Electroplating (Higher Tier)

Purpose of Electroplating

Electroplating is a process used to coat the surface of an object with a thin layer of metal using electrolysis. The main purposes of electroplating are:

• Prevent corrosion: The metal coating protects the underlying metal from reacting with air or moisture, reducing rust and corrosion.
• Improve appearance: Electroplating can give objects a shiny, attractive finish, often used for jewellery and decorative items.
• Provide wear resistance: A hard metal coating can make surfaces more durable and resistant to scratches or wear.

Electroplating Process

Electroplating involves passing an electric current through an electrolyte solution containing metal ions. The object to be plated acts as the cathode (negative electrode), and the metal to be deposited is often the anode (positive electrode).

Key points of the process:

• The electrolyte is a solution containing ions of the metal to be plated, for example, a silver nitrate solution for silver plating.
• The object to be plated is connected to the negative terminal of the power supply, making it the cathode.
• Metal ions in the electrolyte gain electrons (are reduced) at the cathode and deposit as a thin metal layer on the object.
• The anode is usually made of the plating metal itself. It loses metal atoms which dissolve into the electrolyte as ions, maintaining the concentration of metal ions.
• The electric current drives the movement of ions and the plating process.

For instance, when electroplating a key with copper:

• The key is the cathode.
• The electrolyte contains copper ions (Cu²⁺).
• Copper ions gain electrons and form copper metal on the key’s surface.
• The copper anode dissolves to replace the copper ions in the solution.

This ensures a continuous supply of copper ions for plating.

Electrochemical Reactions

Electroplating involves two key half-reactions:

• Reduction at the cathode: Metal ions in solution gain electrons and deposit as solid metal on the object.
• Oxidation at the anode: Metal atoms from the anode lose electrons and enter the solution as metal ions.

For example, in copper electroplating:

• Cathode (reduction): $\mathrm{Cu^{2+} (aq) + 2e^- \rightarrow Cu (s)}$
• Anode (oxidation): $\mathrm{Cu (s) \rightarrow Cu^{2+} (aq) + 2e^-}$

The electric current provides the energy needed to drive these non-spontaneous reactions. Electrons flow from the power supply to the cathode, where metal ions gain electrons and deposit. At the anode, metal atoms lose electrons and replenish the metal ions in the electrolyte.

This continuous flow maintains the electrolyte concentration and allows a steady plating process.

For example, if a silver object is electroplated with copper, copper ions in the electrolyte are reduced at the silver cathode, forming a copper coating. The copper anode dissolves to maintain ion concentration.

Learning example: Electroplating a silver spoon with copper using a copper sulfate solution.

At the cathode (silver spoon): $\mathrm{Cu^{2+} (aq) + 2e^- \rightarrow Cu (s)}$ (copper deposits on the spoon)

At the anode (copper strip): $\mathrm{Cu (s) \rightarrow Cu^{2+} (aq) + 2e^-}$ (copper dissolves into solution)

For instance, if a current of 2.0 A flows for 30 minutes during electroplating, the total charge passed through the electrolyte is calculated as $Q = I \times t = 2.0 \times (30 \times 60) = 3600$ coulombs.

Applications of Electroplating

Electroplating is widely used in various industries for different purposes:

• Jewellery and decorative items: Silver or gold plating improves appearance and value.
• Electrical connectors: Metals like gold or silver are plated onto connectors to improve conductivity and prevent corrosion.
• Preventing rust on iron objects: Iron or steel items are often electroplated with metals like chromium or nickel to protect against rust.