In the electrolysis of aqueous solutions only one ion is involved in the selective discharge of ions at each electrode during the electrolysis. The ion which is selected for discharge at an electrode depends on a number of factors, including
- Position of the ions in the electrochemical series,
- For positive ions, the facility of discharge decrease in going from those least electropositive to those most electropositive. For example, if both copper and hydrogen ions are present in solution, it will be the copper ions which take electrons from the cathode to become copper atoms.
For negative ions, the ease of discharge decrease in going from those least electronegative to those most electronegative.
- Concentration of the Ions in Solution
- Irrespective of the position of the ions in the electrochemical series, there is a tendency to promote the discharge of the most concentrated ion present. For example, in concentrated sodium chloride solution (i.e. brine) , the two cations present are the chlorine ion and the hydroxyl ion. Although the hydroxyl ion is more easily oxidised than the chlorine ion, it is the chlorine ion which will be discharged because its concentration is much greater than that of the hydroxyl ion.
- Nature of the Electrode
- This is not as important as either of the other two factors, except in certain cases. For example in the electrolysis of molten sodium chloride using a mercury cathode, sodium ions are discharged in preference to hydrogen ions which are lower in the series.
The electrolysis of an aqueous solution of copper sulphate using copper electrodes (i.e. using active electrodes) results in transfer of copper metal from the anode to the cathode during electrolysis. The copper sulphate is ionised in aqueous solution.
CuSO4 ==> Cu(++) + SO4(-.-)
The positively charged copper ions migrate to the cathode, where each gains two electrons to become copper atoms that are deposited on the cathode.
Cu(++) + 2e(-) ==> Cu
At the anode, each copper atom loses two electrons to become copper ions, which go into solution.
Cu ==> Cu(++) + 2e(-)
The sulphate ion does not take part in the reaction and the concentration of the copper sulphate in solution does not change. The reaction is completed when the anode is completely eaten away. This process is used in electroplating.
The electrolysis of an aqueous solution of dilute sulphuric acid is often carried out in a Hofmann Voltammeter, an apparatus in which the gases evolved at the anode and cathode can be collected in separate graduated tubes. When the solution is electrolyzed hydrogen is produced at the cathode and oxygen at the anode. These gases can be shown to be present in a 2 to 1 ratio and result from the electrolysis of water under acidic conditions.
Sulphuric acid is a strong electrolyte is fully dissociated in aqueous solution.
H2SO4 ==> 2 H(+) + SO4(2 -)
Water is a weak electrolyte and is only slightly dissociated
H2O ==> H(+) + OH(-)
During electrolysis, the hydrogen ions migrates towards the cathode, and are discharged there (i.e. they gain an electron and are converted to hydrogen gas).
2 H(+) + 2 e(-) ==> H2
At the anode the concentration of hydroxyl ions is too low to maintain a reaction and the sulphate ions are not oxidized but remain on in solution at the end. Water molecules must be the species reacting at the anode.
2 H2O ==> O2 + 4 H(+) + 4 e(-)
The overall reaction is
- Cathode Reaction :
2 H(+) + 2e(-) ==> H2
4 H(+) + 4e(-) ==> 2H2
- Anode Reaction :
2 H2O ==> O2 + 4 H(+) + 4 e(-)
- Overall Cell Reaction:
4 H(+) + 2 H2O ==> 2 H2 + O2 + 4 H(+)
For every hydrogen ions discharged at the anode, another hydrogen ion is formed at the cathode. The net result is that the concentration of the sulphuric acid remains constant and this electrolysis consists of the decomposition of water with the overall reaction
2H2O ==> 2H2 + O2
Electrolysis of Molten Lead Bromide
A small quantity of solid lead bromide is taken in a silica crucible and two graphite electrodes (may be obtained from used torch cells) are inserted. A battery consisting of two dry cells is connected to the electrodes through a key and an ammeter.
Fig: 9.11 - Electrolysis of molten lead bromideWhen the key is pressed, no current flows through the system. This is because solid lead bromide does not conduct electricity. But, when the crucible containing lead bromide is heated the solid lead bromide melts. Now, on pressing the key, electricity flows through the system and a red brown gas (bromine) evolves at the anode and metallic lead deposits at the cathode.
The following reactions occur at the two electrodes:
Thus, electrolysis of lead bromide using graphite electrodes produces lead metal at the cathode and bromine gas at the anode.