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Electrolysis: Question 6

Syllabus 4.1

Multiple choice Core 1 mark

A student electrolyses aqueous potassium sulfate solution, K2SO4\text{K}_2\text{SO}_4, using two inert graphite electrodes connected to a d.c. power supply. Steady streams of gas bubbles are observed forming at both electrodes.

Which row correctly gives the products formed at the cathode and at the anode?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify every ion present in the electrolyte

Aqueous potassium sulfate contains ions from the dissolved salt and from the water itself:

  • K+\text{K}^+ and SO42−\text{SO}_4^{2-} (from the dissolved salt)
  • H+\text{H}^+ and OH−\text{OH}^- (from the water)

Four ions are available, but only one positive and one negative ion can actually be discharged at the electrodes.

Step 2: Decide which positive ion is discharged at the cathode

Potassium is a very reactive metal, far more reactive than hydrogen. When a metal is more reactive than hydrogen, its ions are not discharged; hydrogen ions are discharged instead, since the less reactive species is preferentially reduced. So at the negative cathode, H+\text{H}^+ ions gain electrons and are released as hydrogen gas, while K+\text{K}^+ ions remain in solution.

Step 3: Decide which negative ion is discharged at the anode

The sulfate ion, SO42−\text{SO}_4^{2-}, is not a halide ion and is very difficult to discharge. The sulfur atom inside it is already in a high oxidation state, so it is not oxidised further under these conditions. Instead, at the positive anode, OH−\text{OH}^- ions are discharged, releasing oxygen gas, while SO42−\text{SO}_4^{2-} ions remain in solution.

Step 4: Recognise the overall effect

Because neither K+\text{K}^+ nor SO42−\text{SO}_4^{2-} is ever discharged, this electrolysis simply decomposes the water in the solution into hydrogen and oxygen. The potassium sulfate itself is not used up. Its concentration in the remaining solution actually rises slowly as water is removed.

Why the other options are wrong

  • Option B wrongly discharges potassium instead of hydrogen at the cathode; potassium is too reactive to be discharged while hydrogen ions are present.
  • Option C swaps the two gases between the electrodes: hydrogen (from reduction of H+\text{H}^+) must form at the negative cathode, not the positive anode.
  • Option D wrongly discharges sulfate as sulfur dioxide at the anode; sulfate ions are not discharged here, so oxygen forms instead.

Final answers

  • Cathode: hydrogen. Anode: oxygen, option A\boxed{\text{option A}}.