Electrolysis of molten salts

New substances form when a molten or dissolved ionic compound conducts electricity. For example, lead(II) bromide can be used to produce bromine and lead using the following process:

  1. place solid lead(II) bromide in a crucible and heat over a Bunsen burner until it melts
  2. insert two carbon electrodes into the molten electrolyte and pass a direct current between them

Carbon electrodes are chosen because they have a high melting point and are inert (they will not react with the reactants and products during electrolysis). The observations are summarised in the table:

ObservationExplanation
Anode (positive)Bubbles of brown gasBromine vapour (Br2) is produced
Cathode (negative)Silvery liquid metal present at this electrode underneath the remaining molten electrolyteLead (Pb) is produced

It is easy to predict the products of electrolysis of molten electrolytes because they simply split into their elements:

  • the metal is formed at the negative electrode because that is where the positive metal ions are attracted
  • the non-metal element is formed at the positive electrode where the negative non-metal ions are attracted

For example:

CompoundAnode (positive)Cathode (negative)
Zinc chloride (ZnCl2)Chlorine (Cl2)Zinc (Zn)
Aluminium oxide (Al2O3)Oxygen (O2)Aluminium (Al)