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1.9.2States of Matter

Electrolysis Experiments — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 21 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 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 5Electrolysis Basics

Electrolysis uses an electric current to decompose an ionic compound that is molten or dissolved.

Electrolyte
  • Electrolyte The molten or dissolved ionic compound that conducts electricity because its ions can move freely.
Cathode
  • Cathode The negative electrode, where positive ions gain electrons and are reduced.
Anode
  • Anode The positive electrode, where negative ions lose electrons and are oxidised.
Inert electrodes
  • Inert electrodes Graphite electrodes that do not react with the products, so they only carry the current into the electrolyte.
Ion movement
  • Ion movement Positive ions move towards the cathode and negative ions move towards the anode, because opposite charges attract.

Part 2 of 5Electrolysis of Molten Lead(II) Bromide

1
Solid lead(II) bromide is heated in a crucible until it melts, releasing Pb²⁺ and Br⁻ ions that can move freely.
2
Graphite electrodes are placed in the melt and connected to a power supply, which sets up a potential difference.
3
Pb²⁺ ions move to the cathode, where each gains two electrons to form grey lead metal that collects at the bottom.
4
Br⁻ ions move to the anode, where each loses one electron, and pairs of bromine atoms join to form Br₂ molecules.
5
Brown bromine vapour is given off at the anode, which can be identified by its colour and its smell.

Part 3 of 5Predicting Products

Predicting Products
Binary compound
  • Binary compound A compound of just two elements, such as NaCl, MgO or PbBr₂, which yields the two elements when it is melted and electrolysed.
Cathode product
  • Cathode product Always the metal, because the positive metal ion gains electrons to form the metal atom.
Anode product
  • Anode product Always the non-metal, because the negative ion loses electrons to form the non-metal element.
Molten magnesium oxide
  • Molten magnesium oxide Mg²⁺ ions give magnesium at the cathode, and O²⁻ ions give oxygen gas at the anode.

Part 4 of 5Worked Example

Question Predict the products at the cathode and anode when molten copper(II) chloride is electrolysed.

  1. Copper(II) chloride contains Cu²⁺ and Cl⁻ ions.
  2. Cu²⁺ ions move to the cathode, where each gains two electrons to form copper metal: Cu²⁺ + 2e⁻ → Cu.
  3. Cl⁻ ions move to the anode, where each loses one electron, and two chloride ions form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻.
  4. So copper is deposited at the cathode and chlorine gas is given off at the anode.

Answer Cathode: copper. Anode: chlorine.

Part 5 of 5Exam Checklist & Tips

1.9.2 – Key Points for the Exam

  • Electrolysis The decomposition of a molten or dissolved ionic compound by an electric current.
  • Electrolyte The molten or dissolved ionic compound that is decomposed in electrolysis.
  • Cathode The negative electrode, where reduction takes place.
  • Anode The positive electrode, where oxidation takes place.
  • Binary compound A compound containing only two different elements.

Common Mistakes

  • Naming the anode as the negative electrode; in electrolysis the anode is positive.
  • Saying electrons travel through the electrolyte; electrons move only in the external circuit.
  • Predicting hydrogen at the cathode for a molten compound; hydrogen forms only from aqueous solutions.
  • Sending the positive ions to the anode; positive ions move to the cathode.

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