Electrolysis: O Level & A Level Chemistry

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    Electrolysis is an important topic in both O Level and A Level Chemistry. It involves using electricity to drive a non-spontaneous chemical reaction, usually the decomposition of an ionic compound.

    
    

    Basics of Electrolysis

    
    
    
    
    • Definition: Electrolysis is the chemical decomposition of a substance using an electric current.
      
    
    
    • Key components:
      
      
      
      
      1. Electrolyte: The ionic compound in molten form or dissolved in water that conducts electricity.
         
      
      
      2. Electrodes: Conductors placed in the electrolyte where oxidation and reduction occur.
         
         
         
         • Anode: Positive electrode; oxidation occurs here.
           
         
         
         • Cathode: Negative electrode; reduction occurs here.
           
         
         
         
         
      
      
      3. Power supply: Provides the electrical energy needed for the reaction.
         
      
      
      
      
    
    
    • Example: Electrolysis of molten sodium chloride produces sodium metal at the cathode and chlorine gas at the anode.
      
    
    
    
    

    Key Concepts Students Should Know

    
    

Electrolysis of Molten Ionic Compounds

• Only metal cations and non-metal anions are present.
  
• Cations go to cathode (reduction) and anions go to anode (oxidation).
  
• Example: Molten sodium chloride → Na⁺ + Cl⁻
  • Cathode: Na⁺ + e⁻ → Na
    
  • Anode: 2Cl⁻ → Cl₂ + 2e⁻
    

Electrolysis in Aqueous Solutions

• Water is present and can compete with ions.
  
• Discharge rules:
  
  • At cathode: If metal is more reactive than hydrogen, H₂ is produced.
    
  • At anode: If halide ions are present, halogen is released; otherwise, O₂ is produced.
    
• Example: Electrolysis of aqueous copper(II) sulfate
  • Cathode: Cu²⁺ + 2e⁻ → Cu
    
  • Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻
    

Faraday’s Laws of Electrolysis (A Level)

1. Mass of substance deposited ∝ Charge passed
   
2. 1 mole of electrons deposits/frees 1 mole of monovalent ion
   

• Equation:
  M = (Q x M) / (n x F)
  • m = mass deposited (g)
    
  • Q = charge (C) = I × t
    
  • M = molar mass (g/mol)
    
  • n = number of electrons per ion
    
  • F = Faraday constant (~96485 C/mol)
    
• Tip: Learn the triangle: m↔Q↔n↔Mm
  

3. Practical Considerations

Electrolysis setups are common in school labs: simple cells, molten salts, or aqueous solutions.

• Safety precautions:
  • Wear goggles, gloves, and work under a fume hood if halogens are produced.
    
  • Handle molten salts with care.
    
• Common observations: bubbles, metal deposits, color changes.
  
• Tips for exams: Always label electrodes, indicate products, and write half-equations.
  

4. Applications of Electrolysis

1. Extraction of metals:
   
   • Electrolysis is used to extract reactive metals like aluminium from alumina.
     
2. Electroplating:
   
   • Coating objects with metals (silver, gold, chromium) to prevent corrosion or improve appearance.
     
3. Production of chemicals:
   
   • Chlorine, hydrogen, and sodium hydroxide from brine.
     
4. Purification of metals:
   
   • Electrorefining of copper, where impurities settle as anode sludge.
     

5. Exam Tips for O & A Level

• O Level: Focus on products at cathode/anode for molten salts and aqueous solutions. Learn simple rules for discharge.
  
• A Level: Be confident in writing half-equations, calculating mass using Faraday’s law, and applying electrolysis in industrial contexts.
  
• Always: Include state symbols (s, l, aq, g) in answers, especially for aqueous solutions.
  
• Practice diagrams: Show electrodes, ions in solution, and electron flow.
  

Example Question

Q: Electrolyse molten lead(II) bromide. Write the products at each electrode.
A:

• Pb²⁺ + 2e⁻ → Pb (cathode)
  
• 2Br⁻ → Br₂ + 2e⁻ (anode)