12th Physics Report

Topic : Effect of concentration of an electrolyte on the e.m.f. and internal resistance of a primary cell BY : CLASS : ROLL No.:

AIM To study the effect of concentration of an electrolyte on electromotive force and internal resistance of a primary cell .

APPARATUS • Primary Cell (Lechlanche Cell) • Carbon rod

(Positive Electrode)

• Porous Pot • Glass Vessel • Electrolyte

(Ammonium Electrode)

• Zinc Rod

(Negative Electrode)

• Battery • Potentiometer • Connecting Wire • 2 Volt Battery

THEORY Electromotive force is defined as the maximum potential difference that can exist between the terminals of a cell when the cell is in open circuit. The unit of electromotive force is Volts Dimensional formula of electromotive force is MLT

E = I (r + R) Where:E R r I

– – – –

Electromotive force of the cell Resistance Internal Resistance Current passing through the cell

PROCEDURE 1.

2.

3.

First make the circuit connections accordingly.

diagram.

Make

the

To test the correctness of the connection, insert the plug in the key K1. Now, by making the sliding contact at P & Q. Note the direction of the deflection in the galvanometer. If the deflection in the galvanometer is in mutually opposite direction then the connections are correct. If the deflection in the two cases is in same direction then either the connections are wrong or E.M.F of auxillary battery is less. Correct it. Now take out 2000 ohm resistance from the resistance box. Join along with galvanometer, adjust the rheostat, so that a null point is obtained on the forth wire of potentiometer. Introduce the 2000 ohm resistance plug in its position in the resistance box and by slightly adjusting the jockey

near the previous obtained position of the null point, obtain a sharp position of null point. Measure the balancing length from the end (P). Let it be l1. Again take out plug it into K2. Lower down the resistance, slide the jockey along the wire, and obtain the null point. Introduce 2000 ohm resistance, length from end be l2. 4. Make simultaneous readings. 5. Remove the plug.

6.

Wait some time; repeat the experiment, in every part different resistances must be taken.

PRIMARY CELL LECHLANCHE CELL: The electrolyte is a paste of ammonium chloride containing some zinc chloride. Positive electrode Carbon rod is surrounded by mixture of manganese dioxide, which prevents the formation of gaseous hydrogen, which could polarize the cell, thus acting as anti polarizing agent and powder carbon is used as internal resistance of the cell. Negative electrode is zinc rod. When external circuit is connected across cell, Zinc atom in contact with electrode ionizes, losing two electrons per atom, the electrons flow into the solution, the Zinc is thus Cathode. Zn → Zn2+ + 2eThe Ammonium ions of the electrolyte remove electrons from the carbon anode to which the electrons flow in from the external circuit, one thus has: 2NH4+ + 2e- → 2NH3 + H2

At the anode the hydrogen is neutralized by manganese dioxide at the anode and in the body of electrolyte. The Zinc ion combines with chloride ions to form chloride, so that the overall chemical reaction is: Zn + 2NH4Cl + 2MnO2 → ZnCl2 + 2NH3 + H2O +Mn2O3

Q being the energy released per formula units of reactance .This is the finally the chemical reaction, which releases the energy (and dissolved ammonia as well as water into the electrolyte). The reaction involves migration of ions into and out of electrolyte and thus the energy is lost of course in making the flow inside the cell be reduced by proper choice of material as well as of the size and shape of cell. The e.m.f of the dry cell is 1.5 volts. Its internal resistance can vary from 0.1 to 10 ohms.

OBSERVATIONS

S.No. Amount Concentration E.M.F(Volts)

Internal Resistance (Ohms)

1.

150 ml 1.00 M

2.328

5.37

2.

150 ml 1.00 M

1.987

6.28

3.

150 ml 0.50 M

1.564

8.54

4.

150 ml 0.20 M

1.478

9.8

5.

150 ml 0.05 M

1.264

14.6

of Water of Electrolyte

RESULT The electromotive force is inversely proportional to the change in concentration of electrolyte.

PRECAUTIONS • Adjust the rheostat for null point. • E.M.F. (Battery) >> E.M.F. (Cell). • During on set, Ammeter reading should be constant. • Current should be for short time. • Jockey should not be rugged.