( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
We have learnt that ionic compounds conduct electricity when
molten or aqueous.
When an electric current passes through such compounds, the
compounds are decomposed in a chemical reaction. This is
known as electrolysis.
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The ionic compound is called an electrolyte.
In this lesson, we will learn about the electrolysis of
• Molten ionic compounds
• Aqueous ionic compounds
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
Let us look at the electrolysis of molten lead (II) bromide. The experimental
setup is shown below.
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Carbon electrodes
Porcelain crucible
Molten
lead(II) bromide
Clay triangle
Heat
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
ELECTRODES
( Worksheet ) ( Exit )
Anode
Cathode
•The electrode attached to the
positive terminal of the cell.
•The electrode attached to the
negative terminal of the cell.
•Anions are attracted to it.
•Cations are attracted to it.
Electrodes are usually made of carbon or platinum, as they are
unreactive or inert. They do not react with the compounds in
electrolysis.
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
At the cathode
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The lead ions take electrons from the cathode to become lead
atoms (reduction).
Pb2+
+
Pb2+
+
ee-
2e-
Pb
Pb
Lead forms molten globules at the bottom of the crucible. We
say the lead ions have been discharged.
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
At the anode
The bromide ions donate electrons to the anode to become
bromide molecules (oxidation).
Br Br
Br
+
e-
e-
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Br -
2Br -
Br 2
+
2e-
The bromine is seen as a reddish-brown gas around the anode
during the electrolysis.
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( Introduction )
( Molten electrolyte )
The overall chemical reaction is
( Aqueous electrolyte ) ( Summary )
PbBr2
Pb
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cathode
Lead ions are
attracted to the
cathode
+
Br2
anode
Pb2+
Br -
Pb2+
Br Molten lead (II) bromide
Bromide ions are
attracted to the
anode
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
cathode
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Lead ions move
near to cathode
anode
Pb2+
Br -
Pb2+
Br Molten lead (II) bromide
Bromide ions
move near to
anode
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
cathode
anode
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ePb2+
Br -
Pb2+
Br -
e-
Molten lead (II) bromide
Bromide ions
donate electrons
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
ecathode
e-
anode
Electrons flow
from anode to
battery
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ePb2+
Br -
Pb2+
Br -
e-
Molten lead (II) bromide
Bromide ions
donate electrons
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
Battery pumps electrons
cathode
e-
ee-
anode
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ePb2+
Br -
Pb2+
Br -
e-
Molten lead (II) bromide
Bromide ions
donate electrons
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( Introduction )
( Molten electrolyte )
cathode
e-
( Aqueous electrolyte ) ( Summary )
ee-
e-
Electrons flow
from battery to
cathode
anode
( Worksheet ) ( Exit )
ePb2+
Br -
Pb2+
Br -
e-
Molten lead (II) bromide
Bromide ions
donate electrons
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( Introduction )
( Molten electrolyte )
cathode
Electrons flow
from battery to
cathode
e
ee-
( Aqueous electrolyte ) ( Summary )
ee-
ee-
e-
( Worksheet ) ( Exit )
Lead ions accept
electrons
anode
Pb2+
Pb2+
ee-
Br -
e-
Bromide ions
donate electrons
Br Molten lead (II) bromide
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
cathode
( Worksheet ) ( Exit )
Lead atoms are
produced
anode
Br
Mg
Pb
Cl
Br
Mg
Pb
Cl
Br
Br
Molten lead (II) bromide
Bromine atoms
combine to form
bromine gas
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
QUIZ 1
( Worksheet ) ( Exit )
1. The electrolysis of molten iron (III) chloride yields iron metal
and a gas. What is the colour of this gas produced?
A.
Reddish-brown.
B.
Colourless.
C.
Yellowish-green.
Click on the correct answer
Next question
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
QUIZ 1
( Worksheet ) ( Exit )
2. With reference to the previous question, at which electrode is
the gas formed?
A.
Anode.
B.
Cathode.
Click on the correct answer
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
Solutions can be electrolysed using the apparatus shown
below.
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test-tubes
electrolyte (solution)
carbon electrodes
-
+
to battery
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
Let us look at the electrolysis of dilute hydrochloric acid. The
experimental setup is shown below.
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test-tubes
dilute hydrochloric acid
carbon electrodes
-
+
to battery
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
At the cathode
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The hydrogen ions take electrons from the electrode to become
hydrogen atoms (reduction).
H+
+
e-
H
H+
+
e-
H
When two of the newly formed atoms combine, hydrogen gas
is produced.
H
+
H
H H
H
+
H
H2
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
At the anode
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There are two anions in the electrolyte: hydroxide and chloride
ions. Since hydroxide ions are easier to discharge, oxygen gas
is produced at the anode.
H H
OH e
O
OH
O O
+
+
eOH eeOH -
-
4OH
O2
+
2H2O
+
4e4
( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
The electrolysis of dilute hydrochloric acid is will result in the production of
hydrogen gas and oxygen at the cathode and anode respectively.
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hydrogen
oxygen
dilute hydrochloric acid
carbon electrodes
-
+
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
What gas do you think will be found at the anode when
aqueous copper(II) sulphate is electrolysed?
Sulphur dioxide?
No, not quite. Oxygen gas is evolved at the anode instead and
copper metal is deposited at the cathode.
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How do you explain this phenomenon?
The products can come from the electrolyte or from the water
present. The product that is discharged depends on the nature
of the ions.
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
Negative ions from the electrolyte are discharged if they are
chloride, bromide or iodide ions. For sulphates and nitrates,
oxygen from water is discharged.
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Positive ions from the electrolyte that are below Ni2+(aq) in the
electrochemical (or reactivity) series are discharged at the
negative cathode.
If the positive ions are those of reactive metals above Ni2+(aq)
(e.g.. Na+, K+ and Ca2+), hydrogen gas from water is
discharged.
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( Worksheet ) ( Exit )
( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
Anions
Cations
K+ (aq)
SO42- (aq)
Na+ (aq)
NO3- (aq)
Ca2+ (aq)
Cl - (aq)
Mg2+ (aq)
Br - (aq)
Zn2+ (aq)
I - (aq)
Fe2+ (aq)
OH - (aq)
Pb2+ (aq)
SO42- (aq)
Difficulty of
discharge
decreases
H+ (aq)
Cu2+ (aq)
Ag+ (aq)
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
QUIZ 2
( Worksheet ) ( Exit )
1. When we electrolyse a solution, what takes place at the
cathode?
A.
Oxidation.
B.
Reduction.
Click on the correct answer
Next question
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
QUIZ 2
( Worksheet ) ( Exit )
2. During the electrolysis of sulphuric acid, hydrogen gas and
another gas are produced. What is the other gas?
A.
Oxygen.
B.
Sulphur dioxide.
Click on the correct answer
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
•The decomposition of a substance by electricity is
called electrolysis.
( Worksheet ) ( Exit )
•An electrolyte is an ionic compound, in the molten or
aqueous state, that conducts electricity and is
decomposed by the current.
•The rods through which the direct current enters and
leaves the cell are known as electrodes. Electrodes
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are usually inert.
( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
( Worksheet ) ( Exit )
•The anode is the electrode which is connected to the
positive terminal of a cell. Anions are attracted to it.
Oxidation occurs at the anode.
•The cathode is the electrode connected to the
negative terminal of the cell. Cations are attracted to
the cathode. Reduction occurs at this electrode.
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
When a molten electrolyte is electrolysed,
•a metal (from the positive ions) is discharged at the
cathode.
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•a non-metal (from the negative ions) is discharged at
the anode.
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
When a aqueous electrolyte is electrolysed,
•the products come from either the electrolyte or water
present.
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•The product at the cathode is a metal or hydrogen
gas.
•The product at the anode is a non-metal.
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
•Reactive metals are not discharged at the cathode.
Instead, hydrogen from water is evolved.
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•Sulphate and nitrate ions are not discharged at the
anode. Instead, oxygen from water is produced.
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( Introduction )
( Molten electrolyte )
( Aqueous electrolyte ) ( Summary )
Credits
Prepared by
Anita Anusha
and
Tang Yun Kwee Rosemary
20 October 1999
DIVISION OF INSTRUCTIONAL SCIENCE
copyright 1999
return to
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( Worksheet )
( Exit )
national institute of education
micro
lessons
Redesigned and edited by Adeline Yong Yean Pin
Supervised by Mr Ashley Tan