4.3.4 Electrolysis of Aqueous Solutions

In the process of electrolysis, when an aqueous solution is electrolysed using inert electrodes, the ions that are discharged at the electrodes depend on the relative reactivity of the elements involved. The reactions occurring at the cathode (negative electrode) and the anode (positive electrode) can be understood as follows:

Cathode (Negative Electrode)

• If the metal in the aqueous solution is more reactive than hydrogen, hydrogen gas (H₂) is produced at the cathode.
• This occurs because hydrogen ions (H⁺) from the water molecules in the solution are attracted to the cathode and gain electrons, forming hydrogen gas.

Anode (Positive Electrode)

• If the solution does not contain halide ions (such as chloride, bromide, or iodide), oxygen gas (O₂) is produced at the anode.
• This is because water molecules in the solution break down into hydrogen ions (H⁺) and hydroxide ions (OH^-). The hydroxide ions are attracted to the anode and lose electrons, forming oxygen gas.

However, if the aqueous solution contains halide ions, such as chloride ions (Cl^-), bromide ions (Br^-), or iodide ions (I^-), a different reaction occurs at the anode:

• The halide ions are preferentially discharged at the anode, rather than the hydroxide ions. This means that chlorine gas (Cl₂) is produced if chloride ions are present, bromine gas (Br₂) is produced if bromide ions are present, and iodine gas (I₂) is produced if iodide ions are present.