Oxidation and reduction in electrolysis

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

Reduction happens at the negative cathode because this is where positive ions gain electrons.

Oxidation happens at the positive anode because this is where negative ions lose electrons.

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

Half equations - Higher

A half equation is used to model 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

Example

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).

Question

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. These are half equations for some reactions at the anode:

2Cl- → Cl2 + 2e-

2O2- → O2 + 4e-

Question

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

2Br- → Brl2 + 2e-