1. Chapter 20: Electrochemistry
Chemistry 140 Fall 2002
General Chemistry
Principles and Modern Applications
Petrucci • Harwood • Herring
10th Edition
Chapter 20: Electrochemistry
Mamdouh Abdelsalam
King Faisal University
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2. General Chemistry: Chapter 21
Chemistry 140 Fall 2002
Contents
20-1 Electrode Potentials and Their Measurement
20-2 Standard Electrode Potentials
20-3 Ecell, ΔG, and Keq
20-4 Ecell as a Function of Concentration
20-5 Batteries: Producing Electricity Through Chemical Reactions.
20-6 Corrosion: Unwanted Voltaic Cells
20-7 Electrolysis: Causing Non-spontaneous Reactions to Occur
20-8 Industrial Electolysis Processes
Focus On Membrane Potentials
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General Chemistry: Chapter 21

3. 20-1 Electrode Potentials and Their Measurement
Cu(s) + 2Ag+(aq)
Cu2+(aq) + 2 Ag(s)
Cu(s) + Zn2+(aq)
No reaction
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General Chemistry: Chapter 20

4. An Electrochemical Half Cell
Anode
Cathode
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General Chemistry: Chapter 20

5. An Electrochemical Cell
Chemistry 140 Fall 2002
An Electrochemical Cell
Cu ⇢ Cu2+ + 2e-
2Ag+ + 2e-⇢ 2Ag
Anode: oxidation
Cathode: Reduction
Section 1 Sunday
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General Chemistry: Chapter 20

6. General Chemistry: Chapter 21
Salt bridge
Salt bridge:
To provide ions to the solution to keep the electro neutrality
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General Chemistry: Chapter 21

7. General Chemistry: Chapter 20
Terminology
Electromotive force, Ecell.
The cell voltage or cell potential.
Cell diagram.
Shows the components of the cell in a symbolic way.
Anode (where oxidation occurs) on the left.
Cathode (where reduction occurs) on the right.
Oil (oxidation is loss), Rig (Reduction is gaining)
Boundary between phases shown by |.
Boundary between half cells (usually a salt bridge) shown by ||.
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General Chemistry: Chapter 20

8. General Chemistry: Chapter 20
Terminology
Zn ⇢Zn2+ + 2e-
Cu2+ + 2e- ⇢Cu
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s) Ecell = V
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General Chemistry: Chapter 20

9. General Chemistry: Chapter 20
Terminology
Galvanic cells.
Convert chemical energy to electricity as a result of spontaneous reactions.
Electrolytic cells.
Convert electricity into chemical energy
Non-spontaneous chemical change driven by electricity.
Couple, M|Mn+
A pair of species related by a change in number of e-.
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General Chemistry: Chapter 20

10. 20-2 Standard Electrode Potentials
Cell voltages, the potential differences between electrodes, are among the most precise scientific measurements.
The potential of an individual electrode is difficult to establish.
Arbitrary zero is chosen.
The Standard Hydrogen Electrode (SHE)
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General Chemistry: Chapter 20

11. Standard Hydrogen Electrode
Chemistry 140 Fall 2002
Standard Hydrogen Electrode
2 H+(a = 1) + 2 e- ⇄ H2(g, 1 bar) E° = 0 V
Pt|H2(g, 1 bar)|H+(a = 1)
The two vertical lines indicate three phases are present.
For simplicity we usually assume that a = 1 at [H+] = 1 M and replace 1 bar by 1 atm.
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General Chemistry: Chapter 20

12. Standard Electrode Potential, E°
Chemistry 140 Fall 2002
Standard Electrode Potential, E°
E° defined by international agreement.
The tendency for a reduction process to occur at an electrode.
All ionic species present at a=1 (approximately 1 M).
All gases are at 1 bar (approximately 1 atm).
Where no metallic substance is indicated, the potential is established on an inert metallic electrode (ex. Pt).
Tuesday 12/12/2017
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General Chemistry: Chapter 20

13. General Chemistry: Chapter 20
Reduction Couples
Cu2+(1M) + 2 e- → Cu(s) E°Cu2+/Cu = ?
Pt|H2(g, 1 bar)|H+(a = 1) || Cu2+(1 M)|Cu(s) E°cell = V
anode
cathode
Standard cell potential: the potential difference of a cell formed from two standard electrodes.
E°cell = E°cathode - E°anode
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General Chemistry: Chapter 20

14. Standard Cell Potential
Chemistry 140 Fall 2002
Standard Cell Potential
Pt|H2(g, 1 bar)|H+(a = 1) || Cu2+(1 M)|Cu(s) E°cell = V
E°cell = E°cathode - E°anode
E°cell = E°Cu2+/Cu - E°H+/H2
0.340 V = E°Cu2+/Cu - 0 V
E°Cu2+/Cu = V
Wednesday (13/12/2017)
H2(g, 1 atm) + Cu2+(1 M) → H+(1 M) + Cu(s) E°cell = V
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General Chemistry: Chapter 20

15. Measuring Standard Reduction Potential
Chemistry 140 Fall 2002
Measuring Standard Reduction Potential
anode
cathode
cathode
anode
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General Chemistry: Chapter 20

16. Standard Reduction Potentials
Chemistry 140 Fall 2002
Standard Reduction Potentials
Section 2
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General Chemistry: Chapter 20

17. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
20-3 Ecell, ΔG, and Keq
Cells do electrical work.
Moving electric charge.
Faraday constant, F = 96,485 C mol-1
elec = -nFE
ΔG = -nFE
ΔG° = -nFE°
Thursday section 1 (14/12/2017)
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General Chemistry: Chapter 20

18. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
Spontaneous Change
ΔG < 0 for spontaneous change.
Therefore E°cell > 0 because ΔGcell = -nFE°cell
E°cell > 0
Reaction proceeds spontaneously as written.
E°cell = 0
Reaction is at equilibrium.
E°cell < 0
Reaction proceeds in the reverse direction spontaneously.
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General Chemistry: Chapter 20

19. The Behavior of Metals Toward Acids
M(s) → M2+(aq) + 2 e- E° = -E°M2+/M
2 H+(aq) + 2 e- → H2(g) E°H+/H2 = 0 V
2 H+(aq) + M(s) → H2(g) + M2+(aq)
E°cell = E°H+/H2 - E°M2+/M = -E°M2+/M
When E°M2+/M < 0, E°cell > 0. Therefore ΔG° < 0.
Metals with negative reduction potentials react with acids
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General Chemistry: Chapter 20

20. Relationship Between E°cell and Keq
Chemistry 140 Fall 2002
Relationship Between E°cell and Keq
ΔG° = -RT ln Keq = -nFE°cell
E°cell = nF RT
ln Keq
Section 1 Tuesday
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General Chemistry: Chapter 20

21. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
Summary of Thermodynamic, Equilibrium and Electrochemical Relationships.
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General Chemistry: Chapter 20

22. 20-4 Ecell as a Function of Concentration
ΔG = ΔG° -RT ln Q
-nFEcell = -nFEcell° -RT ln Q
Ecell = Ecell° ln Q nF RT
Convert to log10 and calculate constants
Ecell = Ecell° log Q n V
The Nernst Equation:
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General Chemistry: Chapter 20

23. General Chemistry: Chapter 20
Example 20-8
Applying the Nernst Equation for Determining Ecell.
What is the value of Ecell for the voltaic cell pictured below and diagrammed as follows?
Pt|Fe2+(0.10 M),Fe3+(0.20 M)||Ag+(1.0 M)|Ag(s)
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General Chemistry: Chapter 20

24. General Chemistry: Chapter 20
Example 20-8
Ecell = Ecell° log Q n V
Ecell = Ecell° log n V
[Fe3+]
[Fe2+] [Ag+]
Ecell = V – V = V
Pt|Fe2+(0.10 M),Fe3+(0.20 M)||Ag+(1.0 M)|Ag(s)
Fe2+(aq) + Ag+(aq) → Fe3+(aq) + Ag (s)
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General Chemistry: Chapter 20

25. 20-5 Batteries: Producing Electricity Through Chemical Reactions
Primary Cells (or batteries).
Cell reaction is not reversible. (not rechargeable)
Secondary Cells.
Cell reaction can be reversed by passing electricity through the cell (rechargeable).
Flow Batteries and Fuel Cells.
Materials pass through the battery which converts chemical energy to electric energy.
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General Chemistry: Chapter 20

26. Lead-Acid (Storage) Battery
Chemistry 140 Fall 2002
Lead-Acid (Storage) Battery
The most common secondary battery
Section 1 Thursday
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General Chemistry: Chapter 20

27. General Chemistry: Chapter 20
Lead-Acid Battery
Reduction:
PbO2(s) + 3 H+(aq) + HSO4-(aq) + 2 e- → PbSO4(s) + 2 H2O(l)
Oxidation:
Pb (s) + HSO4-(aq) → PbSO4(s) + H+(aq) + 2 e-
PbO2(s) + Pb(s) + 2 H+(aq) + 2HSO4-(aq) → 2 PbSO4(s) + 2 H2O(l)
E°cell = E°PbO2/PbSO4 - E°PbSO4/Pb = 1.74 V – (-0.28 V) = 2.02 V
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General Chemistry: Chapter 20

28. General Chemistry: Chapter 20
Fuel Cells
O2(g) + 2 H2O(l) + 4 e- → 4 OH-(aq)
2{H2(g) + 2 OH-(aq) → 2 H2O(l) + 2 e-}
2H2(g) + O2(g) → 2 H2O(l)
E°cell = E°O2/OH- - E°H2O/H2
= V – ( V) = V
 = ΔG°/ ΔH° = 0.83
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General Chemistry: Chapter 20

29. 20-6 Corrosion: Unwanted Voltaic Cells
Chemistry 140 Fall 2002
20-6 Corrosion: Unwanted Voltaic Cells
In neutral solution:
O2(g) + 2 H2O(l) + 4 e- → 4 OH-(aq)
EO2/OH- = V
2 Fe(s) → 2 Fe2+(aq) + 4 e-
EFe/Fe2+ = V
2 Fe(s) + O2(g) + 2 H2O(l) → 2 Fe2+(aq) + 4 OH-(aq)
Ecell = V
In acidic solution:
Cancelled sem 1 ( )
O2(g) + 4 H+(aq) + 4 e- → 4 H2O (aq) EO2/OH- = V
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General Chemistry: Chapter 20

30. General Chemistry: Chapter 20
Corrosion Protection
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General Chemistry: Chapter 20

31. General Chemistry: Chapter 20
Corrosion Protection
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General Chemistry: Chapter 20

32. 20-7 Electrolysis: Causing Non-spontaneous Reactions to Occur
Galvanic Cell:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) EO2/OH- = V
Electolytic Cell:
Zn2+(aq) + Cu(s) → Zn(s) + Cu2+(aq) EO2/OH- = V
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General Chemistry: Chapter 20

33. Faraday’s law
The amount of chemical change is proportional to the amount of current passed.
m mass of substance
M molar mass
I current A
T time for which the current passe (s)
n number of electrons transferred
F Faraday constant (96485 C mol-1)
M Abdelsalam