- The tendency of ions to be selected to be discharged at an electrode depends on their position in a series called the electrochemical series.
- The ions at the top of the list is more difficult to be discharged, but as we go down the table, they become easier to be discharged.
- During electrolysis, the ion in the lower position will be selected to be discharge.
- Figure below shows the electrochemical series for the positive and negative ions.
Example: Electrolysis of Aqueous Sulphuric Acid
- Figure above shows the set up of apparatus used to investigate the electrolysis of aqueous sulphuric acid.
- Carbon electrodes are used as the anode and cathode.
- The molecules of sulphuric acid ionise to form hydrogen ions and sulphuric ions.
H2SO4 → 2H+ + SO42-
- In an aqueous solution, water molecules will also ionise to form hydrogen ions and hydroxide ions.
H2O → H+ + OH-
- Therefore, the ions that present in the solution are H+ , SO42-, H+ and OH-
At anode (Positive Electrode)
- The negative ions (SO42- and OH-) will be attracted to the anode.
- OH- ions is lower than SO42- ions in electrochemical series, hence the OH- ions will be selected to be discharged at anode.
- The OH- ions discharged to form water and oxygen gas.
4OH- → 2H2O + O2 + 4e
Colourless gas bubbles are released around anode. When a glowing wooden splinter is inserted into the test tube that contain the gas, the wooden splinter light up.
At cathode (Negative Electrode)
- The positive ions (H+) are attracted to cathode.
- The H+ ions are discharged to form hydrogen molecule.
2H+ + 2e → H2
Colourless gas bubbles are released around cathode. When a lighted wooden splinter is brought close to the mouth of the test tube that contain the gas, a "pop" sound is produced.
Selective discharge occur at anode when there are more than one type of ions present. The ions located lower in the electrochemical series is selected to be discharge.