1. ELECTRICITY AND
CHEMISTRY
LEARNING OBJECTIVES.
To define electrolysis
To predict that metals are formed at the negative
electrode (cathode) and that nonmetals (other
than hydrogen) are formed at the positive
electrode
(anode )
To predict the products of the electrolysis of a
specified binary compound in the molten state.
Give examples of electrode products in the
electrolysis of
(1) molten lead (II) bromide.
2. WARM UP
What are condctors?
Give example.
What are insulators?
Give example.
4. CONDITIONS FOR
ELECTROLYSIS
1. ELECTROLYTE
2. Direct current
3. Electrodes
Anode :- The electrode connected the positive
terminal of the power supply.
Cathode:-The electrode connected the
negative terminal of the power supply.
The cations moves to cathode.
The anions moves to anode.
5. Electrolysis of lead (II) bromide
Lead (II) bromide (PbBr2) is ionically bonded and
contains Pb2+ ions and Br- ions. When the solid is
melted and a voltage is applied the ions are able
to move . The positive lead ions move to cathode
and the negative bromide ions move to the
positive electrode anode. The electrodes are
usually made of carbon which is inert. The
products of the electrolysis are lead and bromine.
Silvery deposits of lead form near the the bottom
of the dish and brown bromine vapour the anode
Cathode :-Pb 2+ + 2e- → Pb
Anode :- 2Br- (I) → Br2 +2e-
7. ASSESMENT
What is meant by electrolysis?
What is the name given to positive electrode?
What two conditions must exist for a substance
to be an electrolyte and allow an electric current
to pass through it ?
Write the half reactions in the anode and cathode.
9. WARM UP
What are cations and anions?
What is cathode? Which ions will move to
cathode?
What is anode ? Which ion will move to anode?
10. Electrolysis of concentrated HCl
When concentrated HCl is electrolysed
Hydreogen forms as a colourless gas at the
caathode and chlorine as a pale green gas at the
anode.
HCl →H+ + Cl-
Inert graphite is used as electrodes.
At cathode
H+ ions will be discharged
2H+ + 2e- →H2
At anode
2Cl- → Cl2 +2e-
12. ELECTROLYSIS OS
CONCENTRATED AQUEOUS NaCl
When ionic compound NaCl dissolves in water the
sodium and chloride ion separates and are free to
move . In additon the water provides H+ and OH- ions
NaCl →Na+ +Cl-
H2O ↔ H+ +OH-At
cathode :- Two ions Na+ and H+ moves but only
H+ ions discharge . The sodium remains in solution.
2H+ +2e- → H2 (g)
At anode :- Two ions Cl- and OH- . If the solution is
concentrated Cl – ions will be discharged.( If the
solution is dilute OH- ions will be discharged in anode
and oxygen will be liberated )
15. 2Cl- →Cl2 +2e-
The chloride ion is give up electrons and form
chlorine molecules.
The sodium ion and hydroxide remains in solution
an dit becomes highly alkaline.( NaOH is formed )
16. Assement
1. What is an inert electrode?
2. What are the products obtained during the
electrolysis of HCl?
3(a). Write the half reactions taking place in
anode and cathode during the electrolysis of aq
concentrated NaOH solution.
(b) Why the solution becomes alkaline ?
17. Learning objectives
The electroplating of metals : To predict the
anode and cathode.
To give the uses of electroplating
The uses of copper and aluminium in cables
18. WARM UP
Have you seen silver plated spoon?
What is the use of that?
19. ELECTROPLATING
It is the process of coating an object with a thin film of
metal using electrolysis. For example cheap metals like
steel or nickel can be coated with silver or gold.
Uses (1) To improve the appearance of of metals
(2) Prevents the corrosion of the metals.
Silver plating of a spoon.
Silver is used as anode and the spoon to be coated as
cathode.
At the anode silver loses electrons and forms silver ions
Ag →Ag + +e-
At the cathode each silver ions accepts an electron and
gets deposited as silver .Ag+ +e-→Ag
Silver nitrate is used as electrolyte. The mas of cathode
increases, which is equal to the decreases in mass at
anode.
21. Uses of copper and aluminum in
cables
Metals like copper and aluminum are good
conductors of electricity. They have free electrons
in their structure which allows them conduct
electricity.
Copper is used in electrical wiring because it is
ductile, a flexible plastic material is used as an
insulator to surround copper.
Aluminum is often used than copper due its low
density but it is not so strong ,so for its use in
overhead cables it is wrapped in a central core of
steel.
22. ASSESMENT
What is meant by electroplating?
What are the uses of electroplating?
In silver plating of a spoon what is used as cathode
and what is the anode?
23. Assesment
What is meant by ductile?
Which is the metal used in electrical wiring?
Why aluminium is used more than copper?
Why Cu and Al can conduct electricity?
24. Learning objectives
Outline the manufacture of
(1) aluminum from pure aluminum oxide in molten
cryolite
.(ii) To findout where aluminium is formed
(iii) Why the carbon electrodes are replaced?
25. WARM UP
What are the uses of aluminium?
What are minerals?
What is an ore?
26. Manufacture of aluminium
Aluminium is extracted from the ore bauxite.
Aluminum oxide is extracted from bauxite by
purification.
It is insoluble in water and has high melting point
2045 C
It is dissolved in cryolite at about 950 C
This allows the ions to move when an electric
current is passed.
Anode :- carbon
Cathode :- carbon lined steel case.
27. What Rproceesdseos oxccuer atq thue ealecttrioodens dsur in–g aluminium
the electrolysis of aluminium oxide (Al2O3)?
At the negative electrode:
Al3+ + 3e- Al (reduction)
At the positive electrode:
2O2- O2 + 4e- (oxidation)
What is the overall equation for this extraction by electrolysis?
aluminium oxide
aluminium + oxygen
2Al2O3 (l) 4Al (l) + 3O2 (g)
28. reactions
At cathode aluminium is formed.
Al3+ + 3e-→Al
At anode oxygen is formed
2O2- →O2 +4e-
The over all equation
2 Al2O3 →4Al +3O2
The oxygen reacts with the carbon anodes to form
carbon dioxide which escapes , so the rods need to
be replaced .
This method is very expensive because large amount
of electricity is used.
In this aluminium ion is reduced and oxide ion is
oxidised.
29. Assesment
In the manufacture of Al why cryolite is used?
At which electrode Al is formed?
Why is it necessary to replace carbon electrodes
regularly?
Write the half reactions for the formation of Al and
oxygen.
30. LEARNING OBJECTIVES
TO IDENTIFY THE PRODUCTS ON THE
CATHODE AND ANODE ON THE
ELECTROLYSIS OF BRINE SOLUTION.
TO FIND OUT THE ARRANGEMENT OF
ELECTROLYSIS OF BRINE SOLUTION
31. What is product obtained in the cathode when
concentrated NaCl is electrolysed.
Why hydrogen is displaed at the cathode not
sodium?
32. Manufacture of sodium hydroxide
and chlorine
NaOH and Cl2 are manufactured by the electrolysis of
concentrated NaCl (brine ) in a diaphragm cell
This is called chlor alkali process.
Four ions are involved here
Na+ ,Cl-, H+ ,OH-Na+
and H+ are attracted to the cathode.
Cl- and OH- are attracted towards anode.
At cathode :- Sodium is more reactive than hydrogen ,so only the
hydrogen ions change to form molecule.
2H+ + 2e- → H2
At anode:- Both OH- and Cl- are attracted to the anode but only
Cl ions gets changed to form molecule.
2Cl- →Cl2 +2e-
The remaining solution contains the ions Na+ and OH- so it is
sodium hydroxide solution.
34. Uses of sodium hydroxide and
chlorine.
sodium hydroxide ↓
bleach
Soap
paper
36. ASSESMENT
What is brine solution?
Where is chlorine gas collected ? What is the
colour of
the gas?
Why a diaphragm cell is used?
What are the uses of sodium hydroxide ?
Give two uses of chlorine.