3B Scientific Electrochemistry Kit - Manual (Page 21)

22
Experiment 8 - Measuring voltage Teacher's instructions
Measuring the voltage for various electrolyte concentrations
Chemicals Hazard
symbols
R phrases S phrases Equipment
Meter
Silver nitrate 34-50/53 26-45-60-61 Electrodes:
2 Ag
Distilled water --- --- 2 Experiment cables
2 Pipettes
Warning: Be careful when handling silver nitrate! Silver nitrate is corrosive!
Experiment procedure:
1. Silver nitrate solutions of various concentrations should be given to the students.
2. Assemble the battery block as described.
3. Fill two chambers located opposite one another in the battery block with 0.1 molar silver nitrate solution and insert a
silver electrode into both of them.
4. Fill two more chambers with 1 molar and 0.01 molar silver nitrate solution.
5. First measure the voltage for the Ag / AgNO
3
(0.1 mol/l) //Ag / AgNO
3
(0.1 mol/l) cell. Both silver electrodes should
be connected to the meter.
6. Remove one silver electrode from the 0.1 molar silver nitrate solution, rinse thoroughly with water and insert into the
1 molar silver nitrate solution. The voltage produced by the
Ag / AgNO
3
(0.1 mol/l) //Ag / AgNO
3
(1.0 mol/l) cell can now be read off from the meter.
7. Next remove the silver electrode from the 1 molar silver nitrate solution, rinse thoroughly with water again and insert
into the 0.01 molar silver nitrate solution. The voltage from the Ag / AgNO
3
(0.1 mol/l) //Ag / AgNO
3
(0.01 mol/l) cell
can now be read off from the meter.
8. For the next experiment remove the silver electrode from the 0.1 molar silver nitrate solution, rinse thoroughly with
water and insert into the 1.0 molar silver nitrate solution. The voltage from the Ag / AgNO
3
(1.0 mol/l) //Ag / AgNO
3
(0.01 mol/l) cell can now be read off from the meter.
Observation and evaluation:
Galvanic cell Measured voltage (V)
Ag / AgNO
3
(0,1 mol/l) // Ag / AgNO
3
(0,1 mol/l) --
Ag / AgNO
3
(0,1 mol/l) // Ag / AgNO
3
(1.0 mol/l) 0.058 V
Ag / AgNO
3
(0,1 mol/l) // Ag / AgNO
3
(0.01 mol/l) 0.058 V
Ag / AgNO
3
(1,0 mol/l) // Ag / AgNO
3
(0,01 mol/l) 0.116 V
Measuring a voltage from chains of electrolyte of equal concentration is impossible. To measure a voltage the electrolyte concentrations
have to be different. If the electrolyte concentrations differ by a factor of 10, the voltage measured is 0.058 V, regardless of whether
measurement is made between 1.0 mol/l and 0.1 mol/l or 0,1 mol/l and 0.01 mol/l concentrations. Each further increase by a factor of 10
increases the measured voltage by another 0.058 V. Due to the diffusion of the more concentrated electrolyte solution through the paper
diaphragm, the concentrations in the two half cells tend to become more equal so that the potential difference decreases over time.
In the more concentrated silver nitrate solution, silver ions are reduced to silver and in the more dilute solution the silver goes into
solution creating silver ions.. This means that the silver electrode in the more concentrated solution is the cathode (+ pole) and the one
in the more dilute solution is the anode (- pole).
+ +
0,1 mol/ l Meter
-
1.0 mol/l 0.01 mol/l 0.1 mol/l
To make 1 liter of the required solutions:
1.
To make a 1 molar solution of AgNO
3
solution, 169.8 g of AgNO
3
should be dissolved in a liter of water.
2. To make a 0.1 molar solution of AgNO
3
solution, 16.98 g of AgNO
3
should be dissolved in a liter of water.
3. To make a 0.01 molar solution of AgNO
3
solution, 1.69 g of AgNO
3
should be dissolved in a liter of water.