How to explain electroplating

Electroplating is one of the most satisfying topics in redox and electrochemistry because it is electrolysis put to a real, everyday use. It is how a cheap metal spoon gets a shiny silver coat, and how steel car parts are given a protective layer of chromium. To explain it well in SPM you need four things: why we do it, how the cell is arranged, the reactions at each electrode, and why the solution does not run out. This guide takes them in turn.

Why we electroplate

Electroplating coats the surface of one metal object with a thin layer of another metal. There are three main reasons to do it:

• To prevent corrosion, a coating of a less reactive metal protects the object underneath from rusting or tarnishing.
• To improve appearance, a layer of silver, chromium or gold makes an object look more attractive and shiny.
• To improve the surface, plating can give a harder, more hardwearing or more conductive surface.

Naming a purpose is often the opening mark of an electroplating question, so start there.

How to set up the cell

The setup is the part students most often muddle, so learn the three rules exactly. Suppose you want to electroplate an iron key with copper.

• The object to be plated is the cathode, the negative electrode. So the iron key is the cathode.
• The plating metal is the anode, the positive electrode. So a piece of pure copper is the anode.
• The electrolyte is a solution containing ions of the plating metal. To plate with copper you use a copper(II) salt solution, such as copper(II) sulphate.

The rule to memorise is simple: object at the cathode, plating metal at the anode, and a solution of the plating metal’s ions in between. The electroplating with copper experiment shows the arrangement in the lab.

The reactions at each electrode

Electroplating is electrolysis with a reactive (active) anode, so the anode itself takes part in the reaction. Using copper plating as the example:

• At the anode (copper): the copper atoms lose electrons and dissolve into the solution as copper ions. This is oxidation:

  Cu → Cu²⁺ + 2e⁻

• At the cathode (the object): copper ions from the solution gain electrons and are deposited as a layer of copper metal on the object. This is reduction:

  Cu²⁺ + 2e⁻ → Cu

So copper leaves the anode, travels through the solution as ions, and lands on the object at the cathode. Over time the copper anode gets thinner while the object gains its copper coat.

Why the electrolyte concentration stays constant

This is the point examiners love, and it is worth stating clearly. As copper ions are removed from the solution at the cathode, they are replaced by new copper ions formed at the anode. The rate at which Cu²⁺ is deposited at the cathode equals the rate at which Cu²⁺ enters the solution at the anode, so the concentration of copper ions in the electrolyte stays the same, and the blue colour of the copper(II) sulphate does not fade. Contrast this with using inert carbon electrodes, where the blue colour would fade because nothing replaces the copper ions.

A second example: silver plating

The same three rules apply if you plate an object with silver. The object is the cathode, a piece of pure silver is the anode, and the electrolyte is a solution containing silver ions.

• At the anode: Ag → Ag⁺ + e⁻, silver dissolves.
• At the cathode: Ag⁺ + e⁻ → Ag, silver is deposited on the object.

This is how items such as cutlery and jewellery are given a silver finish.

How to answer an electroplating question

1. State the purpose, usually to prevent corrosion or improve appearance.
2. Identify the cathode (the object) and the anode (the plating metal).
3. Name the electrolyte, a solution of the plating metal’s ions.
4. Write the half-equations: oxidation at the anode, reduction at the cathode.
5. If asked, explain why the electrolyte concentration and colour stay constant.

Common mistakes to avoid

• Putting the object at the anode, it must be the cathode.
• Using the wrong electrolyte, for example a solution that does not contain the plating metal’s ions.
• Saying the anode is inert; in electroplating the anode dissolves.
• Forgetting to explain the constant concentration when the question clearly asks for it.

Electroplating rewards a clear, ordered answer more than almost any other electrochemistry question, because the marks follow the setup. If the electrode rules keep slipping, a teacher can fix that in a single focused lesson. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; the how it works page shows what a lesson looks like.