Electrolysis for IGCSE chemistry
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Conductors are those substances which can pass current through them. Eg: Copper wire,
Sodium chloride solution.
Insulators do not conduct electric current. Eg: Plastics, Rubber, wood, water
In metal structure, there are free mobile electrons. These electrons carry electric current in
metals.
So metals are conductors
Ionic compounds are those compounds made up of positive and negative ions. Ionic
compounds pass electric current when they are in the molten state or in solution. In the
solid state, these ions are not free to move and electric current is not passed. By melting or
dissolving in the water, the ions become free and then ionic compounds conduct electric
current. They are called ionic conductors. Ionic conductors while passing electric
current, undergo chemical change.
The above process is called electrolysis.
Electronic conductors (metals/graphite)
Electrons are responsible to pass current
Conduct current both in solid and molten state
No chemical change takes place when electric
current is passed through
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Ionic conductors (ionic compounds in liquid state)
Ions are responsible to pass current
Conduct current only when in molten state or in
solution
Chemical change takes place
Electrolysis is the process of passing electric current through electrolyte to cause a
chemical change.
Electrolyte is the substance which undergoes chemical change during electrolysis.
Electrodes are used to send the electric current into the electrolyte and to take electric
current out of the electrolyte
Anode is the electrode connected to the positive terminal of a battery (power source)
Cathode is the electrode connected to the negative terminal of a battery.
Positive ions move towards cathode during electrolysis.
Negative ions move towards the anode during electrolysis.
Always reduction (gain of electrons) takes place at cathode.
Always oxidation (loss of electrons) takes place at anode.
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The products formed at the electrodes depend on the type of electrodes used and the
kind of electrolyte used in the electrolysis.
Using inert electrodes (Graphite/Platinum)
1. Molten sodium chloride NaCl
Ions present: Na+ (sodium ions) and Cl-(chloride ions). Sodium ions move to cathode
and chloride ions to anode. (attraction of opposite charges)
Cathode: sodium ions receive one electron each and become sodium atom (reduction)
Na+ + e-  Na
Anode: Chloride ions lose one electron each and become greenish chlorine gas
(oxidation)
2Cl-  Cl2 + 2e2. Molten lead bromide PbBr2
Ions present: Pb2+ (lead ions) and Br-(bromide ions). Lead ions move to cathode and
bromide ions to anode. (attraction of opposite charges)
Cathode: Lead ions receive two electrons each and become lead atoms (reduction) –
grey metal
Pb2+ + 2e-  Pb
Anode: Bromide ions lose an electron each and become bromine (oxidation) red-brown
gas
2Br-  Br2 + 2e3. Concentrated acids (hydrochloric acid)
Ions present: H+ (hydrogen ions), Cl- (chloride ions), OH- (hydroxide ions) –Hydrogen
ions and hydroxide ions come from water; hydrogen ions and chloride ions come from
hydrochloric acid)
Hydrogen ions move to cathode and chloride ions and hydroxide ions move to anode.
Cathode: Hydrogen ions accept an electron each and become hydrogen gas
(reduction)
2H+ + 2e-  H2
Anode: chloride ions lose an electron each become chlorine gas (oxidation)
2Cl-  Cl2 + 2eIf sulphuric acid or nitric acid used, the product at anode is Oxygen.
2O2 O2 + 4e4. Sodium chloride solution
Ions present: H+ (hydrogen ions), OH- (hydroxide ions), Na+ (sodium ions), Cl(chloride ions)
(hydrogen ions and hydroxide ions are from water; sodium ions and chloride ions from
sodium chloride)
Hydrogen ions and sodium ions move to cathode; hydroxide ions and chloride ions
move to anode.
Cathode: Hydrogen ions gain an electron each and become hydrogen gas (reduction)
2H+ + 2e-  H2
Anode: Chloride ions lose an electron each and become chlorine.
2Cl Cl2 + 2eThe remaining solution contains sodium ions and hydroxide ions. This is a method to
make sodium hydroxide
Note: Metal or hydrogen will be always collected at the cathode
If Sulphate, hydroxide, nitrate at anode, hydroxide get the chance to be collected and
oxygen gas from it will be collected. If chlorine or bromine present, they get the chance
to be collected as chlorine gas or bromine vapour. If the solution is too dilute, oxygen
gas is collected at cathode.
Using metal electrodes other than platinum
5. Electrolysis of copper(II)sulphate solution using copper electrodes.
Copper atoms from the anode lose two electrons each, become copper ions Cu 2+ and
goes into the electrolyte. At cathode, Copper ions Cu2+ accept two electrons each and
become copper atoms.
Anode:
Cu  Cu2+ + 2e- (oxidation)
Cathode:
Cu2+
Summary
Electrolyte
Molten sodium
chloride
Molten
Lead(II)bromide
Sodium
chloride
solution
Acids (HCl)
Copper(II)
sulphate
solution
Copper
(II)sulphate
solution
Silver nitrate
solution
Silver nitrate
solution
+ 2e-  Cu (reduction)
Electrode
Inert
Ions
Na+ Cl-
Anode
Chlorine
Cathode
Sodium
Inert
Pb2+ Br-
Bromine
Lead
Inert
Na+ ClH+ OH-
Chlorine
Hydrogen
Cl- (SO4;; NO -)
3
H+ OHCu2+ SO42H+
OH-
Chlorine
(oxygen)
Hydrogen
Oxygen
Copper
Copper
Cu2+ SO42H+
OH-
Copper
Inert
Ag+ NO3H+
OH+
Ag NO3H+
OH-
Cu atoms
come out of
electrode as
Cu2+
Oxygen
Ag atoms come
out of electrode
as Ag+
Silver
Inert
Inert
Silver
H+
Silver
Uses of electrolysis
1. Highly reactive metals such as sodium, potassium calcium, magnesium and
aluminium are made from their ores by electrolysis. When electrolyse the molten
ore, the metal will be collected at the cathode.
2. Some industrially important substances are made by electrolysis:
Electrolysis of concentrated sodium chloride gives chlorine gas (used to kill germs,
for bleaching, as an antiseptic, making plastics, dyes, medicines etc.,)is collected at
anode; Hydrogen gas (used to manufacture ammonia, to make margarine, in
hydrogen fuel cell etc.,) is collected at cathode and the solution left will be sodium
hydroxide (used in plastics, making soaps, detergents, paper making, removal of
paint, food processing etc.,).
3. Purification of metals
Copper must be pure to make electric cable. Copper extracted from its ore is not
pure. To purify the copper:
Use copper(II)sulphate solution as electrolyte, impure copper as anode and a pure
piece of copper as cathode. During electrolysis, impure copper from anode go to the
solution as copper ions and deposit as pure copper atoms on the cathode.
Anode:
Cu  Cu2+ + 2e- (oxidation)
Cathode:
Cu2+
+ 2e-  Cu (reduction)
4. Electroplating
Electroplating is method to put a coating of a metal over another. Electroplating
helps to protect the inner metal from corrosion (stainless steel) and makes thing
more attractive (silver/gold plated ornaments)
To electroplate silver over an iron ring we can use electroplating. Take a solution of
silver nitrate as electrolyte, a pure piece of silver as anode and the iron ring as
cathode. Silver comes out of anode as silver ions and form a coating of silver on the
cathode (iron ring).