Reactions of the Halogens (Edexcel GCSE Chemistry: Combined Science)

Metal Halides

• The halogens react with some metals to form ionic compounds which are metal halide salts
• The halide ion carries a -1 charge so the ionic compound formed will have different numbers of halogen atoms, depending on the valency of the metal
• E.g., sodium is a group 1 metal:
  • 2 Na + Cl₂ → 2 NaCl

• Calcium is a group 2 metal:
  • Ca + Br₂ → CaBr₂

• The halogens decrease in reactivity moving down the group, but they still form halide salts with some metals including iron
• The rate of reaction is slower for halogens which are further down the group such as bromine and iodine

Sodium donates its single outer electron to a chlorine atom and an ionic bond is formed between the positive sodium ion and the negative chloride ion

• The halogens react with nonmetals to form simple molecular covalent structures
• For example, the halogens react with hydrogen to form hydrogen halides (e.g., hydrogen chloride)

Hydrogen chloride is a simple covalent molecules made by direct combination of hydrogen and chlorine

• Hydrogen halides are steamy acidic gases that dissolve very well in water to form strongly acidic solutions
• For example, hydrogen chloride gas dissolves in water to form hydrochloric acid:

HCl (g) ⟶ HCl (aq)

• The other hydrogen halides will do the same, although strangely enough, hydrofluoric acid is actually a weak acid in water

Trends in reactivity and stability

• Reactivity decreases down the group, so iodine reacts less vigorously with hydrogen than chlorine (which requires light or a high temperature to react with hydrogen)
• Fluorine is the most reactive (reacting with hydrogen at low temperatures in the absence of light)
• The hydrogen halides becomes less stable as you go down the group, so much so that hydrogen iodide decomposes quite readily on heating:

2HI (g)  ⇌ H₂ (g) + I₂ (g)

• This pattern illustrates an important principle in chemistry about stability and reactivity: the more vigorous and energetic a reaction forming a compound is, the more stable in the product, and vice versa

• A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide
• The reactivity of group 7 elements decreases as you move down the group
• 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 Bromides & Iodides

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

• 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 + 2KBr → 2KCl + Br₂

chlorine + potassium bromide →  potassium chloride + bromine

Cl₂ + 2KI → 2KCl + I₂

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 + 2KI → 2KBr + I₂

• This table shows a summary of the displacement reactions of the halogens: chlorine, bromine and iodine 

• From this pattern of reaction we can predict that:
  • Fluorine will displace all other halogens from their compounds
  • Astatine will be displaced by all the halogens from its compounds

• Having said that, astatine is the rarest naturally occurring element so there is not enough around to actually test!