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3.4.4 Half Equations

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Half Equations

  • Half equations can be used to demonstrate the oxidation and reduction reactions that occur at each electrode 
  • It is important that the number of atoms/ions on each side of the equation is balanced, as well as the charges 

Electrolysis of molten lead(II)bromide

  • Lead ions are discharged at the negative electrode (cathode) and gain electrons (reduction) to produce the metal
    • The half equation is:

Pb2+ + 2e ⟶ Pb

  • Bromide ions are discharged at the positive electrode (anode) and lose electrons (oxidation) to produce bromine gas 
    • The half equation is: 

2Br- – 2e ⟶ Br2

OR

2Br- ⟶ Br2 + 2e

Electrolysis of molten aluminium oxide

  • Aluminium ions are discharged at the negative electrode (cathode), gain electrons (reduction) and the aluminium is collected at the bottom of the cell
    • The half equation is:

Al3+ + 3e ⟶ Al

  • Oxide ions are discharged at the positive electrode (anode), lose electrons (oxidation) and oxygen gas is produced
    • The half equation is:

2O2- – 4e ⟶ O2

OR

2O2- ⟶ O2  + 4e

  • The table below shows the half equations for a number of common electrolytes, dilute and concentrated where applicable:

Electrode Half Equations Table 3 1, downloadable IGCSE & GCSE Chemistry revision notesElectrode Half Equations Table 3 2, downloadable IGCSE & GCSE Chemistry revision notes

Exam Tip

Don't forget to make sure the charges on the ions and electrons are balanced.

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