Chemistry - 6 Electrolysis - 6.3 The Extraction of Aluminium & 6.4 Electrolysis of Aqueous Solutions

Uses of aluminium (and its alloys) [7]:

- pans
- overhead power cables
- aeroplanes
- cooking foil
- drink cans
- window and patio door frames
- bicycle frames and car bodies

What compound is electrolysed to procure aluminium?

aluminium oxide, Al₂O₃

Where is aluminium oxide found?

bauxite ore

Aluminium oxide melting point

2050°C

How do we reduce the melting point of aluminium oxide?

mix it with cryolite

Cryolite-aluminium oxide mixture melting point

850°C

Aluminium oxide electrolysis reaction

2Al₂O₃ → 4Al + 3O₂

Aluminium forms at the ... electrode

negative

Oxygen is produced at the ... electrode

positive

Aluminium electrolysis cell [4]

- lined with carbon negative electrode
- molten aluminium is tapped or siphoned off
- carbon dioxide and oxygen gas emitted from anodes
- steel case

At the cathode (aluminium extraction):

Al³⁺ + 3e⁻ → Al

At the anode (aluminium extraction):

2O²⁻ → O₂ + 4e⁻

Reaction of oxygen with hot carbon anodes:

C + O₂ → CO₂

... have to be replaced regularly

Carbon anodes

Brine

water saturated with salt (sodium chloride)

Products of electrolysis of brine [3]

- chlorine gas
- hydrogen
- sodium hydroxide

At the anode (brine electrolysis):

2Cl⁻ → Cl₂ + 2e⁻

At the cathode (brine electrolysis):

2H⁺ + 2e⁻ → H₂

What is left after brine electrolysis?

sodium hydroxide, NaOH