Swap the inert anode for a lump of metal and electrolysis stops making gases. Instead the anode dissolves and the same metal builds up on the cathode. That is how cutlery is silver-plated and how copper for electrical wiring is purified.
In electroplating the object to be plated is the cathode, the plating metal is the anode, and the electrolyte is a solution containing ions of the plating metal. Metal ions gain electrons at the cathode and coat the object, while the anode dissolves to replace them. Metals are electroplated to improve appearance and to resist corrosion. Extended: copper is refined the same way, with an impure copper anode and a pure copper cathode in copper(II) sulfate solution.
Fig. 1: copper-plating an object
Section 01
Setting up electroplating.
In the simulator, watch copper ions pick up electrons and become copper atoms on a metal surface. At the cathode of a plating cell exactly the same change happens, driven by the power supply.
Cathode (−): the object to be plated, for example a steel spoon or a key.
Anode (+): a piece of the plating metal, for example silver, nickel or copper.
Electrolyte: a solution containing ions of the plating metal, for example silver nitrate for silver plating or copper(II) sulfate for copper plating.
Why plate? To improve the appearance (a shiny silver or gold finish) and to protect the object from corrosion (chromium or zinc on steel).
Examiner trap
The object being plated must be the cathode. Metal ions are positive, so they are attracted to the negative electrode. Connect the object to the positive terminal and it dissolves instead of being coated.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In electroplating, the object to be plated is made the:
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A steel spoon is to be silver-plated. Which electrolyte should be used?
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Which is a reason for electroplating a metal object?
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In copper-plating, what is the anode made of?
Section 02
Copper electrodes.
Extended. Electrolyse copper(II) sulfate solution with copper electrodes instead of carbon ones and the products change completely.
Electrodes
Cathode (−)
Anode (+)
Solution
carbon (inert)
copper deposited
oxygen given off
blue colour fades
copper
copper deposited, gains mass
copper dissolves, loses mass
blue colour stays the same
Anode: Cu → Cu2+ + 2e− (the copper atoms are oxidised and go into solution).
Cathode: Cu2+ + 2e− → Cu.
Why the colour stays: one Cu2+ ion enters the solution at the anode for every one that leaves at the cathode, so the concentration does not change.
Section 03
Refining copper.
Extended. Copper for electrical wiring must be very pure, because impurities increase its resistance. Electrolysis refines it.
Anode: a large block of impure copper. It dissolves as copper ions.
Cathode: a thin sheet of pure copper. Pure copper builds up on it.
Electrolyte: copper(II) sulfate solution.
Impurities: do not form copper ions; they fall to the bottom of the cell as anode sludge, which can contain valuable metals such as silver and gold.
Mass change: the anode loses mass and the cathode gains mass. The cathode gains slightly less than the anode loses, because the impurities end up in the sludge.
Stage 2 · Exam
Exam-style questions
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(Extended) Aqueous copper(II) sulfate is electrolysed using copper electrodes. What happens to the blue colour of the solution?
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(Extended) Which half-equation shows the reaction at the anode when copper is refined?
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(Extended) During the refining of copper, the impure anode loses 50.0 g. Which is most likely for the pure cathode?
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A student tries to copper-plate a key but connects the key to the positive terminal. What happens?
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Electroplating and refining copper, mastered.
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