sec/min
100 10-3 C/sec
1.24 10-4 mol e-
2) Ag e- ? Ag ? 1.24 10-4 mol e- 1 mol
Ag / 1 mol e? 1.24 10-4 mol Ag

• 3) 1.24 10-4 mol Ag 107.9 g/mol Ag

? 0.0134 g Ag
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Lead Storage battery

• Pb(s)2 HSO4-(aq)PbO(s)2 H3O(aq) ? 2
  PbSO4(s)4 H2O(l)
• E? 2.05 V
• rechargeable, high capacity, contains toxic Pb

From Olmstead and Williams, 3rd ed.
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NiCd Batteries
Cd(s) 2 OH-(aq) ?
Cd(OH)2(s) 2 e - Eºox0.860 V
2?NiO(OH)(s)H2O(l) e - ?Ni(OH)2 (s) OH-(aq)
Eºred0.490 V 2 NiO(OH)(s) 2 H2O(l) Cd(s)
? 2 Ni(OH)2(s) Cd(OH)2(s) Eº 1.35 V

• rechargeable, delivers a fairly constant voltage
• has recharge memory cause by overcharging
• effect is reversed by a deep discharge and full
  recharge
• Cd is toxic

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NiCd Memory
Cd(s) 2 OH-(aq) ?
Cd(OH)2(s) 2 e - Eºox0.860 V
2?NiO(OH)(s)H2O(l) e - ?Ni(OH)2 (s) OH-(aq)
Eºred0.490 V 2 NiO(OH)(s) 2 H2O(l) Cd(s)
? 2 Ni(OH)2(s) Cd(OH)2(s) Eº 1.35 V
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NiMH Batteries
Alloy(H)(s) OH-(aq) ? Alloy
H2O(l) e - Eºox0.760 V
NiO(OH)(s)H2O(l) e - ?Ni(OH)2 (s) OH-(aq)
Eºred0.490 V NiO(OH)(s) MH(s)
? Ni(OH)2(s) M(s) Eº 1.25 V

• Ni-Metal Hydride batteries (same Ni rxn as NiCd)
• low toxicity (Cd is not present)
• rechargeable, delivers a fairly constant voltage
• has recharge memory too, only less than NiCd
• can be recharged hundreds of times
• 40 greater capacity than NiCd because Cd is
  heavy

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Alkaline Battery
Non-rechargeable

• 2 MnO(s) 2 H2O(l) Zn(s) ? Mn2O3(s)
  Zn(OH)2(s) E? 1.50 V
• Non-rechargeable
• low toxicity

From Olmstead and Williams, 3rd ed.
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Mercury Battery

• HgO(s) Zn(s) ? ZnO(s) Hg(l) E? 1.35 V
• non-rechargeable
• delivery a constant voltage over its lifetime
• contains toxic Hg

From Olmstead and Williams, 3rd ed.
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Corrosion
O2(g) 2 H2O(l) 4 e - ? 4 OH-(aq)
Eºred0.401 V O2(g) 4 H(aq) 4 e - ? 2
H2O(l) Eºred1.23 V

• both of these half-reaction scavenge electrons
  from metals
• any metal with Eºox gt -0.4 V (Eºred lt 0.4 V) will
  rust if exposed to moisture and the air

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Hydrogen Fuel Cells
H2 gas in
water out
air in
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Hydrogen Fuel Cells
H2 gas in
air in
water out
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Fuel Cells
H2 ? 2 H 2 e -
½ O2 2 H 2 e - ? H2O
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Hydrogen Fuel Cells