Redox (Oxford AQA IGCSE Chemistry)

Oxidation & Reduction

• As well as understanding oxidation and reduction in terms of oxygen, you need to understand these reactions in terms of electrons

• When a substance loses electrons it is oxidised

• When a substance gains electrons it is reduced

• If this occurs in the same reaction, the reaction is a redox reaction

• For example, when iron reacts with Cu²⁺ ions oxidation and reduction take place

The redox reaction between Fe and Cu²⁺

Electron Gain & Loss

• Oxidation and reduction take place during the process of electrolysis at the anode (positive electrode) and the cathode (negative electrode) 

• Positive ions are attracted towards the cathode

  • Reduction (gain of electrons) takes place here

  • E.g. Positively charged lead ions will gain electrons at the cathode

• Negative ions are attracted towards the anode

  • Oxidation (loss of electrons) takes place here

  • E.g. Negatively charged bromide ions will lose electrons at the anode

Reactions at Electrodes

• Half equations show the reactions that occur at each electrode

• In the electrolysis of molten lead(II) bromide, lead is produced at the negative electrode (cathode)

  • The half equation for the reaction at this electrode is:

Pb²⁺ + 2e^– ⟶ Pb

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

  • The half equation for the reaction at this electrode is:

2Br^- ⟶ Br₂ + 2e^–

• During the electrolysis of molten aluminium oxide aluminium ions are discharged at the negative electrode (cathode) and aluminium is formed:

Al³⁺ + 3e^– ⟶ Al

• At the positive electrode (anode), oxide ions lose electrons to form oxygen gas:

2O^2- ⟶ O₂  + 4e^–