Action of a simple cell. Zinc rod (-) Copper rod (+) Dilute sulphuric acid H 2 SO 4 Zn 2+ H+H+ H+H+ H+H+ SO 4 2- e e.

Action of a simple cell.

Zinc rod (-)Copper rod (+)

Dilute sulphuric acid H2SO4

Zn 2+H+

H+

H+

SO42-

e

e

Action of a Simple cell.

To make a simple cell in a Laboratory, the following are required:

Copper rod.


To make a simple cell in a Laboratory, the following are required :

Copper rod. Zinc rod.


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

Copper rod. Zinc rod. dilute sulphuric acid.



Copper rod. Zinc rod. dilute sulphuric acid. Connecting wire.



Copper rod. Zinc rod. dilute sulphuric acid. Connecting wire Beaker 100 cm3.


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.



The Zinc rod slowly dissolves in the sulphuric acid.



Bubbles of Hydrogen are formed on the copper plate.


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


dilute sulphuric acid ionises according to the equation.

H2SO4(aq) 2H+(aq) + SO42-(aq)


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.


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

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)

Conventional direction of electric current.

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

Conventional direction of electric current.

The electrons are driven by ELECTROMOTIVE FORCE emf.

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.

Emf of a simple cell.

The emf of a simple cell is about 1.0 volts.

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.




Zn 2+H+

H+

H+

SO42-

e

e




Zn 2+

H+

H+

H+

SO42-

e

e




Zn 2+H+

H+

H+

SO42-

e

ee




Zn 2+H+

H+

H+

SO42-

e

eee




Zn 2+H+

H+

H+

SO42-

e

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Zn 2+H+

H+

H+SO4

2-

e

eeeee




Zn 2+H+H+

H+

SO42-

e

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Zn 2+H+

H+

H+

SO42-

e

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e




Zn 2+H+

H+

H+

SO42-

e

eeeeee

e

e




Zn 2+H+

H+

H+

SO42-

e

eeeeee

e

e

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.

Polarisation.

The defect results from formation of a layer

of Hydrogen bubbles on the Copper Plate and is called Polarisation.

Polarisation.

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

Polarisation.

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

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.

Local Action.

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

Local Action.

This is called Local Action

Local Action.

Local action is produced by the presence of impurities such as

carbon

Local Action.

Local action is produced by the presence of impurities such as

carbon iron

Local Action.

Which set up tiny local cells at the Zinc surfaces.

Local Action.

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

Local Action.

This merely wastes the Zinc.

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.

To prevent Local Action.

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

To prevent Local Action

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

That’s the end of the lesson.

Get ready to answer some revision questions

Revison Questions.

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

i. .............................

ii. ...............................

iii. ..............................

iv. ..............................

v. ..............................

Revision questions.

2 When Copper and Lead are joined by a wire when dipped in sulphuricacid

i. ..................... Slowly dissolves into the sulphuric acid.

Revision questions.


ii .................... ........................ are formed on the Copper plate.

Revision questions.


iii a current of electrons drifts through the wire from the ................... To the ...........

Revision questions.

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

Revision questions.

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

Revision questions.

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

Revision questions.

6 Emf of a simple cell is

A 0.2 V

B 1.0 V

C 1.5 V

D 2.5 V

Revision questions.

7 Presence of Emf is shown by ..................... .....................

Revision questions.

8 Emf of a Simple cell quickly falls to a small value due to formation ................. ........... on the copper plate

9 This defect is called.................................

Revision questions.

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

Revision questions.

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

Revision questions.

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

The end

This work was Produced and presented by

John Ssemmondo

P O Box 1901

Kampala

tel 075630283

[email protected]

Faults of a Simple cell

Action of a simple cell



Zn 2+H+

H+

H+

SO42-

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Zn 2+H+

H+

H+

SO42-



















Action of a simple cell. Zinc rod (-) Copper rod (+) Dilute sulphuric acid H 2 SO 4 Zn 2+ H+H+ H+H+ H+H+ SO 4 2- e e.

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