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).