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Chemistry 06204.1

Electrolysis

Electrolytes, electrodes, products at the anode and cathode, and half equations.

Review these first

Learning objectives

What you need to be able to do

Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.

  • 4.1.1Define electrolysis and identify the electrolyte, anode and cathode.
  • 4.1.2Predict the products of electrolysis of molten binary compounds.
  • 4.1.3Predict the products of electrolysis of aqueous solutions using the reactivity series and concentration.Supplement
  • 4.1.4Write ionic half equations for reactions at each electrode.Supplement

8 minute read

Electrolysis explained

Electrolysis is the breakdown of an ionic compound, when molten or in aqueous solution, by the passage of electricity.

The set-up

  • The electrolyte is the molten or dissolved ionic compound. It conducts because its ions are free to move.
  • The cathode is the negative electrode. Cations (positive ions) go there.
  • The anode is the positive electrode. Anions (negative ions) go there.

A memory hook that survives exam pressure: PANIC — Positive is Anode, Negative Is Cathode.

What happens at each electrode

At the cathode, positive ions gain electrons (reduction): Pb²⁺ + 2e⁻ → Pb

At the anode, negative ions lose electrons (oxidation): 2Br⁻ → Br₂ + 2e⁻

Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Molten compounds

Simple: the metal forms at the cathode, the non-metal at the anode. Molten lead(II) bromide gives lead and bromine.

Aqueous solutions

Water also supplies H⁺ and OH⁻ ions, so there is competition.

At the cathode: hydrogen is produced unless the metal is less reactive than hydrogen (such as copper or silver), in which case the metal is deposited.

At the anode: if a halide ion is present in reasonable concentration, the halogen is produced. Otherwise oxygen is produced from OH⁻ ions.

So concentrated sodium chloride solution gives hydrogen at the cathode and chlorine at the anode, leaving sodium hydroxide behind — the industrial chlor-alkali process.

Think of it like this

Electrolysis is a tug of war for electrons that the power supply referees: it forces electrons onto the cathode and pulls them off the anode, driving reactions that would never happen on their own.

Worked examples

Method, step by step

Molten zinc chloride is electrolysed. Give the products and the half equation at each electrode.

  1. 1The electrolyte contains Zn²⁺ and Cl⁻ ions.
  2. 2Cathode (negative): Zn²⁺ ions gain electrons — Zn²⁺ + 2e⁻ → Zn
  3. 3Anode (positive): Cl⁻ ions lose electrons — 2Cl⁻ → Cl₂ + 2e⁻

Zinc at the cathode, chlorine at the anode.

Common misconceptions

  • Mixing up anode and cathode. In electrolysis the anode is positive — PANIC.
  • Forgetting that water contributes ions in aqueous electrolysis, so the products are often not just the two elements in the salt.
  • Writing half equations that do not balance for charge. Count the electrons carefully.

In the exam

  • Always state the electrode *and* whether it is oxidation or reduction — both can be worth marks.
  • Half equations must balance for both atoms and charge; include the electrons explicitly.
  • For aqueous electrolysis, justify your prediction with the reactivity of the metal relative to hydrogen.