Electrolysis: Chemistry 0620 (Cambridge O Level / IGCSE)
Syllabus 4.1, 4.2 · Strand 4 Electrochemistry
- Questions
- 10
- Total marks
- 49
- Tier mix
- 6 Core · 4 Extended
0 of 10 questions completed
Syllabus coverage
- 4.1 10 questions completed
- 4.2 4 questions completed
Electrolysis (syllabus sections 4.1 and 4.2) is the decomposition of an ionic compound, molten or in solution, by passing an electric current through it. The vocabulary matters: electrolyte, inert electrodes (carbon or platinum), the anode (positive electrode) and the cathode (negative electrode). For molten binary compounds the products are simple, metal at the cathode, non-metal at the anode.
Aqueous solutions are where marks are won and lost, because water supplies competing H⁺ and OH⁻ ions. The standard results to learn include dilute sulfuric acid, concentrated aqueous sodium chloride and aqueous copper(II) sulfate; the general rules are that hydrogen forms at the cathode unless the metal is less reactive than hydrogen, and halogens are discharged at the anode from concentrated halide solutions, otherwise oxygen.
Extended candidates write ionic half-equations for each electrode, explain electron flow through the external circuit and ion migration through the electrolyte, and describe the electrolysis of copper(II) sulfate with copper electrodes (refining). Applications include electroplating for appearance and corrosion resistance, plus the hydrogen–oxygen fuel cell producing only water. All questions below are original with full worked solutions.
Question 1
A technician melts solid zinc bromide, , until it is completely liquid, then passes a direct current through the molten compound using two inert graphite electrodes.
Which row correctly gives the products formed at the cathode and at the anode?
Question 2
A student sets up an electrolysis cell containing dilute sulfuric acid, using two platinum electrodes connected to a d.c. power supply. Gas bubbles form steadily at both electrodes, and the student collects each gas in an inverted measuring cylinder filled with water.
(a) State the name of the gas collected at the cathode, and describe a test that would confirm its identity. [2]
(b) State the name of the gas collected at the anode, and describe a test that would confirm its identity. [2]
(c) After several minutes, the volume of gas collected at the cathode is . Hydrogen and oxygen are always produced in the ratio Calculate the volume of gas collected at the anode in the same time. [1]
(d) Suggest why platinum electrodes, rather than reactive metal electrodes, are used in this electrolysis. [1]
Question 3
A school chemistry club investigates two uses of electrolysis.
In the first investigation, the club electrolyses concentrated aqueous sodium chloride solution using two inert graphite electrodes connected to a d.c. supply.
(a) Identify the gas produced at the anode, and describe a test that would confirm its identity. [2]
(b) Identify the gas produced at the cathode. [1]
(c) Name the compound that remains dissolved in the solution once electrolysis is complete. [1]
In the second investigation, the club's teacher demonstrates how to electroplate a brass belt buckle with a thin, even layer of nickel.
(d) Describe how the buckle could be electroplated with nickel, referring to the electrodes used, the electrolyte, and how the buckle is connected in the circuit. [2]
(e) State one reason why manufacturers electroplate objects such as this buckle with nickel. [1]
Question 4
A metal recycling company purifies impure copper recovered from scrap electrical cable by electrolysis. A large block of impure copper and a thin strip of pure copper are placed in a solution of copper(II) sulfate and connected to a direct current supply.
(a) State which electrode, the impure copper block or the pure copper strip, is connected as the anode. Describe what happens to the mass of each electrode as electrolysis proceeds. [2]
(b) Write ionic half-equations, including state symbols, for the reaction at each electrode.
(i) the anode [1]
(ii) the cathode [1]
(c) Describe, in terms of the movement of electrons in the external circuit and the movement of ions in the electrolyte, how charge is transferred around the complete circuit during this electrolysis. [2]
(d) The company then tests a fresh sample of the same copper(II) sulfate solution, but using two inert graphite electrodes instead of copper electrodes.
(i) State the products formed at each electrode in this case. [2]
(ii) Describe one visible change to the colour of the solution as electrolysis proceeds. [1]
Question 5
A water-treatment company investigates the electrolysis of aqueous calcium bromide, , using inert electrodes, and separately trials a hydrogen–oxygen fuel cell to power its delivery vans.
(a) A concentrated solution of calcium bromide is electrolysed using inert electrodes.
(i) State the product formed at the anode. [1]
(ii) Write an ionic half-equation for this reaction at the anode. [2]
(b) The company repeats the electrolysis using a dilute solution of calcium bromide instead.
(i) State the product now formed at the anode. [1]
(ii) Explain why this product differs from the one formed when the concentrated solution was used. [1]
(c) State one advantage and one disadvantage of using a hydrogen–oxygen fuel cell, rather than a petrol engine, to power a delivery van. [2]
Question 6
A student electrolyses aqueous potassium sulfate solution, , using two inert graphite electrodes connected to a d.c. power supply. Steady streams of gas bubbles are observed forming at both electrodes.
Which row correctly gives the products formed at the cathode and at the anode?
Question 7
A student electrolyses a concentrated aqueous solution of copper(II) chloride, , using two inert carbon electrodes connected to a d.c. power supply.
(a) State the name of the substance formed at the cathode, and describe what would be seen on the electrode as electrolysis proceeds. [2]
(b) State the name of the gas formed at the anode, and describe a test that would confirm its identity. [2]
(c) Give one precaution the student should take when carrying out this electrolysis, given that the gas from part (b) is toxic. [1]
Question 8
As part of an industrial process to extract magnesium metal, a furnace melts magnesium chloride, , and a direct current is passed through the molten compound using two inert electrodes.
(a) State the products formed at the cathode and at the anode. [2]
(b) Write ionic half-equations, including state symbols, for the reaction at
(i) the cathode [1]
(ii) the anode [1]
(c) State the type of reaction, in terms of electron transfer, occurring at the cathode, and at the anode. [2]
Question 9
A jeweller electroplates a steel bracelet with a thin, even layer of silver. The bracelet is connected as the cathode in an electrolytic cell containing aqueous silver nitrate solution, with a strip of pure silver connected as the anode.
(a) Write an ionic half-equation for the reaction that occurs at the cathode. [1]
(b) Write an ionic half-equation for the reaction that occurs at the anode. [1]
(c) Using your half-equations from (a) and (b), explain why the mass of the pure silver anode decreases while the mass of the bracelet increases as electroplating proceeds. [2]
(d) Suggest why the bracelet is thoroughly cleaned and degreased before electroplating begins. [1]
(e) State one property of silver, other than its appearance, that makes it a suitable metal to electroplate onto jewellery. [1]
Question 10
A hydrogen–oxygen fuel cell is used to generate electricity in an experimental delivery van, combining hydrogen and oxygen gas to produce electrical energy.
Which balanced chemical equation correctly represents the overall reaction taking place in the fuel cell?