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Chemistry (Electrolysis) Note on Electrolysis, created by lmg719 on 05/05/2013.

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Created by lmg719 almost 6 years ago
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ElectrolysisElectrolysis is the process by which ionic substances are broken down into simpler substances using electricity.During electrolysis, metals and gases may form at the electrodes.

What is electrolysis?Ionic substances contain charged particles called ions. For example, lead bromide contains positively charged lead ions and negatively charged bromide ions.Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or when melted. For example, if electricity is passed through molten lead bromide, the lead bromide is broken down to form lead and bromine.

Here is what happens during electrolysis: Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced. Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised. The substance that is broken down is called the electrolyte.

ElectroplatingElectrolysis is used to electroplate objects. This is useful for coating a cheaper metal with a more expensive one, such as copper or silver.How it works The negative electrode should be the object that is to be electroplated The positive electrode should be the metal that you want to coat the object with The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate

Electroplating with silverThe object to be plated, such as a metal spoon, is connected to the negative terminal of the power supply. A piece of silver is connected to the positive terminal. The electrolyte is silver nitrate solution.Electroplating with copperThe object to be plated, such as a metal pan, is connected to the negative terminal of the power supply. A piece of copper is connected to the positive terminal. The electrolyte is coppersulfate solution.This arrangement can also be used to purify copper during copper manufacture. In this case, both electrodes are made from copper. The negative electrode gradually gets coated with pure copper as the positive electrode gradually disappears. The animation shows how this works:

Aluminium extractionAluminium is the most abundant (found in large quantities) metal on Earth. But it is expensive, largely because of the amount of electricity used up in theextraction process.Aluminium ore is called bauxite. The bauxite is purified to yield a white powder -aluminium oxide - from which aluminium can be extracted.The extraction is done by electrolysis. But first the aluminium oxide must be melted so that electricity can pass through it. Aluminium oxide has a very high melting point (over 2000°C) so it would be expensive to melt it. Instead, it is dissolved in moltencryolite - an aluminium compound with a lower melting point than aluminium oxide. The use of cryolite reduces some of the energy costs involved in extracting aluminium.

The diagram shows an aluminium oxide electrolysis tank. Both the negative electrode (cathode) and positive electrode (anode) are made of graphite, a form of carbon.

Aluminium metal forms at the negative electrode and sinks to the bottom of the tank, where it is tapped off.Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away. As a result, the positive electrodes have to be replaced frequently. This adds to the cost of the process.

Predicting the products of electrolysisIonic substances in solution break down into elements during electrolysis. Different elements are released depending on the particular ionic substance.At the negative electrodeAt the negative electrode, positively charged ions gain electrons. This is reduction, and you say that the ions have been reducedMetal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series: The metal will be produced if it is less reactive than hydrogen Hydrogen will be produced if the metal is more reactive than hydrogen



Aluminium Extraction

Predicting the Products of Electrolysis