AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Using electrolysis to extract metals: 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.
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Thinking the anode dissolves because positive ions are attracted to it.
Put it right: Positive ions move to the cathode, while negative ions discharge at the anode.
Writing that the anode reacts with the metal being extracted.
Put it right: 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.
Put it right: State the chemical reaction, such as C + O₂ → CO₂, that consumes the carbon.
Exam tips
- 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.
Try the lesson
The start of the Using electrolysis to extract metals: explain why the positive electrode must be continually replaced. lesson, made from checked teaching notes for this specification point.
Have a go first
In the electrolysis of molten aluminium oxide, the positive electrode is made of carbon. Why does this carbon electrode have to be replaced regularly?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Thinking the anode dissolves because positive ions are attracted to it. Instead, positive ions move to the cathode, while negative ions discharge at the anode. Exam tip: Name the anode as the positive electrode and link it to negative ion discharge.
Key words
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Anode oxidation and discharge of negative ions
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Reactivity of carbon with oxygen at high temperature
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See every definition
- Anode oxidation and discharge of negative ions
- Negative ions such as oxide ions are attracted to the positive electrode and lose electrons there.
- Reactivity of carbon with oxygen at high temperature
- The oxygen produced reacts with the hot carbon anode to form carbon dioxide, so the carbon anode is gradually burned away.
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