4CH1

Electrolysis

Principles of Chemistry · 2 question types

PractiseFlashcards

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable
Low
Stable7%

Electrode products, half-equations and OILRIG tested in most series.

Why most covalent substances do not conduct

  • An electric current is a flow of charged particles
    • In a metal wire the moving charges are electrons
    • In a molten or aqueous ionic substance the moving charges are ions
  • Most covalent compounds have no charged particles that are free to move, so they do not conduct
  • This makes them useful as electrical insulators in a wide range of forms
    • Solid: the plastic sheathing on household wiring (poly(ethene), PVC)
    • Liquid: silicone oils inside transformer cores
    • Gas: sulfur hexafluoride (SF6) inside high-voltage switchgear

Why ionic compounds conduct only when molten or dissolved

  • In the solid state, the ions sit in fixed positions within the giant ionic lattice and cannot move past one another, so the solid does not conduct
  • In the molten state, the lattice has broken down and the ions are free to move and carry charge through the liquid
  • In aqueous solution, the water separates the ions from each other and the dissolved ions are again free to move and carry charge
  • An ionic substance set up so that current can pass through it is called the
Electrical conductivity of ionic compounds — in the solid the ions are locked in a fixed lattice and cannot move, but when molten or in solution the ions are free to move and carry charge
Source: Electronic conductivity by Save My Exams
Exam tip

Why an ionic compound must be molten or dissolved to conduct

What comes up: "Explain why lead(II) bromide does not conduct electricity in the solid state but does conduct when molten."

Write (two marks): (1) In the solid state the ions are in fixed positions and cannot move. (2) When molten, the ions are free to move and carry charge through the liquid.

Watch out: The mark scheme credits "ions can move" for M2 and explicitly rejects any reference to electrons moving as the reason an ionic compound conducts — ionic conductivity comes from ions, not electrons.

Cations, anions, and the electrodes

  • A cation is a positively charged ion (Na+, Mg2+, H+)
  • An anion is a negatively charged ion (Cl−, O2−, SO42−)
  • In an electrolysis cell, two electrodes dip into the electrolyte and are connected to a d.c. supply
    • The cathode is the negative electrode
    • The anode is the positive electrode
  • Once the supply is switched on, the ions migrate toward the oppositely charged electrode
    • Cations move to the
    • Anions move to the
Electrolysis cell — a d.c. supply connected to two electrodes dipping into the electrolyte, with hydrogen forming at the negative cathode and oxygen at the positive anode
Source: Electrolysis cell by Save My Exams
  • Electrons flow in the external wire from the anode to the cathode, completing the circuit