Question 2 – Electrolysis of dilute sulphuric acid

A student carried out an electrolysis of dilute sulphuric acid and collected the gases formed.

  1. Draw a labelled diagram to show the apparatus used. Show the gases collected in their correct ratio.
  2. Give the formulae of all the ions present in the electrolyte.
  3. Write half equations for the reactions at the anode and cathode. Write down an overall equation for the reaction.
  4. Describe the tests and results for any gases evolved.

Solution 2

  1. Electrolysis of dilute sulphuric acid

    Take note of the ratio of hydrogen to oxygen. Hydrogen produced is twice as much as the oxygen produced.

  2. H+, SO42- and OH.
  3. Anode reaction: 4OH(aq) – 4e → 2H2O(l) + O2(g)

    OH is lower than SO42- in the order of preferential discharge so is discharged first.

    Cathode reaction: 2H+(aq) + 2e → H2(g)

    Overall reaction: 2H2O(l) → 2H2(g) + O2(g)

  4. Hydrogen gas is produced at the cathode.

    Test for hydrogen gas: Bring a burning splint to the mouth of the test-tube containing hydrogen gas.

    Result The gas burns with a loud pop.

    Oxygen gas is produced at the anode.

    Test for oxygen: Dip a glowing splint in the test-tube containing the gas.

    Result: Oxygen gas relights a glowing splint.