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4.3.5Half equations at electrodes

AQA GCSE Chemistry (8462), Higher tier · Chemical changes › Electrolysis

Practise Half equations at electrodes. 13 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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Reactions at electrodes during electrolysis can be represented by half equations. Complete the half equation.
2H+ + ........ → H2
2e−
Balance the half equation for the production of oxygen at the positive electrode.
....OH− → O2 + ....H2O + ....e−
4OH− → O2 + 2H2O + 4e−
During electrolysis, ions gain or lose electrons at the electrodes. What type of reaction happens at the positive electrode?
oxidation
Complete the half equation for the formation of oxygen at the positive electrode.
4OH− → O2 + 2H2O + ....e−
4e−
Complete the half equation for the formation of bromine at the positive electrode.
2Br− → Br2 + ....e−
2e−

Sample questions

Written for this site in the style of AQA exam questions. They are not taken from real past papers.

Question 1Easy4 marks
During electrolysis, ions gain or lose electrons at the electrodes.
(a) At which electrode do positive ions gain electrons?
Tick (✓) one box.[1]
  • The negative electrode (cathode)
  • The positive electrode (anode)
(b) What type of reaction happens at the positive electrode?[1]
(c) Complete the half equation.
Zn2+ + 2e− → ....................[1]
(d) How many electrons are needed to change one aluminium ion, Al3+, into one aluminium atom?[1]
Show the answer and mark scheme
(a) Answer: The negative electrode (cathode)
(b) Answer: oxidation
  • oxidation
(c) Answer: Zn
  • Zn
(d) Answer: 3
  • 3
Question 2Medium4 marks
Reactions at electrodes during electrolysis can be represented by half equations.
(a) Complete the half equation.
2H+ + ........ → H2[1]
(b) Balance the half equation.
....Cl− → Cl2 + ....e−[1]
(c) Complete the half equation.
Cu2+ + ....e− → Cu[1]
(d) Explain why the reactions at the negative electrode (cathode) are reductions.[1]
Show the answer and mark scheme
(a) Answer: 2e−
  • 2e−
(b) Answer: 2Cl− → Cl2 + 2e−
  • 2Cl− → Cl2 + 2e−
(c) Answer: 2
  • 2
(d)
  • positive ions gain electrons at the cathode (and reduction is gain of electrons)
Question 3Hard6 marks
Half equations can be written for the reactions at both electrodes during electrolysis.
(a) Balance the half equation for the production of oxygen at the positive electrode.
....OH− → O2 + ....H2O + ....e−[1]
(b) Is the production of oxygen at the positive electrode an oxidation or a reduction? Give a reason.[1]
(c) Potassium iodide solution is electrolysed using inert electrodes.
Write the half equation for the reaction at each electrode.[2]
(d) Molten aluminium oxide is electrolysed.
Write the half equation for the reaction at each electrode.[2]
Show the answer and mark scheme
(a) Answer: 4OH− → O2 + 2H2O + 4e−
  • 4OH− → O2 + 2H2O + 4e−
(b)
  • oxidation, because the hydroxide ions lose electrons
(c) Answer: cathode: 2H+ + 2e− → H2; anode: 2I− → I2 + 2e−
  • negative electrode: 2H+ + 2e− → H2
  • positive electrode: 2I− → I2 + 2e−
(d) Answer: cathode: Al3+ + 3e− → Al; anode: 2O2− → O2 + 4e−
  • negative electrode: Al3+ + 3e− → Al
  • positive electrode: 2O2− → O2 + 4e−

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