Half Equations (OCR GCSE Combined Science A (Gateway): Chemistry): Revision Note

Half Equations

Higher Tier Only

• 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:

2O^2- – 4e^– ⟶ O₂

OR

2O^2- ⟶ O₂  + 4e^–

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