Electrolytic Cells
Lesson 8
Electrolytic
Cells
ELECTROLYSIS
Electrolysis is a method of using a direct electric
current (DC) to drive an otherwise nonspontaneous chemical reaction.
Electrolysis is commercially highly important as a
stage in the separation of elements from naturally
occurring sources such as ores using
an electrolytic cell
Electrolytic Cells
Characteristics …
1.
Nonspontaneous redox reaction
2.
Produces chemicals from electricity
3.
Forces electrolysis to occur
An electrolytic cell is a system of two inert (nonreactive)
electrodes (C or Pt) and an electrolyte connected to a power
supply.
It is just ONE cell…
Electrolytic Cell
Oxidation always occurs at the anode and reduction at the
cathode
Electrons flow through the wire and go from anode to
cathode
Anions (- ions) migrate to the anode and cations (+ions)
migrate towards the cathode.
The electrode that is connected to the -ve terminal of the
power supply will gain electrons and therefore be the site of
reduction. Oxidation is connected to +ve terminal.
For electrolysis to work, you need a liquid sample:
1) molten cells
Melt the crystals (produce a "molten" sample )
This is the only way to electrolyse insoluble salts, and is the only
way to produce, by electrolysis, pure metals.
2) aqueous cells
Dissolve the ionic substance in water (this is the most common)
Type 1 electrolytic cell
• Inert electrodes immersed in a molten ionic
compound
1.
Draw and completely analyze a molten NaBr electrolytic cell.
Molten or (l) means ions but no water
e-
Electrons go from anode to cathode.
DC Power Source
-
The positive is oxidation
+
e-
The negative is reduction
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
Na+
Br
cations to cathode
anions to anode
2Na+ + 2Br- → Br2(g) + 2Na(l)
E0 = -3.80 v
The MTV is the minimum theoretical voltage required to start a reaction
MTV = +3.80 v
Type 1 electrolytic cell
• Inert electrodes immersed in a molten ionic
compound
• Things are reversed…meaning:
• The oxidation half reaction is ABOVE the
reduction half-reaction!
• Non-spontaneous…
Type 2 electrolytic cell
• Inert electrodes immersed in an aqueous
ionic compound
• Things are different now, you have water to
consider!
Soooo…what do you do if there is
water in the cell?
Pb2+ undergoes reduction in
water
Reduction of water
Treat as if it were here
Water will undergo reduction above
anything that is below this line.
Below this line, they will undergo oxidation
Water will undergo
reduction in a K+ solution
Reduction
If you have water present
(aq)
Consider the overpotential effect
Take the higher reaction on the left
The strongest oxidizing agent
1.0 M
solution
Water (lower) undergoes oxidation
before F-
Treat as if here
Oxidation of water
Br- (lower) will undergo
oxidation before water
Oxidation
If you have water present
(aq)
1.0 M
Consider the overpotential effect
Take the lower reaction on the right side of the table
The strongest reducing agent
solution
1.
Draw and completely analyze an aqueous KI electrolytic cell.
The negative is reduction
DC Power Source
-
+
Pt
Pt
Cation
or water
Reduction
Cathode
Consonants
The positive is oxidation
K+
H2 O
I-
For Reduction take the highest
highest
1.
Draw and completely analyze an aqueous KI electrolytic cell.
The negative is reduction
DC Power Source
-
The positive is oxidation
+
Pt
Pt
K+
H2 O
I-
Anion
or water
Oxidation
Anode
Vowels
For Oxidation take the lowest
lowest
1.
Draw and completely analyze an aqeuous KI electrolytic cell.
The negative is reduction
e-
Electrons go from anode to cathode.
DC Power Source
-
The positive is oxidation
+
ePt
Pt
_
reduction
cathode
2H2O+2e- → H2(g) + 2OH-0.41 v
anions to anode
K+
H2 O
I-
+
oxidation
anode
2I- → I2(s) + 2e-0.54 v
cations to cathode
2H2O + 2I- → H2 + I2(s) + 2OH-
E0 = -0.95 v
MTV = +0.95 v
Overpotential: voltage actually required to drive electrolytic cells.
Overpotential effect : a higher than normal voltage required for the half reaction. It
can be caused by different reasons. In aqeous solutions, it is due to extra voltage
required to produce a gas bubble such as (O2)
Final comments on electrolytic cells:
Just like an electrochemical cell, they depend on the thermodynamic data of the
reaction but ALSO….
They depend on the kinetic energy, activation energy and localized concentrations
of the reactions involved and different designs of cells.
Homework
Page242
65 ( a and c) ,66, 67, 70