Reactivity of the Halogens (Oxford AQA IGCSE Combined Science Double Award)

Displacement Reactions of the Halogens

• Reactivity of Group 7 non-metals decreases as you go down the group

• This trend in reactivity can be observed in displacement reactions

• A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt

• For example:

  sodium bromide  +  chlorine  →  sodium chloride  +  bromine

• You only need to learn the displacement reactions with chlorine, bromine and iodine

  • Chlorine is the most reactive and iodine is the least reactive

Chlorine with Bromine & Iodine

• Chlorine is more reactive than bromine and iodine

• Chlorine will displace bromine or iodine from an aqueous solution of the metal halide

• When chlorine solution is added to colourless potassium bromide solution :

  • The solution becomes orange as bromine is formed or

  • The solution becomes brown as iodine is formed

chlorine + potassium bromide →  potassium chloride + bromine

Cl₂ + 2KBr → 2KCl + Br₂

chlorine + potassium iodide →  potassium chloride + iodine

Cl₂ + 2KI → 2KCl + I₂

Bromine with Iodine

• Bromine is more reactive than iodine

• Bromine will displace iodine from an aqueous solution of the metal iodide

• The solution becomes brown as iodine is formed

bromine + potassium iodide →  potassium bromide + iodine

Br₂ + 2KI → 2KBr + I₂

Observations during displacement reactions

Reactivity of the Halogens

• Going down Group 7, reactivity decreases

• Group 7 elements react in a similar way because they all have 7 electrons in their outer shell

  • However, some elements are more reactive than others

• The reactivity of the halogens is determined by how easily it will gain one electron to achieve a full outer shell

• Reactivity decreases because:

  • Going down the group, the number of shells increases

  • The outer shell is therefore further away from the nucleus

  • There is less attraction between an incoming electron and the nucleus

  • It is harder to gain an electron