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1.9.2Principles of Chemistry

Electrolysis Experiments — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 23 of 25 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Molten Binary Compounds

Only the ions from the compound are present, so predicting the products is straightforward.

Rule
  • Rule A molten binary ionic compound always gives the metal at the cathode and the non-metal at the anode.
Lead(II) bromide
  • Lead(II) bromide Pb²⁺ ions go to the cathode and form grey lead metal;
    • Br⁻ ions go to the anode and form brown bromine gas.
Other examples
  • Other examples Molten copper chloride gives copper and chlorine, and molten magnesium oxide gives magnesium and oxygen.
Apparatus
  • Apparatus Heat the compound in a crucible, insert two graphite electrodes and connect them to a power supply.

Part 2 of 5Aqueous Solutions: the Rules

Aqueous Solutions: the Rules
Extra ions
  • Extra ions Water ionises slightly, so H⁺ and OH⁻ ions are present as well as the ions of the dissolved compound.
Cathode rule
  • Cathode rule Hydrogen forms at the cathode unless the metal is less reactive than hydrogen, in which case the metal is deposited.
Anode rule
  • Anode rule A halide ion (Cl⁻, Br⁻, I⁻) is discharged to give the halogen;
    • if none is present, oxygen forms from OH⁻ or water.
Order
  • Order Write down all the ions first, then decide which ion at each electrode is discharged.
ElectrolyteCathode productAnode product
Concentrated sodium chlorideHydrogenChlorine
Dilute sulfuric acidHydrogenOxygen
Copper(II) sulfateCopperOxygen

Part 3 of 5Explaining the Three Examples

Explaining the Three Examples
Sodium chloride
  • Sodium chloride Na⁺ is more reactive than hydrogen so H₂ forms, and Cl⁻ is discharged at the anode, leaving NaOH in solution.
Dilute sulfuric acid
  • Dilute sulfuric acid H⁺ ions gain electrons to form hydrogen, and sulfate stays in solution while oxygen forms at the anode.
Copper(II) sulfate
  • Copper(II) sulfate Copper is less reactive than hydrogen so copper coats the cathode;
    • oxygen forms at the anode as before.
Electrodes
  • Electrodes Inert graphite electrodes are used so they do not react with the products.

Part 4 of 5Testing the Products

1
Collect gases in inverted test tubes placed over each electrode.
2
Hydrogen is identified because it burns with a squeaky pop when a lit splint is applied.
3
Oxygen is identified because it relights a glowing splint.
4
Chlorine is identified because it bleaches damp litmus paper.
5
A pink-brown deposit on the cathode shows that copper metal has formed.

Part 5 of 5Exam Checklist & Tips

1.9.2 – Key Points for the Exam

  • Binary compound A compound made of just two elements.
  • Inert electrode An electrode, such as graphite, that does not take part in the reaction.
  • Discharge The gain or loss of electrons by an ion at an electrode to form a neutral species.
  • Reactivity series A ranking of metals used to decide whether metal or hydrogen forms at the cathode.

Common Mistakes

  • Predicting a metal at the cathode from a sodium or potassium solution; hydrogen forms instead.
  • Forgetting that water supplies H⁺ and OH⁻ ions in aqueous electrolysis.
  • Saying oxygen forms from sulfate ions; sulfate stays in solution.
  • Mixing up which gas gives a squeaky pop and which relights a glowing splint.

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