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

Higher Tier Only

• Displacement reactions can be analysed in terms of redox reactions by studying the transfer of electrons
• For the example of magnesium and copper sulfate, a balanced equation can be written in terms of the ions involved:

Mg (s) + Cu²⁺ (aq) + SO₄^2- (aq) → Mg²⁺ (aq) + SO₄^2- (aq) + Cu (s)

• The sulfate ions, SO₄^2-, appear on both sides of the equation unchanged hence they are spectator ions and do not participate in the chemistry of the reaction so can be omitted:

Mg (s) + Cu²⁺ (aq)  → Mg²⁺ (aq)  + Cu (s)

• This equation is an example of a balanced ionic equation which can be further split into two half equations illustrating oxidation and reduction individually:

Mg → Mg²⁺ + 2e^–

Cu²⁺ + 2e^–→ Cu

• The magnesium atoms are thus oxidised as they lose electrons
• The copper ions are thus reduced as they gain electrons

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

Answer

1. 1. Write the full equation
      • Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
   2. Write the ionic equation
      • Zn (s) + Cu²⁺ (aq) + (aq) → Zn²⁺ (aq) + (aq) + Cu (s)
   3. Use the ionic equation to rule out / ignore spectator ions that are present as reactants and products
      • is present as a reactant and a product so it can be ignored
   4. Use the ionic equation to identify the species that is oxidised (OIL)
      • Zn (s) → Zn²⁺ (aq) + 2e^– 
   5. Use the ionic equation to identify the species that is reduced (RIG)
      • Cu²⁺ (aq) + 2e^– → Cu (s)