Electroplating Exam Questions

(a) The iron key / the object to be plated [1]

(b) A piece of copper / the plating metal [1]

(c) Copper sulfate solution / copper(II) sulfate solution / any soluble copper salt [1]

(d) To prevent rusting / to improve appearance / to make it more attractive [1]

Examiner note: The key rule: the object to be plated is always the cathode (negative electrode), and the plating metal is the anode (positive electrode). The electrolyte must contain ions of the plating metal.

(a) Ag+ + e- → Ag [1]

(b) Ag → Ag+ + e- [1]

(c) Silver ions are removed from solution at the cathode but replaced at the same rate as the silver anode dissolves [1]

Examiner note: The anode dissolves — it is not inert. This is what maintains the concentration of metal ions in solution. If an inert electrode were used instead, the solution would gradually lose its metal ions and the plating would stop.

Any three from:

• To prevent corrosion/rusting (e.g. tin plating on steel cans) [1]
• To improve appearance / make the object more attractive (e.g. silver plating cutlery) [1]
• To increase hardness or durability of the surface (e.g. chromium plating) [1]
• To reduce cost (e.g. plating a cheap metal with a thin layer of an expensive metal instead of using solid expensive metal) [1]

Examiner note: Each reason should be distinct. “To make it look nice and shiny” is one reason, not two. Giving a specific example alongside each reason strengthens your answer.

• Steel bracelet labelled as cathode, connected to negative terminal of d.c. power supply [1]
• Nickel metal labelled as anode, connected to positive terminal [1]
• Electrolyte labelled as nickel sulfate solution (or other soluble nickel salt) in a beaker [1]

Examiner note: Diagrams must be clearly labelled. The cathode and anode must be correctly identified with the correct polarity. A common error is connecting the object to the positive terminal — this would dissolve the object instead of plating it.

(a) Metal atoms at the anode lose electrons and dissolve into solution as metal ions [1]

(b) Metal ions from solution gain electrons at the cathode and are deposited as metal atoms on the surface [1]

(c) The mass lost at the anode equals the mass gained at the cathode (assuming no side reactions) [1]

Examiner note: This is conservation of mass in action. The plating metal transfers from anode to cathode via the solution. The total mass of the system does not change.

(a) Any two from:

• Increase the time of electroplating [1]
• Increase the current [1]
• Increase the concentration of the copper sulfate solution [1]

(b) If the current is too high, the copper may be deposited too quickly and form a rough, uneven, or powdery layer that does not adhere well [1]

Examiner note: More charge passed = more metal deposited. Charge = current x time, so increasing either increases the amount of metal deposited. However, too-rapid deposition produces a poor-quality coating.

• The electrolyte must contain ions of the plating metal so that these ions can be discharged at the cathode [1]
• The metal ions gain electrons and are deposited as a layer of metal on the object [1]
• If the wrong electrolyte were used (e.g. sodium sulfate for copper plating), the metal ions of the plating metal would not be present in solution [1]
• Instead, hydrogen would be discharged at the cathode (if the solution contained H+ ions) and no metal coating would form [1]

Examiner note: The electrolyte is the source of the metal ions that form the coating. Using the wrong electrolyte means the wrong ions are discharged. For example, if you tried to copper-plate using sodium chloride solution, you would get hydrogen at the cathode, not copper.