1. negative electrode (anode) positive electrode (cathode)
Electrolysis
negative electrode (anode)
positive electrode (cathode)
anode
cathode
electrolyte
electron flow + -

2. 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

3. 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)

4. B. Electrolysis of dilute solution by using inert electrode
rubber bands
graphite electrodes
wooden piece
electrolysis cell
very dilute sodium chloride
solution
partition e– – +
6 V
4OH-(aq)  O2(g) + 2H2O(l)+ 4e-
2H+(aq) + 2e-  H2(g)
Cl-
OH-
Na+ H+

5. 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)

6. C. Electrolysis of conc solution by using inert electrode
2H+(aq) + 2e-  H2(g)
2Cl-(aq)  Cl2(g) + 2e-

7. 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.

8. D. Electrolysis of dilute solution by using non-inert electrode
Cu2+(aq) + 2e-  Cu(s)
Cu(s)  Cu2+(aq) + 2e-
OH-
SO4-
Cu2+ H+

9. The solution finally becomes……

10. 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)

11. e.g.1 Electrolysis of dil Zn(NO3)2
2H+ +2e-  H2 Zn Pt
Zn  Zn2++2e-
OH-
NO3- H+
Zn2+
dil Zn(NO3)2

12. e.g.2 Electrolysis of conc sodium chloride using mercury as cathode
graphite (as anode)
concentrated sodium chloride solution (electrolyte)
bare platinum wire
water
plastic-covered wire
mercury (as cathode) + -
OH-
Cl- ?
Na+ H+
Reason: 1) Mercury prefers to form alloy with the metal formed. 2) conc effect

13. Reduction Oxidation At the cathode (mercury) At the anode (graphite)
Na+(aq) + e-  Na(s)
Na(s) + Hg(l)  Na/Hg(l)
sodium amalgam (alloy)
Reduction
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.

14. Polarity of electrode (and change) Type of reaction involved
+ (oxidation)
- (oxidation)
anode
- (reduction)
+ (reduction)
cathode
Polarity of electrode (and change)
A redox reaction is forced to take place
A spontaneous redox reaction takes place
Type of reaction involved
A device for bringing about chemical changes by electricity
A device for generating electricity by chemical changes
Function
Electrolytic cell
Chemical cell

15. Decrease the resistance
FACTORS AFFECTING THE RATE OF ELECTROLYSIS
Increase the voltage
Increase the current
Speed up electrolysis
Decrease the resistance

16. Molten chlorides of metals Molten oxide of aluminium
Uses of electrolysis
A. Extraction of some reactive metals
Metals high in the E.C.S
Molten chlorides of metals
electrolysis
Aluminium
Molten oxide of aluminium
electrolysis

17. e.g. Extraction of Aluminium
2O2-  O2 + 4e-
O2-
O2-
O2-
O2-
Al3+ Al3+ Al3+ Al3+ Al3+ Al3+
Al3+ +3e-  Al

18. B. Purification of metals
e.g. Purify copper metal - +
Cu(s)  Cu2+(aq) + 2e-
Cu2+(aq) + 2e-  Cu(s)
Cu2+ H+
OH-
SO42-
What material is – electrode made up of?
What is the assumption?

19. C. Electroplating e.g Electroplating of nickel - + Ni2+ H+ OH- SO42-
Ni(s)  Ni2+(aq) + 2e- - +
Ni2+(aq) + 2e-  Ni(s)
Ni2+ H+
OH-
SO42-
Can a plastic object be electroplated?

20. Ag(s)  Ag+(aq) + e- – + A Ag+(aq) + e- Ag(s) iron spoon (as cathode)
(b) A + –
silver anode
iron spoon (as cathode)
plating bath
silver nitrate solution
(a)
Criterions:
Objects to be plated always place at - pole
The electrolyte should contain the plated metal ions.
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.

21. D. Production of Cl2(g), H2(g), NaOH(aq) by electrolysis of conc. brine
graphite (as anode)
concentrated sodium chloride solution (electrolyte)
bare platinum wire
water
plastic-covered wire
mercury (as cathode) + -
OH-
Cl-
Na+ H+

22. 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)
USEFUL CHEMICALS
Due to concentration effect,
2Cl-(aq)  Cl2(g) + 2e-

23. 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??

24. Liquid wastes of electroplating
WATER POLLUTION PROBLEMS IN HONG KONG
WATER POLLUTION PROBLEMS ASSOCIATED WITH INDUSTRIAL EFFLUENTS
Liquid wastes of electroplating
Acids
Toxic chemicals
Organic solvents
Alkalis
Metal salts
Plating sludge
27.10 Water pollution problems in Hong Kong

25. (a) Industrial effluents
(b) Polluted water stream becomes coloured.
Figure Water pollution due to industrial effluents.

26. (c) Polluted water stream flows into the sea
(d) Dark blue water stain in harbour.
Figure Water pollution due to industrial effluents.

27. 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

28. 1) Reducing the volume of waste solution
(e.g. use less water for rinsing the object)
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.

29. 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 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.