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Grade 10 Advanced About 20 min Version 1 Awaiting review Premium

Electrolysis: splitting compounds with electricity

Electrolysis uses electricity to break down a compound. The liquid that conducts and is broken down is the electrolyte; it contains ions that can move. The negative electrode is the cathode and the positive electrode is the anode. Positive ions (such as H⁺ and Cu²⁺) move to the cathode and gain electrons; negative ions move to the anode and lose electrons. Electrolysing acidified water gives hydrogen at the cathode and oxygen at the anode, twice as much hydrogen as oxygen, because water is H₂O. Pure water hardly conducts, because it has very few ions.

What you will learn

  • Identify the cathode, anode and electrolyte.
  • Describe the products of electrolysing acidified water and copper(II) sulfate solution.
  • Explain why the volume of hydrogen is twice that of oxygen.

Before you start

  • Know that compounds can contain ions.

Materials

  • Water with a little dilute sulfuric acid, pure (distilled) water, copper(II) sulfate solution

Equipment

  • Electrolysis cell (Hofmann voltameter or a beaker with carbon electrodes and test tubes)
  • Low-voltage d.c. supply and ammeter

Procedure

  1. Electrolyse acidified water at 6 V for 10 minutes and record the gas in each tube.
  2. Try pure water at 6 V.
  3. Electrolyse copper(II) sulfate solution at 6 V for 10 minutes and look at the cathode.
  4. Increase the voltage and compare how quickly gas collects.

How this simulation is simplified

  • Gas volumes are at room conditions (1 mole of gas = 24 L). Each electrolyte has a fixed resistance: acidified water 10 Ω, copper(II) sulfate 8 Ω, pure water 10 000 Ω.
  • Faraday constant 96 500 C/mol; no gas dissolves.