The object being copper-plated is connected to the negative terminal so it acts as the cathode. Copper(II) ions gain electrons at its surface and form solid copper there.
The 2027 SEC G3 Chemistry K324 syllabus includes describing electroplating and giving a use for it. A copper-plating question can be answered by tracking what happens to copper ions at the object and at the copper electrode.
- Connect the object to the negative terminal in the diagram.
- Identify copper(II) ions in the electrolyte.
- Write reduction at the object’s surface.
- Write oxidation at the copper anode when the question uses one.
Assign the two electrodes
Consider a school exam diagram showing a conducting object, a copper electrode and aqueous copper(II) sulfate. The object is the cathode and connects to the negative terminal of the direct-current supply. The copper electrode connects to the positive terminal and acts as the anode.
At the cathode, copper(II) ions accept electrons supplied to the surface. The ionic half-equation is Cu²⁺(aq) + 2e⁻ → Cu(s). The copper atoms deposit on the object. A continuous deposit can give the object a copper finish.
Track the copper at the anode
At a copper anode, copper atoms lose electrons and enter the solution as copper(II) ions. The half-equation is Cu(s) → Cu²⁺(aq) + 2e⁻. This replenishes the copper ions being removed at the cathode.
If both reactions are the only electrode reactions, each mole of copper deposited at the cathode is matched by a mole dissolving at the anode. The ideal mass gain and mass loss are equal. The equation also shows why the copper(II) ion concentration can stay approximately steady in this model.
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Predict what reversing the leads would do
Imagine that both electrodes are copper and that the object is intended to gain a fresh copper layer. With the leads reversed, the object is positive and becomes the anode. Copper atoms on its surface can oxidise to copper(II) ions, so its mass falls. Copper deposits on the electrode that is now the cathode.
In an exam answer, give the change and explain it with electron transfer. “The object loses mass because copper atoms on its surface lose electrons and enter the solution as Cu²⁺ ions” links the observation to the reaction.
Try a mass comparison. A copper anode loses 0.32 g and the question specifies that the two copper reactions are the only reactions. The cathode gains 0.32 g.
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Frequently asked questions
Which electrode is the source of the coating? The coating forms from copper(II) ions at the cathode. With a copper anode, dissolving copper replaces ions in the electrolyte.
Which electrode carries out reduction? Reduction occurs at the cathode because the ions gain electrons there.
What is an example of a use? A copper coating can give a conducting object a decorative copper finish.