lectrochemistry: Using Standard Cell Potentials
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Ox amp; Red |
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Half-reactions |
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Voltaic Cells |
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Cell Voltage |
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Calc. Potentials |
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Batteries |
| Electrolytic Cells |
Goal: to understand standard reduction potentials and to calculate the emf of a voltaic cell
Working Definitions:
Standard reduction potentials are potentials for electrodes in which all components are in a standard state at 25ºC, with ion concentrations of 1 M and gas pressures of one atm.
Negative (–) vs. Positive (+):
The values of electrode potentials are customarily reported for the reduction processes (an arbitrary decision). The standard reduction potential for a metal electrode will be negative when referenced to the standard hydrogen electrode if the metal ion is more difficult to reduce than the hydrogen ion. The standard reduction potential for the zinc electrode is –0.76 V:
2e– + Zn2+(aq) 
Zn(s)
| Note: a negative reduction potential tells us that the zinc ion is harder to reduce (is a worse oxidizing agent) than is the hydrogen ion; this also tells us that zinc metal is a better reducing agent than is hydrogen gas. |
The standard reduction potential for a metal electrode will be positive when referenced to the standard hydrogen electrode if the metal ion is more easily reduced than the hydrogen ion. The standard reduction potential for the copper electrode is +0.337 V:
2e– + Cu2+(aq) Cu(s)
| Note: a positive reduction potential tells us that the copper ion is easier to reduce (is a better oxidizing agent) than is the hydrogen ion; this also tells us that copper metal is a worse reducing agent than is hydrogen gas. |
Did we get that?:
Consider the reduction potentials for the following metal ions:
| Au (gold): E°=1.50 V |
3e– + Au3+(aq) Au(s)
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| Li (lithium): E°=–3.05 V |
1e– + Li+(aq) Li(s)
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Au
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Li
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Nicely done!
Which metal ion has the MOST positive reduction potential?
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Au
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Li
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Great job!
This is the same question asked in the previous problem! Better oxidizing agent = more easily reduced...
Put That in a Table:
Consider the following table of standard reduction potentials:
| In aqueous solutions at 25°C | |
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2e– + Hg2+ (aq) Hg(l)
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E° = 0.86 V |
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2e–+ I2 (s) 2 I– (aq)
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E° = 0.54 V |
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2e– + 2 H+ (aq) H2 (g)
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E° = 0 V |
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4e– + Zr4+ (aq) Zr (s)
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E° = –1.53 V |
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1e– + Rb+ (aq) Rb (s)
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E° = –2.93 V |
Use the table above to rank the following agents ability to reduce: ( "1" is highest)
Super job!
You will need to reverse the signs for the reduction potentials; the most positive of these will indicate the best reducing agent!
Before moving to the next page, you should understand what is represented by the magnitude and sign ( + or – ) of a standard reduction potential.
Using Standard Cell Potentials 
