Using standard electrode potentials to calculate electrochemical cell voltages

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Using standard electrode potentials to calculate
electrochemical cell voltages

• Chapter 19 Pages 768-773

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• An electrochemical (voltaic) cell consists of an
  oxidation reaction and a reduction reaction to
  produce a voltage for the cell.
• Ex Batteries
• Need to know which electrode is the cathode,
  which is the anode, and whether the chemical
  reaction is spontaneous.

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• Anode negative electrode where oxidation
  occurs
• Cathode positive electrode where reduction
  occurs
• Electrons travel from the anode to the cathode,
  along a conductive wire voltage electricity.

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Anode
Cathode
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When choosing your anode and cathode follow this
rule

• Look at the reduction potential for the ions, the
  cathode will be the more positive reduction
  potential (p.11)
• Reason higher reduction potential means more
  easily reduced, therefore reduction takes place
  there, therefore the cathode

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Tableof standard reduction potentials

• Shows the relative ability for a substance to be
  reduced (gain electrons). It expresses this
  potential to gain electrons by assigning a
  voltage for each reduction half reaction in the
  table.
• Page 11 of our data booklet

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Eonet Eooxidation Eoreduction

• Eonet total net voltage of the cell (electric
  potential energy of the cell)
• Must be greater than zero for the reaction to be
  spontaneous
• If less than zero, than non-spontaneous reaction
  and no electricity produced.

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Eonet Eooxidation Eoreduction

• Eooxidation oxidation half reaction taking
  place in the cell
• Oxidation half reactions are the reverse reaction
  from the table on p.11, so their sign changes!
  If the reduction potential of Ag 0.80V, then
  the oxidation potential of Ag(s) - 0.80V

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Eonet Eooxidation Eoreduction

• Eoreduction reduction half reaction taking
  place in the cell
• The larger the reduction potential (the more
  positive), the greater the tendency for the
  reaction to occur.
• The smaller the reduction potential (the most
  negative), the least likely for a reaction to
  occur.

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Example Problem 1

• Design an electrochemical cell that uses the
  reactions of the metals Zn and Ag, and solutions
  of their ions, Zn2 and Ag.
• Identify the anode and cathode
• Write the oxidation and reduction half reactions
• Calculate the cell voltage

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Solution

• a), b) Zn2 2e- ? Zn(s) -0.76V
• Ag1 1e- ? Ag(s) 0.80V
• Ag more easily reduced, therefore Ag(s) is the
  cathode, so oxidation takes place at the anode
  (Zn) and the oxidation potential half reaction
  is
• Zn(s) ? Zn2 2e- 0.76V
• c) Eonet Eooxidation Eoreduction
• (0.76V) ( 0.80V)
• 1.56V

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• http//www.chem.iastate.edu/group/Greenbowe/sectio
  ns/projectfolder/flashfiles/electroChem/volticCell
  .html
• Look at this electrochemical cell demonstration.
  Do the same as our example with Zinc and Silver

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Exercise

• An electrochemical cell was designed using the
  metals Mg and Al and solutions containing Mg2
  and Al3.
• Identify the anode and cathode
• Write the oxidation and reduction half reactions
• Calculate the cell voltage
• An electrochemical cell was designed using the
  metals Zn and Pb and solutions containing the
  positive ions of these metals, Zn2 and Pb2.
• Identify the anode and cathode
• Write the oxidation and reduction half reactions
• Calculate the cell voltage