Determine the oxidation number for each atom in the following molecules 1.H 2 S 2.P 2 O 5 3.S 8 4.SCl 2 5.Na 2 SO 3 6. SO 4 -2 7. NaH 8.Cr 2 O 7 -2 9.SnBr.

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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