Aluminium Flashcards
What are the impurities of aluminium?
Fe2O3
SiO2
Characteristics of aluminium
Exists as bauxite
Amphoteric in nature
White bauxite
1-4% Silica
Little iron
Red bauxite
3-25% iron (ii) oxide
1-7% silica
Purification of alumina
The impure bauxite is ground and heated with concentrated NaOH.
NaOH will react with the aluminium oxide and silicon dioxide, converting them to soluble compounds.
NaOH will not react with the iron (III) oxide.
Al2O3 (s) + 2NaOH (aq) + H2O (l) → 2NaAl(OH)4 (aq)
Sodium aluminate
SiO2 (s) + 2NaOH (aq) → Na2SiO3 (aq) + H2O (l)
Sodium silicate
The mixture is then filtered. The residue will be the iron (III) oxide.
This residue is called red mud.
The filtrate is then acidified by passing carbon dioxide gas through.
During this reaction, the aluminium is precipitated as aluminium hydroxide.
2NaAl(OH)4 (aq) + CO2 (g) → Na2CO3 (aq) + H2O (l) + Al(OH)3 (s)
The mixture is filtered and the residue is washed.
The aluminium hydroxide is then heated to produce pure aluminium oxide.
2Al(OH)3 (s) → Al2O3 (s) + 3H2O (g)
Electrolysis of aluminium oxide
The aluminium oxide is dissolved in a mixture of molten cryolite and calcium fluoride.
Al2O3 (s) → 2Al3+ + 3O2-
The electrolysis is carried out in a steam tank in which graphite is used as both the cathode and the anode.
Aluminium ions are discharged at the graphite cathode, forming molten aluminium.
Al3+ + 3e → Al (l)
The molten aluminium collects in the tank and is extracted from an opening at the base.
Oxide ions are discharged at the graphite anodes to form oxygen gas. The oxygen gas will then burn the carbon (graphite) anodes, forming carbon dioxide.
Hence, the graphite anodes must be replaced from time to time.
2O2- - 4e → O2 (g)
O2 + C → CO2
Electrolytic cell setup
Cathode (-): Carbon lining of the steel cell (where aluminium is collected).
Anode (+): Blocks of graphite (carbon) electrodes dipping into molten electrolyte, which react with oxygen.
Molten Aluminium: Forms at the bottom of the cell as a liquid.
Takes place in tanks lined with carbon.
Uses 5V and 40 000 amps
Requires a lot of energy to keep the temperature of the electrolyte stable for a long period of time.
Reduction at cathode
Al³⁺ ions gain electrons to form aluminium atoms:
The molten aluminium sinks to the bottom and is siphoned off (removed).
Oxidation at the anode
O²⁻ ions lose electrons and form oxygen gas, which reacts with the carbon anodes:
The oxygen gas reacts with the carbon anodes, therefore producing carbon dioxide (led away through the fume hood).
This reaction gradually burns away the carbon anodes, so they must be regularly replaced.
Why is aluminium used as an alloy for aircraft fuselages, car bodies and parts of ships?
Low density and cheaper than copper
Good strength/ mass ratio
Corrosion resistant (because a thin surface area of aluminium oxide forms on freshly made metal when exposed to air)
Why is aluminium used to make ladders?
Good strength/ mass ratio
Low density → makes it lighter
Why is aluminium used to make drink cans and roofing materials?
Malleability (easily shaped and flexible)
Cheap and non-toxic
Corrosion resistant (because a thin surface area of aluminium oxide forms on freshly made metal when exposed to air)
Why is aluminium used in electrical wiring?
Good electrical conductivity
Low density
Cheaper than copper, gold and silver
Why is aluminium used to make silver oloured paints, mirror reflectors etc
High reflectivity (92% of light is reflected) when finely powdered
Effects of production of alumina
Breaking bauxite ore → dust and fumes
Waste products produces red mud → drains into the soil → gets into waterways → poisons plant and animals because of materials in sludge
Kilns produce greenhouse gas and dust
Effects of electrolysis of alumina
Uses lots of electricity → releases GHG to produce
Reaction of carbon anodes + oxygen produces GHG (CO2)
Breaking up pots produces dusts containing cyanide, arsenic and other toxic compounds
Fluorine is produced more and more as the amount of aluminium in the cell decreases
Produces perfluorocarbons (powerful GHG)
Effects of quarrying bauxite ore
Destroys land that can be used for agriculture or area of natural beauty
Unsightly, noisy, and produce dust and fumes from explosives
Waste rocks from the ore can produce unsightly spoil heaps
Method of recycling aluminium
Aluminum scrap is collected and sorted to separate it from other materials.
The aluminum is cleaned to remove any coatings or contaminants.
The aluminum is then shredded or crushed into smaller pieces to increase surface area and facilitate melting.
The shredded aluminum is melted in a furnace at high temperatures.
The molten aluminum is then cast into ingots or other shapes for use in manufacturing new products.
