3I- Group 7, halogens

• What’s the trend in electronegativity down group 7


  Decreases

  Increase in number of protons but valance electrons become further from the nucleus as the number of energy levels increase, due to increasing shielding also electronegativity decreases
• What’s the trend in boiling point going down the group


  Increase, larger atoms have more electrons, stronger van der waals between molecules
• What’s the trend in oxidising ability (oxidising agent)/displacement down the group


  Oxidising ability decreases going down the group, ability to receive an electron decreases as electronegativity decreases down the group

  
  

  More reactive halogens (top) will displace a less reactive one (bottom) in an aqueous solution
• What’s the trend in reducing ability (reducing agent) of a halogen


  Reducing ability increases down the group, ability to lose electrons increases as valance electrons receives less attraction to the nucleus
• What happens in the reaction of solid sodium halides with sulphuric acid


  Not a redox reaction, chloride ion is a weak reducing agent and can’t reduce sulphur

  Forms hydrogen chloride gas
• What does silver nitrate test for, why and how is it used


  Tests for halide ions, silver ions combine with halide ions producing silver halide precipitates in different colours

  Add nitric acid to the halide solution, removes carbonate or hydroxide impurities

  Add a few drops of silver nitrate, halide precipitate forms

  AgF- no precipitate

  AgCl- white precipitate

  AgBr- cream precipitate

  AgI- yellow precipitate
• Why is nitric acid used before the silver nitrate test and not hydrochloric or sulphuric acid

  Sulphuric acid would produce silver sulphate, white precipitate

  Hydrochloric acid would produce silver chloride, chlorine ions, white precipitate

  Invalidates the test
• What’s the trend in solubility of halides in ammonia and why is it used as well as silver nitrate to test for halides


  AgCl- dissolves in dilute ammonia

  AgBr- dissolves in conc ammonia

  AgI- insoluble in conc ammonia

  
  

  Silver nitrate produces precipitates similar in colour, cream and white, ammonia confirms observations
• Reaction of chlorine and water to form chloride ions and chlorate (I) ions

  Cl2 (g) + H2O (l) —> HClO (aq) + HCl (aq)
• Reaction of chlorine with water to form chloride ions and oxygen

  2Cl2 (g) + 2H2O (l) —> 4HCl (aq) + O2 (g)

  
  

  In sunlight
• How is chlorine used in water treatment


  Disproportionation reaction forming chloric acid and hydrochloric acid

  Chloric acid is an oxidising agent killing bacteria
• Reaction of chlorine with cold, dilute aqueous NaOH and uses of solution produced


  Cl2 (g) + 2NaOH (aq) —> NaClO (aq) + NaCl (aq) + H2O (l)

  Produces sodium chlorate, an oxidising agent used in bleach