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IV. 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)
-
NaCl
800C
+
+
+
+
+
Substances present: Na+, ClCathode: Na+ + e-  Na(s) E = -2.71V
Anode: 2Cl-  Cl2(g) + 2e- E = -1.36V
Ecell = -4.07V
 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)
-
NaI(aq)
+
+
+
+
+
Substances present: Na+, I- , H2O

Must determine which reactions take
place.
 Two choices for reduction:
a)
Na+ + e-  Na
b) 2H2O + 2e-  H2 + 2OH-
E = -2.71V
E = -0.41V
b) will occur because it requires the least voltage.
 Two choices for oxidation:
a)
b)
2I-  I2 + 2eH2O  ½O2 + 2H+ + 2e-
E = -0.53V
E = -0.82V
a) will occur because it requires the least voltage.
Anode:
2I-  I2 + 2eCathode: 2H2O + 2e-  H2 + 2OH-
Ecell = -0.94V
 This electrolytic cell requires at least 0.94V
 The products are H2(g) & I2(s). (also NaOH)
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)
Practice:
1. Predict anode and cathode half reactions during
electrolysis of these 1.0M solutions.
a) KI
b) HCl
c) NaF
d) NaBr
e) HF
2. Calculate the minimum amount of voltage required
to power each of the above. (Ecell)
3. Draw a labeled diagram of 1a).