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Determine the oxidation number for each atom in the following molecules
1.H2S
2.P2O5
3.S8
4.SCl2
5.Na2SO3
6. SO4-2
7. NaH
8. Cr2O7-2
9. SnBr4
10. Ba(OH)2
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Intersection 14
12/05/06
Electrochemistry
19.9-19.13 p 941-955
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December in Studio
S M Tu W Th F S12/5
Exam 3
12/6
Studio
12/8 Polymers; check out
12/11 Poster session, paper due
12/12 final IS
12/13 In-class assignment
12/17 Review session 7-9 pm
12/19 Final exam 8-10 am
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Watershed Poster Session
• Monday, December 11 in USB 2165
• Board (4 ft x 4ft), easel, pins
• Set up by 1:10 and 3:10
• One person stationed at poster; others evaluate
• Rubric available
• Paper due same time
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Last In-Class Assignment
• Wednesday, December 13th in studio
• Available on-line
• Read papers before coming to class; bring them with you.
• May make any notes you like on the papers
• Goal: to evaluate scientific method and data
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Outline
• Ed’s demos
• Balancing Redox Reactions
• Electrochemistry– Electrochemical cells and Standard Hydrogen
Electrodes– Nernst– Quantifying current
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Ed’s Demos
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Oxidation States of Vanadium: Reduction of V5+ to V+2
• Reaction 1– Zn (s) + 2 VO3
- (aq) + 8 H3O+ (aq) ↔ 2 VO2+ (aq) + Zn+2 (aq) + 12 H2O (l)
• Reaction 2– Zn (s) + 2 VO2
+ (aq) + 8 H3O+ (aq) ↔ 2 V3+ (aq) + Zn+2 (aq) + 6 H2O (l)
• Reaction 3– Zn (s) + 2 V3+ (aq) ↔ 2 V2+ + Zn+2 (aq)
V+5 (aq) → V+4 (aq) yellow to greenV+4 (aq) → V+3 (aq) green to blueV+3 (aq) → V+2 (aq) blue to violet
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Oxidation States of Manganese: Mn+7, Mn+6, Mn+4, and Mn+2
• +7 (purple) to +2 (colorless)– 2 MnO4
- (aq) + H+ (aq) + 5 HSO3- (aq) ↔ 2 Mn+2 (aq) + 5 SO4
-2 (aq) + 3 H2O(l)
• + 7 (purple) to +4 (brown)– OH- + 2 MnO4
- (aq) + 3 HSO3- (aq) ↔ 2 MnO2 (s) + 3 SO4
-2 (aq) + 2 H2O(l)
• + 7 (purple) to + 6 (green)– 2 MnO4
- (aq) + 3 OH- + HSO3- (aq) ↔ 2 MnO4
-2(aq) + SO4-2 (aq) + 2 H2O(l)
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Thinking back….
• What happened when Na(s) was added to water?
Na(s) + H2O(l) Na+ (aq) + H2(g) + OH-
(aq)
• Determine the oxidation state of each reactant and product
• What was oxidized?• What was reduced?
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Balancing Redox Reactions
When KMnO4 (potassium permanganate) is mixed with Na2C2O4
(sodium oxalate) under acidic conditions, Mn+2(aq) ions and CO2(g) form.
The unbalanced chemical equation is:
KMnO4(aq) + Na2C2O4(aq) Mn+2(aq) + CO2(g) + K+
(aq) + Na+(aq)
K+ and Na+ are spectator ions, so we can ignore them at this point.
MnO4- (aq) + C2O4
-2(aq) Mn+2
(aq) + CO2(g)
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Half-Reactions
• Reduction reaction
• Oxidation reaction
MnO4- (aq) + C2O4
-2(aq) Mn+2
(aq) + CO2(g)
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Reduction reaction
Step 1: Balance all elements other than oxygen and hydrogen.
Step 2: Balance the oxygens by adding water.
Step 3: Balance the hydrogens using H+
Step 4: Balance the electrons
Mn+7 on reactants side
Mn+2 on products side
Step 5: Check charge balance and elemental balance
MnO4- Mn+2
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Oxidation reaction
C2O4-2 CO2
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Combine Half Reactions
5 e- + 8H+ + MnO4- Mn+2 + 4 H2O
C2O4-2 2 CO2 + 2e-
We are assuming the reaction takes place under acidic conditions!
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Balancing in Base
5 e- + 8H+ + MnO4- Mn+2 + 4 H2O
C2O4-2 2 CO2 + 2e-
Change H+ to water by adding OH- to each side
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NO2-(aq) + Cr2O7
-2 → Cr+3(aq) + NO3
-(aq) acidic soln
Electrochemical Cells
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Definitions• Electrochemical cell: A combination of anode,
cathode, and other materials arranged so that a product-favored redox reaction can cause a current to flow or an electric current can cause a reactant-favored redox reaction to occur
• Voltaic cell (battery): An electrochemical cell or group of cells in which a product-favored redox reaction is used to produce an electric current.
• Galvanic cell: A cell in which an irreversible chemical reaction produces electrical current
• Electrolytic cell: electrochemical reactions are produced by applying electrical energy
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A Copper-Zinc battery – What Matters?
Consider reduction potentials:
Cu+2 + 2e- → Cu(s) 0.3419 VZn+2 + 2e- → Zn(s) -0.7618 V
Place Zn electrode in copper sulfate solution – What happens?
Cu+2 + 2e- → Cu(s) 0.3419 VZn(s) → Zn+2 + 2e- 0.7618 V
Cu+2 + Zn(s) → Zn+2 + Cu(s) 1.1 V E > 0, spontaneous
Note, no need for electron to flow external to cell for reaction to occur!!
Copper is plated on Zn electrode
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A Copper-Zinc battery – What Matters?
Consider reduction potentials:
Cu+2 + 2e- → Cu(s) 0.3419 VZn+2 + 2e- → Zn(s) -0.7618 V
Place Cu electrode in zinc sulfate solution – What happens?
Cu(s) → Cu+2 + 2e- -0.3419 VZn+2 + 2e- → Zn(s) -0.7618 V
Zn+2 + Cu(s) → Cu+2 + Zn(s) -1.1 V E < 0, not spontaneous
No reaction occurs !!
Zn doesn’t plate on copper electrode?!
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Fig. 19-3, p.918
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What are the ½ reactions?What is the overall reaction?
Identify the oxidation, reduction, anode, and cathode
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Fig. 19-7, p.922
SHE: Standard Hydrogen Electrode
2 H3O+(aq, 1.00 M) + 2e- <-> H2(g, 1 atm) + 2H2O(l)
Eo = 0V
Standard conditions:1M, 1atm, 25oC
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Measuring Relative Potentials
Table of Standard Reduction Potentials
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Standard Reduction Potentials
What is the standard potential of a Au+3/Au/Mg+2/Mg cell?
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The half-reaction with the more positive standard reduction potential occurs at the cathode as reduction.
The half-reaction with the more negative standard reduction potential occurs at the anode as oxidation.
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