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.
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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