Electrolysis diagram

An electrolysis diagram shows a direct current (d.c.) power supply connected to two electrodes that dip into an electrolyte: a melted or dissolved ionic compound. The electrode joined to the positive terminal is the anode; the one joined to the negative terminal is the cathode. Positive ions move to the cathode and negative ions to the anode, where they are discharged as elements.

Labelled electrolysis diagram

How to draw an electrolysis diagram

1. Draw a beaker, and a line across it for the level of the electrolyte. Label the electrolyte with its name and state, for example "copper sulfate solution".
2. Draw two electrodes as rectangles dipping into the electrolyte, not touching each other.
3. Join each electrode with a wire to a d.c. power supply. Use the cell symbol: a long line and a short line.
4. Label the electrode connected to the long line (positive terminal) as the anode (+) and the other as the cathode (−).
5. Show the products: label what forms at each electrode, and draw an upside-down test tube of electrolyte over an electrode if a gas is being collected.

Use a pencil and ruler, and keep labels outside the drawing with lines that don't cross.

Check your understanding.

Electrolysis diagrams for common setups

A molten ionic compound: lead bromide

Solid lead bromide is heated until it melts, so its ions can move. With inert electrodes, the metal forms at the cathode and the non-metal at the anode: lead collects at the cathode, and brown bromine vapour is given off at the anode. The experiment is done in a fume cupboard because bromine is toxic. AQA suggests zinc chloride as a safer alternative.

An aqueous solution: collecting gases

For a solution, both electrodes stand in the electrolyte, often with a test tube full of the solution upside down over each electrode to collect any gas. Water provides extra hydrogen ions (H⁺) and hydroxide ions (OH⁻), so the products depend on reactivity:

• At the cathode: hydrogen is produced if the metal is more reactive than hydrogen. Otherwise the metal forms.
• At the anode: oxygen is produced, unless the solution contains halide ions (chloride, bromide or iodide), in which case the halogen is produced.

With dilute sulfuric acid, the volume of hydrogen collected is twice the volume of oxygen, because water (H₂O) is being broken down. You can test the gases: hydrogen burns with a squeaky pop, oxygen relights a glowing splint and chlorine bleaches damp litmus paper. See the tests for gases guide.

Extracting aluminium

Aluminium is made by the electrolysis of aluminium oxide dissolved in molten cryolite. The mixture melts at a lower temperature than aluminium oxide alone, which saves energy. Aluminium forms at the cathode. Oxygen forms at the carbon anodes and reacts with them to make carbon dioxide, so the anodes wear away and must be replaced regularly.

Half equations at the electrodes

Higher tier students write half equations for what happens at each electrode. Reduction (gain of electrons) happens at the cathode; oxidation (loss of electrons) happens at the anode.

A memory aid for the electron transfer is OIL RIG: Oxidation Is Loss, Reduction Is Gain. The what is electrolysis guide explains the theory in full.

Common mistakes

• Swapping anode and cathode. The anode is positive; the cathode is negative.
• Drawing the electrolyte as a solid. The ions must be free to move, so it is molten or dissolved.
• Using an a.c. supply. Electrolysis needs a direct current.
• Saying sodium forms at the cathode in salt solution. Hydrogen is discharged instead, because sodium is the more reactive of the two.
• Saying electrons flow through the electrolyte. In the liquid the current is carried by moving ions; only the wires carry electrons.
• Leaving out labels. Label the power supply, both electrodes with their charges, and the electrolyte.

Practice questions

1. Which electrode do negative ions move towards?
2. Name the products at each electrode when copper chloride solution is electrolysed with inert electrodes.
3. Why must an ionic compound be molten or dissolved before electrolysis?
4. Write the half equation for the formation of bromine at the anode.

Answers: 1) The anode (positive electrode). 2) Copper at the cathode; chlorine at the anode. 3) So the ions are free to move to the electrodes. 4) 2Br⁻ → Br₂ + 2e⁻.

Practise with a short timed paper

Electrolysis comes up on both tiers of GCSE Chemistry. The daily GCSE Chemistry Higher and GCSE Chemistry Foundation papers are short timed papers of 8 questions, with a new one every day.

Read next

Chemistry · Reactions and electrolysisWhat is electrolysis? How it works, with examplesElectrolysis uses electricity to split ionic compounds. How it works, the products at each electrode, real uses and calculations, with examples.