Author: J R Reid

Electrolysis

DefinitionThe ProcessExamples

What is Electrolysis?

All oxidation and reduction reaction involve the transfer of electrons. Some of these reactions are spontaneous – they happen ‘naturally’ without any help from external forces. For example:

Sodium oxidises to sodium ions in water (Na → Na+ + e-)Fluorine gas reduces to fluoride ions in water ( F2 + 2e- → 2F-)

But sometimes we want to get the reverse reaction to occur i.e. we want to make sodium and fluorine. This does not happen spontaneously so we have to force the electrons to travel in the opposite direction. To force electrons to move we use electricity. This is a process called electrolysis

The Process

Electrolysis is a very useful way of making elements from ionic compounds. The process involves placing two electrodes into a dissolved or molten ionic compound. A current is passed through the liquid and the ions are attracted to the electrodes:

Cathode – this electrode attracts cations. It is negatively charged. It gives electrons to the cations, therefore reducing themAnode – this electrode attracts anions. It is positively charged. It takes electrons from the anions, therefore oxidising them

As the ions get to each electrode the anions have their electrons removed and the cations have electrons added.The ions end up losing their charges and are converted back to atoms

Examples of Electrolysis I

Electrolysis of water:Because water contains ions it can be electrolysed. It breaks up into Hydrogen and OxygenThe diagram to the right shows a Hoffman voltameter – an apparatus where to electrodes (at the bottom) are used to generate hydrogen and oxygen gas. The two graduated tubes can be used to measure the volume of gas being produced.What are the volume ratios of hydrogen and oxygen?

Examples of Electrolysis II

The Extraction of AluminiumAlumina (aluminium oxide) is poured into pot which contains a carbon cathode baseCarbon anodes are placed in and about 20000 amps of current is passed through the alumina.The result is CO2 (from the production of O2 at the carbon anode) and molten aluminium which can be poured off

From: www.genisim.qc.ca/website/ansys91.htm

See: http://www.comalco.com/freedom.aspx?pid=260

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