Electrolysis
positive electrode
(cathode)
negative electrode
(anode)
+
electron
flow
anode
cathode
electrolyte
What is electrolysis?
• By using electricity to decompose
chemical substances in which a redox
reaction is forced to take place
What is cathode and anode?
• Red cat, An ox
What is positive and negative pole?
• According to the battery
A. Electrolysis of molten substance by
using inert electrode
• No preference of discharge of ions
• + pole attracts negative ions, vice versa
2Br-(l)  Br2(g) +2e-
Pb2+(l) + 2e-  Pb(l)
B. Electrolysis of dilute solution by using
inert electrode
4OH-(aq)  O2(g) + 2H2O(l)+ 4e-
rubber bands
wooden piece
electrolysis cell
2H+(aq) + 2e-  H2(g)
graphite electrodes
Na+
Cl-
H+
OH-
very dilute sodium chloride
solution
partition
e–
e–
–
+
6V
B. Electrolysis of dilute solution by using
inert electrode
• Preference of discharge of ions (according to the
E.C.S)
• negative ions go to anode and discharge in the order:
OH->I->Br->Cl- >>>>SO42-, NO3• e.g. OH- ions discharge , as it is more readily to give
electrons than other negative ions
• positive ions go to anode and discharge in the order :
Ag+>Cu2+>H+ >>>>other metal ions
• e.g. H+ ions discharge, as it is more readily to accept
electrons.( Cu2+/Ag+ discharges if they are present)
C. Electrolysis of conc solution by using
inert electrode
2H+(aq) + 2e-  H2(g)
-
2Cl (aq)  Cl2(g) + 2e-
C. Electrolysis of conc solution by using
inert electrode
• By concentration effect, the ions to be discharge may be
different from the dilution solution.
• e.g. negative ions discharge according to the order of ease
to lose e-(s): OH->I->Br->Cl• although OH- is more readily to give electrons, however due
to concentration effect (I-/Br-/Cl- are present in larger
amount), I-/Br-/Cl- are discharged instead.
• e.g. positive ions discharge according to the order of ease
to gain e-(s): : Ag+>Cu2+>H+ >>>>other metal ions
• The ion discharge is same as in dilute solution unless
Mercury is used as anode.
D. Electrolysis of dilute solution by using
non-inert electrode
Cu2+(aq) + 2e-  Cu(s)
Cu(s)  Cu2+(aq) + 2e-
Cu2+
OH-
H+
SO4-
The solution finally becomes……
D. Electrolysis of dilute solution by using
non-inert electrode
• Metals ALWAYS give electrons more readily than
OH- ion.
• + pole: metal electrode discharges, and dissolves,
but not OH- ion.
• Would the preference of discharge of ions be
affected at the – pole by using non-inert electrode?
• NO (metal solids always give electrons)
e.g.1 Electrolysis of dil Zn(NO3)2
2H+ +2e-  H2
Zn
Pt
Zn  Zn2++2e-
H+
OH-
Zn2+
NO3-
dil Zn(NO3)2
e.g.2 Electrolysis of conc sodium
chloride using mercury as cathode
plastic-covered
wire
?
water
-
+
graphite (as anode)
Clconcentrated
sodium chloride
solution (electrolyte)
bare platinum wire
Na+
H+
OH-
mercury (as cathode)
Reason: 1) Mercury prefers to form alloy with the metal formed.
2) conc effect
At the cathode (mercury)
Na+(aq) + e-  Na(s)
Reduction
Na(s) + Hg(l)  Na/Hg(l)
sodium amalgam (alloy)
Overall reaction at cathode: 2Na/Hg(l) + 2H2O(l)  2NaOH(aq) +H2(g) + 2Hg(l)
At the anode (graphite)
Oxidation
Due to concentration effect,
-
2Cl (aq)  Cl2(g) + 2e-
Overall reaction: 2Hg(l) + 2Na+(aq) + 2Cl (aq)  2Na/Hg(l) + Cl2(g)
The sodium chloride solution becomes more and more dilute.
Chemical cell
Electrolytic cell
A device for
A device for
bringing about
Function
generating electricity
chemical changes
by chemical changes
by electricity
A spontaneous redox A redox reaction is
Type of reaction
involved
reaction takes place forced to take place
Polarity of
cathode + (reduction)
- (reduction)
electrode (and
anode
+ (oxidation)
- (oxidation)
change)
FACTORS AFFECTING THE RATE OF
ELECTROLYSIS
Increase
the current
Speed up
electrolysis
Uses of electrolysis
A. Extraction of some reactive metals
Molten
chlorides of
metals
electrolysis
Molten oxide of
aluminium
electrolysis
Metals
high in the
E.C.S
Aluminiu
m
e.g. Extraction of Aluminium
2O2-  O2 + 4e-
O2-
O2-
O2-
O2-
O2-
O2-
O2-
O2-
Al3+ Al3+ Al3+ Al3+ Al3+ Al3+
Al3+ +3e-  Al
B. Purification of metals
e.g. Purify copper metal
-
Cu(s)  Cu2+(aq) + 2e-
+
Cu2+(aq) + 2e-  Cu(s)
Cu2+ OHH+
SO42-
What material is – electrode made up of?
What is the assumption?
C. Electroplating
e.g Electroplating of nickel
-
Ni(s)  Ni2+(aq) + 2e-
+
Ni2+(aq) + 2e-  Ni(s)
Ni2+
OH-
H+
SO42-
Can a plastic object be electroplated?
Ag(s)  Ag+(aq) + eA
+ –
iron
spoon
(as
cathode)
silver
anode
plating
bath
(b)
silver nitrate
solution
(a)
Criterions:
+
Ag (aq) +
e- Ag(s)
Figure 27.18
(a)
A set-up for electroplating silver on a spoon.
(b)
Electroplating silver on metal cups.
Can we use this method to plate zinc on a coin?
This setting only suits for silver, copper, nickel plating.
1) Objects to be
plated always
place at - pole
2) The electrolyte
should contain
the plated
metal ions.
D. Production of Cl2(g), H2(g), NaOH(aq)
by electrolysis of conc. brine
plastic-covered
wire
-
+
water
OHNa+
bare platinum wire
mercury (as cathode)
H+
graphite (as anode)
concentrated
sodium chloride
solution (electrolyte)
Cl-
At the cathode (mercury)
Na+(aq) + e-  Na(s)
Na(s) + Hg(l)  Na/Hg(l)
sodium amalgam (alloy)
Overall reaction at cathode: 2Na/Hg(l) + 2H2O(l)  2NaOH(aq) +H2(g) + 2Hg(l)
At the anode (graphite)
Due to concentration effect,
-
2Cl (aq)  Cl2(g) + 2e-
USEFUL CHEMICALS
E. Anodizing aluminium
The resistance to corrosion of aluminium can be enhanced
by anodizing it using electrolysis. A thicker protective layer of
aluminum oxide is formed on the surface.
+
2H+(aq) + 2e-  H2
4OH-  O2 + 2H2O + 4e-
?
What are the advantages of anodization of aluminium??
WATER POLLUTION PROBLEMS IN HONG KONG
WATER POLLUTION PROBLEMS ASSOCIATED WITH
INDUSTRIAL EFFLUENTS
Liquid wastes of
electroplating
27.10
Acids
Toxic
chemicals
Organic
solvents
Alkalis
Metal salts
Plating sludge
Water pollution problems in Hong Kong
(a) Industrial effluents
(b) Polluted water stream becomes
coloured.
Figure 27.24 Water pollution due to industrial effluents.
(c) Polluted water stream flows into
the sea
(d) Dark blue water stain in harbour.
Figure 27.24 Water pollution due to industrial effluents.
1)acids and alkalis e.g. H2SO4 [remove oxides];
NaOH [removes oil]
 kill water plants and animals
increase rusting rate of metal pipes and ships
2)heavy metal ions e.g. Cu2+ , Ni2+, Cr(VI), Pb2+
toxic to water plants and animals
3)cyanide (CN -)  very toxic
1) Reducing the volume of waste solution
(e.g. use less water for rinsing the object)
2) Recycling of the waste electrolyte.
3) Removal of toxic substances before disposing
Common methods are :
a) Adding Na2CO3 to remove the acids. e.g.
Na2CO3 + H2SO4  Na2SO4 + CO2 + H2O
b) Adding NaOH to precipitate the metal ions. e.g.
2NaOH(aq) + NiSO4(aq)  Na2SO4(aq) + Ni(OH)2(s)
4) Reduce very toxic Cr(VI) compounds into less toxic Cr(III) ions.
CONTROL OF CHEMICAL WASTE DISPOSAL IN
HONG KONG
Chemical waste producers such as electroplating factories are
required to register with the Environmental Protection
Department (EPD) for proper treatment and disposal of chemical
wastes.
Figure 27.26 The Chemical
Waste
Treatment
Centre
(CWTC) at Tsing Yi Island of
Hong Kong. The Centre,
having operated since May
1993, is capable of treating a
great variety of chemical
wastes.