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Unit 21 Electrolysis
Teaching Notes
21.5 Electrolysis of acidified water using platinum electrodes
Electrolysis of acidified water using platinum electrodes
The ratio of Volume of H2 : Volume of O2 may not be exactly 2:1. It is because oxygen is
more soluble in water than hydrogen. Therefore the volume of oxygen collected would be
smaller than expected.
Carbon electrodes used in electrolysis
Carbon is commonly used as electrodes in electrolysis because

it is a good conductor of electricity;

it does not form ions and is not attacked by chlorine;

it can be used in the electrolysis of hot molten compounds because of its very high
melting point;

it is cheap.
Electrolysis of very dilute sodium chloride solution using carbon electrodes
The ratio of Volume of H2 : Volume of O2 may not be exactly 2:1. It is because

oxygen is more soluble in water than hydrogen;

some of the oxygen liberated would react with the carbon electrode to give carbon
dioxide which is fairly soluble in water.
The volume of oxygen collected would thus be much smaller than expected.
21.6
Factors affecting the electrolysis of aqueous solutions
Rate of electrolysis
Electrolysis, in general, can be speeded up by increasing the electric current flowing
through the circuit. Some possible methods are:
1
to increase the voltage applied for electrolysis.
2
to decrease the total resistance of the circuit. This can be done by:

moving the electrodes closer together;

pushing the electrodes deeper into solution so that more surface area will be in
contact with the solution;

using electrodes of larger surface area;

decreasing the resistance in the rheostat, if any;

increasing the concentration of the electrolyte.
Electrolysis of dilute or concentrated sodium chloride solution using carbon electrodes
If universal indicator is added to the solution, the solution near the cathode would turn blue
and that near the anode would turn red for a while and then colourless. This is because
some chlorine gas liberated dissolves in water to give hydrochloric acid and hypochlorous
acid . The whole solution turns colourless due to the bleaching action of hypochlorous acid.
Cl2(g) + H2O(l)
HCl(aq) + HOCl(aq)
hydrochloric
acid
hypochlorous
acid
Electrolysis of dilute copper(II) sulphate solution using copper electrodes
When copper or metals more reactive than copper are made the anode in electrolysis,
they usually dissolve and form ions. These electrodes are called active electrodes as they
take part in the electrolysis. For example, during the electrolysis of dilute zinc nitrate
solution with zinc as anode, the zinc anode would dissolve to form zinc ions.
Zn(s)
Zn2＋(aq) + 2e－
Electrolysis of concentrated sodium chloride solution using a mercury cathode
Hydrogen has a high over-voltage at the mercury cathode. It is much more difficult for H ＋
(aq) to discharge at a mercury cathode than at a cathode made of platinum. This together
with the concentration effect of Na＋(aq), would make the Na＋(aq) to be preferentially
discharged.
21.7
Industrial uses of electrolysis
Factors contributing to the quality of the metal coating formed
A number of factors contribute to the quality of the metal coating formed in electroplating.

In the plating solution, the concentration of the cations to be reduced is carefully
controlled and unwanted side reactions must be avoided.

The type and concentration of electrolyte also needs careful consideration.

The solution must contain compounds to control the acidity and increase the
conductivity.

In order to obtain a smooth and bright deposit of metal and reduce the grain size of
the deposit, additives known as brighteners and leveling agents are added to the
electrolytic bath.

In many electroplating cells the anode must be shaped like the object at the cathode
in order to achieve an even metal coating.
Silver plating a teapot
Refer to the figure below. The electrolyte is silver diammine chloride [Ag(NH3)2]Cl(aq).
The species present in the electrolyte are:
[Ag(NH3)2]＋(aq), Cl－aq), H2O(l), Ag(s)
At the anode
Ag(s) + 2NH3(aq)
[Ag(NH3)2]＋(aq) + e－
This means that the electrode gradually dissolves and must be periodically replaced.
At the cathode
[Ag(NH3)2]＋(aq) + e－
Ag(s) + 2NH3(aq)
The teapot is coated with silver crystals. In fact, a completely direct current is not
used. A current that spends a small fraction of the time in the reverse direction in repeated
cycles is used. This causes the uneven ends of the crystals to dissolve, as the teapot
temporarily becomes the anode. Being longer, the undesired ends are the first to be
oxidized back to silver ions. This levels out the crystal growth.
Chrome-plating
Steel bumper bars are often plated with chromium metal to give them a more attractive
appearance, in addition to providing them with a protective coating so that the underlying
metal would not be exposed to corrosion.
To get a good chrome-plating it is often necessary to pre-plate. Bumper bars are
plated with copper or nickel before being chromed, since chromium adheres better to
copper or nickel than to iron.
There are usually four stages in the chromium-plating of a steel object:

The steel object, which is made the cathode, is cleaned with sulphuric acid and then
washed with deionized water.

The cathode is plated with copper.

The object is then plated with nickel to prevent corrosion.

Finally, the object is chromium-plated.
Properties and uses of electroplating of some metals
Metal
Properties of coating
Uses
Chromium  Very bright and persistent
surface
 Thin coatings for decorative purposes,
e.g. automobile bumpers, watch cases,
pens
 Corrosion and wear resistant
 Thick plating used for corrosion and
wear resistance in moving parts of
machines (e.g. rotating shafts) and
engine cylinders
Copper
 Good electrical conductivity
 Good adhesion with other
metals
 Cheap
 Used as undercoats for decorative
nickel-chromium plates
 Copper plated boards for printed circuits:
a plastic board is first coated with
copper, then electroplated with copper
 Alloy plating with other metals for
decorative purposes, e.g. brass
(copper-zinc), imitation gold
(copper-zinc-tin)
Nickel
 Resistant to alkali
 Not readily oxidized
 Bright surface
 Used as protective undercoats for
articles to be chromium-plated (i.e.
protect the articles from corrosion after
the chromium plate is scratched off)
 For decorative purposes on non-metal
articles
Tin
 Not readily corroded
 For plating steel sheet which is
extensively used in canned food industry
 Resistant to common organic
acids
 Coated on surface of printed circuit
boards and leads of electronic parts to
 Non-toxic
facilitate soldering
Zinc
 Not readily corroded
 Cheap
 Used on steel to make galvanized steel