alkanes chemical properties

Higher and foundation tier

Aluminium uses and extraction

Aluminium is an attractive, lightweight, corrosion resistant metal. It is about one third the weight of steel. It is a good thermal and electrical conductor which makes it ideal for use in electrical appliances, where it is often used as a heat sink. Aluminium is a much better electrical conductor than copper which is why it is often used to make the power lines which carries our electricity across the country. It has many uses including in the aircraft industries, construction and building, motor manufacture and for many household items such as cooking foil, drinks cans, pots and pans and mobile phones. The image below shows a few of the uses of this versatile metal.

uses of aluminium metal

Aluminium is the third most abundant element in the Earth, its main ore is called bauxite. Bauxite contains a high proportion of aluminium compounds, chiefly aluminium oxide. It is named after a village in southern France called Les Baux, where it was first discovered in 1821.

Despite the fact that aluminium is the third most common element on Earth in the late 19th century it was more valuable than silver or gold and it was considered a rare metal. The reason for this is that before the discovery of electrolysis aluminium was extracted by carrying out a displacement reaction using sodium or potassium on aluminium compounds, this made it a very expensive metal indeed.

However with the discovery of the electrolytic process the cost of aluminium fell dramatically. However the extraction of aluminium by electrolysis was not as straightforward as might have been expected at first. In order to extract a metal from its ore by electrolysis the metal ore needs to be molten (melted to form a liquid) or dissolved to form a solution. However bauxite is not soluble in many solvents and it also has a very high melting point, in excess of 20000C. This made the extraction of aluminium from its ore difficult and expensive.

The solution to the problem was found by two scientists working independently from each other, Charles Hall an American scientist and Paul Heroult a French scientist found that aluminium oxide would dissolve in cryolite (Na3AlF6), a rare mineral found in Greenland.

If cryolite is heated to around 8500C then aluminium oxide will dissolve in it and form a solution which conducts electricity. This made it possible to extract aluminium by electrolysis from its ore cheaply and following this the cost of aluminium metal plummeted. Aluminium oxide (Al23+O32-) dissolves in the molten cryolite inside the cell where temperatures are around 8500C. When it dissolves its giant lattice structure is broken down and the aluminium and oxide ions are free to move. This is exactly the same as what would happen if an ionic compound was dissolved in water, only in this case it is dissolved in molten cryolite. The image below shows a typical cell used to extract aluminium.

cell for extracting aluminium

The cell is lined with a layer of graphite, this graphite is made the cathode and so will attract the aluminium ions. At the cathode the aluminium ions will be reduced (gain electrons) and form aluminium metal. Aluminium melts at 6600C and since the cell temperature is maintained around 8500C the aluminium produced will be a liquid. It collects at the bottom of the cell where it will be siphoned out when required. The half-equation for the cathode reaction is:

Al3+ + 3e → Al

At the positively charged anode the negatively charged oxide ions are attracted and oxidised (have electrons removed from them). The half equation is:
O2- → O + 2e
however oxygen gas is a diatomic gas so the equation must be multiplied by two. This gives:
2O2- → O2 + 4e
 
two oxide ions are oxidised on the anode surface and the two oxygen atoms produced join to form a molecule of oxygen. However to slightly complicate this we have to bear in mind that the anodes are made of graphite (a form of carbon) and they are at 8500C and so with oxygen gas produced on their surface they will simply react with it, they slowly burn away and have to be regularly replaced.
graphite (carbon) (s) + oxygen (g) → carbon dioxide(g)
C + O2 → CO2

Overall equation:

cathode: Al3+ + 3e → Al
anode: 2O2- → O2 + 4e
The aluminium ions gain 3e and the oxide ions lose 4 electrons, to balance these equations in terms of electrons simply multiply one equation by x3 and the other equation by x4 to give 12 electrons in each equation:
cathode: 4Al3+ + 12e → 4Al
anode: 6O2- → 3O2 + 12e
The electrons cancel out, there are 12 on each side of the equations, so combining both equations this leaves.
4Al3+ + 6O2- 4Al + 3O2
this equation is the same as:
2Al2O34Al + 3O2
This equation represents the decomposition of aluminium oxide during electrolysis.

Costs

One of the major costs in extracting aluminium is the cost and energy required to continually make new anodes as the old one burn away and turn into carbon dioxide gas. The other major consideration in aluminium extraction is the vast amount of electricity required to extract aluminium from its ore. Huge electrical currents in excess of 150 000 amps are used and so a cheap source of electricity nearby is required. A hydro-electric power station would be ideal!

Key points

Practice questions

Check your understanding - Questions on aluminium extraction.

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