Copper Plating a Steel Key
A student tries to copper-plate a steel key for a technology project. She uses a 250 cm³ beaker containing 0.50 mol/dm³ copper(II) sulfate solution, a copper strip anode and the key as cathode, connected to a 3.0 V supply. The key was wiped with a tissue but not polished. After 20 minutes at about 0.50 A, the coating is thin, patchy and dull, and one side of the key has no copper at all. The solution is still blue. A friend suggests replacing the copper anode with a graphite pencil lead to 'make the copper run out faster', and another suggests increasing the voltage to 12 V to speed up the process. The teacher warns that the key must be clean and that the anode must be the same metal as the coating. The student wants a bright, even coating and asks which suggestions to accept.
1. Explain why the key must be the cathode and why a copper anode keeps the copper sulfate solution blue. Include electrode equations.
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At the cathode, the negative electrode, electrons are supplied to Cu2+ ions: Cu2+(aq) + 2e- -> Cu(s), so copper is deposited on the key. At a copper anode, copper atoms lose electrons: Cu(s) -> Cu2+(aq) + 2e-. The anode therefore replaces the Cu2+ removed at the cathode, so the concentration of copper sulfate stays roughly constant and the blue colour remains. If the key were the anode, it would dissolve instead of being plated. The steel key conducts, so it can be the cathode, but it must make good electrical contact.
2. Account for the patchy, dull coating and the bare side. Suggest three specific improvements.
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The key was only wiped, so grease and oxide remained; copper cannot deposit evenly on an insulated surface. One side may have faced away from the anode, giving a low current density and little copper. Poor contact or a loose clip would also stop plating. Improvements: polish with emery paper, degrease with detergent or propanone and rinse; connect the clip to a clean area and keep the key centrally facing the anode; use a lower current for longer, stir the solution gently or reposition the key. A brighter coating needs a clean surface and moderate current, not just more voltage.
3. Evaluate the two suggestions: use a graphite anode, or raise the voltage to 12 V. What would you recommend?
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Graphite is a conductor but not copper. At a graphite anode, water or hydroxide is oxidised instead of copper: 2H2O -> O2 + 4H+ + 4e- (or 4OH- -> O2 + 2H2O + 4e-). Cu2+ is still removed at the cathode, so the blue colour fades and plating slows; oxygen bubbles may disturb the deposit. It does not make copper run out faster in a useful way. 12 V is too high for this small cell: current and gas evolution increase, giving a rough, powdery, dark deposit, heating and spattering. Recommend keeping the copper anode and using about 3 V with a clean key, good contact and perhaps 30 minutes. Monitor and stir; do not exceed the supply rating.