Replenish the copper ions lost from the solution.
Explanation
Electroplating is the chemical process of depositing a layer of a desired metal onto another material using an electric current. In the context of copper electroplating, the setup involves an electrolyte (copper sulphate solution), a cathode (the object to be plated), and an anode (a pure copper plate).
Mechanism of Action:
- Electrolyte Dissociation: When electric current passes through the copper sulphate ($CuSO_4$) solution, it dissociates into positively charged copper ions ($Cu^{2+}$) and negatively charged sulphate ions ($SO_4^{2-}$).
- Deposition at Cathode: The free copper ions are attracted to the negative terminal (cathode). Here, they gain electrons and deposit onto the surface of the object being plated.
- Replenishment at Anode: As copper ions are consumed from the solution to plate the cathode, the concentration of the electrolyte decreases. To compensate for this loss, copper atoms from the copper anode lose electrons and dissolve into the solution as copper ions ($Cu \rightarrow Cu^{2+} + 2e^-$).
Thus, the copper anode serves to continuously restore the copper ions in the solution, ensuring the process continues effectively.
Key Takeaway:
In an electroplating cell, the metal to be deposited is always made the anode (positive terminal) to replenish the electrolyte, while the object receiving the coating is made the cathode (negative terminal).