Redox & Electron Transfer (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Chemistry): Revision Note

Redox & electron transfer

Extended tier only

• Redox reactions can also be defined in terms of electron transfer

• Oxidation is a reaction in which an element, ion or compound loses electrons

  • The oxidation number of the element is increased

  • This can be shown in a half-equation, e.g. when silver reacts with chlorine, silver is oxidised to silver ions:

Ag → Ag⁺ + e^-

• Reduction is a reaction in which an element, ion or compound gains electrons

  • The oxidation number of the element is decreased

  • This can be shown in a half-equation, e.g. when oxygen reacts with magnesium, oxygen is reduced to oxide ions:

O₂ + 4e^- → 2O^2-

• 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

• We can then remove the spectator ions to see the overall change

Fe + Cu²⁺→ Fe²⁺ + Cu

• The iron atom 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)