Transcript Matter - Hong Kong True Light College

Electrolysis
Terms used in electrolysis
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Electrolysis is the decomposition of an electrolyte
in molten state or aqueous solution by electricity.
An electrolyte is a substance which conducts an
electric current in molten state or aqueous solution,
and is decomposed by electricity.
The anode is the electrode where oxidation occurs.
It is the electrode connected to the positive
terminal of the d.c. supply.
The cathode is the electrode where reduction
occurs. It is the electrode connected to the
negative terminal of the d.c. supply.
Terms used in electrolysis
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An anion is a negative ion and is attracted to the
anode.
A cation is a positive ion and is attracted to the
cathode.
An ammeter is an instrument used to measure the
electric current passing through a circuit. Electric
current is measured in ampere (A).
A variable resistor (or rheostat) is used to vary the
resistance and then regulate the current.
Electrolysis
Factors affecting electrolysis
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The position of ions in the electrochemical
series.
The concentration of ions in the solution.
The nature of the electrodes.
Position of cations in the e.c.s.
Position of anions in the e.c.s.
Case 1: Electrolysis of molten
lead(II) bromide
Nichrome wire
Electrode (-)
Molten lead(II)
bromide
Electron flow
Nichrome wire
Electrode (+)
Case 1: Electrolysis of molten
lead(II) bromide
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Solid lead(II) bromide does not conduct
electricity because the ions are not mobile.
Molten lead(II) bromide contains mobile ions.
Cation
Anion
Pb2+()
Br-()
Case 1: Electrolysis of molten
lead(II) bromide
Electron flow
Nichrome wire
Electrode (-)
Molten lead(II)
bromide
Nichrome wire
Electrode (+)
Case 1: Electrolysis of molten
lead(II) bromide
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At cathode, lead(II)
cations receive
electrons, they undergo
reduction and discharge
to form lead atoms.
Pb2+() + 2e–  Pb()
Case 1: Electrolysis of molten
lead(II) bromide
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At anode, bromide
anions give up
electrons, they undergo
oxidation and
discharge to form
bromine atoms.
2Br-()  Br2() + 2e–
Case 1: Electrolysis of molten
lead(II) bromide
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Bromine atoms
then join in pair to
form bromine
molecules.
Case 2: Electrolysis of acidified
water using platinum electrodes
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Although water is known to be poor
electrical non-conductor, it actually ionizes
slightly to give hydrogen ions and hydroxide
+
ions. H2O()
H (aq) + OH–(aq)
Pure acids are covalent compounds.
However they ionize in water.
HCl(g) + water  HCl(aq)
+
–
HCl(aq)  H (aq) + Cl (aq)
Case 2: Electrolysis of acidified
water using platinum electrodes
Cation
Anion
H+(aq)
OH-(aq)
Case 2: Electrolysis of acidified
water using platinum electrodes
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At cathode, hydrogen
ions receive electrons,
they undergo reduction
and discharge to form
hydrogen gas.
2H+(aq) + 2e–  H2(g)
Case 2: Electrolysis of acidified
water using platinum electrodes
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At anode, hydroxide
ions give up
electrons, they
undergo oxidation
and discharge to
form oxygen gas.
4OH-(aq)  O2(g) +
2H2O() + 4e–
Case 2: Electrolysis of acidified
water using platinum electrodes
2H+(aq) + 2e–  H2(g)
(1)
4OH-(aq)  O2(g) + 2H2O() + 4e– (2)
(1)x2: 4H+(aq) + 4e–  2H2(g)
(3)
(2)+(3): 4OH-(aq) + 4H+(aq)  O2(g) + 2H2O() + 2H2(g)
4H2O()  O2(g) + 2H2O() + 2H2(g)
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Overall equation: 2H2O()  O2(g) + 2H2(g)
Case 2: Electrolysis of acidified
water using platinum electrodes
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Dilute acid is added to provide more mobile
ions so as to increase the conductivity of the
water.
The concentration of dilute acid increases at
the end as water is consumed in the
electrolysis.
Case 3: Electrolysis of dilute sodium
chloride solution using carbon electrodes
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
Cl-(aq)
Case 3: Electrolysis of dilute sodium
chloride solution using carbon electrodes
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The sodium ions and hydrogen ions move
towards the cathode.
At the cathode, the position of hydrogen
ions in the electrochemical series is lower
than that of sodium ions. Hydrogen ions are
preferentially discharged (reduced) to form
colourless hydrogen gas.
+
–
2H (aq) + 2e  H2(g)
Case 3: Electrolysis of dilute sodium
chloride solution using carbon electrodes
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The chloride ions and hydroxide ions move
towards the anode.
At the anode: the position of hydroxide
ions in the electrochemical series is higher
than that of chloride ions. Hydroxide ions
are preferentially discharged (oxidized) to
form colourless oxygen gas.
–
4OH (aq)  O2(g) + 2H2O() + 4e
Case 3: Electrolysis of dilute sodium
chloride solution using carbon electrodes
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Overall reaction: 2H2O()  O2(g) + 2H2(g)
Water ionizes continuously to replace the
hydrogen ions discharged at the cathode. Thus
there is an excess of hydroxide ions near the
cathode and the solution there becomes alkaline.
Water ionizes continuously to replace the
hydroxide ions discharged at the anode. Thus
there is an excess of hydrogen ions near the anode.
The solution there becomes acidic.
Case 3: Electrolysis of dilute sodium
chloride solution using carbon electrodes
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If a few drops of universal indicator are added to
the sodium chloride solution, the solution near the
cathode will turn blue while that near the anode
will turn red.
The sodium chloride becomes more concentrated
as water is consumed in the electrolysis.
Case 4: Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes
Cation
Anion
H+(aq)
OH-(aq)
Cu2+(aq) SO42-(aq)
Case 4: Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes
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The copper(II) ions and hydrogen ions
move towards the cathode.
At the cathode, the position of copper(II)
ions in the electrochemical series is lower
than that of hydrogen ions. Copper(II) ions
are preferentially discharged (reduced) to
form brown copper metal.
2+
–
Cu (aq) + 2e  Cu(s)
Case 4: Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes
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The sulphate ions and hydroxide ions move
towards the anode.
At the anode: the position of hydroxide
ions in the electrochemical series is higher
than that of sulphate ions. Hydroxide ions
are preferentially discharged (oxidized) to
form colourless oxygen gas.
–
4OH (aq)  O2(g) + 2H2O() + 4e
Case 4: Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes
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Overall reaction:
2Cu2+(aq) + 4OH–(aq)  2Cu(s) + O2(g) + 2H2O()
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Water ionizes continuously to replace the
hydroxide ions discharged at the anode. Thus
there is an excess of hydrogen ions near the
anode. The solution there becomes acidic.
If a few drops of universal indicator is added
into the solution, red colour appears around
anode.
Case 4: Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes
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The blue colour of the solution fades out
because the concentration of copper(II)
ions decreases.
Copper(II) ions and hydroxide ions are
consumed in the electrolysis. Hydrogen
ions and sulphate ions remain in the
solution. Thus the solution eventually
becomes sulphuric acid.
Case 4: Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes
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After a few minutes, cathode is coated with
copper.
If the polarities of cells are then reversed,
anode is coated with copper. The factor of
electrode should be considered as in case 8.
Case 5: Electrolysis of dilute sodium
iodide solution using carbon electrodes
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
I-(aq)
Case 5: Electrolysis of dilute sodium
iodide solution using carbon electrodes
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The sodium ions and hydrogen ions move
towards the cathode.
At the cathode, the position of hydrogen
ions in the electrochemical series is lower
than that of sodium ions. Hydrogen ions are
preferentially discharged (reduced) to form
colourless hydrogen gas.
+
–
2H (aq) + 2e  H2(g)
Case 5: Electrolysis of dilute sodium
iodide solution using carbon electrodes
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The iodide ions and hydroxide ions move towards
the anode.
At the anode: the position of hydroxide ions in the
electrochemical series is higher than that of
chloride ions. However, the concentration of
iodide ions is much greater than that of hydroxide
ions. Iodide ions are preferentially discharged
(oxidized) to form iodine.
2I-(aq)  I2(aq) + 2e–
Case 5: Electrolysis of dilute sodium
iodide solution using carbon electrodes
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Overall reaction:
2H+(aq) + 2I–(aq)  H2(g) + I2(aq)
The solution near the cathode becomes
alkaline.
The iodine produced at the anode dissolves in
the solution. Therefore a brown colour
develops around the anode.
Case 5: Electrolysis of dilute sodium
iodide solution using carbon electrodes
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Hydrogen ions and iodide ions are consumed
in the electrolysis. Sodium ions and
hydroxide ions remain in the solution. The
solution eventually becomes sodium
hydroxide solution.
Case 6: Electrolysis of conc. sodium
chloride solution using carbon electrodes
Conc.
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
Cl-(aq)
Case 6: Electrolysis of conc. sodium
chloride solution using carbon electrodes
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The sodium ions and hydrogen ions move
towards the cathode.
At the cathode, the position of hydrogen
ions in the electrochemical series is lower
than that of sodium ions. Hydrogen ions are
preferentially discharged (reduced) to form
colourless hydrogen gas.
+
–
2H (aq) + 2e  H2(g)
Case 6: Electrolysis of conc. sodium
chloride solution using carbon electrodes
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The chloride ions and hydroxide ions move
towards the anode.
At the anode: the position of hydroxide ions in the
electrochemical series is higher than that of
chloride ions. However, the concentration of
chloride ions is much greater than that of
hydroxide ions. Chloride ions are preferentially
discharged (oxidized) to form chlorine gas.
2Cl-(aq)  Cl2(g) + 2e–
Case 6: Electrolysis of conc. sodium
chloride solution using carbon electrodes
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Overall reaction:
2H+(aq) + 2Cl–(aq)  H2(g) + Cl2(aq)
Water ionizes continuously to replace the
hydrogen ions discharged at the cathode. Thus
there is an excess of hydroxide ions near the
cathode. The solution there becomes alkaline.
The chlorine gas formed at the anode dissolves in
the solution. The solution there becomes acidic
and has a bleaching effect.
Case 6: Electrolysis of conc. sodium
chloride solution using carbon electrodes
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Hydrogen ions and chloride ions are consumed in
the electrolysis. Sodium ions and hydroxide ions
remain in the solution. Eventually, the solution
becomes sodium hydroxide solution.
Case 7: Electrolysis of conc. sodium chloride
solution using mercury electrodes
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
Cl-(aq)
Hg()
Hg()
Case 7: Electrolysis of conc. sodium chloride
solution using mercury electrodes
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The sodium ions and hydrogen ions move towards
the cathode.
At the cathode, the position of hydrogen ions in
the electrochemical series is lower than that of
sodium ions. However, sodium ions are
preferentially discharged (reduced) to form sodium
metal. The sodium metal formed dissolves in the
mercury to form a sodium amalgam.
Na+(aq) + e– + Hg(l)  Na/Hg(l) sodium amalgam
Case 7: Electrolysis of conc. sodium chloride
solution using mercury electrodes
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The sodium amalgam then reacts with water to
form sodium hydroxide and hydrogen.
2Na/Hg(l) + 2H2O(l)  2NaOH(aq) + H2(g) + 2Hg(l)
Case 7: Electrolysis of conc. sodium chloride
solution using mercury electrodes
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The chloride ions and hydroxide ions move
towards the anode.
At the anode: the position of hydroxide ions in the
electrochemical series is higher than that of
chloride ions. However, the concentration of
chloride ions is much greater than that of
hydroxide ions. Chloride ions are preferentially
discharged (oxidized) to form chlorine gas.
2Cl-(aq)  Cl2(g) + 2e–
Case 7: Electrolysis of conc. sodium chloride
solution using mercury electrodes
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Overall reaction:
2Na+(aq) + 2Cl–(aq) + 2Hg(l)  2Na/Hg(l) + Cl2(g)
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Sodium ions and chloride ions are consumed in
the electrolysis. Thus the sodium chloride solution
becomes more and more dilute.
This reaction is very important in the manufacture
of chlorine bleaching solution.
Case 8: Electrolysis of dilute copper(II)
sulphate solution using copper electrodes
Cation
Anion
H+(aq)
OH-(aq)
Cu2+(aq) SO42-(aq)
Cu(s)
Cu(s)
Case 8: Electrolysis of dilute copper(II)
sulphate solution using copper electrodes
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The copper(II) ions and hydrogen ions
move towards the cathode.
At the cathode, the position of copper(II)
ions in the electrochemical series is lower
than that of hydrogen ions. Copper(II) ions
are preferentially discharged (reduced) to
form brown copper metal.
2+
–
Cu (aq) + 2e  Cu(s)
Case 8: Electrolysis of dilute copper(II)
sulphate solution using copper electrodes
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The sulphate ions and hydroxide ions move
towards the anode.
At the anode: the position of hydroxide ions in the
electrochemical series is higher than that of
sulphate ions. However, copper is a stronger
reducing agent than hydroxide ions and thus more
easily oxidized. The copper anode dissolves to
form copper(II) ions (oxidized).
Cu(s)  Cu2+(aq) + 2e–
Case 8: Electrolysis of dilute copper(II)
sulphate solution using copper electrodes
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Overall reaction:
Cu(s)  Cu(s)
anode
cathode
The net effect is the transfer of copper from the
anode to the cathode. The rate at which copper
deposits on the cathode is equal to the rate at which
the copper anode dissolves.
Increase in mass of cathode = decrease in mass of anode
Case 8: Electrolysis of dilute copper(II)
sulphate solution using copper electrodes
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The concentration of copper(II) ions in the solution
remains the same. The blue colour of the solution
does not change.
Comparing a chemical cell and
an electrolytic cell
Comparing a chemical cell and
an electrolytic cell
Chemical cell
Electrolytic cell
Function
A device for generating
electricity from
chemical reactions.
A device for bringing out
chemical changes by
electricity.
Direction of
electricity
Electrons flow from
negative electrode to the
positive electrode
through the external
circuit.
Circuit is completed by
the movement of mobile
electrons.
Cations discharge and gain
electrons at cathode, while
anions discharge and give
up electrons at anode.
Circuit is completed by the
movement of mobile ions.
Comparing a chemical cell and
an electrolytic cell
Chemical cell
Electrolytic cell
Reactions at
positive
electrode
Reduction (Cathode)
Oxidation (anode)
Reactions at
negative
electrode
Oxidation (anode)
Reduction (Cathode)
Commercial uses of electrolysis
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Manufacture of hydrogen, chlorine and
sodium hydroxide and bleaching solution
Refining of copper
Electroplating
Extracting reactive metals
Aluminium anodization
Manufacture of bleaching solution
Manufacture of bleaching solution
At the anode: 2Cl–(aq)  Cl2(g) + 2e–
+
At the cathode: Na (aq) + e– + Hg(l)  Na/Hg(l)
sodium amalgam
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The sodium amalgam then flows into a second
cell and reacts with water to form sodium
hydroxide, hydrogen and mercury.
Mercury is then recovered and then pumped back
into the reaction chamber.
2Na/Hg(l) + 2H2O(l)  2NaOH(aq) + H2(g) + 2Hg(l)
Manufacture of bleaching solution
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This process also produces waste which contains
poisonous mercury compounds. These waste
products will cause serious pollution problems if
they are discharged into rivers and seas.
Refining of copper
Refining of copper
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Copper ore contains a few impurities —
mostly silver, gold, platinum, iron and
zinc — reduce the electrical conductivity
of copper significantly.
anode: impure copper
cathode: very pure copper
electrolyte: copper(II) sulphate solution and
sulphuric acid
Refining of copper
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Iron and zinc are more reactive than copper.
They form ions more readily than copper.
At anode, iron and zinc give up electrons
first. Then copper gives up electrons to
form copper(II) ions.
Zn(s)  Zn2+(aq) + 2e–
Fe(s)  Fe2+(aq) + 2e–
2+
–
Cu(s)  Cu (aq) + 2e
Refining of copper
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Impurities such as silver, gold and platinum
settle at the bottom of the container.
At the cathode, the position of copper(II)
ions in the electrochemical series is lower
than that of hydrogen ions. Copper(II) ions
are preferentially discharged (reduced) to
form brown copper metal.
2+
–
Cu (aq) + 2e  Cu(s)
Refining of copper
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Overall reaction:
Cu(s)  Cu(s)
anode
cathode
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Refer to case 8
Electroplating
Electroplating
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Electroplating is the coating of an object
with a thin layer of a metal by electrolysis.
Cathode: object to be plated
Anode: plating metal
Electrolyte: a solution of a compound of
the plating metal
Electroplating
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Objects may be electroplated with copper,
nickel, chromium, gold or silver.
Typical example: electroplating of copper
Pollution problems of
electroplating
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The electroplating industry produces many
toxic waste by-products.
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acids and alkalis
cumulative poisons of heavy metals and ions
(such as nickel, chromium and mercury)
toxic cyanides.
Solutions
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Controlling the pH value of effluents
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The pH value of acidic effluents can be
controlled by adding sodium carbonate.
The pH value of alkaline effluents can be
controlled by adding sulphuric acid.
Treatment of heavy metal compounds
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Add sodium hydroxide solution to the effluents
to form insoluble metal hydroxides. The solid is
then filtered off.
Solutions
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Treatment of poisonous chromium waste
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Poisonous chromium(VI) compounds are
reduced to non-toxic chromium(III) compounds
by sodium sulphite. Sodium hydroxide solution
is then added to the chromium(III) compounds
to form solid chromium(III) hydroxide. The
solid is then filtered off.
Extraction of reactive metals
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Reactive metals such as K, Na, Ca, Mg and
Al are extracted from its ores by
electrolysis of molten metal ores.
Metal ions are attracted to the cathode and
reduced to form metal.
Mn+(l) + ne-  M(s)
Anodization of aluminium
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Aluminium oxide is a protective oxide layer. It
does not react with acids and alkalis.
However, natural occurring aluminium oxide
layers are thin and unevenly distributed.
Anode: Al
Cathode: circular sheet of steel
Electrolyte: dilute sulphuric acid
Anodization of aluminium
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At anode:
4OH-(aq)  O2(g) + 2H2O() + 4e–
Oxygen is then reacted with aluminium
anode to form a thick protection oxide layer.
4Al(s) + 3O2(g)  2Al2O3(s)
Aluminium oxide can be dyed.