Oxidation and reduction in electrolysis - Higher

Half equations

A half equation 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:

  • electrons are shown as e-
  • the numbers of atoms of each element must be the same on both sides
  • the total charge on each side must be the same (usually zero)

Cathode reactions

Positively charged ions gain electrons at the cathode. These are half equations for some reactions at the cathode:

Na+ + e- → Na

Pb2+ + 2e- → Pb

2H+ + 2e- → H2


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).


Write a balanced half equation for the formation of calcium from a calcium ion, Ca2+.

Ca2+ + 2e- → Ca

Anode reactions

Negatively charged ions lose electrons at the anode. This is a half equation for a reaction at the anode:

2Cl- → Cl2 + 2e-


Write a balanced half equation for the formation of oxygen from oxide ions in the electrolysis of molten aluminium oxide.

2O2- → O2 + 4e-

Oxygen is often the anode product from the electrolysis of an aqueous solution. There are two common ways of writing an equation for its formation.

One way is to show hydroxide ions losing electrons to form water:

4OH- → 2H2O + O2 + 4e-

The other way is to show water molecules losing electrons to form water:

2H2O → O2 + 4H+ + 4e-


Write a balanced half equation for the formation of bromine, Br2, from bromide ions, Br-.

2Br- → Br2 + 2e-

Oxidation and reduction

Oxidation and reduction can be described in terms of electrons:

  • oxidation is the loss of electrons
  • reduction is the gain of electrons

This means that:

  • reduction happens at the cathode
  • oxidation happens at the anode

This can be seen by looking at the half equations above.

It helps to remember OIL RIG - Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

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