Electrolysis — Chemistry, 14–17 years
Electrolysis uses electrical energy to force a chemical change that would not happen readily on its own. It is a practical way to split compounds or coat objects with metal.
Electricity pushes particles apart
In electrolysis, a power supply pushes charged particles towards opposite electrodes in a liquid or molten substance. At the electrodes, ions gain or lose electrons and become new substances. Electrical energy is changed into chemical change, rather than the reaction producing electricity.
Why use electricity to split substances?
Some compounds are too stable to split by simply heating or mixing them. Electrolysis supplies the energy and directs the products to different electrodes. This solved practical problems such as obtaining reactive metals from their ores and producing important chemicals in controlled amounts.
Splitting molten lead bromide
Molten lead bromide contains Pb²⁺ and Br⁻ ions. At the negative electrode, Pb²⁺ gains two electrons: Pb²⁺ + 2e⁻ → Pb, so lead metal forms. At the positive electrode, two bromide ions lose electrons: 2Br⁻ → Br₂ + 2e⁻, so bromine forms. The electrical supply drives this otherwise difficult separation.
The electrode signs can confuse
A common mistake is to think the positive electrode must attract positive ions and the negative one must attract negative ions. That reverses the charges. Positive ions move towards the negative electrode, while negative ions move towards the positive one; their electron changes then determine the products.
Metal coating and aluminium
Electroplating uses electrolysis to put a thin layer of one metal onto an object. This can improve appearance or protect against corrosion, as with chromium or nickel coatings. Large-scale electrolysis also produces aluminium from its compound, although the process needs a great deal of electricity.
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