Electrolysis of Solutions and Getting Reactive Metals Out of Ores
In a solution the water competes with the dissolved ions, so the products follow reactivity rules. Aluminium extraction and electroplating are the industrial cases, and both are energy-hungry.
What a learner can do afterwards
- Predicts the products of electrolysing a named aqueous solution using the reactivity of the ions present
- Explains why aluminium is extracted by electrolysis while iron is reduced with carbon
- Explains why cryolite is added to the aluminium oxide and what it saves
- Explains why recycling a metal uses far less energy than extracting it from ore
1 · Read
In a solution the water competes with the dissolved ions, so two rules pick the products. At the cathode, hydrogen forms if the metal is more reactive than hydrogen, and the metal plates out if it is less reactive. At the anode, oxygen forms from water unless halide ions are concentrated, in which case the halogen is released.
Try the rules on named solutions. Copper sulfate gives copper at the cathode, since copper is less reactive than hydrogen. Sodium chloride solution gives hydrogen, since sodium is more reactive. Concentrated brine gives chlorine at the anode because the chloride is concentrated, leaving sodium hydroxide in solution.
Extraction is the same battle against reactivity. Iron sits low enough that carbon can snatch the oxygen from its oxide in a blast furnace. Aluminium clings to oxygen far too tightly for carbon, so only electrolysis of molten aluminium oxide can free it. The oxide is dissolved in cryolite, which lowers the melting point and saves enormous energy.
Recycling a metal uses far less energy than extracting it from ore. Melting clean scrap skips mining, purifying the ore and the huge electrical bill of electrolysis. That is why aluminium cans are worth collecting.
Reactivity rules pick the products, cryolite cheapens aluminium, and recycling beats extraction on energy.
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