Today, we will be talking about electrolysis of molten ionic compounds. Some ionic compounds are binary compound. A binary compound is a compound containing only two elements, usually a metal cation and a non-metal anion.

When a molten binary compound undergoes electrolysis, a metal and a non-metal are formed as products. Today, we will be talking about a binary compound such as Sodium Chloride. Molten sodium chloride contains mobile sodium ions (Na⁺) and chloride ions (Cl⁻). The figure below shows the chemical reaction occurring at the electrodes during the electrolysis of sodium chloride.


Sodium Chloride diagram



One thing important to note that electrolysis of compounds can only happen in molten or aqueous state. We can not decompose solid sodium chloride into its elements with electrolysis. This is because in solids, the ions of ionic compounds are held in fixed positions and are unable to move, hence they cannot carry charge.



Sodium chloride ions image





At the anode:

  • Negatively-charged Cl⁻ ions are attracted to the anode.
  • Cl⁻ ions lose electrons to form Chlorine gas. They are oxidized.
  • Cl⁻ ions are now discharged.

2Cl⁻(l)     →       Cl₂(g)  +   2ⅇ-

At the cathode:

  • Positively-charged Na⁺ ions ar attracted to the cathode.
  • Each Na⁺ ions gain one electron to form sodium atom. It is reduced.
  • Na⁺ ions are now discharged.

Summary:

The overall equation for the reaction is: 

2NaCl(l)     →     2Na(l)  +  Cl₂(g)

Why are inert electrodes used in the electrolysis of molten sodium chloride?

During the electrolysis, chlorine gas produced is very reactive. Inert carbon electrodes are thus used to prevent the reaction between the chlorine and the electrode. And by inert electrodes, we mean the electrodes that don’t react with the products of electrolysis or the electrolyte. Inert electrodes are usually made from Carbon and Platinum because they are rarely involved in the electrolytic reactions.