Using electrolysis to extract metals — AQA GCSE Combined Science
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Using electrolysis to extract metals explained
Electrolysis extracts a metal from its molten compound by passing direct current through the melt.
Read the full explanation
The positive metal ions move to the cathode and form the metal, while negative non-metal ions move to the anode. (HT only: Metal ions gain electrons at the cathode, while non-metal ions release electrons at the anode). This route is chosen when carbon reduction is unsuitable: either the metal is more reactive than carbon, so carbon cannot displace it, or the metal reacts with carbon. For example, aluminium is extracted from molten aluminium oxide because aluminium is above carbon in the reactivity series. The process is energy-intensive: melting the compound requires heating to a high temperature, and maintaining the current requires a large electrical supply. Both demands make electrolysis expensive compared with reduction by carbon.
Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode (anode).
Aluminium is extracted from a molten mixture of aluminium oxide and cryolite. Aluminium oxide has a very high melting point, so cryolite is added to lower the melting point and reduce the energy needed to melt the electrolyte. The mixture is electrolysed using carbon anodes, which are chosen because carbon conducts electricity and has a high melting point. At the cathode, aluminium metal forms. At the carbon anode, oxygen gas forms. (HT only: Al³⁺ ions gain electrons at the cathode, and oxide ions lose electrons at the anode). The oxygen produced reacts with the hot carbon anodes to form carbon dioxide gas. This explains why the anodes gradually burn away and must be replaced regularly, making the process continuous and energy-intensive.
explain why a mixture is used as the electrolyte
A mixture is used as the electrolyte because a pure ionic compound often has a very high melting point. Melting it alone would require a great deal of energy, making the process expensive. By mixing the compound with another substance, the melting point is lowered, so the electrolyte can be molten at a lower temperature. The mixture still contains mobile ions, so it conducts electricity and allows electrolysis to happen. For example, aluminium oxide is mixed with molten cryolite. This lowers the operating temperature and saves energy, while the aluminium oxide still provides aluminium ions that are discharged at the cathode. The mixture must not introduce ions that would be preferentially discharged instead of the desired metal ions.
explain why the positive electrode must be continually replaced.
During electrolysis of a molten or dissolved metal compound, the positive electrode (anode) is often made of carbon. At the anode, negative ions such as oxide ions, O²⁻, are attracted and discharged. For example, in the extraction of aluminium from molten aluminium oxide, oxide ions lose electrons: 2O²⁻ → O₂ + 4e⁻. The oxygen produced reacts with the hot carbon anode to form carbon dioxide: C + O₂ → CO₂. This means the carbon anode is gradually burned away, so it must be continually replaced to keep the electrolysis cell working efficiently. If the anode were not replaced, it would crumble and the circuit would break, stopping the extraction process.
Your focus
- Describe how electrolysis extracts a metal from a molten compound, including the role of the cathode and anode.
- Explain why electrolysis is chosen instead of reduction with carbon for certain metals.
- Identify and explain the two main energy demands of electrolysis extraction.
Show all 12 objectives
- Describe the electrolysis of molten aluminium oxide and cryolite to manufacture aluminium.
- Explain the function of cryolite and why carbon anodes must be continually replaced.
- (HT only) Describe the formation of aluminium and oxygen in terms of electron transfer.
- Explain that a mixture is used to lower the melting point of the electrolyte.
- Relate the lower melting point to reduced energy costs in electrolysis.
- Describe how mobile ions in the mixture allow the electrolyte to conduct electricity.
- Identify the positive electrode as the anode and state that negative ions move towards it.
- Write the anode half-equation for oxide ion discharge and the equation for carbon reacting with oxygen.
- Explain how the reaction of oxygen with carbon causes the anode to be consumed and why replacement is necessary.
Using electrolysis to extract metals exam tips
Marking Points
- Electrolysis uses a direct electrical current passed through a molten compound, causing positive metal ions to move to the cathode to form the metal.
- Negative non-metal ions move to the anode, producing the non-metal element.
- Electrolysis is selected when the metal is too reactive to be extracted by reduction with carbon, because carbon cannot displace a more reactive metal from its compound.
- Electrolysis is also selected when the metal reacts with carbon, so carbon reduction would not yield the pure metal.
- Melting the compound requires a large energy input to overcome the lattice and produce mobile ions.
- Producing and maintaining the electrical current requires a large energy input, making electrolysis an expensive extraction method.
- (HT only) Explain that reduction occurs at the cathode (electron gain) and oxidation at the anode (electron loss).
- Aluminium is manufactured by electrolysis of a molten mixture of aluminium oxide and cryolite.
- Cryolite is added to lower the melting point of the electrolyte, reducing the energy required and making the process more economical.
- Carbon is used as the positive electrode (the anode) because it conducts electricity and has a high melting point.
- At the cathode, aluminium ions form aluminium metal, and at the anode, oxide ions form oxygen gas.
- The oxygen produced reacts with the hot carbon anode to form carbon dioxide.
- Because the carbon anodes react and burn away, they are gradually used up and need regular replacement.
- (HT only) Explain that Al³⁺ ions gain electrons at the cathode and oxide ions lose electrons at the anode.
- A pure ionic compound may have a very high melting point, so melting it alone would need a lot of energy.
- Mixing the compound with another substance lowers the melting point of the electrolyte.
- A lower melting point means less energy is needed to keep the electrolyte molten, which reduces costs.
- The mixture must still contain mobile ions so that it conducts electricity and electrolysis can occur.
- In aluminium extraction, aluminium oxide is dissolved in molten cryolite to lower the working temperature.
- The ions from the main compound must be the ones discharged to give the desired metal at the cathode.
- Negative ions, such as O²⁻, are attracted to the positive electrode (anode).
- At the anode, negative ions lose electrons and are discharged, for example 2O²⁻ → O₂ + 4e⁻.
- The non-metal product, such as oxygen, reacts with the hot carbon anode.
- Carbon dioxide is formed, for example C + O₂ → CO₂, so the carbon anode is used up.
- Continual replacement maintains the anode and keeps the electrolysis circuit complete.
Examiner Tips
- 💡When explaining why electrolysis is used, name the metal and compare its reactivity with carbon rather than giving a general statement.
- 💡Link each energy cost to a specific stage: melting the compound and generating the current, so both parts of the statement are addressed.
- 💡Avoid saying electricity 'breaks down' the compound without describing ion movement to the electrodes.
- 💡Name both substances in the electrolyte and state the role of cryolite when explaining the extraction.
- 💡Link the reaction at the anode to the need for regular replacement of carbon electrodes, showing understanding of the industrial process.
- 💡(HT only) Be prepared to describe the electron transfer at each electrode, ensuring atoms and charges balance.
- 💡Structure your answer as: pure compound has high melting point, so mixing lowers it, so less energy is needed.
- 💡Name the industrial example if the question is about aluminium: aluminium oxide mixed with molten cryolite.
- 💡Use the term 'mobile ions' to show why the mixture can conduct electricity.
- 💡Avoid saying 'it makes it easier' without explaining that the melting point is lowered and energy is saved.
- 💡Name the anode as the positive electrode and link it to negative ion discharge.
- 💡Include a balanced equation for the anode reaction and for the reaction with carbon.
- 💡Explain the consequence: without replacement, the anode is destroyed and electrolysis stops.
Common Mistakes
- Thinking electrolysis can use a solid ionic compound: the ions are fixed in the lattice and cannot move, so the compound must be molten or dissolved; correct by stating that the compound is melted to allow ions to move.
- Assuming carbon reduction works for any metal: carbon can only displace metals below it in the reactivity series, and some metals react with carbon; correct by checking the reactivity series.
- (HT only) Confusing oxidation and reduction: correct by linking cathode to electron gain (reduction) and anode to electron loss (oxidation).
- Saying cryolite is added to increase the melting point: it lowers the melting point, so less energy is needed; correct by linking cryolite to reduced energy cost.
- Confusing the electrodes: aluminium forms at the cathode, not the anode; correct by stating that positive aluminium ions move to the negative electrode.
- Forgetting that the carbon anode is consumed: oxygen reacts with carbon to form carbon dioxide, so anodes must be replaced; correct by including this reaction in explanations of the process.
- Saying the mixture is used to increase the current. Correction: the main purpose is to lower the melting point and reduce energy costs, though conductivity must still be sufficient.
- Writing that the mixture makes the electrolyte solid. Correction: the electrolyte must be molten or dissolved so its ions are free to move.
- Forgetting to mention mobile ions. Correction: a good answer links the mixture to both a lower melting point and the presence of mobile ions for conduction.
- Claiming that mixing always makes the reaction faster. Correction: the key benefit is the reduced energy demand, not a guaranteed increase in reaction rate.
- Thinking the anode dissolves because positive ions are attracted to it; correction: positive ions move to the cathode, while negative ions discharge at the anode.
- Writing that the anode reacts with the metal being extracted; correction: the anode reacts with the non-metal product, such as oxygen, not the metal.
- Ignoring the role of carbon and saying the anode simply wears away; correction: state the chemical reaction, such as C + O₂ → CO₂, that consumes the carbon.