Electrolysis Principles (Cambridge (CIE) IGCSE Chemistry)

Electrolysis: general principles

• 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

• Covalent compounds cannot conduct electricity hence they do not 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

Particles in ionic compounds are in fixed position in the solid state but can move around when molten or in solution

Key terms used in a simple electrolytic cell

• 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

The basic set-up of an electrolytic cell

• Metals and hydrogen form positively charged ions and so either a metal or hydrogen gas is formed at the cathode 

• Non-metals form negatively charged ions and 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