Electrochemistry

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Electrochemistry

• Marriage of redox and thermo
• Spontaneous electron-transfer reactions can
  result in spontaneous electric current if the
  reactants are separated by a wire
• Voltaic (Galvanic) cells
• We can use the spontaneity of the reaction to
  do electrical work

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• We can push electrons through a cell in order
  to make a nonspontaneous redox reaction occur.
• Electrolysis cell
• Doing work to force chemical reaction to occur
  opposite of voltaic cell

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Balancing Redox Equations

• Deferred until later
• For now just know that
• A half reaction has electrons written as a
  reactant or a product
• Oxidation half reaction A reactant gets
  oxidized (loses electrons) electrons appear as a
  product
• Reduction half reaction A reactant gets reduced
  (gains electrons) electrons appear as a
  reactant
• A balanced redox equation does not show electrons
  explicitly. e-s lost e-s gained (called
  n)

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Voltaic Cells

• Recall lab

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• The spontaneous rxn occurs in the cell
• e-s flow from to (get to go where they want
  to go)
• Anode where ox occurs
• Cathode where red occurs
• Salt bridge prevents charge buildup (which would
  stop flow)

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You used graphite in place of Pt in lab for
Fe2/Fe3 and I2/I-cells. A cheaper inert
electrode.
Used when neither redox species in a half
reaction (or electrode) is a neutral metal.
Neither is a neutral metal
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Standard Reduction Potentials (Ered)

• Recall lab
• Make a bunch of different cells, get different
  Ecell values (Eºcell if at standard state).
• Clearly some reductions are more favorable than
  others
• How do you know? Which direction did e-s
  flow?
• By how much?
• Rank them? (Must pick a zero as reference.)

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Quick quiz

• NOTE Every electrode compartment has one
  oxidizing agent and one reducing agent (this pair
  is called the redox couple)
• If an electrode has Ni(s) and Ni2 ions in it,
  which species is the oxidizing agent and which
  the reducing agent (of the pair)?

Ni2 (b/c its more positive it has room for
an electron
Ox agent is ___
Ni (b/c its more negative it has an
electron to give)
Red agent is ___
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Revisit Earlier CellLook at this as a
Competition for the electrons. Which oxidizing
agent wants them more?
Who is the (possible) oxidizing agent on the
left? _____
Who is the (possible) oxidizing agent on the
right? _____
Zn2
Cu2
Hint The two players are Zn and Zn2
Hint The two players are Cu and Cu2.
Who wins? (Which one got the electrons?) ____
Cu2
? Cu2 pulled harder
Cu2 2 e- ? Cu(s)
Sowhich of the half reactions shown at the right
is more favorable (greater tendency to happen)?
Zn2 2 e- ? Zn(s)
By how much?.....
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Reducing Cu2 is more favorable than reducing
Zn2 by 1.10 V! (Measure it w/voltmeter!)
We define a standard reduction potential,
Ered, for every reduction half reaction such
that
Ecell Ered(cathode) - Ered(anode)
The more positive the E (Ecell, Ered, or Eox),
the more favorable the process
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Reducing Cu2 is more favorable than reducing
Zn2 by 1.10 V! (Measure it w/voltmeter!)
Ecell Ered(cathode) - Ered(anode)
1.10 V Ered(Cu2/Cu) - Ered(Zn2/Zn)
NOTE
If Ered(Zn2/Zn) were 0 V, Ered(Cu2/Cu) would
be 1.10 V
If Ered(Zn2/Zn) were -1.0 V, Ered(Cu2/Cu)
would be 0.10 V
If Ered(Zn2/Zn) were 1.0 V, Ered(Cu2/Cu)
would be 2.10 V
?The zero is arbitrary, but must be chosen /
agreed upon!
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This was chosen to be the zero of potential.
2 H 2 e- ? H2 (g) Ered 0.0 V
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0.76 V Ered(SHE) - Ered(Zn2/Zn)
? 0.76 V 0 - Ered(Zn2/Zn)
? Ered(Zn2/Zn) - 0.76 V
Both as reductions
Ecell Ered Eox
OR Ecell Ecathode - Eanode
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Table 18.1 (continued)
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Calculate E?cell (see board)
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Lab interlude

• See overhead / board
• The lab manual initially asks you to pretend that
  the Ag/Ag reduction potential is 0.0 V just to
  show you the arbitraryness of this.
• Then it tells you that in reality, Ag/Ag
  reduction potential is 0.80 V if the H/H2
  potential (SHE) is 0.0 V

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Cu/Cu2 Fe3/Fe2
Cu/Cu2
0.45 V
-0.13 V
0.32 V
Cu ? Cu2 2e-
Fe3 e-? Fe2
Zn/Zn2 Ag/Ag
1.50 V
Zn/Zn2
Ag e-? Ag
0.0 V
1.50 V
Zn ? Zn2 2e-
Cu/Cu2 Zn/Zn2
1.05 V
Zn/Zn2
1.50 V
-0.45 V
Cu2 2e-? Cu
Zn ? Zn2 2e-
Circle the species that is the better oxidizing
agent
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From Text Table
0.80 V
Ag e-? Ag
0.0 V
0.80 V
0.77 V
0.67 V
-0.13 V
Fe3 e-? Fe2
Fe3 e-? Fe2
-0.13 V
0.34 V
0.35 V
Cu2 2e-? Cu
-0.45 V
Ag e-? Ag
0.0 V
-0.76 V
-1.50 V
-0.70 V
Zn2 2e- ? Zn
Cu2 2e-? Cu
-0.45 V
Zn ? Zn2 2e-
1.50 V
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Table 18.1 (continued)
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Excerpt from Voltaic Cell lab reading
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Calculate E?cell (see board)
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Calculate E?cell (see board)
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Nernst Equation

• See handout

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Standard vs. Nonstandard Cell
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Explain in detail an in a conceptual way why the
cell potential goes up when the NH3 is added. Is
the driving force for the cell rxn greater or
smaller after the NH3 is added?
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Relationship between variables (at any
conditions)
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Relationship between variables (at standard
state conditions)
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What mass of gold is plated in 25 minutes if the
current is 5.5 A?
Au3 (aq) 3 e- ? Au(s)
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