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SingaporeChemistrySyllabus dot point

How is electrolysis used to electroplate objects and purify metals, and what happens when the electrodes take part?

Describe electroplating and the purification of copper using reactive electrodes, explain the electrode reactions, and state industrial uses of electrolysis

A focused answer to the O-Level Chemistry outcome on electroplating and metal purification. How reactive electrodes dissolve and deposit metal, the electrode reactions in electroplating and copper refining, and the industrial uses of electrolysis.

Generated by Claude Opus 4.88 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe how electrolysis is used to electroplate objects with a metal and to purify copper, explain the electrode reactions when the electrodes are reactive (take part in the reaction), and state some industrial uses of electrolysis. This dot point builds on the principles of electrolysis by introducing reactive electrodes, where the anode itself dissolves.

The answer

Reactive electrodes take part

In the earlier dot points the electrodes were inert. In electroplating and copper purification the electrodes are reactive: the anode is made of the plating or pure-source metal and dissolves during electrolysis, while the cathode is the object to be coated (or pure metal to be grown). The metal effectively transfers from the anode, through the solution as ions, to the cathode.

Electroplating

Electroplating coats an object with a thin layer of metal (for example silver, chromium or nickel) to make it more attractive or to protect it from corrosion. The setup:

  • The object to be plated is the cathode (negative).
  • The anode is made of the plating metal.
  • The electrolyte is a solution of a salt of the plating metal.

At the cathode, metal ions are deposited onto the object: Ag++e−→Ag\text{Ag}^+ + e^- \rightarrow \text{Ag} for silver-plating. At the anode, the plating metal dissolves to replace the ions used up: Ag→Ag++e−\text{Ag} \rightarrow \text{Ag}^+ + e^-. So the anode loses mass, the object gains a coating, and the concentration of the solution stays roughly constant.

Purification of copper

Copper for electrical wiring must be very pure. Electrolysis refines it:

  • The anode is the impure copper.
  • The cathode is a thin sheet of pure copper.
  • The electrolyte is copper(II) sulfate solution.

At the anode, copper atoms (including the impure copper) dissolve: Cu→Cu2++2e−\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-, so the anode gets smaller. At the cathode, only copper(II) ions are deposited as pure copper: Cu2++2e−→Cu\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}, so the cathode grows. Impurities do not dissolve; they fall below the anode as an anode sludge, which often contains valuable metals such as silver and gold.

Industrial uses of electrolysis

Electrolysis is widely used in industry:

  • Extracting reactive metals such as aluminium from molten aluminium oxide.
  • Purifying copper to the high purity needed for wiring.
  • Electroplating for decoration and corrosion protection.
  • Manufacturing chemicals such as chlorine, hydrogen and sodium hydroxide from brine.

Examples in context

Example 1. Chrome-plated and silver-plated objects. Bathroom taps are chromium-plated to resist corrosion and look shiny, and cheaper cutlery is silver-plated for appearance at lower cost than solid silver. Both use electroplating with the object as cathode and the plating metal as anode, a direct everyday application.

Example 2. High-purity copper for cables. Copper refined by electrolysis reaches over 99.99%99.99\% purity, which is essential because even small impurities greatly reduce electrical conductivity. The valuable silver and gold recovered from the anode sludge help offset the cost of the process, showing chemistry and economics combined.

Try this

Q1. State which electrode an object should be when it is electroplated. [1 mark]

  • Cue. The cathode (the negative electrode).

Q2. In the purification of copper, write the half-equation for the reaction at the impure copper anode. [1 mark]

  • Cue. Cu→Cu2++2e−\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-.

Q3. Explain what happens to the impurities when copper is purified by electrolysis. [2 marks]

  • Cue. The impurities do not dissolve; they fall below the anode as a sludge (which may contain valuable metals such as silver and gold).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksA steel spoon is to be electroplated with silver. (a) State which electrode the spoon should be and which substance the other electrode and the electrolyte should be. (b) Write the half-equations at each electrode. (c) Explain why the mass of the silver electrode decreases.
Show worked answer →

(a) The spoon is the cathode (negative electrode). The other electrode (anode) is made of silver, and the electrolyte is a solution of a silver salt (such as silver nitrate).

(b) Cathode (spoon): Ag++e−→Ag\text{Ag}^+ + e^- \rightarrow \text{Ag}. Anode (silver): Ag→Ag++e−\text{Ag} \rightarrow \text{Ag}^+ + e^-.

(c) At the anode, silver atoms lose electrons and dissolve into the solution as silver ions, so the silver electrode loses mass. These ions are replaced at the cathode, where they are deposited on the spoon.

Markers reward the spoon as cathode with a silver anode and silver-salt electrolyte, both half-equations, and the anode losing mass because silver dissolves into solution as ions.

Original4 marksIn the purification of copper by electrolysis, the impure copper is the anode and a thin sheet of pure copper is the cathode, in copper(II) sulfate solution. (a) Describe what happens at each electrode. (b) State what happens to the impurities.
Show worked answer →

(a) At the anode (impure copper), copper atoms lose electrons and dissolve into the solution as copper(II) ions (Cu→Cu2++2e−\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-), so the impure anode gets smaller. At the cathode (pure copper), copper(II) ions gain electrons and are deposited as pure copper (Cu2++2e−→Cu\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}), so the pure cathode grows.

(b) The impurities do not dissolve; they fall to the bottom below the anode as a sludge (anode sludge), which may contain valuable metals.

Markers reward copper dissolving at the anode and depositing as pure copper at the cathode (with half-equations), and the impurities collecting as a sludge below the anode.

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