Download 7.5 The Electrolytic Cell

7.5 The Electrolytic Cell
•
The Structure and Function
of the Electrolytic Cell
•
Comparing Electrochemical
and Electrolytic Cells
•
Electrolytic Cell Types
•
Electrolytic Applications
7.5 The Electrolytic Cell
The Structure and Function of the
Electrolytic Cell
A chemical is an electrolyte if its aqueous solution conducts
electricity.
Whenever electricity is conducted through a molten electrolyte
or an electrolyte solution, the process is called electrolysis and
the apparatus is called an electrolytic cell.
Electrolytic cells transform electrical energy into chemical
energy.
7.5 The Electrolytic Cell
The Structure and Function of the
Electrolytic Cell
(Graphite)
Ni2+ + 2e → Ni
(Graphite)
2I ͞ → I2 + 2e
7.5 The Electrolytic Cell
The Structure and Function of the
Electrolytic Cell
The circuit is completed as cations in the molten electrolyte or
electrolyte solution pick up the DC source’s electrons while anions
or the anode itself give up electrons that return to the DC source.
The reactions of electrolytic cells are non-spontaneous (negative
Eo). The strongest available reducing agent passes an electron(s)
down to the strongest available oxidizing agent in the SRP table.
7.5 The Electrolytic Cell
The Structure and Function of the
Electrolytic Cell
The net Eo provides an estimation of how much voltage will be
required to drive the electrolytic cell but this value will normally
be less than the voltage actually required. The difference between
the voltages the SRP Table predicts will be necessary to drive
electrolytic cells and the voltages actually required to drive those
cells is referred to as overpotential.
7.5 The Electrolytic Cell
Comparing Electrochemical and Electrolytic Cells
Electrochemical Cell
Electrolytic Cell
exothermic
‘makes’ electricity
transforms chemical energy into electrical energy
endothermic
‘takes’ electricity
 transforms electrical energy into chemical
energy
is a DC voltage source
requires a DC voltage source
2 half-cells
1 cell
spontaneous redox reaction
E° is positive
non-spontaneous redox reaction
E° is negative
salt bridge or equivalent
no salt bridge
7.5 The Electrolytic Cell
V
Comparing Electrochemical and Electrolytic Cells
Electrochemical Cell
Electrolytic Cell
V
DC
The oxidation half-reaction is below the reduction
half-reaction in the SRP table.
Think of the electrons floating upward.
The oxidation half-reaction is above the reduction
half-reaction in the SRP table.
Think of the electrons being pushed downward.
The half-reactions which are the furthest apart in
the SRP table will occur, generating the greatest
possible voltage.
The half-reactions which are the closest together in
the SRP table will occur, requiring the least
possible voltage to drive the cell.
text pages 466-467
6.5 The Electrolytic Cell
Electrolytic Cell Types
Type 1 cells - inert electrodes immersed in a molten ionic compound.
Only the molten ions (1 type of thing) can be oxidized and reduced.
Type 2 cells - inert electrodes immersed in an aqueous ionic solution.
Only the ions or H2O (2 types of things) can be oxidized and reduced.
Be mindful of the overpotential effect.
Type 3 cells - non-inert electrodes immersed in an aqueous ionic solution.
The ions, H2O, or the electrodes themselves (3 types of things) can be
oxidized and the ions or H2O can be reduced. Be mindful of the
overpotential effect.
7.5 The Electrolytic Cell
Electrolytic Applications
Electrowinning is a metallurgical term for the electrolytic
recovery of a metal from a solution containing its ions.
Electroplating is a form of electrowinning in which a
conductive material, usually a metal, is coated with a thin layer
of a different metal.
Electrorefining is the electrolytic purification of a metal.
7.5 The Electrolytic Cell
Electrolytic Applications
The Héroult-Hall Process produces aluminum by electrolyzing
a molten mixture of aluminum oxide and cryolite which melts at
a much lower temperature than the aluminum oxide alone.
The chloralkali industry refers to the industrial electrolysis of
aqueous sodium chloride to produce sodium hydroxide and
chlorine.
In impressed current cathodic protection the metal being
protected is the cathode of an electrolytic cell, i.e. its hooked up
to the negative electrode of a DC source.