Electrolysis Principles (Cambridge (CIE) IGCSE Chemistry) : Revision Note

Electrolysis: general principles

• Electrolysis is the process in which a molten ionic compound is broken down by an electric current

• The process also occurs for aqueous solutions of ionic compounds

• Covalent compounds do not conduct electricity, so they cannot undergo electrolysis

• Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that can move and carry the charge

In solids, ions are fixed in place. When molten or in solution, they are free to move and carry charge.

Key terms used in a simple electrolytic cell

• Electrode is a rod (usually metal or graphite) that conducts electricity 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

The basic set-up of an electrolytic cell

• Metals and hydrogen form positive ions, so a metal or hydrogen gas is formed at the cathode

  • The production of a metal or hydrogen gas depends on the reactivity series

  • If the metal is less reactive than hydrogen (e.g. copper, silver), then the metal is produced

  • If the metal is more reactive than hydrogen (e.g. sodium, magnesium), then hydrogen gas is produced instead

• Non-metals form negative ions, so non-metals (except hydrogen) are formed at the anode

Electrolysis: charge transfer

Extended tier only

• During electrolysis, current needs to flow around the circuit

• In order for this to occur, charge must be transferred around the circuit (current is a measure of the rate of flow of charge) by charge carriers

• The power supply provides the cathode with a supply of electrons, causing it to become negatively charged

• Positive ions (cations) in the electrolyte move towards the cathode where they gain electrons

• Negative ions (anions) in the electrolyte move towards the anode where they lose electrons

• The electrons move from the anode back towards the power supply

• So, in a complete circuit:

  • Electrons are the charge carriers in the external circuit

  • Ions are the charge carriers in the electrolyte

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