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Oxygen is produced at the anode unless halide ions are present, and hydrogen at the cathode for reactive metals. Free notes and 9 questions answered.
Products at the Cathode and Anode is part of Electrolysis in GCSE AQA Higher Chemistry.
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Oxygen is produced at the anode unless halide ions are present, and hydrogen at the cathode for reactive metals. Free notes and 9 questions answered.
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Use this lesson to stay inside Electrolysis while connecting the detail back to the wider Topic 4: Chemical changes topic.
Read the core explanations from Products at the Cathode and Anode before testing yourself from memory.
When you pass electricity through a solution, different substances appear at each electrode. But which ones? This isn't random — there's a pattern you can predict every time.
Think of electrolysis like a competition. At each electrode, different ions are fighting to react. The winners follow specific rules, and once you know these rules, you'll never guess wrong again.
The cathode is the negative electrode where reduction happens. Positive ions (cations) rush towards it, desperate to gain electrons. But here's the key question: which ion actually gets the electrons?
This happens because reactive metals have a stronger tendency to remain as ions, so hydrogen ions are preferentially reduced at the cathode instead. So when electricity flows, hydrogen ions grab the electrons first, leaving the metal ions in solution.
Worked example: Predicting Cathode Products
What is produced at the cathode when sodium chloride solution is electrolysed?
Answer: Hydrogen gas (H₂) is produced at the cathode
**Trap**: Many students think the metal in the compound name always gets produced. Actually, it depends on reactivity. Sodium chloride electrolysis produces hydrogen, not sodium metal.
The anode is where oxidation occurs — negative ions lose electrons here. The default product is oxygen gas from water molecules. But there's an important exception that catches many students out.
Why do halogens get priority? Halide ions are easier to oxidise than the OH⁻ ions from water. It's like they push to the front of the queue.
Worked example: Predicting Anode Products
What is produced at the anode when potassium bromide solution is electrolysed?
Answer: Bromine gas (Br₂) is produced at the anode
For solutions without halide ions, like sodium sulphate, oxygen gas bubbles off at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻
Now you can predict both electrodes. Let's see how this works for different solutions and understand why these patterns exist.
Worked example: Complete Electrolysis Prediction
Predict all products when dilute hydrochloric acid is electrolysed.
Answer: Hydrogen gas at cathode, chlorine gas at anode
Notice how the solution becomes less acidic during electrolysis — you're removing H⁺ and Cl⁻ ions, leaving behind water.
Electrolysis questions are mark-heavy in exams, but they follow predictable patterns. Here's how to nail them every time.
**Common Exam Mistakes**: Students often forget that aqueous solutions contain H⁺ and OH⁻ ions from water. These compete with the dissolved compound's ions. Also, don't assume the metal in the compound name is always produced — check the reactivity series first.
Exam precision note: In Products at the Cathode and Anode - Electrolysis of aqueous solutions - Electrolysis, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
In aqueous electrolysis, the cathode product depends on the reactivity series. If the metal ion is more reactive than hydrogen, hydrogen is usually produced instead because hydrogen ions are discharged more easily. Less reactive metal ions, such as copper ions, can be discharged to form the metal.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-19.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Hydrogen is produced at the cathode when the metal in the solution is more reactive than hydrogen, because reactive metals tend to stay as ions and hydrogen ions are reduced instead: 2H⁺ + 2e⁻ → H₂. If the metal is less reactive than hydrogen, such as copper, the metal is produced.
Electrolysing dilute hydrochloric acid produces hydrogen gas at the cathode and chlorine gas at the anode: 2HCl(aq) → H₂(g) + Cl₂(g). The solution becomes less acidic because H⁺ and Cl⁻ ions are removed.
The half equation for bromide ions forming bromine at the anode is 2Br⁻ → Br₂ + 2e⁻. Bromide is a halide ion, so it is oxidised more easily than OH⁻ ions, as in the electrolysis of potassium bromide solution.
Oxygen is produced at the anode unless the solution contains halide ions (Cl⁻, Br⁻ or I⁻), in which case the halogen is produced instead. For a solution without halide ions, such as sodium sulphate, the half equation is 4OH⁻ → O₂ + 2H₂O + 4e⁻.
Electrolysing sodium sulphate solution gives hydrogen at the cathode and oxygen at the anode. Sodium is more reactive than hydrogen, so H⁺ ions are reduced, and with no halide ions present, oxygen bubbles off at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻.
Electrolysing potassium bromide solution produces bromine at the anode, because bromide is a halide ion that is oxidised more easily than OH⁻: 2Br⁻ → Br₂ + 2e⁻. Potassium is more reactive than hydrogen, so hydrogen is produced at the cathode.
Halide ions (Cl⁻, Br⁻, I⁻) are discharged instead of OH⁻ ions because they are easier to oxidise than the OH⁻ ions from water. They push to the front of the queue, so the halogen is produced at the anode if halide ions are present.
Dilute hydrochloric acid becomes less acidic during electrolysis because H⁺ and Cl⁻ ions are removed, leaving behind water. H⁺ ions are reduced to hydrogen gas at the cathode and Cl⁻ ions are oxidised to chlorine gas at the anode.
A metal is produced at the cathode in aqueous electrolysis when it is less reactive than hydrogen, such as copper. If the metal is more reactive than hydrogen, hydrogen gas is produced instead and the metal ions stay in solution.