Electrochemical Cells (Galvanic, Voltaic, Electric)

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Electrochemical Cells(Galvanic, Voltaic,
Electric)
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• the half cell reactions are occurring
  separately and are joined by a wire
• the e- transfer occurs through this external
  circuit
• the PE difference (voltage) is manifested as
  usable energy

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As the cell proceeds
  Cathode Anode
mass of electrode
Solution electrical charge
Ions from salt bridge
increases ?
decreases ?
-ve as Y ? Y
ve as X ? X
-ve anions
ve cations
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Cell Shorthand Notation

• eg. For the Zn/Cu cell Zn Zn2 Cu2 Cu
• Conventions
• the notation indicates a phase boundary where
  the electrode and electrolyte are in physical
  contact.
• the notation represents the salt bridge or
  porous barrier
• if additional reactants are required or specific
  products are formed, they are written with the
  solution separated by a comma or a semicolon.

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• if the cell has no metal for the electrode, inert
  electrodes C (s) or Pt (s) are used
•   Mg Mg2 MnO4 , H Mn2 C (s)  
• standard cells are 1.0 M at SATP 
• all three sets of information REDOX equation,
  cell diagram and the cell notation are related
  and if 1 is provided, the other 2 should be able
  to be created.

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Calculating Cell Potential, Ecell

• for each half reaction the reduction potential is
  listed on the chart on p. 805 or the reference
  sheet. 
• the complete cell is a combination of the
  reduction and oxidation half reactions - the sum
  of the two is the Cell potential, E cell 
• this is the usable energy given by 
• E cell E red (cathode) E ox (anode)

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• as all half reactions are listed as reduction
  rxns, the oxidation reaction flips the sign, so
• E cell E red (cathode) - E red (anode)
• eg. Calculate the E cell for
• Br2 Cu ? 2 Br Cu

ox Cu ? Cu e E ox -(0.52)
V
2x
2
2
2
red Br2 2 e ? 2 Br E red 1.07 V
Br2 2 Cu ? 2 Br 2Cu E cell
0.55 V
? Spontaneous ( ve)
Or E cell 1.07 V (0.52 V) 0.55 V
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• eg. a) Determine the anode, cathode and calculate
  the standard cell potential produced by a
  galvanic cell consisting of a Sn electrode in
  contact with a solution of Sn2 ions and a Cr
  electrode in contact with a solution of Cr3
  ions.  

Sn2 2e- ? Sn E -0.14 V
Cr3 3e- ? Cr E -0.74 V
(lesser ? flip)
Anode Cr ? Cr3 3e- E 0.74 V
Cathode Sn2 2e- ? Sn E - 0.14 V
Ecell 0.60 V
b) Write the shorthand cell notation.



Cr
Cr3 (aq)
Sn2 (aq)
Sn
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• eg. Use complete half-reactions and potentials to
  predict whether the following reaction is
  spontaneous or non-spontaneous in aqueous
  solutions. If the cell is spontaneous, write the
  cell shorthand notation.

O2 2 SO2 4 OH- ? 2 SO42- 2
H2O
0
-2
4
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an(ox) 2 SO2 2 H2O ? 2 SO42- 4 H
4 e- E -0.18 V
cat(red) O2 4 H 4 e- ? 2 H2O
E 1.23 V
Ecell 1.05 V


Pt (s)

SO2 (g)
SO42- (aq)
O2 (g),
4 H
Pt (s)