CIE IGCSE Chemistry

Revision Notes

4.1.1 Electrolysis Principles

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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

Molten ionic substances conduct electricity, IGCSE & GCSE Chemistry revision notes

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

 

Basics of electrolysis, IGCSE & GCSE Chemistry revision notes

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 

Exam Tip

Use the PANIC mnemonic to remember which electrode is the positive and which is the negative:

Positive (is) Anode Negative Is Cathode

Electrolysis: Charge Transfer

EXTENDED

  • 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

Transfer of Charge, IGCSE & GCSE Chemistry revision notes

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

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