Edexcel A Level Chemistry:复习笔记2.3.2 Halogen Displacement Reactions

• The reactivity of halogens is also shown by their displacement reactions with other halide ions in solutions
• A more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen
• Eg. The addition of chlorine water to a solution of bromine water:

Cl2 (aq) + 2NaBr (aq) → 2NaCl (aq) + Br2 (aq)

• The chlorine has displaced the bromine from solution as it is more reactive which can be summarised in the following ionic equation:

Cl2 (aq) + 2Br- (aq) → 2Cl- (aq) + Br2 (aq)

Chlorine with Bromides & Iodides

• If you add chlorine solution to colourless potassium bromide or potassium iodide solution a displacement reaction occurs:
  • The solution becomes yellow-orange as bromine is formed
  • The solution becomes brown as iodine is formed

   

• If an organic solvent is added, such as cyclohexane, the following observations are seen:
  • The organic layer will appear yellow-orange as bromine is formed
  • The organic layer will appear purple as iodine is formed
  • The organic solvent is useful as the the halogens are more soluble in this layer which helps observe the colour changes more easily
• Chlorine is above bromine and iodine in Group 7 so it is more reactive
• Chlorine will displace bromine or iodine from an aqueous solution of the metal halide:

Cl2 (aq) + 2KBr (aq) → 2KCl (aq) + Br2 (aq)

chlorine + potassium bromide →  potassium chloride + bromine

Cl2 (aq) + 2KI (aq) → 2KCl (aq) + I2 (aq)

chlorine + potassium iodide →  potassium chloride + iodine

Bromine with Iodides

• Bromine is above iodine in Group 7 so it is more reactive
• Bromine will displace iodine from an aqueous solution of the metal iodide

bromine + potassium iodide →  potassium bromide + iodine

Br2 (aq) + 2KI (aq) → 2KBr (aq) + I2 (aq)

• The reaction will turn brown as iodine is formed
• If an organic solvent is added the organic layer will appear purple as iodine is formed