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K324 / 2027
Redox Chemistry

Lesson 5 of 6

Purifying and plating copper

Copper electrodes behave differently from inert ones.

O-Level 6092 (2026) / SEC G3 K324 (2027)

Pure only
  • I can explain copper purification.Syllabus

    Syllabus K324, 7.2(h). Explain copper purification

  • I can explain electroplating.Syllabus

    Syllabus K324, 7.2(i). Explain electroplating

Purifying and plating copper

Copper electrodes behave differently from inert ones.

Make a guess. You are not marked.

Impure copper is the anode and pure copper is the cathode in copper(II) sulfate solution. What happens to the mass of the anode?

  1. It decreases, as copper dissolves
  2. It stays the same, as copper is inert
  3. It increases, as copper deposits
Show the answer

It decreases, as copper dissolves

Copper atoms at the anode lose electrons and go into solution as copper(II) ions, so the anode dissolves away.

To purify copper, use impure copper as the anode, pure copper as the cathode, and aqueous copper(II) sulfate as electrolyte. At the anode, Cu(s) -> Cu2+(aq) + 2e-. At the cathode, Cu2+(aq) + 2e- -> Cu(s). Copper transfers from the impure electrode to the pure one; some impurities collect as anode sludge. Detailed industrial operation is not required.

Label the purification cell

An impure copper anode on the left and a pure copper cathode on the right dip into blue aqueous copper(II) sulfate. Copper dissolves from the anode and impurities fall below it as sludge. Copper deposits on the cathode. The solution stays blue.

Impure copper is the anode (+). Pure copper is the cathode (-). The electrolyte is copper(II) sulfate solution. The anode gets thinner, the cathode gets thicker and the solution stays blue.

The anode loses mass and the cathode gains mass. Copper ions removed at the cathode are replenished at the copper anode, so concentration remains approximately constant in the simple model. This contrasts with an inert anode, where oxygen forms and the blue solution fades.

Mass changes. The same electrons pass through both electrodes, so for every copper atom that leaves the anode, one copper atom joins the cathode. The cathode gains mass and the anode loses mass. The anode loses a little more mass than the cathode gains, because the impurities also leave it: unreactive ones such as silver fall as sludge, and more reactive ones such as zinc dissolve as ions and stay in solution.

For copper electroplating, make the object to be coated the cathode, use copper as the anode, and choose a solution containing Cu2+. Clean the object first so the coating adheres. Electroplating can improve appearance or provide a protective surface; the coating metal and intended use determine the benefit.

Label the plating cell

A copper anode on the left and a metal key on the right dip into blue aqueous copper(II) sulfate. Copper dissolves from the anode and deposits as a copper coat on the key, which is the cathode.

The object to be plated is the cathode (-). The plating metal is the anode (+). The electrolyte contains ions of the plating metal.

Check your understanding

A student makes a key the anode when trying to copper-plate it. What should change?

  1. Keep the key as the anode but raise the current so copper deposits faster.
  2. Use sodium sulfate solution instead, so that no copper ions compete at the key.
  3. Make the key the cathode, so Cu2+ ions gain electrons and deposit on it.
Show the answer

Make the key the cathode, so Cu2+ ions gain electrons and deposit on it.

Plating is reduction: Cu2+ + 2e- -> Cu happens at the cathode. A copper anode dissolves to replace the Cu2+ ions that the key removes.

Exam-style question

Impure copper can be purified by electrolysis.
(a) Draw a labelled diagram of the apparatus. [2]
(b) Write the half-equation for the reaction at each electrode. [2]
(c) During the electrolysis the pure copper electrode gains 3.20 g. Explain why the impure electrode loses more than 3.20 g. [1] [5]

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