Halogen Displacement & Redox (Edexcel GCSE Chemistry: Combined Science)

• Recall that oxidation and reduction take place together at the same time in the same reaction
• Oxidation is loss of electrons and reduction is the gain of electrons
• Halogen displacement reactions are redox reactions
• This can be seen if we analyse in more detail the displacement of iodine by chlorine in the following reaction:

CI₂ + 2Kl ⟶ 2KCI + l₂

chlorine + potassium iodide  ⟶ potassium chloride + iodine

• The full ionic equation is:

CI₂  +  2K⁺  +  2l^- ⟶ 2K⁺ +  2CI^- +  l₂

• The K⁺ ions are spectator ions as they appear on both sides of the equation unchanged so can be eliminated. The net ionic equation is thus:

CI₂   +  2l^- ⟶  2CI^- +  l₂ 

• If we now analyse each half equation, we can see that each chlorine atom has been reduced as an electron is gained on changing from the chlorine molecule to chloride ions:

CI₂   +  2e^- ⟶  2CI^-    REDUCTION

• Each iodide ion has been oxidised as an electron is lost on changing from iodide ions to the iodine molecule:

2l^- ⟶  I₂    +  2e^-   OXIDATION

• This shows that halogen displacement reactions are redox processes