The Process of Electroplating
Electroplating is a practical application of electrolysis where a conductive object is coated with a thin layer of a different metal. This is done for decorative purposes, like silver-plating cutlery, or for corrosion protection, like chrome-plating car parts.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The object to be plated is always made the cathode (the negative electrode).
- The anode (the positive electrode) is made from the pure metal that will form the coating.
- The electrolyte is a solution containing ions of the plating metal (e.g., using copper(II) sulfate solution to plate an object with copper).
- At the anode (oxidation), the pure metal dissolves to form positive ions, replenishing the electrolyte.
- At the cathode (reduction), metal ions from the electrolyte gain electrons and are deposited as a solid metal layer onto the object.
- During the process, the anode loses mass, the cathode gains mass, and the concentration of the electrolyte remains constant.
Diagram
Formulae
Anode (+, Oxidation): M(s) → Mn+(aq) + n e- This half-equation shows the plating metal (M) dissolving from the positive anode into the electrolyte as ions (Mn+).
Cathode (-, Reduction): Mn+(aq) + n e- → M(s) This half-equation shows the metal ions (Mn+) from the electrolyte depositing as a solid metal layer (M) onto the negative cathode.
Definitions
- Electroplating
- The process of depositing a thin metallic coating onto an object by passing a direct current through a solution containing ions of the metal.
- Active Anode
- An electrode that participates in the electrolytic reaction by being oxidised. In electroplating, the anode is active as it dissolves to supply metal ions.
- Cathode
- The negative electrode in an electrolytic cell where reduction (gain of electrons) occurs. The object being plated is the cathode.
Worked example
A student wants to electroplate an iron nail with a layer of zinc. Which of the following setups is correct?
- 1
Identify the object to be plated:
the iron nail.
This must be the cathode (negative electrode).
- 2
Identify the plating metal:
zinc.
The anode (positive electrode) must be made of pure zinc.
- 3
Identify the required electrolyte:
a solution containing zinc ions, such as zinc sulfate (ZnSO4).
- 4
Evaluate the options based on these three rules.
The correct setup must have a zinc anode, an iron nail cathode, and a zinc salt solution as the electrolyte.
- 5
Option D matches all criteria:
Positive electrode = Zinc, Negative electrode = Iron nail, Electrolyte = Zinc sulfate solution.
Answer: A setup with a pure zinc anode (+), the iron nail as the cathode (-), and an aqueous solution of zinc sulfate as the electrolyte.
Common mistakes
- ×Reversing the electrodes: Placing the object to be plated at the anode. Remember, positive metal ions are attracted to the negative cathode.
- ×Using an inert anode (like graphite or platinum): For electroplating, the anode must be 'active' and made of the plating metal to keep the ion concentration in the electrolyte constant.
- ×Choosing the wrong electrolyte: The electrolyte must contain ions of the metal you are plating with, not ions of the metal the object is made from.
No-calculator tips
- ✓Use the mnemonic 'PANIC': Positive Anode, Negative Is Cathode. This helps you correctly assign the electrodes in an electrolytic cell.
- ✓Remember the flow: The anode metal dissolves into ions, these ions travel through the electrolyte, and then they plate onto the cathode. Visualise this mass transfer from anode to cathode.
- ✓Focus on the purpose: You want to ADD metal TO the object. This means metal ions (cations, +) must go TO the object. For that to happen, the object must be negative (the cathode).