The Process of Electrolysis (AQA GCSE Chemistry: Combined Science)

• 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

An electrolytic cell

The basic set-up of an electrolysis cell

Electrical Conductivity of Ionic Compounds

• Ionic compounds in the solid state cannot conduct electricity since they have no free ions that can move and carry the charge
• The ions must be able to move and can only do so in the molten state or when dissolved in a solution, usually aqueous
• When the cell is turned on and an electric current is passed through an electrolyte the ions in the solution start to move towards the electrodes

Movement of ions during electrolysis

• During electrolysis the electrons move from the power supply towards the cathode
• Electron flow in electrochemistry thus occurs in alphabetical order as electrons flow from the anode to the cathode
• Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode
• Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode

The flow of electrons and ions in electrolysis

Diagram showing the direction of movement of electrons and ions in the electrolysis of NaCl