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Difficulty: MediumIndustrial Applications of Electrolysis

In the industrial refining of copper by electrolysis, a block of impure copper is used as the anode in an aqueous copper(II) tetraoxosulfate(VI) electrolyte. Which of the following half-equations correctly represents the primary reaction taking place at the anode?

  1. Cu(s)→Cu(aq)2++2e−Cu_{(s)} \rightarrow Cu^{2+}_{(aq)} + 2e^-Answer
  2. B
    Cu(aq)2++2e−→Cu(s)Cu^{2+}_{(aq)} + 2e^- \rightarrow Cu_{(s)}
  3. C
    4OH(aq)−→2H2O(l)+O2(g)+4e−4OH^-_{(aq)} \rightarrow 2H_2O_{(l)} + O_{2(g)} + 4e^-
  4. D
    2H(aq)++2e−→H2(g)2H^+_{(aq)} + 2e^- \rightarrow H_{2(g)}

Answer

The primary reaction occurring at the anode is the oxidation of copper metal into copper(II) ions: Cu(s)→Cu(aq)2++2e−Cu_{(s)} \rightarrow Cu^{2+}_{(aq)} + 2e^-.
During the industrial electrorefining of copper, an active copper anode is oxidized by losing two electrons per atom, dissolving into the electrolyte as copper(II) ions (Cu(s)→Cu(aq)2++2e−Cu_{(s)} \rightarrow Cu^{2+}_{(aq)} + 2e^-). This maintains the concentration of copper(II) ions in solution while transferring pure copper onto the cathode.

Step-by-Step Solution

1
Identify the general electrode process at the positive terminal (anode).
Oxidation (loss of electrons) always occurs at the anode.
Electrons are removed from species at the anode during electrolysis.
2
Determine whether the anode electrode material is inert or active (reactive).
The anode is made of impure copper metal, which is an active electrode.
Active metal anodes dissolve into solution more readily than anions in the solution can discharge.
3
Write the oxidation half-reaction for the dissolution of the copper anode.
Cu(s)→Cu(aq)2++2e−Cu_{(s)} \rightarrow Cu^{2+}_{(aq)} + 2e^-
Neutral copper atoms in the impure block lose two electrons each to enter the solution as soluble copper(II) cations.

Key Concept

Electrorefining of copper and active anode oxidation
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Topics

Industrial Applications of Electrolysis

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