1. ELECTROCHEMICAL CELLS Chapter 20 : D8 C20
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2. Terms Used for Voltaic Cells
Figure 20.3

3. What determines the measured
If Al and Cu are separated work is done by the electrons. Voltmeter is used to measure the energy.
What determines the measured
value?

4. What causes the electrical potential of an electrochemical cell?
The electrical potential of a voltaic cell is a measure of the cell’s ability to produce an electric current.
Electrical potential is usually measured in volts (V).
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5. The Cu|Cu2+ and Ag|Ag+ Cell

6. The potential of an isolated half-cell cannot be measured.
Electrical Potential
The potential of an isolated half-cell cannot be measured.
You cannot measure the electrical potential of a zinc half-cell or of a copper half-cell separately.
When these two half-cells are connected to form a voltaic cell, however, the difference in potential can be measured.
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7. CELL POTENTIAL, E
1.10 V
1.0 M
Zn and Zn2+,
anode
Cu and Cu2+,
cathode
For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn2+] and [Cu2+] = 1.0 M.

8. CELL POTENTIAL, E STANDARD CELL POTENTIAL, Eo
a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C.
STANDARD CELL POTENTIAL, Eo
is when [Zn2+] and [Cu2+] = 1.0 M.
For Zn/Cu cell, potential is V at 25 ˚C

9. Electrical Potential
The standard hydrogen electrode is used with other electrodes so the reduction potentials of the other cells can be measured.
The standard reduction potential of the hydrogen electrode has been assigned a value of 0.00 V.
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10. 2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm)
CELL POTENTIALS, Eo
Can’t measure 1/2 reaction Eo directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL.
SHE
Eo = 0.0 V
2 H+(aq, 1 M) e- <----> H2(g, 1 atm)

12. Electrical Potential
The difference between the reduction potentials of the two half-cells is called the cell potential.
cell potential = –
reduction potential of half-cell in which reduction occurs
reduction potential of half-cell in which oxidation occurs –
or Ecell = Ered – Eoxid
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13. Supplier of electrons Acceptor of electrons
Zn/Zn2+ half-cell hooked to a SHE.
Eo for the cell = V
Negative electrode
Positive electrode
Supplier of electrons
Acceptor of electrons
Zn --> Zn2+ + 2e-
Oxidation
Anode
2 H+ + 2e- --> H2
Reduction
Cathode

14. Reduction of H+ by Zn
No current

15. Eo for Zn ---> Zn2+ (aq) + 2e- is +0.76 V
Overall: reduction of H+ by Zn metal.
Zn(s) + 2H+(aq) --> Zn2+ + H2(g) Eo =+0.76 V
Therefore, oxidation potential
Eo for Zn ---> Zn2+ (aq) + 2e- is V

16. Cu/Cu2+ and H2/H+ Cell Eo = +0.34 V Positive Negative
Acceptor of electrons
Supplier of electrons
Cu2+ + 2e- --> Cu
Reduction
Cathode
H2 --> 2 H+ + 2e-
Oxidation
Anode

17. Cu/Cu2+ and H2/H+ Cell
Overall reaction is reduction of Cu2+ by H2 gas.
Cu2+ (aq) + H2(g) ---> Cu(s) H+(aq)
Measured Eo = V
Therefore, reduction potential
Eo for Cu e- ---> Cu is V

18. Calculating Cell Voltage
Balanced half-reactions can be added together to get overall, balanced equation.
Zn(s) ---> Zn2+(aq) + 2e-
Cu2+(aq) + 2e- ---> Cu(s)
Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
If we know Eo for each half-reaction, we could get Eo for net reaction.

19. Zn/Cu Electrochemical Cell+
Anode, negative, source of electrons
Cathode, positive, sink for electrons
Zn(s) ---> Zn2+(aq) + 2e- Eo = V
Cu2+(aq) + 2e- ---> Cu(s) Eo = V
Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
Eo (calc’d) = V

20. Electrical Potential
The tendency of a given half-reaction to occur as a reduction is called the reduction potential.
The half-cell in which reduction occurs has a greater reduction potential than the half-cell in which oxidation occurs.
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21. Using Standard Potentials, Eo Table 20.1
Which is the best oxidizing agent (reduced): O2, H2O2, or Cl2? _________________
Which is the best reducing agent (oxidized): Hg, Al, or Sn? ____________________

22. Electrical Potential
The standard cell potential (E0cell) is the measured cell potential when the ion concentrations in the half-cells are 1M, any gases are at a pressure of 101 kPa, and the temperature is 25°C.
E0cell = E0red – E0oxid
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23. Calculating Standard Cell Potentials
How can you determine if a redox reaction is spontaneous?
Positive Cell Potentials
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24. Calculating Standard Cell Potentials
If the cell potential for a given redox reaction is positive, then the reaction is spontaneous as written.
If the cell potential is negative, then the reaction is non-spontaneous.
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25. Fe3+(aq) + 3e– → Fe(s) E0cell = –0.036 V
Determine the cell reaction for a voltaic cell composed of the following half-cells.
Fe3+(aq) + 3e– → Fe(s) E0cell = –0.036 V
Al3+(aq) + 3e– → Al(s) E0cell = –1.66 V
Oxidation: Al(s) → Al3+(aq) + 3e– (at anode)
Reduction: Fe3+(aq) + 3e– → Fe(s) (at cathode)
Al(s) + Fe3+(aq) → Al3+(aq) + Fe(s)
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26. Zn(s) + 2Ag+(aq) → Zn2+(aq) + 2Ag(s)
Sample Problem 21.1
Determining Reaction Spontaneity
Zn(s) + 2Ag+(aq) → Zn2+(aq) + 2Ag(s)
Show that the following redox reaction between zinc metal and silver ions is spontaneous.
If E0cell is positive, the reaction is spontaneous.
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27. First identify the half-reactions.
Sample Problem 21.1
First identify the half-reactions.
Oxidation: Zn(s) → Zn2+(aq) + 2e–
Reduction: Ag+(aq) + e– → Ag(s)
Write both half-cells as reductions with their standard reduction potentials.
Zn2+(aq) + 2e– → Zn(s) E0Zn2+ = –0.76 V
Ag+(aq) + e– → Ag(s) E0Ag+ = V
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28. Calculate the standard cell potential.
Sample Problem 21.1
Calculate the standard cell potential.
E0cell = E0red – E0oxid = E0Ag+ – E0Zn2+
= V – (–0.76 V) = V
E0cell > 0, so the reaction is spontaneous.
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30. REDUCTION
POTENTIALS

31. More About Calculating Cell Voltage
Assume I- ion can reduce water.
2 H2O e- ---> H OH Cathode: Red
2 I > I e Anode: Ox
2 I H2O --> I OH H2
Assuming reaction occurs as written,
E˚net = E˚cathode - E˚anode or = E˚red + E˚ox
= ( V) - ( V) = V
Minus E˚ means rxn. occurs in opposite direction

32. Using Standard Potentials, Eo Table 20.1
In which direction do the following reactions go?
Cu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)
What is Eonet for the overall reaction?

33. Standard Redox Potentials, Eo
E˚net = “distance” from “top” half-reaction (cathode) to “bottom” half-reaction (anode)
E˚net = E˚cathode - E˚anode
Eonet for Cu/Ag+ reaction = V

34. All ingredients are present. Which way does reaction proceed?
Eo for a Voltaic Cell
Cd --> Cd2+ + 2e- or
Cd2+ + 2e- --> Cd
Fe --> Fe2+ + 2e- or
Fe2+ + 2e- --> Fe
All ingredients are present. Which way does reaction proceed?

35. B. A. 2H+ + 2e– → H2 B. Al3+ + 3e– → Al C. Br2 + 2e– → 2Br–
In a voltaic cell in which one half-reaction is Fe3+ + 3e– → Fe, which of the following would occur as oxidation?
A. 2H+ + 2e– → H2
B. Al3+ + 3e– → Al
C. Br2 + 2e– → 2Br–
D. Fe3+ + e– → Fe2+ B.
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36. Key Concepts
The electrical potential of a cell results from a competition for electrons between two half-cells.
You can determine the standard reduction potential of a half-cell by using a standard hydrogen electrode and the equation for standard cell potential.
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37. Key Concepts & Key Equation
If the cell potential for a given redox reaction is positive, then the reaction is spontaneous as written. If the cell potential is negative, then the reaction is nonspontaneous.
E0cell = E0red – E0oxid
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38. Glossary Terms
electrical potential: the ability of a voltaic cell to produce an electric current
reduction potential: a measure of the tendency of a given half-reaction to occur as a reduction (gain of electrons) in an electrochemical cell
cell potential: the difference between the reduction potentials of two half-cells
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39. Glossary Terms
standard cell potential (E0cell): the measured cell potential when the ion concentrations in the half-cells are 1.00M at 1 atm of pressure and 25°C
standard hydrogen electrode: an arbitrary reference electrode (half-cell) used with another electrode (half-cell) to measure the standard reduction potential of that cell; the standard reduction potential of the hydrogen electrode is assigned a value of 0.00 V
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41. REDUCTION
POTENTIALS