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mohd faisol mansor/chemistry form 4/chapter 6
100
CHAPTER 6 ELECTROCHEMISTRY
Electrolytes are
substances that can
conduct electricity when
they are in molten state
and aqueous solution.
This due to the present of
free moving ions in the
electrolytes.
Electrolysis is a process
whereby compounds in
molten or aqueous state
are broken down into
their constituent
elements by passing
electricity through them.
Non-electrolytes are
substances that can not
conduct electricity when
they are in all state. This
because non-electrolyte
exist as molecule which
means contain no ions.
Example
2NaCl (l) 2Na (s) + Cl2 (g)
Sodium Metal
Chlorine Gas
mohd faisol mansor/chemistry form 4/chapter 6
101
Electrolytic Cell (molten state)
a) Electrode attach to positive terminal (battery) =
b) Electrode attach to negative terminal (battery) =
c) Anion (negative ion) discharged at electrode =
Anion will _______________ electrons.
d) Cation (positive ion) discharged at electrode =
Cation will ______________ electrons.
e) Electron flow from ___________________ to ___________________
f) Electrolytic Cell will change the _______________ energy to
________________ energy.
[ Draw the apparatus of electrolysis molten sodium chloride ]
mohd faisol mansor/chemistry form 4/chapter 6
102
Product of Electrolysis
Ion
discharged
at Cathode
Observation Half-equation Product
All ion metal
except
Copper ion
Copper ion
Hydrogen
ion
Ion
discharged
at Anode
Observation Half-equation Product
Oxide ion
Chloride ion
Bromide ion
Iodide ion
Hydroxide
ion
Gas Test
Hydrogen
gas
Oxygen gas
Chlorine gas
mohd faisol mansor/chemistry form 4/chapter 6
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Electrolysis Molten Lead(II) Bromide
1. State the ion consists in the electrolyte.
2. Which electrode is
a) Cathode =
b) Anode =
3. Which ion will be discharged at
a) Cathode =
b) Anode =
4. State the observation at
a) Cathode =
b) Anode =
5. State the product formed at
a) Cathode =
b) Anode =
6. Write the half equation at
a) Cathode =
b) Anode =
7. Draw the electron flow on the diagram above.
P
Q
mohd faisol mansor/chemistry form 4/chapter 6
104
1. State the ion present in the following electrolyte. Predict the
products from the electrolysis of some molten compound and
write the ionic equation involved.
a) Magnesium oxide
b) Copper(II) chloride
c) Lead(II) iodide
Exercise
mohd faisol mansor/chemistry form 4/chapter 6
105
2. State the meaning of the following terms.
a) Anode b) Cathode c) Electrolysis
3. A molten oxide, R2O3 is electrolysed using carbon electrodes.
a) Draw a labeled diagram to show the set-up of apparatus for
the electrolysis.
b) What ions are present in the electrolyte? Write the formulae for
the ions present in the electrolyte.
c) State the ions move to each of the electrodes during
electrolysis.
mohd faisol mansor/chemistry form 4/chapter 6
106
d) Write half equation of the reaction at each of the electrodes.
e) Name the substances formed at each of the electrodes.
f) Label the flow of electron in the diagram (a).
mohd faisol mansor/chemistry form 4/chapter 6
107
mohd faisol mansor/chemistry form 4/chapter 6
108
Electrolysis of Aqueous Solution
Aqueous solution consists of four types of ions. Two ions from the
compound and two ions from the water.
Example:
Molten sodium chloride Sodium chloride solution
Generally, there are 3 factors that may influence the selective of ions
during electrolysis of an aqueous solution
1. Position of ions in the electrochemical series
2. Concentration of ions in the electrolytes
3. Types of electrodes used in the electrolysis
mohd faisol mansor/chemistry form 4/chapter 6
109
Position of ions in the electrochemical series (ECS)
The ions that are lower in the ECS will selected to be discharged.
1. State the ion consists in the electrolyte.
2. Which electrode is
a) Cathode =
b) Anode =
3. Which ion will be discharged at
a) Cathode =
b) Anode =
4. State the observation at
a) Cathode =
b) Anode =
5. State the product formed at
a) Cathode =
b) Anode =
6. Write the half equation at
a) Cathode =
b) Anode =
7. Draw the electron flow on the diagram above.
[ Draw the apparatus of electrolysis sodium chloride solution ]
mohd faisol mansor/chemistry form 4/chapter 6
110
1.
For the electrolysis of copper(II) sulphate solution,
(a) State all the ions that are present in the electrolyte.
(b) State the ions in (a) which discharged to the
i) anode :
ii) cathode :
(c) Write a half equation for the reaction at the
i) anode :
ii) cathode :
(d) The blue colour of copper(II) sulphate solution fades if the
electrolysis is carried for a long period of time. Explain why.
Exercise
Carbon electrode
Copper(II)
sulphate solution
mohd faisol mansor/chemistry form 4/chapter 6
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2.
For the electrolysis of dilute sulphuric acid,
a) State all the ions that are present in the electrolyte
b) State the ion in (a) which discharged to
i) anode
ii) cathode
c) Write half equation for the reaction at the
i) anode
ii) cathode
d) Explain why the concentration of dilute sulphuric acid increases
gradually during the electrolysis
3. Base on the answer 1(c) and 2(c), name the process that occur
at the
a) anode
b) cathode
Dilute sulphuric
acid
Carbon electrode
mohd faisol mansor/chemistry form 4/chapter 6
112
Concentration of ions in the electrolytes
If the concentrations of particular ions are high, the ion is selectively
discharged
1. State the ion consists in the electrolyte.
2. Which electrode is
a) Cathode =
b) Anode =
3. Which ion will be discharged at
a) Cathode =
b) Anode =
4. State the observation at
a) Cathode =
b) Anode =
5. State the product formed at
a) Cathode =
b) Anode =
6. Write the half equation at
a) Cathode =
b) Anode =
7. Draw the electron flow on the diagram above.
[ Draw the apparatus of electrolysis concentrated sodium chloride solution ]
mohd faisol mansor/chemistry form 4/chapter 6
113
1.
Diagram above show the apparatus set up for the experiments of
electrolysis using two different concentration of hydrochloric acid.
a) State all the ions that are present in the electrolyte
i) Experiment A :
ii) Experiment B :
Exercise
Carbon
electrode
Dilute Hydrochloric
acid solution
Concentrated
Hydrochloric acid
solution
Experiment A
Carbon
electrode
Experiment B
mohd faisol mansor/chemistry form 4/chapter 6
114
b) State the ion in (a) which discharged to anode and cathode in
i) Experiment A :
ii) Experiment B :
c) Write half equation for the reaction at the anode and cathode in
i) Experiment A :
ii) Experiment B :
d) State the observation occur at cathode and anode in
i) Experiment A :
ii) Experiment B :
e) State the product formed at cathode and anode in
i) Experiment A :
ii) Experiment B :
mohd faisol mansor/chemistry form 4/chapter 6
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Types of electrodes used in the electrolysis
If using the active electrode at anode, ions that are present in the
electrolytes are not discharge. Instead the active electrode will
corrodes and dissolves in the electrolytes.
1. State the ion consists in the electrolyte.
2. Which electrode is
a) Cathode =
b) Anode =
3. Which ion will be discharged at
a) Cathode =
b) Anode =
4. State the observation at
a) Cathode =
b) Anode =
5. State the product formed at
a) Cathode =
b) Anode =
6. Write the half equation at
a) Cathode =
b) Anode =
7. Draw the electron flow on the diagram above.
[ Draw the apparatus of electrolysis silver chloride solution using silver electrodes ]
mohd faisol mansor/chemistry form 4/chapter 6
116
1.
Diagram above show the apparatus set up for the experiments of
electrolysis using two different electrodes immersed in copper(II)
sulphate solution.
a) State all the ions that are present in the electrolyte
i) Experiment A :
ii) Experiment B :
Exercise
Carbon
electrode
Copper(II) sulphate
solution
Experiment A
Copper
plate
Experiment B
Copper(II) sulphate
solution
mohd faisol mansor/chemistry form 4/chapter 6
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b) State the observation occur at anode and cathode in
i) Experiment A :
ii) Experiment B :
c) Write half equation for the reaction at the anode and cathode in
i) Experiment A :
ii) Experiment B :
d) Explain the observation on the colour of copper(II) sulphate
solution in
i) Experiment A :
ii) Experiment B :
mohd faisol mansor/chemistry form 4/chapter 6
118
Electrolysis in Industry
Most common
application:
i) Extraction of metal
ii) Purification of metal
iii) Electroplating
1) Extraction of Metal
Extraction of aluminium
from aluminium oxide.
2) Purification of Metal
In purification:
The impure metal is made to be the
anode
The cathode is a thin layer of pure metal
3) Electroplating
Electroplating is a process to coat
one metal onto another metal.
The purposes of electroplating
onto metal are:-
i) Make it look more attractive
ii) more resistant to corrosion
In electroplating :
object to be electroplated
as the cathode
anode is the metal used for plating
Electrolyte is a solution of the
compound of the electroplating
metal
Copper nugget
mohd faisol mansor/chemistry form 4/chapter 6
119
1) Ion present in electrolyte =
2) Ion discharged
a) Cathode =
b) Anode =
3) Observation
a) Cathode =
b) Anode =
4) Half equation
a) Cathode =
b) Anode =
5)Function of cryolite, Na3AlF6
Extraction of Metal
[ Draw the apparatus of electrolysis for extraction of aluminium from aluminium oxide ]
mohd faisol mansor/chemistry form 4/chapter 6
120
1) Ion present in electrolyte =
2) Ion discharged
a) Cathode =
b) Anode =
3) Observation
a) Cathode =
b) Anode =
4) Half equation
a) Cathode =
b) Anode =
5) Colour changes of electrolyte
Purification of Metal
[ Draw the apparatus of electrolysis for purification of impure copper ]
mohd faisol mansor/chemistry form 4/chapter 6
121
1) Ion present in electrolyte =
2) Ion discharged
a) Cathode =
b) Anode =
3) Observation
a) Cathode =
b) Anode =
4) Half equation
a) Cathode =
b) Anode =
5) Colour changes of electrolyte
Electroplating
[ Draw the apparatus of electrolysis to electroplate key by using copper as electrode ]
mohd faisol mansor/chemistry form 4/chapter 6
122
Voltaic Cells
A simple voltaic cell can be made by dipping two different
types of metals in an electrolyte
Electron flow from one metal to another metal through the
connecting wire in the external circuit.
More electropositive metal will release electron, thus act as
the negative terminal. Less electropositive metal will accept
electron and act as the positive terminal.
Continuous flow of electron produces an electric current.
Simple zinc-copper
Voltaic
Zinc more reactive than
copper
Zinc will act as terminal
________________, and
copper will act as
terminal ______________.
Zinc will release electron
to form Zn 2+.
Half equation:
Cu 2+ ions from copper(II) sulphate solution receive
electron to form copper metal.
Half equation :
Overall equation:
The further the distance between the position of two
metals is in ECS the bigger the cell voltage.
mohd faisol mansor/chemistry form 4/chapter 6
123
Different Types of Voltaic Cells
Two types of voltaic cell:
1) Primary cells: non rechargeable cell
Example: Daniell cell, dry cells, alkaline cell
2) Secondary cells: rechargeable cells
Example: Lead-acid accumulator, Nickel-cadmium
Daniell Cell 1
1. Used salt bridge
Salt bridge contain inert
ions or salt that does not
react with electrolyte.
Example:
Daniell Cell 2
2. Used porous pot
Porous pot has fine pores
that allow ions flow through.
What is the function of salt
bridge and porous pot in
Daniell Cell?
mohd faisol mansor/chemistry form 4/chapter 6
124
1. For the simple voltaic cell that you see at the diagram
a) State how electricity was produced.
b) What are the chemical changes that occur at the
magnesium ribbon and the copper plate.
c) Write the half equation for the changes that occur at each
the electrode.
d) What is the direction of electron flow from terminal to
another through the external circuit.
Exercise
Mg
Cu
Magnesium sulphate
solution
mohd faisol mansor/chemistry form 4/chapter 6
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Electrochemical series (ECS)
The electrochemical series (ECS) can be constructed by two
method:
a) The potential difference (voltage difference) between pairs of
metal.
b) The ability of metal to displace another metal from its salt solution.
Tendency of
metal to release
electrons to
form ions
increases
Tendency of cation
to receive
electrons to form
metals
increases
mohd faisol mansor/chemistry form 4/chapter 6
126
The potential difference (voltage difference) between pairs of metal.
The bigger the voltage value the further apart their position.
The metal act as negative terminal is placed at higher position
in electrochemical series (ECS).
How to determine the positive/negative terminal?
Example: The voltaic cells are constructed as shown in the figure. The
voltmeter reading of the cell I is 1.1 V while that of cell II is
2.5 V.
Arrange the metals in descending order in the
electrochemical series.
Cell 1
Cell 2
P
P
Q
R
mohd faisol mansor/chemistry form 4/chapter 6
127
The ability of metal to displace another metal from its salt solution
If the M can displace metal N from an aqueous N salt solution, then:
i) Metal M is more electropositive than metal N
ii) Metal M is placed at a higher position than metal N in the ECS
Example: Zinc and copper(II) sulphate sulphate solution
Observation :
Half-equation :
The Important of ECS
ECS can be used to determine:
The terminal of voltaic cell
The standard cell voltage
The ability of a metal to displace another metal from its
salt solution.
mohd faisol mansor/chemistry form 4/chapter 6
128
1. The diagram shows an electrolytic cell. The left section of the cell
(S) is a source of electricity to drive the right section (T) of the
cell.
a) State the change of energy in cell S
b) i) For cell S, state the positive terminal of the cell
ii) Explain your choice for b(i)
c) i) State what has happened at the negative terminal
ii) What process has happened in this electrode
Exercise
Copper Zinc Aluminium
Copper(II) Sulphate Zinc Sulphate
Aluminium
Sulphate
S T
mohd faisol mansor/chemistry form 4/chapter 6
129
d) Explain why the color of copper(II) sulphate remain unchanged
e) Determine the anode of cell T
f) Write down the half equation for the cathode in cell T
g) What will happened if the aluminium in cell S is replaced by copper