Passing electricity through compounds: electrolysis
5 marks · HardDefine what is meant by each of the following terms in relation to electrolysis: (i) anion, (ii) cation.
Describe what happens during the electrolysis of molten potassium bromide (). In your answer, refer to the movement and discharge of ions at the electrodes.
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(a) — · must show anion: negative ion, cation: positive ion
(b) — · must show K+ ions move to cathode, Br- ions move to anode, Ions discharged at electrodes (K+ to K, Br- to Br2)
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Marking points
B1 (a) · 1 mark · Defines anion as a negative ion.
B1 (a) · 1 mark · Defines cation as a positive ion.
M1 (b) · 1 mark · States that K+ ions move to the cathode and Br- ions move to the anode.
M1 (b) · 1 mark · States that ions are discharged at the electrodes: K+ at the cathode and Br- at the anode.
A1 (b) · 1 mark · Describes or names the products: potassium is formed at the cathode, bromine gas at the anode.
Worked solution
a (i) An anion is a negatively charged ion. a (ii) A cation is a positively charged ion.
b During the electrolysis of molten potassium bromide, K+ ions move towards the negative electrode (cathode), where they gain electrons and are discharged as potassium metal. Br- ions move towards the positive electrode (anode), where they lose electrons and are discharged as bromine gas. The process involves the flow of ions towards electrodes where reduction or oxidation occurs, resulting in the formation of new substances.