Electrolysis and electrolytic cells

Chemistry · Electrochemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An electrolyte conducts when it is fused (molten) or dissolved in water, because its ions are free to move. This is electrolytic conduction. Unlike conduction in a metal, it causes chemical changes at the electrodes.

In an electrolytic cell, electrical energy drives a reaction that would not happen on its own. Positive ions move to the cathode and are reduced. Negative ions move to the anode and are oxidised.

A fused salt gives its own elements. In an aqueous solution, water competes: it is reduced or oxidised instead of the ions of very reactive metals or of ions such as sulphate and nitrate.

Rules to remember

Electrolytic cell: cathode is negative, reduction occurs; anode is positive, oxidation occurs.
Fused NaCl: Na⁺ + e⁻ → Na at the cathode; 2Cl⁻ → Cl₂ + 2e⁻ at the anode.
Concentrated aqueous NaCl: hydrogen at the cathode (2H₂O + 2e⁻ → H₂ + 2OH⁻), chlorine at the anode, NaOH left in solution.
Aqueous Na₂SO₄ or NaNO₃ with inert electrodes: hydrogen at the cathode and oxygen at the anode.
Downs cell: fused NaCl gives sodium and chlorine; CaCl₂ is added to lower the melting point; iron cathode, graphite anode.
Nelson's cell: electrolysis of brine gives NaOH, H₂ and Cl₂; graphite anode, perforated steel cathode lined with an asbestos diaphragm.
Electroplating: object = cathode, plating metal = anode, electrolyte = a solution of a salt of the plating metal.

Common mistake

Students give sodium as the cathode product for aqueous sodium chloride. Sodium forms only from the fused salt. In water, hydrogen is produced because water is reduced far more easily than Na⁺ ions.

Worked example

Aqueous copper(II) chloride is electrolysed using graphite electrodes. What forms at each electrode?

Ions present: Cu²⁺ and Cl⁻ from the salt, with water also able to react.
Cathode (negative): Cu²⁺ is reduced more easily than water, so Cu²⁺ + 2e⁻ → Cu.
Anode (positive): chloride ions are oxidised, 2Cl⁻ → Cl₂ + 2e⁻.

Answer: Copper is deposited on the cathode and chlorine gas is given off at the anode.

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