Predicting aqueous halide electrode products
Use the metal, water, halide concentration and electrode material together instead of relying on a molten-salt rule.
Section 4.1 · Electrolysis
Exam scope and assessment notes
Extended only (Supplement) · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Know the required brine products and the molten binary-compound pattern.
- Recognise reduction, oxidation and balanced half-equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why are molten-product rules insufficient for an aqueous salt?
What you will learn
- Predict electrode products for the stated dilute and concentrated aqueous halide examples.
- Construct the corresponding half-equations and explain the role of competing species.
Make the cathode decision independently
Water introduces additional possible electrode reactions. With inert electrodes, aqueous salts of very reactive metals such as sodium or potassium give hydrogen at the cathode rather than those metals. Aqueous copper(II) chloride instead gives copper because its ions are discharged in preference to hydrogen in the school-level prediction. The cathode decision follows the cation and aqueous conditions, not the halide name alone.
For sodium chloride solution, a cathode equation using water is 2H₂O+2e⁻→H₂+2OH⁻. For copper(II) chloride, use Cu²⁺+2e⁻→Cu. A correct anode prediction cannot repair a wrong cathode assumption. Always identify the electrode material first: a soluble metal anode can change the required prediction compared with an inert one.
Read concentration before predicting the anode
At an inert anode, halide species compete with water-derived species. In the syllabus chloride comparison, concentrated aqueous chloride gives chlorine, whereas dilute aqueous chloride gives oxygen as the predicted main anode product. Use 2Cl⁻→Cl₂+2e⁻ for chlorine and 4OH⁻→O₂+2H₂O+4e⁻ for oxygen. Do not treat a formula that contains chlorine as a guarantee of chlorine under every stated concentration.
Concentrated aqueous bromide and iodide examples give bromine and iodine at an inert anode: 2Br⁻→Br₂+2e⁻ and 2I⁻→I₂+2e⁻. Bromine gives a brown colour; iodine gives a brown solution and can be confirmed by the blue-black starch result. These are school predictions for specified conditions; there is no universal numerical concentration threshold in this syllabus. Use the given concentration, electrode and observation evidence rather than inventing one cutoff for every halide or assuming every halogen appears as a gas.
Pause and explain
Which comparison is correct for concentrated and dilute NaCl(aq) with inert electrodes in the syllabus model?
Worked example
Compare concentrated and dilute aqueous NaCl using inert electrodes in the syllabus model. Give products and anode half-equations.
Show the worked solution
- Both stated aqueous cells give hydrogen at the cathode rather than sodium.
- Concentrated chloride gives chlorine at the anode: 2Cl⁻→Cl₂+2e⁻.
- Dilute chloride gives oxygen as the main anode prediction: 4OH⁻→O₂+2H₂O+4e⁻.
Answer Cathode hydrogen in both; concentrated anode chlorine, dilute anode oxygen.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write products and half-equations for concentrated NaCl(aq) with inert electrodes, using water for the hydrogen half-equation.
Give me a hint
Hydrogen gains electrons at the cathode; chloride loses electrons at the anode.
Compare my reasoning
- Cathode: 2H₂O+2e⁻→H₂+2OH⁻.
- Anode: 2Cl⁻→Cl₂+2e⁻.
- Check both atoms and charge: cathode charge −2 each side; anode charge −2 each side.
Cathode H₂: 2H₂O+2e⁻→H₂+2OH⁻. Anode Cl₂: 2Cl⁻→Cl₂+2e⁻.
Look for these in your work
- The aqueous hydrogen source is explicit.
- Both half-equations conserve charge.
Compare dilute NaCl(aq) and concentrated CuCl₂(aq), each with inert electrodes. Name all four electrode products and explain the differences.
Give me a hint
Choose each cathode from its cation and each anode from the stated concentration.
Compare my reasoning
- Dilute NaCl predicts hydrogen at the cathode and oxygen at the anode.
- Concentrated CuCl₂ predicts copper at the cathode and chlorine at the anode.
- The cation changes the cathode competition, while the stated chloride concentration changes the main anode prediction.
Dilute NaCl: H₂ cathode/O₂ anode. Concentrated CuCl₂: Cu cathode/Cl₂ anode.
Look for these in your work
- Both electrolyte identity and concentration are used.
- Cathode and anode decisions are explained separately.
An inert anode in concentrated KI(aq) produces a brown solution with a blue-black starch test. Identify its product, write its half-equation and predict the cathode product.
Give me a hint
Use the observation as evidence for iodine; potassium is a reactive metal in an aqueous electrolyte.
Compare my reasoning
- The anode product is iodine, I₂, supported by the starch result.
- Its half-equation is 2I⁻→I₂+2e⁻.
- The aqueous cathode gives hydrogen rather than potassium metal.
Iodine at the anode: 2I⁻→I₂+2e⁻; hydrogen at the cathode.
Look for these in your work
- Observation evidence identifies the halogen.
- The aqueous cathode prediction remains distinct from molten KI.
Common mistakes
- Predicting the salt's metal at every aqueous cathode.
- Dropping the word dilute or concentrated from a chloride question.
- Inventing one concentration cutoff for every possible electrode and halide.
- Calling iodine a gas in the aqueous observation.
What can you explain now?
Predict both products for concentrated aqueous CuCl₂ with inert electrodes.
Compare with the explanation
Cathode: copper; anode: chlorine.
Copper ions form copper at the cathode, while concentrated chloride gives the chlorine anode product.
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Make it stick
Create a two-electrode product table for a dilute chloride, a concentrated chloride and a stated concentrated bromide or iodide. Explain each decision and balance one half-equation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.