Electrolysis: NonSpontaneous Redox Reactions

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Electrolysis Non-Spontaneous Redox Reactions

• An external source of energy (electrical current)
  is used to drive a nonspontaneous redox reaction.
• Figure 17.19(a) A Standard Galvanic Cell
  (b) A Standard Electrolytic Cell

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• In the electrolysis cell pictured, electrons flow
  from the power source into the zinc electrode.
• Cathode Zn2(aq) 2 e ? Zn(s)
• Anode Cu(s) ? Cu2(aq) 2 e
• Net Cu(s) Zn2(aq) ? Cu2(aq) Zn(s)
• ?E 0.76 0.34 1.10 V
• Therefore, the external power source must supply
  at least 1.10 V in order to make this reaction
  occur.

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Quantitative Aspects of Electrolysis

• Current charge / time or I q/t
• Units 1 A (ampere) 1 C (coulomb) / s
• Faradays Laws of Electrolysis
• The amount of a substance produced or con-sumed
  at an electrode is proportional to
• 1) the amount of electrical charge passed
  through the cell.
• 2) the molar mass of the substance divided by
  the number of electrons in the process.

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• Now, relate current and time to the mass of
  substance reacted.
• Ex A current of 0.450 A is passed through a
  solution of Ni2(aq) for 30.0 m. How much Ni is
  plated out at the cathode?
• q I t (0.450 C/s)(1.80 x 103 s) 810 C
• no. mols e (no. Faradays) 810 C / 96485 C/F
• 8.40 x 103 F

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Some Commercial Electrolysis Processes

• The Hall-Heroult Process (1854) Aluminum
• 1855 100,000 / lb
• 1990 0.74 / lb
• Al ore is bauxite, Al2O3. Cant plate Al out of
  aqueous solution (H2O electrolyzes), so the ore
  must be melted (m.p. 2050C).
• Hall added cryolite, Na3AlF6, to bauxite,
  bringing the melting point down to 1000C.

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Figure 17.22A Schematic Diagram of an
Electrolytic Cell for Producing Aluminum by the
Hall-Heroult Process
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Figure 17.25 The Downs Cell for the Electrolysis
of Molten Sodium Chloride
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Figure 17.26 The Mercury Cell for Production of
Chlorine and Sodium Hydroxide