The shape of the lesson
By the end of the lesson, learners can
- The electrolysis cell: Aluminium oxide melts above 2000 °C.
- Half-equations and costs: Extended.
- Electrolyte: Al2O3 in molten cryolite (lower temperature).
- Cathode: aluminium. Anode: oxygen.
- Anodes burn: C + O2 → CO2; replaced often.
- Extended: Al3+ + 3e− → Al; 2O2− → O2 + 4e−.
One picture every idea returns to
The ore is bauxite, purified to aluminium oxide, Al2O3. It is dissolved in molten cryolite, which lowers the operating temperature and improves conductivity, and electrolysed with carbon (graphite) electrodes. Aluminium forms at the cathode and oxygen at the anode. The oxygen reacts with the hot carbon anodes to form carbon dioxide, so the anodes must be replaced regularly. Extended: Al3+ + 3e− → Al and 2O2− → O2 + 4e−.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Splitting bauxite with current.. Aluminium is too reactive to be reduced by carbon, so it is won by electrolysis. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: The electrolysis cell | Aluminium oxide melts above 2000 °C. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Half-equations and costs | Extended. | Quiz-Quiz-Trade |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Half-equations and costs, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Quiz-Quiz-Trade
Each student holds a card with a question on the front and the answer on the back. They pair up, quiz each other, coach if needed, trade cards and find a new partner. Three rounds covers the whole set.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Cryolite is not a catalyst and not the source of aluminium: it is a solvent that lowers the temperature needed, saving energy. | And it is the anodes, not the cathode, that must be replaced, because oxygen forms there and burns the carbon. |
Extracting aluminium by electrolysis: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Name the ore from which aluminium is extracted, and the compound it is purified to.
Explain why aluminium oxide is dissolved in molten cryolite before electrolysis.
Name the product at each electrode.
Explain why the carbon anodes have to be replaced regularly. Include an equation.
(Extended) Write the half-equations for the reactions at the cathode and the anode.
Suggest two reasons why aluminium extraction plants are often built near hydroelectric power stations.
Answer key and marking notes
- Bauxite.
- Aluminium oxide, Al2O3.
- Aluminium oxide has a very high melting point (over 2000 °C).
- Dissolving it in molten cryolite allows electrolysis at a lower temperature (about 950 °C), saving energy.
- Cathode: aluminium.
- Anode: oxygen.
- Oxygen is produced at the hot anodes and reacts with the carbon.
- C + O2 → CO2, so the anodes are gradually burned away.
- Cathode: Al3+ + 3e− → Al
- Anode: 2O2− → O2 + 4e−
- The process needs very large amounts of electricity.
- Hydroelectric power supplies cheap electricity (and produces no carbon dioxide from fuel burning).
7 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Aluminium oxide must be molten, or dissolved in molten cryolite, before it can be electrolysed. Why can solid aluminium oxide not be used?
- Aits ions are held in fixed positions and cannot move to the electrodes
- Bsolid aluminium oxide contains no ions
- Cthe solid would react with the carbon electrodes
- Dcryolite only works as a catalyst on liquids
The waste gases from an aluminium electrolysis cell contain carbon dioxide. Where does the carbon in this gas come from?
- Athe carbon anodes, which react with the oxygen produced there
- Bthe cryolite, which decomposes during electrolysis
- Cthe bauxite ore, which is a carbonate
- Dthe molten aluminium at the cathode
The diagram shows the cell used to extract aluminium. At which labelled part are aluminium ions reduced?
- AS
- BP
- CQ
- DR
Which describes the reaction at the cathode during the extraction of aluminium?
- AAl3+ + 3e− → Al, a reduction
- BAl3+ + 3e− → Al, an oxidation
- CAl → Al3+ + 3e−, a reduction
- DAl3+ → Al + 3e−, a reduction
At the anode, oxide ions are converted into oxygen gas. What happens to each oxide ion?
- Ait loses two electrons, and the oxygen atoms formed join in pairs as O2 molecules
- Bit gains two electrons to form an oxygen atom
- Cit loses one electron to form O−, which is the gas given off
- Dit gains electrons from the cathode through the electrolyte
The electrode reactions are Al3+ + 3e− → Al and 2O2− → O2 + 4e−. Which ionic equation for the overall reaction has the electrons correctly balanced?
- A4Al3+ + 6O2− → 4Al + 3O2
- BAl3+ + O2− → Al + O2
- C2Al3+ + 3O2− → 2Al + 3O2
- D4Al3+ + 6O2− + 12e− → 4Al + 3O2
Which two changes that happen at the anodes of the aluminium cell are both oxidations?
- Aoxide ions forming oxygen, and carbon forming carbon dioxide
- Baluminium ions forming aluminium, and carbon forming carbon dioxide
- Coxide ions forming oxygen, and carbon dioxide forming carbon
- Doxygen forming oxide ions, and carbon forming carbon dioxide
Quiz key
Aluminium oxide is ionic, but in the solid the Al3+ and O2− ions are locked in a lattice. Only when molten or dissolved can the ions move to the electrodes and carry the current. The solid does contain ions; they just cannot move. Reaction with the electrodes is not the reason. Cryolite is a solvent that lowers the operating temperature, not a catalyst.
Oxygen forms at the hot carbon anodes and reacts with them: C + O2 → CO2, which is why the anodes gradually burn away. Cryolite is sodium aluminium fluoride and contains no carbon. Bauxite is an ore of aluminium oxide, not a carbonate. Aluminium contains no carbon, so it cannot be the source.
Al3+ ions are attracted to the negative electrode, the carbon lining of the cell (S), where they gain electrons: Al3+ + 3e− → Al. The carbon blocks (P) are the anodes, where oxide ions are oxidised to oxygen. Q is the electrolyte, aluminium oxide dissolved in molten cryolite, through which the ions move. R is the layer of molten aluminium that has already formed and collects on the cell floor.
Aluminium ions gain three electrons each to become atoms, and gain of electrons is reduction. Calling the same change oxidation mixes up the definitions. Al → Al3+ + 3e− is the reverse process, an oxidation, and it would destroy the product. Writing the electrons as a product of Al3+ becoming Al gives a charge imbalance of 3+ on the left and 3− on the right.
O2− must lose both of its extra electrons to become a neutral oxygen atom, and two atoms combine to form O2: 2O2− → O2 + 4e−. This is oxidation. Gaining electrons would make the ion more negative. Losing only one electron would leave a charged particle, not oxygen gas. Electrons do not travel through the electrolyte from the cathode; the oxide ion gives its electrons to the anode.
The cathode uses 3 electrons per Al and the anode releases 4 electrons per O2. The lowest common multiple is 12, so multiply the cathode equation by 4 and the anode equation by 3: 4Al3+ + 6O2− → 4Al + 3O2. Adding the half-equations without scaling leaves unbalanced charge and atoms. The equation with 3O2 from only 3O2− has 3 oxygen atoms on the left and 6 on the right. Electrons cancel completely in an overall equation, so they should not appear.
Oxide ions lose electrons to form oxygen, which is oxidation by electron loss. The carbon of the anode then gains oxygen to form carbon dioxide, which is oxidation by oxygen gain. Aluminium ions forming aluminium is reduction and happens at the cathode. Carbon dioxide turning into carbon would be a reduction, and oxygen forming oxide ions would be a gain of electrons, also reduction.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.