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Cambridge IGCSE Chemistry · 0620

Chapter 4: Electrochemistry — Part 1

Topic 4.1a · Fundamentals of electrolysis and charge transfer

Defining Electrolysis

Electrolysis is defined as the decomposition of an ionic compound, when molten or in aqueous solution, by an electric current being passed through it.

Components of an Electrolytic Cell

An electrolytic cell consists of several key components:

  • Anode: The positively charged electrode which attracts negatively charged ions (anions) towards it.
  • Cathode: The negatively charged electrode which attracts positively charged ions (cations) towards it.
  • Electrolyte: An ionic compound in the molten or aqueous state that undergoes electrolysis. In this state, ions are free to move and are attracted toward the electrode with the opposite charge.
  • Power Supply: The electrodes are connected to a direct current (D.C.) supply.
A diagram of an electrolytic cell showing the anode (+), cathode (-), electrolyte, and the movement of ions toward the oppositely charged electrodes
Diagram 1: A diagram of an electrolytic cell showing the anode (+), cathode (-), electrolyte, and the movement of ions toward the oppositely charged electrodes

Exam Traps

  • Do not confuse anode with cathode charge — “an ox red cat” refers to redox, not electrode polarity.
  • Cations move to the cathode and anions to the anode; do not reverse ion movement in diagrams.

Transfer of Charge and Redox at Electrodes

For electrolysis to occur, charge must be transferred around the apparatus by specific charge carriers:

  • In the external circuit: The charge carriers are electrons. The D.C. power supply provides electrons to the cathode, causing it to become negatively charged. Electrons released at the anode transfer back to the power supply.
  • In the electrolyte: The charge carriers are the ions. Cations move toward the cathode, and anions move toward the anode.

The chemical changes at the electrodes are redox reactions:

  • Oxidation: Occurs at the anode, where anions lose electrons. This leads to an increase in oxidation number.
  • Reduction: Occurs at the cathode, where cations gain electrons. This leads to a decrease in oxidation number.
  • OILRIG Mnemonic: Oxidation Is Loss, Reduction Is Gain (of electrons).

Exam Traps

  • Do not say electrons move through the electrolyte — only ions move in the solution or melt.
  • Do not place reduction at the anode; “AN OX RED CAT” means oxidation at anode, reduction at cathode.

Constructing Ionic Half-Equations

Half-equations represent the movement of electrons (e-). The number of atoms and the overall charge must be balanced on both sides.

  • Example (Anode): 2Cl- ? Cl2 + 2e- (Chloride ions are oxidised to chlorine gas).
  • Example (Cathode): Al3+ + 3e- ? Al (Aluminium ions are reduced to aluminium atoms).

Exam Traps

  • Do not forget to balance electrons — 2Cl− → Cl2 needs + 2e− on the product side.
  • Half-equations show electron transfer at one electrode only; do not combine them into a full equation unless asked.

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