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Electrolysis: Question 7

Syllabus 4.1

Structured Core 5 marks

A student electrolyses a concentrated aqueous solution of copper(II) chloride, CuCl2\text{CuCl}_2, using two inert carbon electrodes connected to a d.c. power supply.

(a) State the name of the substance formed at the cathode, and describe what would be seen on the electrode as electrolysis proceeds. [2]

(b) State the name of the gas formed at the anode, and describe a test that would confirm its identity. [2]

(c) Give one precaution the student should take when carrying out this electrolysis, given that the gas from part (b) is toxic. [1]

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Worked solution

Part (a): Substance at the cathode

Copper(II) chloride solution contains Cu2+\text{Cu}^{2+} ions (from the salt) and H+\text{H}^+ ions (from the water). Copper is less reactive than hydrogen, so at the negative cathode, the copper ions are discharged in preference to the hydrogen ions: they gain electrons (reduction) and are deposited as solid copper.

Observation: a pink-brown (orange-brown) solid gradually coats the surface of the cathode.

Part (b): Gas at the anode

The solution also contains Cl−\text{Cl}^- ions (from the salt) and OH−\text{OH}^- ions (from the water). Because the solution is concentrated in chloride ions, the chloride ions are discharged preferentially at the positive anode, releasing chlorine gas.

Test: hold damp blue litmus paper in the gas. Chlorine bleaches it white.

Part (c): Safety precaution

Chlorine gas is toxic, so the electrolysis should be carried out in a fume cupboard (or a well-ventilated area), and the student should avoid breathing in the gas directly.

Final answers

  • (a) Copper; a pink-brown solid deposit builds up on the cathode.
  • (b) Chlorine; damp blue litmus paper is bleached white.
  • (c) Carry out the electrolysis in a fume cupboard or well-ventilated area.