Almost every metal is found joined to oxygen in an ore. To get the metal, you must take the oxygen away. For some metals this is easy. For others it takes a huge amount of electricity.
It matters because aluminium is in aircraft, drinks cans and window frames, and the way it is made explains why recycling it is so worthwhile. It is also a favourite 6 mark exam question.
Course note: the choice of method, the aluminium cell and its products are for both tiers. Half equations, oxidation and reduction in terms of electrons, and the reacting mass calculation are Higher tier.
Choosing the method
Taking oxygen away from a metal oxide is called reduction. Gaining oxygen is oxidation. Carbon is a cheap way to reduce an oxide, but only if carbon is more reactive than the metal. A more reactive element takes the oxygen from a less reactive one.
So look at the reactivity series. Metals below carbon, such as zinc, iron and copper, are made by heating the oxide with carbon. Metals above carbon, such as aluminium, magnesium, calcium, sodium and potassium, hold on to their oxygen too tightly. They are extracted by electrolysis of a molten compound. Metals at the bottom, such as gold, are found as the element itself.
Electrolysis costs more than carbon, because it uses a great deal of electricity. That is why a metal is only made this way when carbon will not work.
The aluminium cell
Aluminium is found as aluminium oxide in an ore called bauxite. The ore is purified first. Aluminium oxide melts above 2000 °C, which would cost far too much energy. So it is dissolved in molten cryolite. The mixture melts at about 950 °C. It is like grit on an icy road, which lets the ice melt at a lower temperature.
Electrolysis needs free ions, so the oxide must be molten or dissolved. The cell is a steel tank lined with carbon. The carbon lining is the cathode, and large carbon blocks dip in from above as the anodes. A very large direct current passes through the molten mixture.
Why not use aluminium chloride solution instead? Aluminium is more reactive than hydrogen, so a solution would give hydrogen at the cathode and no aluminium. The compound has to be molten.
An aluminium extraction cell. A steel tank lined with carbon, which is the cathode, holds molten aluminium oxide dissolved in cryolite at about 950 degrees Celsius. Carbon anodes dip in from above. Molten aluminium collects at the bottom and is tapped off.
What happens at each electrode
Aluminium ions, Al³⁺, are positive, so they move to the cathode. They gain electrons and become aluminium atoms. The molten aluminium is denser than the mixture, so it sinks to the bottom and is tapped off.
Oxide ions, O²⁻, are negative, so they move to the anodes. They lose electrons and become oxygen. At about 950 °C the oxygen reacts with the carbon anodes to make carbon dioxide: C + O₂ → CO₂. The anodes burn away, so they have to be replaced regularly.
Higher tier: cathode, Al³⁺ + 3e⁻ → Al. Anode, 2O²⁻ → O₂ + 4e⁻. At the cathode there is gain of electrons, which is reduction. At the anode there is loss of electrons, which is oxidation.
Higher tier: building the overall equation
Each half equation must lose or gain the same number of electrons before you add them. The cathode half equation, Al³⁺ + 3e⁻ → Al, takes three electrons. The anode half equation, 2O²⁻ → O₂ + 4e⁻, gives four. The smallest number that both divide into is 12.
Multiply the cathode equation by 4 and the anode equation by 3. This gives 4Al³⁺ + 12e⁻ → 4Al and 6O²⁻ → 3O₂ + 12e⁻. Add them. The 12 electrons cancel. The result is 4Al³⁺ + 6O²⁻ → 4Al + 3O₂.
The 4Al³⁺ and 6O²⁻ ions come from 2Al₂O₃, so the overall equation is 2Al₂O₃ → 4Al + 3O₂. Check that the atoms balance: 4 aluminium and 6 oxygen on each side. Aluminium oxide is split by electricity, which is why this change is called decomposition.
Why it costs so much
Extracting aluminium uses a very large amount of electricity. Energy is needed to melt the oxide mixture and keep it at about 950 °C. A huge current is needed to turn the ions into atoms. The carbon anodes also have to be replaced.
To cut costs, plants are built near cheap electricity, such as hydroelectric power. Cryolite lowers the temperature, which saves energy. Recycling aluminium is better still. Melting scrap uses only about 5% of the energy of making new metal, and it also saves mining the ore and dumping waste.
Compare iron. Iron is made by heating its oxide with carbon, which is much cheaper, because the carbon both supplies the energy and removes the oxygen. Aluminium cannot be made this way because it is more reactive than carbon.
Higher tier: reacting masses
Use the overall equation 2Al₂O₃ → 4Al + 3O₂ and the four steps from the moles lesson. Turn mass into moles, use the ratio, then turn moles into mass. The ratio of Al₂O₃ to Al is 2 : 4, which is 1 : 2.
Worked example. What mass of aluminium can be made from 51 tonnes of aluminium oxide? Mr of Al₂O₃ = 2 × 27 + 3 × 16 = 102. Moles of Al₂O₃ = 51 ÷ 102 = 0.50 (in million moles, because tonnes were used). Moles of Al = 2 × 0.50 = 1.0. Mass of Al = 1.0 × 27 = 27 tonnes.
Check that the answer is sensible: the aluminium is lighter than the oxide, as it has lost its oxygen. 27 tonnes is less than 51 tonnes. The oxygen lost has a mass of 24 tonnes, and 27 + 24 = 51.
Exam technique and recap
Command words. State: give one fact. Name: give the substance. Explain: give a reason and use because. Compare: write about both things, with a link word such as whereas. Describe: say what happens, in order.
For a 6 mark answer on aluminium, include: the ore or oxide; dissolved in molten cryolite to lower the melting point; carbon electrodes; aluminium forms at the cathode and oxygen at the anode; the anodes burn away; and a reason for the high energy use.
Recap. Metals above carbon are extracted by electrolysis of a molten compound. Aluminium oxide is dissolved in cryolite. Aluminium forms at the cathode and oxygen forms at the carbon anodes, which burn away. The process is expensive because of the electricity. Recycling saves about 95% of that energy.
Key points
Reduction is loss of oxygen. Metals more reactive than carbon cannot be reduced by carbon, so they are extracted by electrolysis.
Aluminium oxide is dissolved in molten cryolite to lower the melting point and save energy.
Aluminium forms at the carbon cathode. Oxygen forms at the carbon anodes, burns them away, and they are replaced.
Electrolysis is expensive because of the electricity. Recycling aluminium saves about 95% of the energy.
Worked examples
Example 1
Calcium is more reactive than carbon. State how calcium is extracted from its compounds.
Compare calcium with carbon in the reactivity series: calcium is more reactive.
Carbon cannot remove oxygen from a metal that is more reactive than itself.
So calcium is extracted by electrolysis of a molten calcium compound.
Answer: electrolysis of a molten calcium compound
Common slips
Slip: Calcium is less reactive than carbon. Calcium is above carbon in the reactivity series, so it is more reactive.
Slip: Heat calcium oxide with carbon. Carbon is less reactive than calcium, so it cannot take the oxygen away.
Slip: Electrolysis of calcium chloride solution. A solution would give hydrogen at the cathode. The compound must be molten.
Example 2
Explain why the carbon anodes in an aluminium extraction cell have to be replaced regularly.
Oxide ions lose electrons at the anodes and form oxygen.
The cell is at about 950 °C, so the oxygen reacts with the hot carbon.
C + O₂ → CO₂. The carbon burns away as carbon dioxide, so the anodes get smaller.
Answer: The oxygen formed at the anodes reacts with the hot carbon to make carbon dioxide, so the anodes burn away.
Common slips
Slip: Aluminium forms at the anodes and covers them. Aluminium ions go to the cathode, not the anodes.
Slip: The anodes dissolve in the cryolite. Carbon does not dissolve. It reacts with oxygen.
Slip: The carbon is reduced by the oxygen. The carbon gains oxygen, so it is oxidised, and it leaves as carbon dioxide.
Example 3
Higher tier. 2Al₂O₃ → 4Al + 3O₂. Calculate the mass of aluminium that can be made from 51 tonnes of aluminium oxide. Ar of Al = 27 and O = 16.
The ratio Al₂O₃ : Al is 2 : 4, so moles of Al = 0.50 × 2 = 1.0.
Mass of Al = 1.0 × 27 = 27 tonnes.
Answer: 27 tonnes
Common slips
Slip: Moles of Al = 0.50 ÷ 2 = 0.25. The ratio is 2 : 4, so the aluminium moles are double those of the oxide, not half.
Slip: Mr of Al₂O₃ = 27 + 16 = 43. The formula has 2 Al and 3 O: 2 × 27 + 3 × 16 = 102.
Slip: Mass of Al = 1.0 × 102 = 102 tonnes. Use the Ar of aluminium, 27, for the mass of aluminium, not the Mr of the oxide.
Questions and answers
Extracting metals by electrolysis practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Name the ore that aluminium is obtained from.
Show answer
Answer: bauxite
The ore is purified to give aluminium oxide.
It is called bauxite.
Question 2 Easy
Name the compound that is added to aluminium oxide to lower its melting point.
Show answer
Answer: cryolite
It dissolves the oxide.
It is cryolite.
Question 3 Easy
Name the material used for the electrodes in the aluminium cell.
Show answer
Answer: carbon
The electrodes are made of a form of carbon.
It is carbon, or graphite.
Question 4 Easy
At which electrode is aluminium made in the cell?
Show answer
Answer: the cathode
Aluminium ions are positive.
They go to the negative electrode, the cathode.
Question 5 Easy
Is aluminium more reactive or less reactive than carbon?
Show answer
Answer: more reactive
Aluminium is above carbon in the reactivity series.
So it is more reactive.
Question 6 Medium
Zinc is less reactive than carbon. State how zinc is extracted from zinc oxide.
Show answer
Answer: heating with carbon
Carbon is more reactive than zinc.
Heating the oxide with carbon removes the oxygen.
Question 7 Medium
Name the gas formed at the anodes in the aluminium cell.
Show answer
Answer: oxygen
Oxide ions lose electrons.
They form oxygen.
Question 8 Medium
Explain why aluminium is not extracted by heating aluminium oxide with carbon.
Show answer
Answer: aluminium is more reactive than carbon
Carbon is less reactive than aluminium.
So carbon cannot remove the oxygen.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
Iron and aluminium are both extracted from their oxides.
(a) Name the method used to extract iron. [1 mark]
(b) Name the method used to extract aluminium. [1 mark]
(c) Explain why different methods are used for the two metals. [2 marks]
Show answer and marking points
Answer: (a) heating with carbon (b) electrolysis (c) Aluminium is more reactive than carbon, so carbon cannot remove its oxygen. Iron is less reactive than carbon, so carbon can reduce it.
(a) heating with carbon (1)
(b) electrolysis (1)
(c) aluminium is more reactive than carbon (1)
(c) so carbon cannot remove its oxygen, but it can for iron (1)
Exam-style question 26 marks
Compare the extraction of iron from iron oxide with the extraction of aluminium from aluminium oxide. [6 marks]
Show answer and marking points
Answer: Iron is less reactive than carbon, so iron oxide is heated with carbon, which takes the oxygen away and reduces the oxide to iron. Aluminium is more reactive than carbon, so carbon cannot remove its oxygen and electrolysis is used instead. Aluminium oxide is dissolved in molten cryolite to lower its melting point and save energy. Aluminium forms at the cathode. The aluminium process uses a lot of electricity, so it is more expensive than the iron process.
iron is less reactive than carbon (1)
iron oxide is heated with carbon, which removes the oxygen (1)
aluminium is more reactive than carbon, so carbon cannot be used (1)
aluminium oxide is electrolysed (1)
dissolved in cryolite to lower the melting point (1)
electrolysis uses a lot of electricity, so it costs more (1)
Common mistakes
Saying aluminium is extracted by heating its oxide with carbon
Aluminium is more reactive than carbon, so carbon cannot remove its oxygen. Electrolysis is needed.
Saying cryolite is added to make the reaction faster or more reactive
Cryolite lowers the melting point of aluminium oxide, which saves energy.
Saying aluminium forms at the anode
Aluminium ions are positive. They go to the cathode and form aluminium there.
Forgetting why the anodes wear away
Oxygen forms at the anodes and reacts with the hot carbon to make carbon dioxide.
Using an aluminium salt solution instead of the molten oxide
A solution gives hydrogen at the cathode because aluminium is more reactive than hydrogen.
Questions students ask
Is Extracting metals by electrolysis on the Foundation or Higher tier?
Extracting metals by electrolysis is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 7.
What is a common mistake on Extracting metals by electrolysis?
Saying aluminium is extracted by heating its oxide with carbon. Aluminium is more reactive than carbon, so carbon cannot remove its oxygen. Electrolysis is needed.
Where are the answers to the Extracting metals by electrolysis questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.