Electroplating an object with copper

Electroplating with copper turns the electrolysis of copper(II) sulfate into something useful, a thin, even copper coat on an object, laid down by choosing the electrodes deliberately. In the Form 5 Redox Equilibrium chapter it applies selective discharge and the dissolving anode to a real purpose: decoration and protection against corrosion. Follow the method, then use the Paper 3 section to state the variables and the operational definition an examiner wants.

Aim

To electroplate a metal object with copper using copper(II) sulfate solution and a copper anode, and to explain the reactions at the cathode and anode as a redox process.

Apparatus and materials

• Beaker or electrolytic cell
• A clean metal object to be plated, such as an iron key or spoon
• A strip of copper (the anode)
• Battery or d.c. power supply, ammeter, switch and connecting wires
• Copper(II) sulfate solution
• Sandpaper and access to water for rinsing
• Safety goggles

Procedure

1. Clean the metal object with sandpaper and wash it to remove grease and any oxide layer.
2. Pour copper(II) sulfate solution into the beaker.
3. Connect the object to the negative terminal of the supply so that it becomes the cathode.
4. Connect the copper strip to the positive terminal so that it becomes the anode.
5. Lower both the object and the copper strip into the solution, keeping them apart so they do not touch.
6. Close the switch and pass a small, steady current for several minutes.
7. Switch off, remove the object, rinse it in water and dry it gently.
8. Examine the surface of the object and the copper anode.

Expected observations

• A reddish-brown layer of copper is deposited evenly over the surface of the object at the cathode.
• The copper anode gradually becomes thinner as it dissolves.
• The blue colour of the copper(II) sulfate solution stays about the same throughout.

Describe the coating and the change in the anode; do not invent a mass of copper deposited or a plating time.

Inference and conclusion

In electroplating, the object is made the cathode. Copper(II) ions in the solution move to it, gain electrons and are reduced to copper atoms, which build up as an even layer on the object. At the anode, the copper strip is oxidised and dissolves, releasing copper(II) ions into the solution as fast as they are deposited at the cathode. Because the anode replaces the copper ions used up, the concentration of the solution, and its blue colour, stays constant. Reduction happens at the cathode and oxidation at the anode, so electroplating is a redox process. A clean surface and a small, steady current are needed for a smooth, well-attached coat. Electroplating is used to improve appearance and to protect a cheaper metal from corrosion.

Science process skills (Paper 3 style)

• Hypothesis. “A metal object becomes coated with copper when it is made the cathode in copper(II) sulfate solution with a copper anode.” State a testable relationship.
• Variables. The manipulated variable could be the time for which the current flows (or the size of the current); the responding variable is the mass or thickness of copper deposited on the object; the controlled variables are the concentration and volume of the solution, the current (if time is varied), the surface area of the object and the distance between the electrodes.
• Tabulating and plotting. Record the mass of the object before and after plating for several times, then plot the mass of copper deposited against time; a straight line through the origin would show the mass increases steadily with time.
• Making an inference. From copper building only on the electrode joined to the negative terminal, infer that copper is deposited where reduction occurs, at the cathode.
• Operational definition. “Electroplating is coating an object with a metal by making the object the cathode in a solution of a salt of that metal, shown here by copper depositing on the object.”

Safety precautions

• Wear safety goggles, because copper(II) sulfate solution is irritant and harmful if it splashes into the eyes.
• Use only a low-voltage d.c. supply with dry hands, so there is no risk of electric shock.
• Handle the sandpaper carefully and away from others, so grit does not enter the eyes.
• Dispose of the used solution in the labelled waste container, because it contains copper ions harmful to the environment.

Common errors

• Not cleaning the object first, so grease or oxide gives a patchy, poorly attached coat.
• Connecting the object to the positive terminal by mistake, so it becomes the anode and is not plated.
• Using too large a current, which gives a rough, flaky deposit instead of a smooth layer.
• Letting the object and the anode touch, which short-circuits the cell.
• Removing the object without rinsing, so it dries with a stain of solution rather than a clean copper finish.

How we help

In our online one-to-one SPM Chemistry lessons, taught in English, our teachers connect each step of electroplating to the half-equation it obeys, so you can explain why the object must be the cathode and why the blue colour does not fade. Fees start from RM50 per hour, with a paid one-hour trial lesson so you can see the method taught before you commit. Because electroplating builds directly on the electrolysis of copper(II) sulfate you meet just before it in SPM Chemistry, mastering the electrode roles here makes both the theory and the Paper 3 application straightforward.