Electrolysis

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 electrolyte

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 cathode
  • Anions move to the anode
• Electrons flow in the external wire from the anode to the cathode, completing the circuit