Faulty Nickel Electroplating on an Iron Key
A student electroplates an iron key with nickel using a nickel anode, a nickel sulphate electrolyte and a 6 V battery. The key is connected to the negative terminal and the nickel strip to the positive terminal. After 25 minutes at 0.5 A, the key has a dark, powdery deposit and gas bubbles are seen at the key. The green colour of the nickel sulphate solution has become noticeably paler. The key was wiped with cloth but not degreased before use. The nickel strip is small and partly covered with a white crust. The student wants a smooth, adherent, silvery deposit and asks whether increasing the current to 1.5 A would solve the problem. She has a 0–2 A ammeter and a rheostat available.
1. Write the electrode reactions for the intended plating and explain why the nickel sulphate concentration should stay roughly constant.
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At the key (cathode, negative): Ni2+(aq) + 2e− → Ni(s). At the nickel anode (positive): Ni(s) → Ni2+(aq) + 2e−. When the setup is correct, each nickel ion reduced at the key is replaced by one nickel ion formed at the anode, so the concentration of NiSO4 remains approximately constant. The anode loses mass and the cathode gains mass at rates set by the current. The green colour should therefore not fade rapidly. If the colour fades, Ni2+ is being consumed faster than it is replenished, which points to a fault such as a passivated or too-small anode, wrong connections, or too high a current.
2. Diagnose the dark powdery deposit, gas bubbles and fading colour. State the corrections needed.
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The dark powdery deposit and gas bubbles at the key indicate that the current density is too high and/or the electrolyte is too dilute: hydrogen is being evolved alongside nickel deposition, and nickel is depositing too fast to form an adherent layer. The fading green colour shows Ni2+ is being removed faster than the anode can replace it-consistent with a small, partly crusted anode and possibly a passivated surface. The key was not degreased, so the deposit cannot bond evenly. Corrections: clean and degrease the key, use a larger and clean nickel anode, use fresh concentrated NiSO4 electrolyte, lower the current (e.g., 0.2–0.3 A) and plate for longer, and ensure good contact. Increasing current to 1.5 A would worsen the problem.
3. What would happen if the key were connected to the positive terminal instead? Explain.
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The key would become the anode. Iron is more reactive than nickel, so iron would be oxidised: Fe(s) → Fe2+(aq) + 2e−, and the key would dissolve or corrode rather than receive a deposit. At the nickel strip (now cathode), Ni2+ + 2e− → Ni(s), so nickel would plate onto nickel, not onto the key. The electrolyte would become contaminated with Fe2+ and the key would be damaged. This shows that polarity is not a minor detail: electroplating requires the object to be the cathode.