Topic 4 of 8 · Form 4
Electrochemistry
Decomposition of an ionic compound, molten or in aqueous solution, by electricity. Cations move to the cathode (reduction); anions move to the anode (oxidation).
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Electrolysis
Decomposition of an ionic compound, molten or in aqueous solution, by electricity. Cations move to the cathode (reduction); anions move to the anode (oxidation).
- Molten lead(II) bromide: lead at the cathode; bromine at the anode.
- Concentrated aqueous sodium chloride: hydrogen at the cathode; chlorine at the anode; sodium hydroxide remains.
- Dilute sulfuric acid: hydrogen and oxygen.
- Aqueous copper(II) sulfate with copper electrodes: copper transfers from anode to cathode (purification, electroplating).
Half-equations
Cathode: Cu²⁺ + 2e⁻ → Cu; Anode: 2Cl⁻ → Cl₂ + 2e⁻; 4OH⁻ → 2H₂O + O₂ + 4e⁻.
Products from aqueous solutions
At the cathode, hydrogen forms unless the metal is less reactive than hydrogen; at the anode, oxygen forms unless a halide is concentrated.
Industrial applications
Extraction of aluminium from bauxite dissolved in molten cryolite (carbon anodes burn away); electroplating; the chlor-alkali process.
Hydrogen–oxygen fuel cells
Produce electricity with water as the only product; advantages and disadvantages compared with petrol engines.
Check yourself
- Name the products of electrolysis of molten aluminium oxide.
- Why must the carbon anodes be replaced in aluminium extraction?
- Write the cathode half-equation for the electrolysis of dilute sulfuric acid.
Show answers
- Aluminium at the cathode, oxygen at the anode.
- Oxygen reacts with the hot carbon anodes, forming carbon dioxide.
- 2H⁺ + 2e⁻ → H₂
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