What is electrolysis, and how is it used to extract aluminium?
Electrolysis as the breakdown of an ionic compound using electricity, the products at the electrodes, and the extraction of aluminium.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on electrolysis, covering the breakdown of molten and dissolved ionic compounds using electricity, the products formed at each electrode, and the extraction of aluminium.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe electrolysis as the breakdown of an ionic compound using electricity, the products at the electrodes, and the extraction of aluminium.
What electrolysis is
For electrolysis to work, the ionic compound must be molten or dissolved, so the ions are free to move and carry charge to the electrodes.
Why the compound must be molten or dissolved
The products at the electrodes
The electrodes are connected to a power supply:
- The negative electrode (cathode) attracts the positive ions (metal ions or hydrogen). They gain electrons and form the metal (or hydrogen gas).
- The positive electrode (anode) attracts the negative ions (non-metal ions). They lose electrons and form the non-metal (often a gas such as oxygen or chlorine).
The extraction of aluminium
The aluminium oxide is mixed with cryolite to lower its melting point, saving energy. The process uses a lot of electricity, which is why aluminium is expensive to produce.
Electrolysis of solutions
Electrolysis is also used on dissolved compounds, not just molten ones, and the products can differ. When a solution is electrolysed, water is present, so hydrogen is often produced at the negative electrode instead of a reactive metal, and at the positive electrode a non-metal such as oxygen or chlorine forms. For example, electrolysing sodium chloride solution (brine) produces hydrogen at the negative electrode and chlorine at the positive electrode, with sodium hydroxide left in solution. Knowing that solutions can give different products from molten compounds is a useful extension.
Why electrolysis is used despite the cost
Even though electrolysis uses a lot of expensive electricity, it is essential for extracting reactive metals such as aluminium that carbon cannot reduce, and for producing useful chemicals such as chlorine. The high cost is the reason aluminium is more expensive than iron, and why recycling aluminium (which avoids the extraction) saves so much energy. Being able to explain why electrolysis is chosen despite its cost, by linking it to the metal's reactivity, is a common evaluation point that ties this dot to the reactivity series.
Try this
Q1. What forms at the negative electrode during the electrolysis of a molten metal compound? [1 mark]
- Cue. The metal.
Q2. Why is molten aluminium oxide mixed with cryolite? [1 mark]
- Cue. To lower its melting point and save energy.
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 style4 marksExplain why an ionic compound must be molten or dissolved before it can be electrolysed.Show worked answer →
A Unit 5 explain question worth 4 marks. Reward: electrolysis needs the ions to be free to move so they can carry charge to the electrodes (1); in a solid, the ions are held in fixed positions and cannot move (1); when molten or dissolved, the lattice breaks down and the ions are free to move (1); the moving ions can then be attracted to the electrodes and discharged (1). Markers credit the need for free-moving ions and that the solid cannot conduct. A common error is to say electrons must be free (it is the ions that move).
WJEC style4 marksDescribe what is formed at the negative and positive electrodes when molten aluminium oxide is electrolysed.Show worked answer →
A Unit 5 electrolysis question worth 4 marks. Reward: at the negative electrode (cathode), the positive aluminium ions gain electrons and form aluminium metal (2); at the positive electrode (anode), the negative oxide ions lose electrons and form oxygen gas (2). Markers credit aluminium at the negative electrode and oxygen at the positive electrode, with the idea of gaining/losing electrons. A common error is to swap the electrodes.
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