Electrolysing aqueous solutions of ionic compounds can be more complicated than electrolysing molten compounds, because the water molecules can provide hydrogen ions (H+) and hydroxide ions (OH–), in addition to the ions from the ionic compounds.
Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:
For example, the electrolysis of copper(II) chloride solution produces copper at the negative electrode. However, the electrolysis of sodium chloride solution produces hydrogen. Note that the electrolysis of sodium chloride solution produces chlorine gas and hydrogen gas but also leaves a solution of sodium hydroxide as well.
If the negative ion from the ionic compound is simple (eg Cl– or Br–), then that element is produced. If the negative ion is a compound ion (eg NO3–, SO42–, CO32–), then oxygen is produced from the hydroxide ion present instead.
The table summarises some of the elements you should expect to get during electrolysis.
|Negative ion in solution||Element given off at positive electrode|
|Chloride, Cl–||Chlorine, Cl2|
|Bromide, Br–||Bromine, Br2|
|Iodide, I–||Iodine, I2|
This table shows some common ionic compounds (in solution), and the elements released when their solutions are electrolysed using inert electrodes, eg carbon electrodes:
|Ionic substance||Element at –||Element at +|
|Copper(II) chloride, CuCl2||Copper, Cu||Chlorine, Cl2|
|Sodium chloride, NaCl||Hydrogen, H2||Chlorine, Cl2|
|Hydrochloric acid, HCl||Hydrogen, H2||Chlorine, Cl2|
As shown above, the electrolysis of sodium chloride solution will produce chlorine gas at the anode and hydrogen gas at the cathode.
During the electrolysis, hydrogen and chloride ions are removed from solution whereas sodium and hydroxide ions are left behind in solution. This means that sodium hydroxide is also formed during the electrolysis of sodium chloride solution. This process is carried out on an industrial scale using sea water to produce hydrogen gas, chlorine gas and sodium hydroxide solution.