Extracting aluminium
Aluminium is more The tendency of a substance to undergo a chemical reaction. than carbon so it must be extracted from its A substance formed by the chemical union of two or more elements. using The decomposition (breakdown) of a compound using an electric current.. Even though aluminium is more If there is a lot of something, it is described as being abundant. than iron in the Earth's The outer layer of the Earth on top of the mantle., aluminium is more expensive than iron. This is mainly because of the large amounts of Energy transferred by electricity. used in the The process of obtaining a metal from a mineral, usually by reduction or electrolysis. process.
Electrolysis of aluminium oxide
The electrolyte
Aluminium A rock containing enough quantities of a mineral for extraction to be possible. is treated to produce pure aluminium oxide. The A substance which, when molten or in solution, will conduct an electric current. used in electrolysis are An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron).:
- in the A term used to describe a liquid substance (eg rock, glass or metal) formed by heating a solid. state, or
- A substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution. in water
Aluminium oxide is Unable to dissolve in a particular solvent. For example, sand is insoluble in water. in water, so it must be molten to act as an electrolyte. However, the The temperature at which a solid changes into a liquid as it is heated. of aluminium oxide is high. A lot of energy must be transferred to break its strong Ionic bonding forms between two atoms when an electron is transferred from one atom to the other, forming a positive-negative ion pair., and this is expensive. To reduce costs, powdered aluminium oxide is dissolved in molten cryolite. This ionic compound melts at a lower temperature than aluminium oxide, reducing costs. However, significant amounts of energy are required to melt the cryolite.
The electrolysis process
The diagram shows an electrolysis cell used to extract aluminium. Both A conductor used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode. are made of graphite, a form of carbon with a high melting point and which conducts electricity.
During electrolysis:
- at the The negative electrode during electrolysis., aluminium Electrically charged particle, formed when an atom or molecule gains or loses electrons. gain Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. and form aluminium The smallest part of an element that can exist.
- at the The positive electrode during electrolysis., oxide ions lose electrons and form oxygen gas
The oxygen reacts with the carbon anodes, forming carbon dioxide. So the anodes are gradually The gain of oxygen, or loss of electrons, by a substance during a chemical reaction.. They must be replaced frequently, adding to the cost of producing aluminium.
Worked example - Higher
Explain, with the help of a An equation, involving ions and electrons, that describes the process happening at an electrode., how oxide ions are oxidised during the electrolysis of aluminium oxide.
The half equation is: 2O2- 鈫 O2 + 4e-. It shows that oxide ions lose electrons, and oxidation is loss of electrons.
Question
Explain, with the help of a half equation, how aluminium ions are The loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction. during the electrolysis of aluminium oxide.
The half equation is: Al3+ + 3e- 鈫 Al
It shows that aluminium ions gain electrons, and reduction is gain of electrons.