Oxidation & Reduction (Edexcel IGCSE Chemistry: Double Science)

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

• Mg is oxidised as it lose electrons
• Cu²⁺ is reduced as it gain electrons
• Oxidation is the loss of electrons
• Reduction is the gain of electrons

Oxidising agents in terms of electrons

• Oxidising agents will oxidise other species in a reaction 
  • They are themselves reduced therefore will gain electrons
• In the following example Fe has been oxidised and Cu²⁺ has been reduced
• Therefore Cu²⁺ is the oxidising agent

The redox reaction between Fe and Cu²⁺

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

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