Chemical Changes
Electrolysis You must be able to: Explain why ionic compounds conduct electricity when molten or in aqueous solution Predict the products of the electrolysis of simple ionic compounds and explain how electrolysis can be used to extract reactive metals HT Write half equations for the reactions that take place at the electrodes during electrolysis.
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Electrolysis • • • •
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Electrolysis is the use of an electrical current to break down compounds containing ions into their constituent elements. The substance being broken down is called the electrolyte. The electrodes are made from solids that conduct electricity. During electrolysis: – negatively charged ions move to the anode (positive electrode) – positively charged ions move to the cathode (negative electrode). Electrolysis can be used to separate ionic compounds into elements. For example, lead bromide can be split into lead and bromine: – The lead bromide is heated until it melts. – The positively charged lead ions move to the negative electrode (cathode). – Here they gain electrons to form lead atoms – pure lead is produced at this electrode. – The negatively charged bromide ions move to the positive electrode (anode). – Here they lose electrons to form bromine atoms, which join together to form bromine molecules – bromine is released at this electrode. HT
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HT • • • • •
At the cathode: Pb2+ + 2e – At the anode: 2Br –
Ionic substances can only conduct electricity when they are molten or dissolved in water.
Oxidation and Reduction Reduction occurs when positively charged ions gain electrons at the negative electrode. Oxidation occurs when negatively charged ions lose electrons at the positive electrode. In a redox reaction both reduction and oxidation occur. Reactions that take place at the electrodes during electrolysis can be represented by half-equations. For example, in the electrolysis of molten copper chloride: – Copper is deposited at the negative electrode. Cu2+ + 2e –
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Pb Br2 + 2e –
Cu
Power supply Leads
Inert (unreactive) electrodes Beaker
Aqueous solution, e.g. sodium chloride solution
Key Point For the electrolysis of molten ionic compounds, the electrodes used must be inert so that they do not react with the electrolyte or the products.
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Key Point
You can remember this by thinking of the word OILRIG: •
Oxidation Is Loss of electrons (OIL)
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Reduction Is Gain of electrons (RIG).
The copper ions gain electrons so they are reduced.
GCSE Combined Science Revision Guide
227841 AQA Higher Tier Combined Science All-in-One_P110_119.indd 118
6/29/16 11:25 AM