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