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Electrolysis

4.3.1 The process of electrolysis

Electrolysis

  • When ionic substance is molten (electrolyte)
  • Ions move freely & pass electric current
    •      +ve ions to move to -ve electrode (cathode)
    •       -ve ions to move to +ve electrode (anode)
    • Ions are discharged at electrodes producing elements
    • Why is electrolysis used?
      • To extract metals from molten compounds
      • Why electrolysis expensive? (2)
        • Uses large amount of energy
        • To melt compounds
        • To produce electric current
          • 4.3.2 Electrolysis of molten ionic compounds
        • When a simple ionic compound (eg lead bromide) is electrolysed in molten state using inert electrodes
          • Metal (lead) is produced at cathode
          • Non-metal (bromine) is produced at anode

          Students should be able to predict the products of electrolysis of binary ionic compounds in the molten state

          4.3.3 Using electrolysis to extract metalsExtraction of aluminium

          • Aluminium is manufactured by electrolysis of a molten mixture of aluminium oxide & cryolite using carbon as +ve electrode
          • Anode 2O2- → O2 + 4e–­ (oxidation)
            Cathode Al3+ + 3e– → Al (reduction)
            Overall 2Al2O3(l) → 4Al(s) + 3O2(g)
          • Why can aluminium not be extracted by heating aluminium oxide with carbon? (1)
            • Al is more reactive than C so it displaced C

            Explain why aluminium forms at the negative electrode during electrolysis (1)

            • Al3+ is +ve charged so it will attract to -ve charged electrode, where they gain e–
            • Al3+ + 3e– → Al
            • Explain why +ve electrode is used up during the process (3)
              • Electrode is made of carbon
              • O2 is produced at +ve electrode
              • C + O2 → CO2
              • Why does the carbon anode used in the electrolysis cell need to be continually replaced (3)
                • O2 is produced at +ve electrode
                • C + O2 → CO2
                • Carbon anode wears away
                • Why cryolite is used? (1)
                  • Lower melting pt of aluminium oxide

       

4.3.4 Electrolysis of aqueous solutions

In aq solutions

 

Anode Halide present → given off

If not → O2 released      4OH– → O2 + 2H2O + 4e– (oxidation)
Cathode Cation more reactive than H2 → H2 produced

If not → metal produced      2H+ + 2e– → H2 (reduction)

In liquid – ions split up eg NaF → Na + F2 

4.3.5 Representation of reactions at electrodes as half equations (HT only)

Common electrolytes

Aq solution (ions present) Products at Anode Products at Cathode
Concentrated NaCl 2Cl– → Cl2 + 2e– (oxidation)

Cl2 released

 2H+ + 2e– → H2 (reduction)

H2 released
Concentrated CuSO4 4OH– → O2 + 2H2O + 4e– (oxidation)

O2 released

 Cu2+ + 2e– → Cu (reduction)

Cu less reactive than H2 so
Cu produced