Notes on Electrolytic Cells
An electrolytic cell is a system of two inert (nonreactive)
electrodes (C or Pt) and an electrolyte connected to a power
supply. It has the following characteristics
1.
2.
3.
Nonspontaneous redox reaction
Produces chemicals from electricity
Forces electrolysis to occur
When analyzing an electrolytic cell, your first and most
important step is to determine the oxidation and reduction
reactions.
Electrolytic Cell Main Rule
The electrode that is connected to the -ve terminal of the
power supply will gain electrons and therefore be the site of
reduction.
Other Rules: For Electrochemical and Electrolytic Cells
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.
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Draw a beaker,
two inert
electrodes wired
to a power supply.
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Supply
DC
+
Draw a beaker,
two inert
electrodes wired
to a power supply.
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Supply
DC
+
Label the
electrode with Pt
or C.
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Supply
DC
+
Pt
Label the
electrode with Pt
or C.
Pt
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Supply
DC
+
Pt
Add the
electrolyte
Pt
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Supply
DC
+
Add the
electrolyte
Molten or liquid
means no water!
Pt
Pt
Na+
Br-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Supply
DC
+
Label the negative
and positive
electrodes
Pt
Pt
Na+
Br-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Supply
DC
+
Label the negative
and positive
electrodes
Pt
Pt
_
+
Na+
Br-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
The negative is
reduction and the
positive is
oxidation.
Pt
Pt
_
+
Na+
Br-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
The negative is
reduction and the
positive is
oxidation.
Na+
Br-
+
oxidation
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
The anode is
oxidation and the
cathode is
reduction.
Na+
Br-
+
oxidation
anode
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
The anion
Power Source
migrates to the
+
anode and the
cation to the
cathode.
Pt
Pt
_
reduction
cathode
Na+
Br-
+
oxidation
anode
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
The anode
reaction is the
oxidation of the
anion.
Na+
Br-
+
oxidation
anode
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
The anode
reaction is the
oxidation of the
anion.
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
The Cathode
reaction is the
reduction of the
cation.
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
The Cathode
reaction is the
reduction of the
cation.
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
Gas Br2 is
produced at the
anode.
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
Liquid Na is
produced at the
cathode.
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
The potential for
Power Source
each half reaction
+
is calculated and
the oxidation sign
is reversed
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
The potential for
Power Source
each half reaction
+
is listed and the
oxidation sign is
reversed
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
The overall redox
Power Source
reaction is
+
written.
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
1.
Draw and completely analyze a molten NaBr
electrolytic cell.
The overall redox
Power Source
reaction is
+
written.
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
Na+
Br-
2Na+ + 2Br- → Br2(g)+ 2Na(l)
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
E0 = -3.80 v
1.
Draw and completely analyze a molten NaBr
The minimum theoretical
electrolytic cell.
Power Source
+
voltage MTV required to
force this
nonspontaneous reaction
to occur is the negative
of the cell potential.
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
Na+
Br-
2Na+ + 2Br- → Br2(g)+ 2Na(s)
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
E0 = -3.80 v
1.
Draw and completely analyze a molten NaBr
The minimum theoretical
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
2Na+ + 2Br- → Br2(g)+ 2Na(s)
voltage MTV required to
force this
nonspontaneous reaction
to occur is the negative
of the cell potential.
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
E0 = -3.80 v MTV = +3.80 v
1.
Draw and completely analyze a molten NaBr
Electrons flow through
electrolytic cell.
Power Source
+
Pt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
2Na+ + 2Br- → Br2(g)+ 2Na(s)
the wire from anode to
cathode.
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
E0 = -3.80 v MTV = +3.80 v
1.
Draw and completely analyze a molten NaBr
Electrons flow through
electrolytic cell.
e-
the wire from anode to
cathode.
Power Source
+
ePt
Pt
_
reduction
cathode
2Na+ + 2e- → 2Na(l)
-2.71 v
2Na+ + 2Br- → Br2(g)+ 2Na(s)
Na+
Br-
+
oxidation
anode
2Br- → Br2(g)+ 2e-1.09 v
E0 = -3.80 v MTV = +3.80 v
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Add the ions.
(aq) or M
or solution
means water.
Pt
Pt
K+
IH2O
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Label the -, +,
anode, cathode,
oxidation, and
reduction.
Pt
Pt
K+
IH2O
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
reduction
Label the -, +,
anode, cathode,
oxidation, and
reduction.
K+
IH2O
+
Anode
oxidation
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
reduction
The cation and
water migrate to the
cathode
K+
IH2O
+
Anode
oxidation
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
reduction
The cation and
water migrate to the
cathode
K+
IH2O
+
Anode
oxidation
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
reduction
The cation or water
reduces. The higher
one on the chart is
most spontaneous
and occurs.
K+
IH2O
+
Anode
oxidation
Cl2
+
1/2O2 +
2e-
→ 2Cl-
2H+(10-7M)
+ 2e-
2H2O + 2e-
1.36 v
→ H20
→ 2H2 + 2OH-
-0.42 v
Zn2+
+
2e-
→ Zn(s)
-0.76 v
K+
+
1e-
→ K(s)
-2.93 v
0.82 v
Cl2
+
1/2O2 +
2e-
→ 2Cl-
1.36 v
2H+(10-7M)
→ H20
0.32 v
Reduction of water
2H2O + 2e-
→ 2H2 + 2OH-
-0.42 v
Zn2+
+
2e-
→ Zn(s)
-0.76 v
K+
+
1e-
→ K(s)
-2.93 v
Cl2 +
2e-
→ 2Cl-
1/2O2 +
2H+(10-7M)
→ H20
0.32 v
Oxidation of water
1.36 v
Reduction of water
2H2O + 2e-
→ 2H2 + 2OH-
-0.42 v
Zn2+
+
2e-
→ Zn(s)
-0.76 v
K+
+
1e-
→ K(s)
-2.93 v
Cl2 +
2e-
→ 2Cl-
1/2O2 +
2H+(10-7M)
→ H20
0.32 v
Oxidation of water
1.36 v
Reduction of water
2H2O + 2e-
Zn2+
Reduction of K+
K+
+
→ 2H2 + 2OH-
-0.42 v
→ Zn(s)
-0.76 v
+
2e-
1e-
→ K(s)
-2.93 v
Cl2
+
→ 2Cl-
2e-
1.36 v
→ H20
0.32 v
Oxidation of water
1/2O2 + 2H+(10-7M)
strongest oxidizing agent or highest
select most spontaneous reaction
Reduction of water
→ 2H2(g) + 2OH- -0.42 v
2H2O + 2e-
Overpotential Effect- treat water as if it were just below Zn
Zn2+
Reduction of K
K+
+
2e-
→ Zn(s)
-0.76 v
+
1e-
→ K(s)
-2.93 v
The overpotential effect is a higher than normal voltage
required for the half reaction. This is often due to extra
voltage required to produce a gas bubble in solution.
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
Reduction
The cation or water
reduces. The higher
one on the chart is
most spontaneous
and occurs.
K+
IH2O
+
Anode
oxidation
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
Reduction
2H2O+2e- → 2H2+ 2OH-0.41 v
The cation or water
reduces. The higher
one on the chart is
most spontaneous
and occurs.
K+
IH2O
+
Anode
oxidation
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
Reduction
2H2O+2e- → 2H2+ 2OH-0.41 v
The anion + water
goes to the anode.
K+
IH2O
+
Anode
oxidation
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
Reduction
2H2O+2e- → 2H2+ 2OH-0.41 v
For oxidation the
most spontaneous
reaction is found on
the redox chart and
is lowest.
K+
IH2O
+
Anode
oxidation
→ 2Cl-
Cl2 + 2e-
1.36 v
1/2O2 + 2H+(10-7M) + 2e- → H20
0.82 v
Oxidation of water
I2(s)
→
+ 2e-
2I-
0.54 v
→ 2H2 + 2OH-
-0.42 v
Reduction of water
2H2O + 2e-
Zn2+
+
2e-
→ Zn(s)
-0.76 v
K+
+
1e-
→ K(s)
-2.93 v
→ 2Cl-
Cl2 + 2e-
1/2O2(g) + 2H+(10-7M) →
I2(s)
→
+ 2e-
1.36 v
H20
0.82 v
Oxidation of water
2I0.54 v
Oxidation of I-
Reduction of water
2H2O + 2e-
→ 2H2 + 2OH-
-0.42 v
Zn2+
+
2e-
→ Zn(s)
-0.76 v
K+
+
1e-
→ K(s)
-2.93 v
→ 2Cl1.36 v
overpotential effect means water is here
Cl2 + 2e-
1/2O2 + 2H+(10-7M) →
H20
0.82 v
Oxidation of water
→
2I0.54 v
Oxidation of I-
I2(s)
+ 2e-
Reduction of water
2H2O + 2e-
→ 2H2 + 2OH-
-0.42 v
Zn2+
+
2e-
→ Zn(s)
-0.76 v
K+
+
1e-
→
-2.93 v
K(s)
→ 2Cl1.36 v
overpotential effect means water is here
Cl2 + 2e-
1/2O2 + 2H+(10-7M) →
I2(s)
H20
0.82 v
Oxidation of water
→
+ 2e-
2I0.54 v
Oxidation of Ipick strongest reducing agent- lower
Reduction of water
2H2O + 2e-
→ 2H2 + 2OH-
-0.42 v
Zn2+
+
2e-
→ Zn(s)
-0.76 v
K+
+
1e-
→
-2.93 v
K(s)
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
Reduction
2H2O+2e- → 2H2+ 2OH-0.41 v
For oxidation the
most spontaneous
reaction is found on
the redox chart and
is lowest.
K+
IH2O
+
Anode
Oxidation
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
Reduction
2H2O+2e- → 2H2+ 2OH-0.41 v
For oxidation the
most spontaneous
reaction is found on
the redox chart and
is lowest.
K+
IH2O
+
Anode
Oxidation
2I- → I2(s) + 2e-0.54 v
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Pt
Pt
Cathode
Reduction
2H2O +2e- → 2H2+ 2OH-0.41 v
Write the overall
reaction with the
cell potential.
K+
IH2O
+
Anode
Oxidation
2I- → I2(s) + 2e-0.54 v
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
Power Source
+
Write the overall
reaction with the
cell potential.
Pt
Pt
Cathode
Reduction
2H2O+2e- → 2H2+ 2OH-0.41 v
K+
IH2O
2H2O+ 2I- → 2H2+ I2(s) + 2OH-
+
Anode
Oxidation
2I- → I2(s) + 2e-0.54 v
E0 = -0.95 v
2. Draw and completely analyze a 1.0 M KI electrolytic cell.
e-
Power Source
+
Write the overall
reaction with the
cell potential.
ePt
Pt
Cathode
Reduction
2H2O+2e- → H2+ 2OH-0.41 v
+
Anode
Oxidation
K+
IH2O
2H2O+ 2I- → H2+ I2(s) + 2OH-
2I- → I2(s) + 2e-0.54 v
E0 = -0.95 v
MTV = +0.95v