Electrolysis: Halide Concentration Rules
Predicting products of electrolysis of aqueous solutions — concentrated vs. dilute
Interactive Reference: Virtual Electrolysis Cell
Section A: Concentrated Halide Solutions
5 questions · 12 marks
Name the products formed at the anode and cathode when concentrated aqueous potassium bromide (KBr) is electrolysed using inert electrodes.
[2 marks]Write the half-equations for the reactions at each electrode during the electrolysis of concentrated aqueous potassium bromide.
[3 marks]Name the products at the anode and cathode when concentrated aqueous sodium chloride (NaCl) is electrolysed.
[2 marks]State the observation you would make at the anode during electrolysis of concentrated potassium iodide (KI) solution. Suggest a simple test to confirm the product.
[2 marks]Explain, in terms of ion discharge, why bromine is produced at the anode rather than oxygen when concentrated potassium bromide solution is electrolysed.
[3 marks]Section B: Dilute Solutions
5 questions · 9 marks
Name the product formed at the anode when dilute aqueous sodium chloride (NaCl) is electrolysed.
[1 mark]Explain why oxygen is produced at the anode instead of chlorine when dilute sodium chloride solution is electrolysed.
[2 marks]Write the half-equation for the reaction at the anode during electrolysis of dilute sodium chloride solution.
[2 marks]Name the products at both electrodes when dilute sulfuric acid (H₂SO₄) is electrolysed using platinum electrodes.
[2 marks]During electrolysis of dilute copper(II) sulfate solution using inert electrodes, name the products at the anode and cathode. Describe the observation at each electrode.
[2 marks]Section C: Comparisons & Extended Questions
5 questions · 17 marks
A student electrolyses concentrated sodium chloride (NaCl) solution, then slowly adds water until the solution becomes very dilute. Describe the change in observations at the ANODE as the solution becomes more dilute. Explain the chemistry behind each observation.
[4 marks]Compare and contrast the products of electrolysis at the anode for: (a) Concentrated KBr solution, (b) Dilute KBr solution.
[3 marks]The chlor-alkali industry electrolyses concentrated sodium chloride solution. State the three useful products and give one industrial use for each.
[3 marks]A student electrolyses concentrated copper(II) bromide (CuBr₂) solution using inert (graphite) electrodes. Predict the products at each electrode. Justify your prediction by referring to the reactivity/discharge series.
[3 marks][Extended Writing] Explain the complete rules for predicting the products of electrolysis of aqueous solutions. Your answer must address: (i) Rules for the cathode, (ii) Rules for the anode, (iii) The effect of concentration.
[4 marks]The Golden Rules — Halide Discharge Summary
🔵 CATHODE (−) — Reduction Rules:
• Metal ions present (not Group 1/2/Al) → metal deposited
• Group 1/2 metals or Al → H₂(g) produced instead
• Less reactive species discharged preferentially
🟢 ANODE (+) — Oxidation Rules:
• Concentrated halide (Cl⁻, Br⁻, I⁻) → halogen produced
• Dilute halide OR no halide → O₂ produced
• OH⁻ preferentially discharged in dilute solutions
⚡ Concentration is the Deciding Factor at the Anode:
Concentrated [X⁻] ▶ X⁻ preferentially discharged → halogen formed
Dilute [X⁻] ▶ OH⁻ preferentially discharged → O₂ formed
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