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How does electrolysis extract reactive metals, and why is aluminium made this way?

Electrolysis of molten ionic compounds, the reactions at the electrodes, and the extraction of aluminium from molten aluminium oxide.

A focused answer to WJEC GCSE Chemistry topic 2.3 on electrolysis, covering how a molten ionic compound conducts and breaks down, the reactions at the cathode and anode, and how aluminium is extracted from molten aluminium oxide using cryolite.

Generated by Claude Opus 4.810 min answer

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  1. What this topic is asking
  2. Why electrolysis is needed
  3. How electrolysis works
  4. Extracting aluminium
  5. Electrolysis of other molten compounds
  6. Try this

What this topic is asking

WJEC wants you to explain how electrolysis decomposes a molten ionic compound and to describe how aluminium is extracted this way. This is part of topic 2.3 Metals and their extraction in Unit 2 of WJEC GCSE Chemistry (3430).

Why electrolysis is needed

How electrolysis works

Extracting aluminium

The need to replace the anodes adds to the running cost, and the carbon dioxide released is a further environmental drawback of the process. The whole cell runs at a high temperature and draws a very large current, so aluminium smelters are usually built where electricity is cheap, for example near hydroelectric power. This high energy demand is exactly why recycling aluminium, which only needs the metal to be melted, is so much cheaper than extracting it fresh.

Electrolysis of other molten compounds

Try this

Q1. State why molten aluminium oxide conducts electricity but solid aluminium oxide does not. [1 mark]

  • Cue. When molten the ions are free to move and carry charge; in the solid they are held in fixed positions.

Q2. Write the half equation for the formation of oxygen at the anode. [1 mark]

  • Cue. 2O2βˆ’β†’O2+4eβˆ’2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^-.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20204 marksAluminium is extracted by the electrolysis of molten aluminium oxide. Name the products at each electrode and write a half equation for the reaction at the negative electrode.
Show worked answer β†’

A topic 2.3 electrolysis question. At the negative electrode (cathode), aluminium is produced (1 mark); at the positive electrode (anode), oxygen is produced (1 mark). The aluminium ions gain electrons (reduction): Al3++3eβˆ’β†’Al\text{Al}^{3+} + 3e^- \rightarrow \text{Al} (2 marks for the correct half equation, including the three electrons). Markers reward the correct product at each electrode and a balanced half equation. A common error is to forget the 3eβˆ’3e^- needed to balance the 3+3+ charge.

WJEC 20233 marksExplain why aluminium oxide is mixed with cryolite before electrolysis and why the carbon anodes must be replaced regularly.
Show worked answer β†’

A topic 2.3 Explain question. Cryolite is added to lower the melting point of the aluminium oxide, so less energy (and so less cost) is needed to keep it molten (1 mark). At the anode, oxygen is produced (1 mark); at the high temperature the oxygen reacts with the carbon anodes to form carbon dioxide, so the anodes slowly burn away and must be replaced (1 mark). Markers reward the melting-point reason and the carbon-plus-oxygen reason. A common error is to say cryolite is a catalyst.

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