Electrolysis Principles (Cambridge International IGCSE Chemistry)

Electrolysis: general principles

Spec point — 4.1.1 · Define electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current
Identify in simple electrolytic cells: the anode as the positive electrode the cathode as the negative electrode the electrolyte as the molten or aqueous substance that undergoes electrolysis State that metals or hydrogen are formed at the cathode and that non-metals (other than hydrogen) are formed at the anode

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

Exam Hint 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

Spec point — 4.1.1 · Describe the transfer of charge during electrolysis to include: the movement of electrons in the external circuit the loss or gain of electrons at the electrodes the movement of ions in the electrolyte

Electrolysis: charge transfer

Extended tier only

How charge flows in electrolysis

• During electrolysis, current flows around the circuit as electrons and ions move

  • Electrons and ions transfer charge because they are the charged particles

• Electrons flow from the negative terminal of the power supply to the cathode

  • This gives the cathode its negative charge

• The electrons do not pass through the solution

• At the cathode, cations gain electrons to form atoms

• Meanwhile, anions move to the anode and lose electrons

  • These electrons then flow through the external circuit back to the power supply’s positive terminal, completing the circuit

• So, in a complete circuit:

  • Electrons are the charge carriers in the external circuit
  • Ions are the charge carriers in the electrolyte

Exam Hint In electrolysis, we focus on the movement of electrons, not the direction of conventional current:

• Electrons flow from the negative terminal to the positive terminal

  • They flow alphabetically, from the anode to the cathode

• Conventional current flows from positive to negative

Movement of ions in the electrolyte

• Positive ions (cations) in the electrolyte move towards the cathode

  • Reduction occurs at the cathode (gain of electrons)

• Negative ions (anions) in the electrolyte move towards the anode

  • Oxidation occurs at the anode (loss of electrons)

The flow of electrons and ions in electrolysis

Diagram showing the direction of movement of electrons and ions in the electrolysis of an electrolytic solution