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Electrolytic Cells:

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... effect' at the top of the SRP table are more easily oxidized than water. ... (Br- and Cl- always oxidize before H2O due to overpotential effect) ... – PowerPoint PPT presentation

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Title: Electrolytic Cells:


1
  • Electrolytic Cells
  • Non-spontaneous, endothermic redox reactions can
    be made to occur by applying an external voltage
    (in an electrolytic cell).

Type I Electrolytic Cell
  • Electrolysis of a molten substance (ionic
    compound)

eg molten NaCl
External voltage
Inert electrodes (carbon)
Substances present Na, Cl-
Cathode
Na e- ? Na(s)
E? -2.71V
Anode
2Cl- ? Cl2(g) 2e-
E? -1.36V
E?cell -4.07V
2
  • At least 4.07V are required to force reaction to
    occur. (In practice more than this is required)
  • This is how Na(s) and Cl2(g) are produced
    industrially.

Type II Electrolytic Cell
  • Electrolysis of an aqueous solution of an ionic
    compound.

eg NaI
Inert electrodes (platnum or carbon)
Substances present Na, I- , H2O
  • Must determine which reactions take place.

3
  • Two choices for reduction

a)
Na e- ? Na
E? -2.71V
b)
2H2O 2e- ? H2 2OH-
E? -0.41V
b) will occur because it requires the least
voltage.
  • Two choices for oxidation

a)
2I- ? I2 2e-
E? -0.53V
b)
H2O ? ½O2 2H 2e-
E? -0.82V
a) will occur because it requires the least
voltage.
Anode
2I- ? I2 2e-
E?cell -0.94V
Cathode
2H2O 2e- ? H2 2OH-
  • This electrolytic cell requires at least 0.94V
  • The products are H2(g) I2(s). (also NaOH)

4
Overpotential effect
  • In practice, the actual voltage required to power
    an electrolytic cell is always greater than the
    calculated voltage.
  • The difference between the calculated voltage and
    the voltage actually required is called the
    overpotential.
  • This overpotential is higher for reactions
    involving the oxidation or reduction of water.
  • Compounds included in the overpotential effect
    at the top of the SRP table are more easily
    oxidized than water.
  • Compounds included in the overpotential effect
    at the bottom of the SRP table are more easily
    reduced than water.

(Br- and Cl- always oxidize before H2O due to
overpotential effect)
5
Practice
  • Predict anode and cathode half reactions during
    electrolysis of these 1.0M solutions.
  • KI
  • HCl
  • NaF
  • NaBr
  • HF
  • Calculate the minimum amount of voltage required
    to power each of the above. (E?cell)
  • Draw a labeled diagram of 1a).
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