Oxidation & Reduction in Terms of Electrons (AQA GCSE Chemistry: Combined Science)

Higher tier only

• 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 a compound of copper such as copper sulfate a displacement reaction occurs

iron + copper sulfate → iron(II) sulfate + copper 

Fe + CuSO₄ → FeSO₄ + Cu

• We can write this as an ionic equation

Fe + Cu²⁺ + SO₄^2– → Fe²⁺ + SO₄^2– + Cu

• The sulfate ions, SO₄^2-, appear on both sides of the equation unchanged
  • This means that they are spectator ions and do not participate in the chemistry of the reaction
  • So, they can be removed from the equation

Fe + Cu²⁺→ Fe²⁺ + Cu

• This balanced ionic equation can be further split into two half equations illustrating oxidation and reduction individually

Fe → Fe²⁺ + 2e^–

Cu²⁺ + 2e^–→ Cu

• The iron has lost electrons to become a positive ion, so has been oxidised
• The positive copper ion has gained electrons to become an atom, so have been reduced

The redox reaction between Fe and Cu²⁺

The Fe atom is oxidised (loses electrons) and the Cu²⁺ ion is reduced (gains electrons)

Electrolysis

• 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. Pb²⁺ + 2e^– → Pb
• Negative ions are attracted towards the anode
  • Oxidation (loss of electrons) takes place here
    • E.g. 2Br^– → Br₂ + 2e^–

Higher tier only

• Using the principles of electron loss and gain it is possible to identify which species undergo oxidation and reduction in redox reactions