Electrolytic
Cells
An electrolytic cell uses electricity to
force a non-spontaneous redox
reaction to take place.
In some cases, an electrolytic cell can
be set up to reverse the reaction of a
voltaic cell
This can “recharge” the battery
The Daniell cell is a voltaic cell made
up of copper and zinc half-cells
Electrons spontaneously flow from
Zn anode to Cu cathode
To recharge the cell, a power supply is
added which reverses the flow of the
electrons
The power supply “pulls” electrons
from the Cu and “pushes” them onto
the Zn
This forces the oxidation of the Cu and
the reduction of the Zn2+
The power supply forces the Cu to act
as the anode and the Zn to act as the
cathode
Comparison of Voltaic and Electrolytic Cells
Voltaic Cells
Electrolytic Cells
spontaneous reaction
non-spontaneous reaction
converts chemical energy to
electrical energy
oxidation (at anode):
Zn(s) → Zn2+(aq) + 2e−
reduction (at cathode):
Cu2+(aq) + 2e− → Cu(s)
converts electrical energy to
chemical energy
oxidation (at anode):
Cu(s) → Cu2+(aq) + 2e−
reduction (at cathode):
Zn2+(aq) + 2e− → Zn(s)
cell reaction:
Zn(s)+Cu2+(aq) → Zn2+(aq)+Cu(s)
cell reaction:
Zn2+(aq)+Cu(s) →Zn(s)+Cu2+(aq)
standard cell potential:
Eonet = Eocathode - Eoanode
Eonet = Eocopper - Eozinc
Eonet = +1.10 V
standard cell potential:
Eonet = Eocathode - Eoanode
Eonet = Eozinc - Eocopper
Eonet = −1.10 V
A negative value for the Eonet means
that the reaction is non-spontaneous
This represents the minimum energy
that the power supply must provide to
drive the reaction
Most electrolytic reactions require a
minimum amount of electrical energy
that is somewhat larger than indicated
by the Eonet
This additional amount of energy is
known as the cell’s overvoltage
Example:
The Potassium Iodide
Electrolytic Cell
Electrodes: Inert C(s)
Electrolyte: KI(aq)
SOA: H2O(l)
SRA:I−(aq)
Cathode:
2 H2O(l) + 2 e− → H2(g) + 2 OH−(aq)
Anode:
2 I−(aq) → I2(s) + 2 e−
Net Cell:
2 H2O(l) + 2 I −(aq) → H2(g) + 2 OH−(aq) + I2(s)
Cathode Observations
Bubbles of gas appear
Increase in pH
Anode Observations
Reddish-brown color appears
Cell potential calculation:
Eonet = Eocathode − Eoanode
Eonet = Eowater − Eoiodide
Eonet = (−0.83 V) − (+0.54 V)
Eonet = − 1.37 V
A minimum of 1.37 V must be input
before the reaction takes place.
The Chloride Anomaly
In aqueous solutions containing H2O(l)
and Cl−(aq) as the only possible RAs,
H2O(l) should be the substance being
oxidized
However, the overvoltage required for
the oxidation of H2O(l) is so large that
the Cl−(aq) ions are in fact oxidized
more easily
As electricity is supplied to this
aqueous solution, the Cl−(aq) will be
oxidized to Cl2(g) instead of H2O(l)
being oxidized into O2(g) and H+(aq)
Predict the most likely reaction to take
place when electricity is passed into a
solution of cobalt(II) chloride