Electrolysis
Using electricity to split a molten or dissolved ionic compound into its elements. It is used to extract reactive metals such as aluminium.
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Analogy
Electricity acts like a crowbar that pulls apart a compound that chemicals alone cannot split.
Example
Cathode (negative): positive ions gain electronsAnode (positive): negative ions lose electrons
In practice
Aluminium oxide is split by electrolysis to give aluminium metal and oxygen.
Practice
Flashcards
- Which method extracts metals more reactive than carbon?Electrolysis.
- What is the cathode?The negative electrode, where positive ions gain electrons.
Interview questions
- Explain why aluminium is extracted by electrolysis but iron can be extracted with carbon.
- Aluminium is more reactive than carbon
- Carbon cannot displace aluminium from its oxide
- Iron is less reactive than carbon
- Electrolysis needs a lot of electricity so it costs more
- Describe what happens at each electrode in the electrolysis of molten lead bromide.
- Lead ions move to the cathode and gain electrons
- Lead metal forms at the cathode
- Bromide ions move to the anode and lose electrons
- Bromine gas forms at the anode
Questions people ask
Why is electrolysis expensive?
It uses large amounts of electricity. Aluminium production needs a huge current and the compound must be kept molten. To lower costs, aluminium oxide is dissolved in molten cryolite, which melts at a lower temperature than the oxide alone. Recycling aluminium needs only a fraction of that energy.
What happens when you electrolyse a salt solution?
Water also contributes ions, so the products can differ from a molten salt. In sodium chloride solution, hydrogen forms at the cathode, since it is less reactive than sodium, and chlorine forms at the anode. Sodium hydroxide stays in the solution.