Chemistry Form 4: Chapter 6 - Electrolyte
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Electrolyte is a chemical substance which conducts electricity in the
molten or aqueous state.
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Electrolytes are able to conduct electricity because there are freely move
ions in the molten or aqueous state.
Examples of electrolyte are salt solution, dilute acids and dilute alkalis
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Chemistry Form 4: Chapter 6 - Electrolysis of Lead (II) Bromide
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Electrolysis is a chemical process where a substance in its molten state or
in an aqueous solution decomposed by electric current.
This involves conversion of energy from electrical energy to chemical
energy.
Electrodes are the strips of metals inserted into electrolyte for conduction
of electricity. he metal electrode connected to the positive terminal of the
battery is called the anode (+). The metal electrode connected to the
negative terminal of the battery is called the cathode (-).
Electrolytic cell is the complete set-up for electrolysis. This consists of the
vessel containing the electrolyte, anode, cathode, battery and wires.
At the Cathode
Observation
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When electricity is flowing, a silvery
deposit of lead metal forms on the
cathode. In fact, as it is molten, it is
more likely to drip off in a molten
blob.
At the Anode
Observation
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When electricity is flowing,
brown fumes of bromine gas are
seen at the anode.
Half equation
Pb2+ + 2e ---> Pb
Half equation
2Br- ---> Br2 + e
Explanation
Explanation
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The lead(II) ions, as they are positive,
move to the negative cathode, where
each ion gains two electrons to form a
lead atom.
Any reaction at a cathode involved a
gain in electrons. This is called
reduction or more exactly, cathodic
reduction .
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The bromide ions, as they are
negative, move to the positive
anode, where each loses an
electron to form a bromine atom.
Then two of these newly formed
atoms combine to form bromine
gas.
Any reaction at an anode
involves a loss of electrons.
Chemistry Form 4: Chapter 6 - Electrolysis of Concentrated of Sodium
Chloride Solution
Electrolysis of dilute sodium
chloride solution
At
anode :
Anions : OH- and Cl-
Electrolysis of concentrated
sodium chloride solution
At
anode :
Anions : OH- and Cl-
OH- ions are selectively
discharge according to the
position of ions in the
Electrochemical Series, ES.
Cl- ions are discharged
instead of OH- because of
their higher concentration
in the electrolyte.
Gas bubbles are formed which
lighted up a glowing wooden
splinter. This gas is oxygen.
A greenish-yellow gas
with a pungent and
choking smell is released.
The gas turns the blue
litmus paper red and then
white.
4OH- ---> 2 H2O + O2 + 4e
2Cl- ---> Cl2
+
2e
At
cathode:
Cations : H+ dan Na+
At
cathode:
Cations : H+ dan K+
H+ ions are selectively
discharge according to the
position of ions in the ES.
H+ ions are selectively
discharge according to the
position of ions in the ES.
Gas bubbles are formed. When
a lighted wooden splinter is
placed near the mouth of the
test tube, a ‘pop’ sound is
produced. Hydrogen gas is
produced
Gas bubbles are formed.
When a lighted wooden
splinter is placed near the
mouth of the test tube, a
‘pop’ sound is
produced. Hydrogen gas
is produced
2H+ + 2e ---> H2
2H+ + 2e ---> H2
Chemistry Form 4: Chapter 6 - Types of Electrodes Affect Product of
Electrolysis
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Carbon dan platinum electrodes are inert because both of these materials
do not react with the electrolytes or the product of electrolysis.
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Other electrodes except carbon and platinum can react with the
electrolytes.
Chemistry Form Four : Chapter 6 - Electrolysis of copper (II) sulphate
Electrolysis of copper (II) sulphate
using carbon electrode. The bulb is lighted indicates that copper (II) sulphate is an
electrolyte which can conduct electricity. Carbon electrode is inert which does not react
with electrolyte or product of electrolysis.
At anode : Hydroxide ions ions are selectively
discharged. Gas bubbles are formed which lighted up a glowing wooden splinter. This
gas is oxygen.
At cathode : Copper ions are selectively discharged. Shiny brown metal deposited
around cathode. This metal is copper.
Electrolysis copper (II) sulphate solution with copper electrodes
At cathode : Copper ions are selectively discharged. Shiny brown metal deposited,
makes the cathode thicker. This metal is copper.
At anode : Copper electrode eroded and became thinner. Copper anode dissolved to form
copper ions.
Chemistry Form Four: Chapter 6 - Electroplating of copper metal
1) Anode --> plating metal = copper
2) Cathode --> object to be electroplated = iron key
3) Electrolyte --> salt solution that contains electroplating metal ions = copper (II)
sulphate
cathode: A brown metal deposited on the surface of the iron key.
anode: Copper anode becomes thinner.
Chemistry Form 4: Chapter 6 - Purification of Metal
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The anode is a impure copper.
The cathode is a pure copper.
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When electricity is passed through the cell, the copper is dissolved at
anode where Cu2+ ions go into solution. Anode copper becomes thinner.
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At cathode, copper ions are selectively discharge and deposited. Cathode
copper becomes thicker.
The blue intensity of solution remains unchanged because concentration of
copper (II) ions in the solution remains unchanged. The rate at which copper
enter into electrolyte from anode is equal to the rate at which the copper ions
discharged at cathode.
Chemistry Form 4: Chapter 6 - Extraction of Aluminium From its Ore
The ores of aluminium is called bauxite. Bauxite is a composition of aluminium
oxide, (Al2O3).
The position of aluminium in the reactivity series indicates that, it could not be
extracted by reducing the oxide using carbon or carbon monoxide. Electrolysis of
molten aluminium oxide ("alumina") will, however, give aluminium and oxygen.
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The function of cryolite is to lower the melting
point of aluminium oxide.
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Aluminium metal forms at the cathode electrode
and sinks to the bottom of the tank, where it is
tapped off.
Oxygen forms at the anode electrodes. This
oxygen reacts with the carbon of the anode
electrodes, forming carbon dioxide, and they
gradually burn away. Consequently, the anode
electrodes have to be replaced frequently.
Chemistry Form 4: Chapter 6 - Daniell Cell
Daniell cell is also known as Galvanic cell which using
same working principle as voltaic cell. It converts
chemical energy to electrical energy. Daniell cell is a
part of a battery consisting of two metals joined by a
salt bridge or a porous pot.
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More electropositive metal acts as negative terminal which undergoes
oxidation process (release electron)
Less electropositive metal acts as positive terminal which undergoes
reduction (gain electron).
Electron flow from negative terminal to positive terminal.
The function of salt bridge or porous pot is to allow the movement of ions
to pass through it to complete electrical circuit. Another function is to
separate two electrolyte solutions.
Examples of salt bridge are sodium chloride or potassium nitrate.
At Negative Terminal: More electropositive metal ( Zinc )
Observation: Zinc plate dissolves or becomes thinner, its mass decreases
Half chemical equation :
Zn ------>
Zn2+ +
2e
At Positive Terminal: Less electropositive metal ( Copper )
Observation: Copper plate becomes thicker
Half chemical equation :
Cu2+
+
2e  Cu
Chemistry Form Four: Chapter 6 - Displacement of Metal from Salt
Solution
Electrochemical series can be constructed by using displacement reaction of metal from
salt solution. A more electropositive (reactive) metal can displace a less electropositive
metallic ion (less reactive) from its salt solution.
For example: Reaction between magnesium and copper (II) sulphate produces copper
and magnesium sulphate.
Chemistry Form 4: Chapter 6 - Constructing Electrochemical Series based
on the Potential Difference between Two Metals
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In a voltaic cell, two different metals are used to create a potential difference.
A higher metal in the electrochemical series will be made negative terminal.
A lower metal in the electrochemical series will be made positive terminal.
Conversion of energy is chemical energy to electrical energy.
Metals which are further apart in the electrochemical series produce a
higher voltage than metals which are close to each other.
By arranging the metals in order of their voltage readings, you can build up the
electrochemical series.