Science
What is electrolysis, and how does it work?
Quick answer
Electrolysis uses electricity to split a compound that is melted or dissolved. Positive ions move to the negative electrode and negative ions to the positive one.
In one sentence
Electrolysis is splitting a compound into simpler substances by passing an electric current through it while it is melted or dissolved.
Explained
Some compounds hold on to their elements very tightly. You cannot get sodium out of salt or aluminium out of its ore just by heating them. Electrolysis uses electricity to pull them apart. The word means "splitting with electricity".
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You need three things:
- an electrolyte: an ionic compound that is molten (melted) or dissolved in water, so its ions are free to move;
- two electrodes: rods, often of graphite (carbon) or a metal, dipped into the electrolyte;
- a direct current (d.c.) supply, such as a battery or power pack.
The electrode joined to the positive terminal is the anode; the one joined to the negative terminal is the cathode. (A memory trick: PANIC, Positive Anode, Negative Is Cathode.)
Ionic compounds are made of charged particles called ions. When the current flows, opposite charges attract:
- Positive ions (metal ions and hydrogen ions) move to the cathode. There they gain electrons and become atoms. Gaining electrons is called reduction.
- Negative ions (non-metal ions such as chloride, Cl⁻) move to the anode. There they lose electrons. Losing electrons is called oxidation.
Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Molten compounds are the simplest case: you get the metal at the cathode and the non-metal at the anode. Molten lead bromide gives lead and bromine; molten sodium chloride gives sodium and chlorine.
Solutions in water are trickier, because water itself provides a few H⁺ and OH⁻ ions that compete:
- At the cathode: you get hydrogen, unless the metal is less reactive than hydrogen (copper, silver, gold), in which case the metal forms.
- At the anode: you get oxygen, unless the solution contains chloride, bromide or iodide ions, in which case chlorine, bromine or iodine forms.
Electrolysis of water. Pure water hardly conducts, so a little dilute acid or a salt such as sodium sulfate is added. Hydrogen bubbles off at the cathode and oxygen at the anode: 2H₂O → 2H₂ + O₂. You collect twice as much hydrogen as oxygen by volume. When the electricity comes from wind or solar power, the hydrogen is called "green hydrogen".
Electrolysis is a chemical change (new substances form) and it needs a constant input of energy, so it is endothermic. It was first done in 1800, when William Nicholson and Anthony Carlisle split water soon after Alessandro Volta invented the battery. In the 1830s Michael Faraday introduced the words electrolysis, electrode, anode, cathode and ion.
Examples
| Electrolyte | At the cathode (−) | At the anode (+) |
|---|---|---|
| Molten lead bromide, PbBr₂ | lead | bromine |
| Molten sodium chloride, NaCl | sodium | chlorine |
| Aluminium oxide in molten cryolite | aluminium | oxygen (it reacts with the carbon anode to make CO₂) |
| Sodium chloride solution (brine) | hydrogen | chlorine (sodium hydroxide is left in solution) |
| Copper(II) sulfate solution, graphite electrodes | copper | oxygen |
| Dilute sulfuric acid | hydrogen | oxygen |
Uses: extracting reactive metals such as aluminium and sodium, making chlorine, hydrogen and sodium hydroxide from brine, purifying copper, and electroplating: coating a cheap metal with a thin layer of silver, gold, chromium or nickel. To silver-plate a metal spoon, the spoon is made the cathode in a solution of silver ions, so silver builds up on it.
Common mistakes
- Mixing up anode and cathode. In electrolysis the anode is positive and the cathode is negative (PANIC).
- Trying to electrolyse a solid ionic compound. The ions must be free to move, so it must be molten or dissolved.
- Thinking the metal always forms at the cathode in solution. For sodium chloride solution you get hydrogen, because sodium is more reactive than hydrogen.
- Writing chlorine as Cl. Chlorine, bromine, hydrogen and oxygen gases are made of molecules: Cl₂, Br₂, H₂, O₂.
Step by step
Step by step
- Is it molten or a solution?Molten: the metal goes to the cathode and the non-metal to the anode. Solution: use the next two steps.
- Cathode (−)Is the metal less reactive than hydrogen (copper, silver, gold)? Then the metal forms. Otherwise hydrogen forms.
- Anode (+)Are there chloride, bromide or iodide ions? Then the halogen forms. Otherwise oxygen forms.
- Write half equationsCathode: ions gain electrons, e.g. Cu²⁺ + 2e⁻ → Cu. Anode: ions lose electrons, e.g. 2Cl⁻ → Cl₂ + 2e⁻.
- Check what is leftIn solution, the ions that were not discharged stay behind, such as sodium hydroxide from brine.
Practice questions
Predict the products
Try each one, then check your answer.
- Molten potassium bromide.
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Cathode: potassium. Anode: bromine. - Molten zinc chloride.
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Cathode: zinc. Anode: chlorine. - Copper(II) chloride solution.
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Copper is less reactive than hydrogen: copper at the cathode. Chloride ions: chlorine at the anode. - Sodium sulfate solution.
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Sodium is more reactive than hydrogen: hydrogen at the cathode. No halide: oxygen at the anode. - Potassium iodide solution.
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Cathode: hydrogen. Anode: iodine (the solution turns brown). - Silver nitrate solution.
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Cathode: silver. Anode: oxygen. - Which electrode attracts Cl⁻ ions?
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Negative ions go to the positive electrode: the anode. - Complete the half equation: Al³⁺ + ___ → Al.
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Aluminium ions need three electrons: Al³⁺ + 3e⁻ → Al - Complete the half equation: 2Br⁻ → Br₂ + ___.
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Two bromide ions lose one electron each: 2Br⁻ → Br₂ + 2e⁻ - Is the change at the cathode oxidation or reduction?
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Ions gain electrons there: reduction. - 20 cm³ of oxygen is collected while water is electrolysed. How much hydrogen is made?
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2H₂O → 2H₂ + O₂, so twice the volume: 40 cm³ - A current of 2.0 A flows for 965 s through copper sulfate solution. What mass of copper forms? (1 mol of electrons = 96 500 C; Cu = 63.5)
Show answer
Charge = 2.0 × 965 = 1930 C = 1930 ÷ 96 500 = 0.02 mol of electrons. Cu²⁺ + 2e⁻ → Cu, so 0.01 mol Cu × 63.5 = 0.635 g
Word problems
Electrolysis in real life
Try each one, then check your answer.
- Aluminium oxide melts at over 2000 °C. Why is it dissolved in molten cryolite before electrolysis?
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Cryolite lowers the operating temperature (to around 950 °C), which saves a lot of energy and money. - Why do the carbon anodes in an aluminium plant have to be replaced often?
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Oxygen forms at the anodes and reacts with the hot carbon to make carbon dioxide, so the anodes slowly burn away. - A jeweller wants to gold-plate a ring. Should the ring be the anode or the cathode?
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The cathode: gold ions (positive) are attracted to it and become a layer of gold metal. - Why is hydrogen made by electrolysis using solar power called “green”?
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The electricity comes from a renewable source, so making the hydrogen releases little or no carbon dioxide. - Ben tries to electrolyse a dry pile of table salt and nothing happens. What should he do, and why?
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Dissolve it in water (or melt it). In the solid the ions cannot move, so no current flows.
Questions people ask
What is electrolysis in chemistry, in simple words?
It is using electricity to break a compound into simpler substances. The current makes positive ions move to one electrode and negative ions to the other, where they turn into atoms or molecules.
What is electrolysis used for?
Extracting metals like aluminium and sodium, purifying copper, electroplating jewellery and cutlery, and making chlorine, sodium hydroxide and hydrogen.
How does the electrolysis of water work?
With a little sodium sulfate or dilute acid added so it conducts, the current splits water into hydrogen at the negative electrode and oxygen at the positive electrode, two volumes of hydrogen for every one of oxygen.
Is electrolysis a physical or chemical change?
A chemical change. New substances are made, and it is a decomposition reaction: one compound breaks down into two or more simpler substances.
Can you do electrolysis at home?
A grown-up can help you try it safely with a 9 V battery, two pencil leads as electrodes and water with a spoon of bicarbonate of soda. Never use mains electricity, and do not use salt water, because it makes chlorine, a poisonous gas.
Is electrolysis hair removal the same thing?
It uses the same word. Trained professionals pass a tiny current through a very fine probe into a hair follicle to damage the root, so the hair does not grow back. In one method, the current causes electrolysis of the salty fluid in the skin. It is a cosmetic treatment, not a school experiment.
How does electrolysis remove rust?
The rusty object is made the cathode in a solution of washing soda. At the cathode, the rust is reduced and loosened so it can be scrubbed off. It makes hydrogen and should only be done by adults with proper safety precautions.
Quick quiz
Electrolysis
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Question 1 of 4
Why can’t solid sodium chloride be electrolysed?
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