11.7 Quantitative
Electrolysis
The aim of the investigation is to use the electrolysis of dilute
sulphuric acid to confirm the theoretical quantity of electricity
required to produce one mole of hydrogen.
1.
Theoretical Quantity
The ion-electron equation for the formation of hydrogen
gas is:
2H+(aq) + 2e-
H2(g)
The equation shows that two moles of electrons are
needed to produce one mole of hydrogen gas.
The quantity of charge needed
=
=
2 x 96,500
193,000 C
2.
Electrolysis Experiment
The purpose of this activity is to record the time taken to produce a
measured volume of hydrogen during the electrolysis of dilute
sulphuric acid using a constant current.
The apparatus was set up as shown above.
The current was passed through the solution for about 1
minute to allow the electrodes to become saturated with
gas.
The current and timer were switched on at the same time
and left until 20cm3 of hydrogen had been collected.
A current of 0.5 A was maintained by adjusting the variable
resistor throughout the experiment.
Results
Current
0.5 A
Time
318 s
Volume Produced
21.5 cm3
3.
Calculation and Conclusion
Given that the molar volume of hydrogen is 24.08 l mol-1 the
quantity of charge required to produce 1 mole can be
calculated using the following steps:
Step 1: Calculate the charge passed in the experiment.
Q
= It
= 0.5 x 318
= 159 C
Step 2: Work out the charge needed to make 24.08 litres
(24,080 cm3) of hydrogen.
Volume
Charge
21.5 cm3
159 C
1
24,080
159
21.5
159 x 24,080
21.5
Charge needed for 1 mole of H2 = 178,080 C
This agrees with the theoretical value (within experimental
error).