Redox Reactions (CIE A Level Chemistry)

• Oxidation numbers can be used to balance chemical equations
• Roman numerals between brackets are used to show the ox. no. of an atom that can have multiple oxidation states, e.g.:
  • Fe(II) = iron with an oxidation number of +2
  • Fe(III) = iron with an oxidation number of +3

Oxidation

• Oxidation is the gain of oxygen, e.g.:
  • Cu + H₂O → CuO + H₂
  • Cu has gained an oxygen and is oxidised

• Oxidation is also the loss of a hydrogen, e.g.:
  • 2NH₃ + 3Br₂ → N₂ + 6HBr
  • N has lost a hydrogen and is oxidised
• Oxidation is also the loss of electrons, e.g.:
  • Cu²⁺ + Mg → Mg²⁺ + Cu
  • Mg has lost two electrons and is oxidised
• Oxidation causes an increase in oxidation number, e.g:
  • Cu²⁺ + Mg → Mg²⁺ + Cu
  • The oxidation number of Mg changes from 0 to +2, thus Mg is oxidised

Reduction

• Reduction is the loss of oxygen, e.g.:
  • Cu+ H₂O → 2CuO + H₂
  • O has been reduced
• Reduction is also the gain of a hydrogen, e.g.:
  • 2NH₃+ 3Br₂ → N₂ + 6HBr
  • Br has been reduced

• Reduction is also the gain of electrons, e.g.:
  • Cu²⁺ + Mg → Mg²⁺ + Cu
  • Cu has been reduced

• Reduction causes a decrease in oxidation number, e.g.:
  • Cu²⁺ + Mg → Mg²⁺ + Cu
  • The oxidation number of Cu changes from +2 to 0, thus Cu is reduced

OIL RIG acronym

Use the acronym "Oil Rig" to help you remember the definitions of oxidation and reduction

Redox reactions

• Redox reactions are reactions in which oxidation and reduction take place together
  • While one species is oxidising, another is reducing in the same reaction
• For example:

Cu²⁺+ Mg → Mg²⁺ + Cu

• Cu has been reduced from +2 to 0
• Mg has been oxidised from 0 to +2

Disproportionation reactions

• A disproportionation reaction is a reaction in which the same species is both oxidised and reduced at the same time

Example disproportion reaction

Example of a disproportion reaction in which chlorine has been both oxidised and reduced