How does passing electricity through an ionic compound break it down, and what is made at each electrode?
Electrolysis of molten ionic compounds and aqueous solutions, the movement of ions to the electrodes, the products at the cathode and anode, the rules for aqueous electrolysis, and the use of electrolysis to extract reactive metals.
A focused answer to the OCR Gateway GCSE Combined Science A topic C3 on electrolysis, covering electrolysis of molten and aqueous ionic compounds, ion movement to the electrodes, the products at the cathode and anode, the rules for aqueous solutions, and extracting reactive metals.
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What this topic is asking
OCR wants you to describe electrolysis of molten ionic compounds and aqueous solutions, explain how ions move to the electrodes, predict the products at the cathode and anode, apply the rules for aqueous solutions, and explain the use of electrolysis to extract reactive metals.
What electrolysis is
When the current is switched on, the ions move to the oppositely charged electrode: positive ions (cations) move to the negative electrode (the cathode), and negative ions (anions) move to the positive electrode (the anode). At the cathode, positive ions gain electrons (this is reduction); at the anode, negative ions lose electrons (this is oxidation). This is why electrolysis is a redox process driven by electricity.
Electrolysis of molten compounds and aqueous solutions
For a molten ionic compound the products are simple: the metal forms at the cathode and the non-metal at the anode. For example, molten lead bromide gives lead at the cathode and bromine at the anode.
For example, electrolysing copper sulfate solution with inert electrodes gives copper at the cathode (copper is less reactive than hydrogen) and oxygen at the anode (no halide present). Electrolysing sodium chloride solution gives hydrogen at the cathode (sodium is more reactive than hydrogen) and chlorine at the anode (a halide is present).
Extracting reactive metals
Electrolysis is used to extract metals that are too reactive to be reduced by carbon. Metals more reactive than carbon (such as aluminium) cannot be displaced from their oxides by heating with carbon, so they are extracted by electrolysis of the molten compound. Aluminium is obtained by electrolysing molten aluminium oxide; because aluminium oxide has a very high melting point, it is dissolved in molten cryolite to lower the temperature and save energy. Aluminium forms at the cathode and oxygen at the anode (the oxygen reacts with the carbon anodes, which must be replaced regularly). Metals less reactive than carbon (such as iron and zinc) are usually extracted more cheaply by reduction with carbon instead, so electrolysis is reserved for the reactive metals.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksMolten lead bromide is electrolysed. Name the product formed at each electrode and explain why the products form where they do, using the charges on the ions.Show worked answer →
A Chemistry Paper 4 structured question on electrolysis. Reward: at the cathode (negative electrode) lead metal forms; at the anode (positive electrode) bromine forms. The explanation: the positive lead ions () are attracted to the negative cathode, where they gain electrons (are reduced) to become lead atoms; the negative bromide ions () are attracted to the positive anode, where they lose electrons (are oxidised) to become bromine. Markers credit the correct product at each electrode and the link from ion charge to attraction to the oppositely charged electrode, with gain or loss of electrons. A common error is to swap the electrodes.
OCR 20214 marksExplain why aluminium is extracted from its ore by electrolysis rather than by heating with carbon, and state what is produced at the negative electrode.Show worked answer →
A C3 question on extracting reactive metals. Reward: aluminium is more reactive than carbon, so carbon cannot displace it from its compounds (reduction with carbon only works for metals less reactive than carbon); electrolysis is therefore used to extract aluminium from molten aluminium oxide. At the negative electrode (cathode) the aluminium ions gain electrons and aluminium metal is produced. Markers credit the point that aluminium is too reactive to be reduced by carbon, that electrolysis is used instead, and that aluminium forms at the cathode. They may note that the oxide is mixed with cryolite to lower the melting point and save energy.
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