# 4.3.6 Half Equations in Electrolysis

Higher Tier Only

### Half Equations in Electrolysis

• In electrochemistry we are mostly concerned with the transfer of electrons, hence the definitions of oxidation and reduction are applied in terms of electron loss or gain rather than the addition or removal of oxygen
• Oxidation is when a substance loses electrons and reduction is when a substance gains electrons
• As the ions come into contact with the electrode, electrons are either lost or gained and they form neutral substances
• These are then discharged as products at the electrodes
• At the anode, negatively charged ions lose electrons and are thus oxidised
• At the cathode, the positively charged ions gain electrons and are thus reduced
• This can be illustrated using half equations which describe the movement of electrons at each electrode

• In the electrolysis of molten lead(II) bromide the half equation at the negative electrode (cathode) is:

Pb2+ + 2e ⟶ Pb

• At the positive electrode (anode) bromine gas is produced by the discharge of bromide ions:

2Br – 2e– ⟶ Br2

OR

2Br ⟶ Br2 + 2e

#### Electrolysis of molten aluminium oxide

• Aluminium ions are discahrged at the negative electrode (cathode) and the aluminium is collected at the bottom of the cell:

Al3+ + 3e ⟶ Al

• At the positive electrode (anode) oxygen gas is produced:

2O2- – 4e– ⟶ O2

OR

2O2- ⟶ O2  + 4e

• Half equations illustrate the transfer of electrons during a chemical process as they provide a more detailed picture of the redox processes taking place
• Half equations combine to give the ionic equation for an electrolytic cell.
• The example below illustrates how this is done for the sodium chloride:

Electrode Half Equations Table 1

Electrode Half Equations Table 2

• The table below shows the half equations for a number of common electrolytes, dilute and concentrated where applicable:

#### Exam Tip

In electrode half equations the charges on each side of the equation should always balance.

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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