Module 3.1.3- The halogens

• Halogens exist as...

  diatomic molecules at room temperature and atmospheric conditions (s2p5)
• Why do halogens increase in boiling point as you go down the group

  more electrons

  stronger London forcesmore energy required to break the intermolecular forces

  boiling point increases
• Halogens are an oxidising agent because...

  they oxidise another species and reduce themselves
• Halogen-halide displacement reactions


  Reactivity of Halogens decrease down the group The tested halogen solution is added to the aqueous solutions containing the halide ions of the other two and if it displaces it there is a colour change

  GOBGOV

  
• Bromine and iodine appear similar orange-brown in water. What test distinguishes between these 2?

  Add organic non-polar solvent cyclohexane and shake

  Iodine becomes a deep violet

  Bromine remains orange

  
• Halogen displacement reactions


  X2 + 2W- -> 2X- + W2

  halide ion is displaced if it is more reactive

  
• Reaction of chlorine with bromide ions

  Cl2 + 2NaBr -> 2NaCl + Br2

  
• Trend in reactivity as you go down the group for halogens

  Atomic radius increases

  more inner shells so shielding increases

  less nuclear attraction to capture an electron for it to react

  reactivity decreases
• Disproportionation reaction


  the same element is simultaneously oxidised and reduced in the redox reaction
• Examples of disproportionation reactions


  chlorine + water

  chlorine + cold dilute NaOH
• Reaction of chlorine with water

  Cl2 + H2O -> HClO + HCl

  Disproportionation reaction as chlorine is both reduced and oxidised
• What else can HClO (chloric (I) acid) act as rather than killing bacteria and how can you demonstrate this

  act as weak bleach

  demonstrate this by adding some indicator solution to solution of chlorine in water. initially red -> disappears due to bleaching action of HClO acid.
• Reaction of chlorine with cold, dilute aqueous sodium hydroxide

  Cl2 + 2NaOH -> NaClO + NaCl + H2O

  used as bleach

  solution contains a large concentration of chlorate (I) ions from sodium chlorate (I)
• Benefits of chlorine use

  purification

  kills bacteria
• Risks of chlorine use

  very toxic gas

  respiratory irritant in small conc. fatal in large conc.

  cancer causing
• Test for halide ions


  Aqueous halide ions react with aqueous silver ions to form precipitates (silver nitrate)

  Ag+ + X- -> AgX

  white -> cream -> yellow