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Oxidation and Reduction
Mrs Faraziehan Senusi
Corrosion & Prevention
Reference: Chemistry: the Molecular Nature of Matter and Change,
6th ed, 2011, Martin S. Silberberg, McGraw-Hill
• The principle of an electrolytic cell is: electrical
energy from an external source drives a
• This process is called electrolysis.
• In voltaic cell, Sn anode will gradually become oxidized to
Sn2+ ions, and the Cu2+ ions will gradually be reduced and plate
out on the Cu cathode because the cell reaction is spontaneous
in that direction:
• Therefore, the reverse cell reaction is nonspontaneous and
never happens, as the negative E°cell and positive ΔG°
• However, we can make this process happen by supplying
from an external source an electric potential greater than
• In effect, we have converted the voltaic cell into an
electrolytic cell and changed the nature of the electrodesanode is now cathode, and cathode is now anode
• Note that in an electrolytic cell, oxidation takes place at the
anode and reduction takes place at the cathode, but the
direction of electron flow and the signs of the electrodes are
• In a voltaic cell, electrons are generated at the anode, so it is
negative, and electrons are consumed at the cathode, so it is
• In an electrolytic cell, the electrons come from the external
power source, which supplies them to the cathode, so it is
negative, and removes them from the anode, so it is
• Lets consider: THE ELECTROLYSIS OF MOLTEN
• Solid sodium chloride does not conduct electricity.
However, molten (melted) NaCl is an excellent conductor
because its ions are freely mobile. The melting point is
• Consider a cell in which a source of direct current is
connected by wires to two inert graphite electrodes. They
are immersed in a container of molten (melted) sodium
chloride. When the current flows, we observe the following:
A pale green gas, which is chlorine, Cl2, is liberated at one
Molten, silvery white metallic sodium, Na, forms at the other
electrode and floats on top of the molten sodium chloride.
• Chlorine must be produced by oxidation of Cl ions, and
the electrode at which this happens must be the anode.
• Metallic sodium is produced by reduction of Na ions at the
cathode, where electrons are being forced into the cell.
• The formation of metallic Na and gaseous Cl2 from NaCl
is nonspontaneous except at temperatures very much
higher than 801°C.
• The direct current (dc) source must supply electrical
energy to force this reaction to occur.
The Downs Cell …
Industrial method electrolytic cell of molten NaCl electrolysis
The iron screen prevent Na to spontaneously react with Cl2
Most practical, highly effective method of obtaining metallic Na, but
operating cost is very high (construction cost, electricity cost, heating cost)
FARADAY’S LAW OF
• The amount of substance that undergoes oxidation or reduction
at each electrode during electrolysis is directly proportional to
the amount of electricity that passes through the cell.
Faraday’s Law of Electrolysis
• A quantitative unit of electricity is called the faraday.
• A smaller electrical unit commonly used is the coulomb (C).
• One coulomb is defined as the amount of charge that passes a
given point when 1 ampere (A) of electric current flows for 1
FARADAY’S LAW OF
• Problems based on Faraday's law often ask you to calculate
current, mass of material, or time.
FARADAY’S LAW OF
How long does it take to produce 3.0g of Cl2 (g) by
electrolysis of aqueous NaCl using a power supply with a
current of 12A?
From half-reaction, we loss of 2 mol of electrons produces 1 mol of
So, we find the total charge:
Then, use relationship between charge and current to find time needed: