Electrolysis of Molten Compounds (Edexcel GCSE Chemistry: Combined Science)

• Binary ionic compounds consists 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 decompose into their constituent elements
• Lead(II) bromide is an ionic solid with a relatively low melting point and can be used to illustrate the electrolysis of a molten compound

Diagram showing the electrolysis of lead(II) bromide

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

Anode: Brown fumes of bromine gas

Predicting the Products

• 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
• For example,
  • Zinc chloride will produce zinc at the cathode and chlorine at the anode