Chemistry AQA GCSE Core Revision Notes on Electrolysis

Ad Electrolysis

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Ionic compounds have many uses and can provide other substances. Electrolysis is used to produce alkalis and elements such as aluminium, chlorine and hydrogen. Oxidation–reduction reactions do not just involve oxygen.

You need to:

■ predict the products of electrolysing solutions of ions

■ explain and evaluate processes that use the principles described in this unit, including the use of electroplating.

Electrolysis

a) When an ionic substance is melted or dissolved in water, the ions are free to move about within the liquid or solution.

b) Passing an electric current through ionic substances that are molten, for example lead bromide, or in solution breaks them down into elements.

This process is called electrolysis and the substance that is broken down is called the electrolyte.

c) During electrolysis, positively charged ions move to the negative electrode, and negatively charged ions move to the positive electrode.

d) Electrolysis is used to electroplate objects. This may be for a variety of reasons and includes copper plating and silver plating.

e) At the negative electrode, positively charged ions gain electrons (reduction) and at the positive electrode, negatively charged ions lose electrons (oxidation).

f) If there is a mixture of ions, the products formed depend on the reactivity of the elements involved.

g) Reactions at electrodes can be represented by half equations, for example:

2Cl – Cl2 + 2e–

or

2Cl – – 2e– Cl2

h) Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite. Aluminium forms at the negative electrode and oxygen at the positive electrode. The positive electrode is made of carbon, which reacts with the oxygen to produce carbon dioxide.

i) The electrolysis of sodium chloride solution produces hydrogen and chlorine. Sodium hydroxide solution is also produced. These are important reagents for the chemical industry, eg sodium hydroxide for the production of soap and chlorine for the production of bleach and plastics.

Suggested ideas for practical work to develop skills and understanding include the following:

■ the electrolysis of molten lead bromide or zinc chloride

■ investigation of the electrolysis of any solutions of a soluble ionic compound, eg copper chloride, sodium chloride, zinc bromide, zinc iodide

■ a demonstration of the Hoffman voltameter

■ the electroplating of copper foil with nickel in a nickel sulfate solution

■ the movement of ions, eg by the electrolysis of a crystal of KMnO4 on filter paper dampened with sodium chloride solution, or the electrolysis of CuCrO4 in a saturated urea solution using a U-tube

■ using conductivity sensors to monitor conductivity and changes in conductivity.

TIP: Do you know why cryolite is used in this process?

Check yourself:

Are you able to complete and balance half equations for the reactions occurring at the electrodes during electrolysis?