Electrolysis of Molten Ionic Compounds (WJEC GCSE Chemistry: Combined Science)

What is electrolysis?

• When an electric current is passed through a molten ionic compound the compound decomposes or breaks down
• The process also occurs for aqueous solutions of ionic compounds
• Liquids and solutions that are able to conduct electricity are called electrolytes
• Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
• An electrolytic cell is the name given to the set-up used in electrolysis and which consists of the following:
  • Electrode: a rod of metal or graphite through which an electric current flows into or out of an electrolyte
  • Electrolyte: ionic compound in molten or dissolved solution that conducts the electricity
  • Anode: the positive electrode of an electrolysis cell
  • Anion: negatively charged ion which is attracted to the anode
  • Cathode: the negative electrode of an electrolysis cell
  • Cation: positively charged ion which is attracted to the cathode

The basic set-up of an electrolysis cell

The electrolyte must be molten or in solution for a current to flow 

• Lead(II) bromide is an ionic compound meaning that it is a compound consisting of just two elements joined together by ionic bonding
• When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge
• These compounds undergo electrolysis and always produce their corresponding element
• To predict the products of any binary molten compound first identify the ions present
• The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode
• Therefore the cathode product will always be the metal and the product formed at the anode will always be the non-metal

Diagram showing the electrolysis of lead(II) bromide

Lead is formed at the cathode and bromine gas is formed at the anode

Method:

• Add lead(II) bromide into a crucible and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge
• Add two graphite rods as the electrodes and connect this to a power pack or battery
• Turn on the power pack or battery and allow electrolysis to take place
• Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off
• Positive lead ions move to the negative electrode (cathode) and gain electrons to form grey lead metal which deposits on the bottom of the electrode

Electrode Products:

Anode: Bromide ions lose electrons (are oxidised) to form bromine gas

2Br^– → Br₂ + 2e^–

Cathode: Lead ions gain electrons (are reduced) to form lead metal

Pb²⁺ + 2e^– → Pb