3B Scientific Electrochemistry Kit - Manual (Page 19)

20
Experiment 7 - Measuring voltage Teacher's instructions
Measuring the voltage of a zinc-carbon (Leclanché) cell
Chemicals Hazard
symbols
R phrases S phrases Equipment
Meter
Graphite powder --- --- Electrodes:
1 C, 1 Zn
Manganese (IV) oxide 20/22 25 2 Experiment cables
Ammonium chloride 22-36 22 1 Plastic beaker
Distilled water --- --- 2 Pipettes
Warning: Be careful when handling chemicals!
Experiment procedure:
1. To assemble a zinc-carbon or Leclanché cell a 20% ammonium chloride solution and some depolarizing paste are
required. These should be given to the students.
2. Assemble the battery block as described.
3. Fill one chamber each with 20% ammonium chloride solution and insert the zinc electrode. Fill the chamber opposite
with a depolarizing paste consisting of graphite powder, manganese (IV) oxide (battery manganese) and ammonium
chloride solution and insert the carbon electrode. The depolarizing paste is there to prevent hydrogen forming at the
carbon electrode.
4. Connect the two electrodes to the meter using 2 experiment cables with crocodile clips and measure the voltage
generated by the Leclanché cell.
Observation and evaluation:
Batteries are important where there is no electric current. Batteries are galvanic cells that can convert chemical energy directly into
electrical energy.
The dry cell (Leclanché cell) is an economically important type of galvanic cell. It supplies a voltage of between 1.3 and 1.4 V, as can be
read from the meter.
C
+ +
Meter
-
Zn
Calculation of the mass of NH
4
Cl required to prepare 1 liter of a 20% solution:
Put the beaker on a set of scales and adjust the scales to show 0.0 g. Fill the beaker with 200 g of NH
4
Cl then add water until the scales
display a mass 1000 g . This makes up a 20% solution.
To make depolarizing paste for about 30 students:
45g of graphite powder should be well mixed with 225 g of manganese (IV) oxide and 225 ml of 20% ammonium chloride solution. The
resulting paste should not be stored for more than 1 day.
After use, the paste should be disposed of in the container for inorganic substances.