A half equations is used to represent the reaction that happens at an electrode during electrolysis. It shows what happens when ions gain or lose electrons. In half equations:
Positively charged ions gain electrons at the cathode. These are half equations for some reactions at the cathode:
Na+ + e- → Na
Pb2+ + 2e- → Pb
Balance the half equation for the formation of aluminium during electrolysis: Al3+ + e- → Al. The balanced half equation is: Al3+ + 3e- → Al (because three negatively charged electrons are needed to balance the three positive charges on the aluminium ion).
Negatively charged ions lose electrons at the anode. These are half equations for some reactions at the anode:
2Cl- → Cl2 + 2e-
2O2- → O2 + 4e-
Oxidation and Reduction
For the half equation: Al3+ + 3e- → Al, this is described as a reduction reaction as aluminium ions are gaining electrons and the gain of electrons is reduction.
For the half equation 2O2- → O2 + 4e-, this is described an oxidation reaction as the oxide ions are losing electron and loss of electrons is oxidation.