Oxidation & Reduction (Edexcel IGCSE Chemistry)

Oxidation & reduction in terms of oxygen

• The reactions of metals with oxygen, such as in iron rusting can be classified as oxidation
• Oxidation is any reaction in which a substance gains oxygen
• The opposite of oxidation is reduction
• Reduction is a reaction in which a substance loses oxygen
• For example, the displacement reaction between zinc and copper(II)oxide can be classified as a redox reaction

Zn    +     CuO    →    ZnO    +    Cu

zinc + copper(II) oxide → zinc oxide + copper

• Oxidation cannot occur without reduction happening simultaneously, hence these are called redox reactions
• The copper(II)oxide supplies the oxygen, so it is the oxidising agent
• The zinc is the reducing agent because it removes the oxygen

Oxidation & Reduction in terms of electrons

• Displacement reactions can be analysed in terms 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
• These equations illustrate the broader definition of oxidation and reduction in terms of electron transfer:
  • Oxidation is the loss of electrons
  • Reduction is the gain of electrons

'OIL RIG' is a useful mnemonic to help remember the definitions of oxidation and reduction