# Oxidation and reduction in electrolysis - Higher

## Half equations

A is used to represent the reaction that happens at an during . It shows what happens when gain or lose . In half equations:

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

### Cathode reactions

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

Na+ + e- → Na

Pb2+ + 2e- → Pb

2H+ + 2e- → H2

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

2Cl- → Cl2 + 2e-

2O2- → O2 + 4e-

### Example

Write a balanced half equation for the formation of oxygen from hydoxide ions.

4OH- → 2H2O + O2 + 4e-

Question

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

2Br- → Br2 + 2e-

## Oxidation and reduction

and 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.

Move on to Video