oxide mixed with carbon powder dipped in

ammonium chloride paste in a zinc case

(negative terminal).

Leclanché cell

In Leclanché cell In a dry cell the

the electrolyte electrolyte is in

used (ammonium form of paste of

Dry cell

chloride)

in ammonium

liquid form

chloride

mixed

with starch

insulating top seal?

The action of the dry cell

answers

During the working of the cell, zinc is

converted into zinc chloride and hydrogen

The insulating top seal prevents

ammonium

drying up

chloride

jelly

from

gas is produced.

Manganese (IV) oxide

converts hydrogen produced into water.

Hence polarization is minimized. The

carbon powder acts as catalyst, i.e. it

quickens the working of the manganese (IV)

oxide.

(d) What is the function of mixture of

powdered carbon and Manganese

(IV) oxide?

Answers

The manganese

(IV)

oxide

acts

Note

depolarizer; it reacts with hydrogen formed

hence it minimizes polarization

The paste is prevented from drying by

sealing the top of cell with some insulating

materials.

The carbon powder acts as catalyst, i.e. it

quickens the working of the manganese (IV)

oxide.

Example 01

(a) Draw a well labeled diagram of a dry

cell

Example 02

Explain why Leclanché cell is not suitable

for use as a continues source of energy?

Answers

This is because cannot be recharged once it

loses its electrical power

Example 03

When a bulb is connected between the two

plates of a simple cell, the bulb lights up.

(b) How do the components of a dry cell

differ from those of Leclanché cell?

However, the brightness fades about a

minute.

the opposite direction to that in which the

cell usually supplies current in an external

circuit.

(i) Explain why the bulb lights up.

A secondary cell must first be charged with

electricity. The electrical energy is stored in

a chemical form. When the cells is in use,

the stored chemical energy is once again

Answers

Electrical energy is produced by a chemical

action between the plates of the cell – the

zinc and copper electrodes in dilute

sulphuric acid. This energy is converted into

heat and light energy in the bulb. The bulb

then lights up.

changed

electrical

energy.

Also,

secondary cells are called storage cells or

accumulator, some common accumulators

are:

1. Lead Acid accumulators

(ii) Why its brightness fall? How is the

effect minimized in a dry cell?

2. Nickel – Cadmium accumulators

3. Alkaline and chloride accumulators

Answers

ADVANTAGES OF A SECONDARY

CELL

This is due to polarization caused by the

collection of hydrogen bubbles on the

copper plate which prevents further 퐻⁺ions

from reaching the plate. A back e.m.f is set

and the p.d drops. The bulb then becomes

dim, after sometimes it will not light up at

all.

The main advantage of this type of cell is

that it has a very low internal resistance and

can therefore give a large current with very

little

drop

the

terminal

potential

difference.

Polarization

prevented

using

DISTINGUISH BETWEEN PRIMARY AND

SECONDARY CELLS

depolarizers such as potassium dichromate

which will oxidize hydrogen to water.

Primary cell

(i) It cannot be used

after

Secondary cell

It can be useful

after use

Uses of dry cell

Dry cells are used in operating radio's,

(ii) It cannot be

recharged

It can be recharged

electronic

electronic like torches.

SECONDARY CELL

calculator

and

other

small

(iii)Have low voltage

Have high voltage

This is the cell which can be recharged after

it has run down (used).

(iv)Example are dry

cell

Example are lead

acid accumulator

The main advantage of lead – acid

accumulator is its ability to be recharged. It

major disadvantage is its size and weight

Charging of secondary cell

This is done by passing a d.c current from a

dynamo or similar device through the cell in

LEAD-ACID ACCUMULATOR

It consists of positive and negative plates

which are made of lead – antimony alloy.

The plates have a mesh – like grid. The

positive plate is packed with Lead (IV)

oxide while the negative plate is packed

with spongy Lead. The plates are dipped

into sulphuric acid which is the electrolyte.

It is known as Lead acid battery.

The separators prevent the positive and

negative plates from coming into contact

thereby avoiding production of a short

circuit.

Charging the lead – acid battery

Charging the lead acid – battery: -is to

drive all the acid out of the plates and return

it to the electrolyte

During charging, a direct current is passed

through the battery with the positive

terminal of the charger connected to the

positive terminal of the cell while negative

terminal of the charger to the negative

terminal of the cell as shown below.

The cell has a screw cap which can be

removed when checking the density of

sulphuric acid. When fully charged, the

relative density is 1.28 to1.15 when fully

discharged. The cape has a small vent – hole

to allow gases to escape.

As electrical energy is taken from the cell,

the density of sulphuric acid falls. The cell is

said to be discharged if the relative density

is below 1.18. To regain energy, the cell is

recharged by a direct current (d.c) source,

with the positive terminal of the charging

unit connected to the positive terminal of the

cell.

The aim of charging battery is to drive all

acid out of the plates and return it for the

electrolyte.

ACTION OF THE LEAD ACID

BATTERY

The maximum recommended discharged

75% of that maximum voltage. When the

battery is full discharged both electrodes are

completely converted function cannot be

recharged.

It consists more than one lead ferrous cell,

made up of lead peroxide as anode, porous

lead metal as cathode and sulphuric acid as

electrolyte.

An electrode is separated by

insulator called separator.

Cathode electrode: Spongy lead plate (–)

Anode Electrode: Lead (iv) oxide (+)

Note When charging the battery, the caps of

the lead acid accumulator are opened to

allow gases to escape from the battery.

Discharging accumulator

available or when connecting to a

main supply of electricity would be

inconvenient.

This is the process of using charges stored in

the lead – Acid Accumulator. During

discharging, the positive terminal of the

charger is connected to the negative terminal

of the cell while negative terminal of the

charger to the positive terminal of the cell as

shown below.

(ii) Portable radios, torches, calculators

and watches are example of devices

that use primary cells.

(iii)It is possible to buy rechargeable

batteries for these devices. These are

secondary cells to start the engine

and to run all the electrical circuits.

(iv)This cell is recharged by the

alternator when the car is in use.

CARE

FOR

LEAD

ACID

When the accumulator is discharged after

long use both plates become coated with

lead sulphate.

ACCUMULATOR

1. The cell should be charged regularly and

should never be left fully discharged. Cars

have alternators that automatically charge

the battery.

THE ALKALINE ACCUMULATOR

This is the accumulator that uses alkaline

solution as electrolyte

2. The acid level should be maintained by

adding distilled water where necessary.

3. The terminal should be kept clean and

greased.

Advantages of alkaline accumulator over

lead acid accumulator

(i) It is portable

4. Rough handling should be avoided. For

example, dropping battery down

(ii) It requires little attention to maintain as

compared to the lead acid accumulator

(iii)Can be placed in a discharged state for

a long time

5. The cell should not be short circuited, that

is allowed to drive large current or

connected terminal of the battery to each

other.

Disadvantages

(i) It is more expensive

6. When charging the rate specified by

manufacturer should not be exceed.

(ii) It has a lower E.M.F per cell

Uses of electric cells

7. Should be stored in a dry place and not on

wooden supports. Not on the floor

(i) Electric cells are very useful when

no mains supply of electricity is