Electrolysis

O Level / IGCSE Chemistry · Electrochemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Electrolysis uses an electric current to break down an ionic compound that is molten or dissolved in water. This liquid is the electrolyte. It conducts because its ions are free to move.

Positive ions move to the negative electrode, the cathode, and gain electrons (reduction). Negative ions move to the positive electrode, the anode, and lose electrons (oxidation). Electrons flow through the wires of the external circuit, never through the liquid.

A molten compound of two elements has only two kinds of ion, so the metal forms at the cathode and the non-metal at the anode. In water there are also H+ and OH− ions, so hydrogen or oxygen may form instead.

Rules to remember

Common mistake

Students often say sodium forms at the cathode when aqueous sodium chloride is electrolysed. Sodium is more reactive than hydrogen, so in a solution hydrogen forms; sodium metal forms only from the molten compound.

Worked example

Concentrated aqueous potassium bromide is electrolysed using graphite electrodes. What forms at each electrode?

  1. Ions present: K+ and Br− from the salt, H+ and OH− from the water.
  2. Cathode: potassium is more reactive than hydrogen, so hydrogen forms. 2H+ + 2e− → H2
  3. Anode: the solution is a concentrated halide, so the halogen forms. 2Br− → Br2 + 2e−

Answer: Hydrogen gas at the cathode and bromine at the anode (the solution near the anode turns orange). Potassium hydroxide is left in solution.

Practice questions

Try each one, then open the answer.

1. A steel spoon is to be electroplated with silver. Which arrangement is correct?

  1. A
    spoon as cathode, silver as anode, electrolyte dilute sulfuric acid
  2. B
    spoon as anode, silver as cathode, electrolyte copper sulfate
  3. C
    spoon as anode, silver as cathode, electrolyte silver nitrate
  4. D
    spoon as cathode, silver as anode, electrolyte silver nitrate
Show answer

Answer: D. The object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.

2. Which statement about the electrolyte in electrolysis is correct?

  1. A
    it is an ionic compound that is molten or dissolved in water
  2. B
    it is a solid metal whose free electrons carry the current
  3. C
    it conducts electricity by the movement of electrons through it
  4. D
    it is a covalent liquid that contains no charged particles at all
Show answer

Answer: A. An electrolyte is a compound that conducts because its ions are free to move, so it must be molten or dissolved.

3. Aqueous copper(II) sulfate is electrolysed using copper electrodes. What happens to the mass of the electrodes?

  1. A
    both gain mass
  2. B
    the anode loses mass, the cathode gains mass
  3. C
    neither changes
  4. D
    the anode gains mass, the cathode loses mass
Show answer

Answer: B. Copper dissolves from the anode (Cu → Cu2+ + 2e−) and is deposited on the cathode (Cu2+ + 2e− → Cu). This is the basis of copper refining and electroplating.

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