Group 7 (17), Halogens

• Properties of Group 7

  G7 elements are diatomic molecules

  appearance:

  • F2 -> pale green gas
  • Cl2 -> green gas
  • Br2 -> orange liquid
  • I2 -> grey solid/purple vapour/brown in solution
  • At2 -> black solid

  melting/boiling point increases going down group:

  • larger molecules -> more electrons -> more van der Waals forces

  electronegativity decreases going down group

  • atomic radius increases going down group
  • more electron shielding
  • (increased nuclear charge)
• Properties of Group 7

  strength of covalent bond decreases as molecule gets bigger

  • Cl2 is the strongest & gets weaker going down
  • F2 slightly weaker due to electrons beginning to repel each other

  Oxidising agent -> oxidises other elements & reduces itself

  • oxidising ability decreases going down group

  reducing agent -> reducing other elements & oxidises itself

  • reducing ability increases going down group
• Reactions of Group 7

  1. Displacement reactions

  2. Chlorine + water (disproportionation reaction = same species both oxidised & reduced)

  Cl2 + H2O ↔ HCl + HClO

  in swimming pools:

  2HClO + UV energy -> 2HCl + O2 & 2Cl2 + 2H2O ↔ 2HCl + 2HClO

  -> 2Cl2 + 2H2O -> 4HCl + O2

  3. reacting with sodium hydroxide

  2NaOH + Cl2 -> NaClO (bleach) + NaCl + H2O
• 4. Reaction of halides with concentrated H2SO4

  • NaF

  NaF + H2SO4 -> NaHSO4 + HF (not redox) observation: white misty fumes

  • NaCl

  NaCl + H2SO4 -> NaHSO4 + HCl (not redox) observation: white misty fumes
• Reaction of halides with concentrated H2SO4

  • NaBr

  NaBr + H2SO4 -> NaHSO4 + HBr (not redox)

  ALSO 2HBr + H2SO4 -> Br2 + SO2 + 2H2O (redox)

  observation: orange/brown vapour/gas & maybe white misty fumes

  • NaI

  NaI + H2SO4 -> NaHSO4 + HI (not redox)

  ALSO 2HI + H2SO4 -> I2 + SO2 + 2H2O (redox)

  ALSO 6HI + H2SO4 -> 3I2 + S + 4H2O (redox)

  ALSO 8HI + H2SO4 -> 4I2 + H2S + 4H2O (redox)

  observation: white fumes (HI), dark grey/black solid (I), purple vapour (I), yellow solid (S), rotten egg smell (H2S)
• ID tests

  • Test for halides
  • add HNO3 + AgNO3
  • if present: Cl -> white precipitate, Br -> cream precipitate, I -> yellow precipitate

  Ag+(aq) + X-(aq) -> AgX(s) X = halide

  • AgCl dissolves in NH3
  • AgBr only dissolves in concentrated NH3
  • AgI doesn't dissolve in NH3