AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemical changes › Electrolysis
Practise Electrolysis of aqueous solutions. 10 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Predicting the products when a solution of an ionic compound in water is electrolysed with inert electrodes. Water provides H+ and OH− ions, so hydrogen or oxygen can form instead of the metal or non-metal. Required practical 3 is based on this.
Key facts
Water provides H+ and OH− ions
Cathode: metal more reactive than hydrogen → hydrogen; less reactive → the metal
A solution contains the ions of the compound and hydrogen ions (H+) and hydroxide ions (OH−), because a small number of water molecules break down into these ions.
At each electrode only one product forms. Which one depends on the relative reactivity of the elements involved.
At the cathode (negative)
Hydrogen is produced if the metal is more reactive than hydrogen, e.g. with solutions of potassium, sodium, calcium or magnesium compounds.
The metal is produced if it is less reactive than hydrogen, e.g. copper or silver. A coating of the metal forms on the cathode.
At the anode (positive)
diagram
If the solution contains halide ions (Cl−, Br− or I−), the halogen is produced: chlorine, bromine or iodine.
Otherwise, e.g. with sulfate or nitrate ions, oxygen is produced from the hydroxide ions.
Ions that are not discharged stay in the solution: electrolysis of sodium chloride solution leaves sodium hydroxide solution.
Two questions decide the products of electrolysing a solution with inert electrodes.
Required practical 3
diagram
Put two inert electrodes (graphite rods) into the solution and connect them to a low-voltage d.c. supply.
Collect any gas in small test tubes filled with the solution and placed over each electrode, then test it: hydrogen burns with a squeaky pop; oxygen relights a glowing splint; chlorine bleaches damp litmus paper.
Graphite electrodes, each under a test tube of the solution: any gas collects at the top of the tube, ready to test.
Look for a metal coating on the cathode (copper is pink-brown) and colours near the anode (bromine is orange, iodine is brown).
Start with a hypothesis, e.g. about how the reactivity of the metal affects the cathode product, and test several solutions under the same conditions.
Chlorine is toxic: work in a well-ventilated room and run the electrolysis only for a short time.
How to answer each type of question
Predict the products of electrolysing a solution
2 to 4 marksGrade 5
Cathode: is the metal more reactive than hydrogen? Yes → hydrogen. No → the metal.
Anode: are halide ions present? Yes → the halogen. No → oxygen.
Name elements, e.g. chlorine, not chloride.
Example. Predict the product at each electrode when these solutions are electrolysed using inert electrodes. (a) potassium bromide solution (b) silver nitrate solution
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A student electrolysed sodium chloride solution using inert electrodes. Name the gas produced at the negative electrode.
hydrogen
Dilute sulfuric acid is electrolysed using inert electrodes. It contains H+ and SO42− ions, and OH− ions from water. Name the gas formed at the positive electrode.
oxygen
When sodium chloride solution is electrolysed, the gas produced at the positive electrode bleaches damp litmus paper. Name this gas.
Chlorine.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
A student electrolysed sodium chloride solution using inert electrodes.
(a) Name the gas produced at the negative electrode.[1]
(b) Name the gas produced at the positive electrode.[1]
(c) Describe the test for chlorine and give the result.[2]
Show the answer and mark scheme
(a)Answer: hydrogen
hydrogen
(b)Answer: chlorine
chlorine
(c)
(hold) damp litmus paper (in the gas)
the litmus paper is bleached / turns white
Question 2Medium5 marks
A student electrolysed copper(II) chloride solution and then copper(II) sulfate solution, using inert electrodes.
(a) Describe what the student would see at the negative electrode during the electrolysis of copper(II) chloride solution.[1]
(b) Describe what the student would see at the positive electrode during the electrolysis of copper(II) chloride solution.[1]
(c) Oxygen is produced at the positive electrode when copper(II) sulfate solution is electrolysed. Describe the test for oxygen and give the result.[2]
(d) The blue colour of the copper(II) sulfate solution fades during the electrolysis. Explain why.[1]
Show the answer and mark scheme
(a)
a pink / brown / orange-brown solid (copper) coats the electrode
(b)
bubbles (of gas)
(c)
(insert a) glowing splint
it relights
(d)
copper(II) ions (which make the solution blue) are removed from the solution as copper forms at the negative electrode