Electrolysis (Oxford AQA IGCSE Chemistry)

Freely Moving Ions

• Ionic compounds have a giant lattice structure

• The lattices formed by ionic compounds consist of a regular arrangement of alternating positive and negative ions which are fixed in position

• For electrical current to flow there must be freely moving charged particles such as electrons or ions present 

• Ionic compounds are good conductors of electricity in the molten state or in solution as they have ions that can move and carry a charge

• They are poor conductors in the solid state as the ions are in fixed positions within the lattice and are unable to move

Ions in a fixed lattice and in molten / solution

What is Electrolysis?

• Electrolysis means the breaking down a substance using electricity

• An electric current is passed through an ionic compound in the liquid or aqueous state into its elements

  • E.g. Lead bromide will be broken up into lead metal and bromine gas

Key terms used in electrolysis

• Electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte

• Electrolyte is the ionic compound in a molten or dissolved solution that conducts the electricity

• Anode is the positive electrode of an electrolysis cell

• Anion is a negatively charged ion which is attracted to the anode

• Cathode is the negative electrode of an electrolysis cell

• Cation is a positively charged ion which is attracted to the cathode

 A diagram to show the basics of electrolysis

Moving Ions

• During electrolysis:

  • Positively charged ions move to the negative electrode (the cathode)

  • Negatively charged ions move to the positive electrode (the anode)

The electrolysis of lead bromide

• Lead(II) bromide is an ionic compound

  • It is a metal and non-metal joined together by ionic bonding

• When it is heated it becomes molten and can conduct electricity as the ions can move freely and carry the charge

Diagram showing the electrolysis of lead(II) bromide

• Negative bromide ions move to the positive electrode (anode)

  • They lose two electrons (are oxidised) to form bromine molecules

  • There is bubbling at the anode as brown bromine gas is given off

2Br^– → Br₂ + 2e^–

• Positive lead ions move to the negative electrode (cathode)

  • They gain electrons (are reduced) to form grey lead metal

  • This deposits on the bottom of the electrode

Pb²⁺ + 2e^– → Pb