Practise Electrolysis of aqueous solutions. 18 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
4
State the products from sodium chloride solutionHydrogen forms at the cathode and chlorine at the anode, leaving sodium hydroxide solution.
5
Predict the product at the cathodeHydrogen forms if the metal is more reactive than hydrogen; otherwise the metal forms.
5
Predict the product at the anodeThe halogen forms if halide ions are present; otherwise oxygen forms.
6
Explain where hydrogen and hydroxide ions come fromA small number of water molecules break down into hydrogen ions and hydroxide ions.
6
Describe tests for the gases producedHydrogen gives a squeaky pop, oxygen relights a glowing splint and chlorine bleaches damp litmus paper.
7
Plan the required practical investigationUse inert electrodes and a d.c. supply, identify each product, and compare solutions to test a hypothesis.
8
Explain which ions remain in solutionIons that are not discharged stay behind, e.g. Na+ and OH− ions form sodium hydroxide solution.
Notes
Ions in an aqueous solution
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)
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.
Required practical 3
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.
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.
Cheatsheet
Water provides H+ and OH− ions
Cathode: metal more reactive than hydrogen → hydrogen; less reactive → the metal
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
List all the ions present, including H+ and OH− from water.
Cathode: compare the metal's reactivity with hydrogen.
Anode: say whether halide ions are present.
Example. Sodium chloride solution is electrolysed using inert electrodes. Explain why hydrogen, not sodium, is produced at the cathode and why chlorine, not oxygen, is produced at the anode.
Show the model answer
The solution contains hydrogen ions (from water) as well as sodium ions (1) Sodium is more reactive than hydrogen, so hydrogen is produced (1) The solution contains chloride ions, which are halide ions (1) so chlorine is produced instead of oxygen (1)
6-mark: plan an investigation into electrolysing solutions
4 to 6 marks7
Describe the apparatus: beaker of solution, two inert (graphite) electrodes, d.c. supply, test tubes over the electrodes.
Say what you keep the same: voltage, concentration, volume, time.
Say how you identify each product: gas tests, metal coating, colour changes.
Repeat with different solutions and say what result would support the hypothesis.
Example. A student's hypothesis is: 'When a solution of a metal chloride is electrolysed, the metal is produced at the cathode only if the metal is less reactive than hydrogen.' Plan an investigation to test this hypothesis. [6 marks]
Show the model answer
Marked in levels. A top-level answer is a clear, workable plan, such as: Put copper(II) chloride solution in a beaker with two graphite (inert) electrodes connected to a low-voltage d.c. supply (1) Place a small test tube filled with the solution over each electrode to collect any gas (1) Switch on for a few minutes. A pink-brown coating on the cathode shows copper; a gas that burns with a squeaky pop shows hydrogen (1) Test the gas from the anode with damp litmus paper: it is bleached if chlorine is present (1) Repeat with chlorides of metals more reactive than hydrogen, e.g. sodium chloride and potassium chloride, using the same concentration, voltage and time (1) The hypothesis is supported if only the metal less reactive than hydrogen (copper) forms at the cathode (1)
Shortcuts and memory tricks
Cathode rule: metal below hydrogen → metal; metal above hydrogen → hydrogen.
Anode rule: halide → halogen; no halide → oxygen.
In GCSE questions the metals that coat the cathode are usually copper and silver.
Where marks are lost
Saying sodium forms when sodium chloride solution is electrolysed: it is hydrogen.
Forgetting that sulfate and nitrate ions are not discharged: oxygen forms instead.
Testing chlorine with a splint: use damp litmus paper, which is bleached.
Writing 'chloride gas' instead of chlorine.
Forgetting the H+ and OH− ions from water when listing the ions present.
Exam technique
List the ions present first, including H+ and OH− from water, then apply the two rules.
In explanations, compare reactivity directly: 'sodium is more reactive than hydrogen'.
In practical questions, give the observation or test for every product.
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
In aqueous solutions, water molecules break down to produce hydrogen ions and hydroxide ions that can be discharged at the electrodes. Name all the ions present in sodium chloride solution.
Na+, Cl−, H+, OH−
A student electrolysed 200 cm3 of copper(II) sulfate solution using inert electrodes. Copper was deposited at the negative electrode and oxygen was produced at the positive electrode. Relative atomic mass (Ar): Cu = 63.5 The volume of one mole of any gas at rtp is 24 dm3. Write the half equation for the reaction at each electrode.
Cu2+ + 2e− → Cu; 4OH− → O2 + 2H2O + 4e−
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
Question 3Hard6 marks
In aqueous solutions, water molecules break down to produce hydrogen ions and hydroxide ions that can be discharged at the electrodes.
(a) Name all the ions present in sodium chloride solution.[2]
(b) During the electrolysis of sodium chloride solution, the solution becomes alkaline. Explain why.[2]
(c) Explain why oxygen is produced at the positive electrode when copper(II) sulfate solution is electrolysed.[1]
(d) Copper(II) sulfate solution was electrolysed until all the copper ions had been discharged. Name the solution left at the end.[1]
Show the answer and mark scheme
(a)Answer: Na+, Cl−, H+, OH−
Na+ and Cl−
H+ and OH−
(b)
hydrogen ions are discharged (as hydrogen) and chloride ions are discharged (as chlorine)
leaving sodium ions and hydroxide ions in solution / sodium hydroxide solution is left
(c)
the solution contains no halide ions, so hydroxide ions (from water) are discharged instead of sulfate ions