Action of a simple cell'

1
Action of a simple cell.
e
Zinc rod (-)
Copper rod ()
H
SO42-
e
H
Zn 2
Dilute sulphuric acid
H
H2SO4
2
Action of a Simple cell.

• To make a simple cell in a Laboratory, the
  following are required
• Copper rod.

3
Action of a Simple cell.

• To make a simple cell in a Laboratory, the
  following are required
• Copper rod.
• Zinc rod.

4
Action of a Simple cell.

• To make a simple cell in a Laboratory, the
  following are required.
• Copper rod.
• Zinc rod.
• dilute sulphuric acid.

5
Action of a Simple cell.

• To make a simple cell in a Laboratory, the
  following are required.
• Copper rod.
• Zinc rod.
• dilute sulphuric acid.
• Connecting wire.

6
Action of a Simple cell.

• To make a simple cell in a Laboratory, the
  following are required.
• Copper rod.
• Zinc rod.
• dilute sulphuric acid.
• Connecting wire
• Beaker 100 cm3.

7
Action of a Simple cell.

• When the Copper and Zinc plates are joined by
  awire and dipped into dilute sulphuric acid the
  following observations are made
• The Zinc rod slowly dissolves in the sulphuric
  acid.

8
Action of a Simple cell.

• When the Copper and Zinc plates are joined by
  awire and dipped into dilute sulphuric acid the
  following observations are made
• The Zinc rod slowly dissolves in the sulphuric
  acid.

9
Action of a Simple cell.

• When the Copper and Zinc plates are joined by
  awire and dipped into dilute sulphuric acid the
  following observations are made
• Bubbles of Hydrogen are formed on the copper
  plate.

10
Action of a Simple cell.

• A current of electrons drifts through the wire
  from the Zinc to Copper.

11
Action of a Simple cell.

• dilute sulphuric acid ionises according to the
  equation.
• H2SO4(aq) 2H(aq) SO42-(aq)

12
Action of a Simple cell.

• Zinc atoms dissolves from the Zinc plate and go
  into solution as Zn2 ions each of which leaves
  two electrons behind on the Zinc rod.

13
Action of a Simple cell.

• These electrons are the source of electron
  current which goes through the wire from the Zinc
  to the Copper.

14
What happens at the Copper rod?

• At the Copper rod Hydrogen ions are discharged
  to form Hydrogen atoms and molecules.
• H e H atom
• H H H2(g)

15
Conventional direction of electric current.

• Current flows from positive terminal of a cell
  to the negative terminal.

16
Conventional direction of electric current.

• The electrons are driven by ELECTROMOTIVE FORCE
  emf.

17
Emf of a cell.

• Emf of a cell is equal to the potential
  difference across its terminal when it is not
  producing current in a circuit.

18
Emf of a simple cell.

• The emf of a simple cell is about 1.0 volts.

19
Emf of a simple cell.

• If a small pocket bulb is connected to the
  terminals of acell its filament will glow for a
  few minutes.

20
Action of a simple cell.
e
Zinc rod (-)
Copper rod ()
H
SO42-
e
H
Zn 2
Dilute sulphuric acid
H
H2SO4
21
Action of a simple cell.
e
e
Zinc rod (-)
H
Copper rod ()
SO42-
H
Zn 2
Dilute sulphuric acid
H
H2SO4
22
Action of a simple cell.
e
e
e
H
Zinc rod (-)
Copper rod ()
SO42-
H
Zn 2
H
Dilute sulphuric acid
H2SO4
23
Action of a simple cell.
e
e
e
e
Zinc rod (-)
H
Copper rod ()
SO42-
H
Zn 2
Dilute sulphuric acid
H
H2SO4
24
Action of a simple cell.
e
e
e
e
e
Zinc rod (-)
H
Copper rod ()
SO42-
H
Zn 2
H
Dilute sulphuric acid
H2SO4
25
Action of a simple cell.
e
e
e
e
e
e
H
Zinc rod (-)
Copper rod ()
SO42-
H
Zn 2
H
Dilute sulphuric acid
H2SO4
26
Action of a simple cell.
e
e
e
e
e
e
e
Zinc rod (-)
Copper rod ()
SO42-
H
H
Zn 2
Dilute sulphuric acid
H
H2SO4
27
Action of a simple cell.
e
e
e
e
e
e
e
e
Zinc rod (-)
H
Copper rod ()
SO42-
H
Zn 2
Dilute sulphuric acid
H
H2SO4
28
Action of a simple cell.
e
e
e
e
e
e
e
e
e
Zinc rod (-)
H
Copper rod ()
SO42-
H
Zn 2
H
Dilute sulphuric acid
H2SO4
29
Action of a simple cell.
e
e
e
e
e
e
e
e
e
Zinc rod (-)
H
Copper rod ()
SO42-
H
Zn 2
H
Dilute sulphuric acid
H2SO4
30
Faults of a Simple cell.

• The emf of a simple cell quickly falls to a
  small value From about 1.0 V to 0.2 V.

31
Polarisation.

• The defect results from formation of a layer
• of Hydrogen bubbles on the Copper Plate and
  is called Polarisation.

32
Polarisation.

• The hydrogen layer sets up a back emf in the cell
  in opposition to that due to Copper and Zinc.

33
Polarisation.

• The hydrogen gas partially insulates the plate
  hence increases the internal resistance of the
  cell.

34
How to stop Polarisation.

• The original emf of the cell can be restored by
  adding a depolarizing agent Potassium dichromate
  to the cell which oxidises hydrogen to water.

35
Local Action.

• If the Zinc used is impure Bubbles of hydrogen
  will be seen coming off the Zinc.

36
Local Action.

• This is called Local Action

37
Local Action.

• Local action is produced by the presence of
  impurities such as
• carbon

38
Local Action.

• Local action is produced by the presence of
  impurities such as
• carbon
• iron

39
Local Action.

• Which set up tiny local cells at the Zinc
  surfaces.

40
Local Action.

• Bubbles of hydrogen are given off from the
  impurity and the surrounding Zinc slowly
  dissolves in the acid.

41
Local Action.

• This merely wastes the Zinc.

42
To prevent Local Action.

• . To prevent local action the Zinc is cleaned and
  a small globule of mercury is rubbed over the
  surface with a piece of cotton wool.

43
To prevent Local Action.

• The mercury forms a bright coating of Zinc
  amalgam all over the surface.

44
To prevent Local Action

• The amalgam covers up the impurities and
  prevents them from coming into contact with the
  acid.

45
Thats the end of the lesson.

• Get ready to answer some revision questions

46
Revison Questions.

• State five materials required to make a simple
  cell in a lab
• .............................
• ...............................
• ..............................
• ..............................
• ..............................

47
Revision questions.

• When Copper and Lead are joined by a wire when
  dipped in sulphuricacid
• ..................... Slowly dissolves into the
  sulphuric acid.

48
Revision questions.

• When Copper and Lead are joined by a wire when
  dipped in sulphuricacid
• ii .................... ......................
  .. are formed on the Copper plate.

49
Revision questions.

• When Copper and Lead are joined by a wire when
  dipped in sulphuricacid
• iii a current of electrons drifts through the
  wire from the ................... To the
  ...........

50
Revision questions.

• 3 Conventional current direction is from
  .......................terminal of a cell to
  ........................ The terminal.

51
Revision questions.

• 4 Emf means .................................
  .............

52
Revision questions.

• 5 Electrons in a cell are driven by
  ................

53
Revision questions.

• Emf of a simple cell is
• A 0.2 V
• B 1.0 V
• C 1.5 V
• D 2.5 V

54
Revision questions.

• Presence of Emf is shown by .....................
  .....................

55
Revision questions.

• Emf of a Simple cell quickly falls to a small
  value due to formation .................
  ........... on the copper plate
• This defect is called.............................
  ....

56
Revision questions.

• 10 The hydrogen layer sets up a ...........
  .........and partially insulates the plate hence
  increases the .................. ................
  of the cell.

57
Revision questions.

• The original emf of the cell can be restored by
  adding a ........................ called
  .............................. ...................
  ...............

58
Revision questions.

• If impure zinc is used in a simple cell, then
  local action can be prevented by using
  ................... .....................

59
The end

• This work was Produced and presented by
• John Ssemmondo
• P O Box 1901
• Kampala
• tel [phone]
• basemondo_at_hotmail.com