Group 7

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Trends in physical properties

ELECTRONEGATIVITY FALLS DOWN GROUP 7

  • Because as halogens get bigger
  • Bonding pair gets further away from the nucleus
  • So is less attracted to it --> BECOMES LESS ELECTRONEGATIVE

MELTING AND BOILING POINTS INCREASE DOWN GROUP 7

  • As molecules get bigger, more intermolecular forces between molecules
  • Stronger van der waals forces
  • More energy required to turn into liquid or gas
  • MELTING AND BOILING POINTS RISE
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Trends in oxidising abilities of halogens

OXIDISING ABILITY FALLS DOWN GROUP 7

  • Oxidation is LOSS of electrons
  • Fluorine is too powerful an oxidising agent to do solution reactions
  • Chlorine can take electrons from bromide and iodide ions so is more powerful than bromine and iodine
  • Bromine is a more powerful oxidising agent than iodine
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Trends in reducing abilities of halide ions

REDUCING ABILITY OF HALIDE IONS INCREASES DOWN GROUP 7

  • When a halide ion acts as a reducing agent it GIVES electrons to something else
  • The bigger the halide ion the further the electrons are from the nucleus
  • The more they are screened by inner electron shells
  • Weaker attraction between outer electrons and nucleus
  • Easier to lose electrons as you go down the group

Reactions of halide salts and conc. sulfuric acid

  • NaCl(s) + H2SO4(l) ==> NaHSO4(s) + HCl(g)
  • 2NaBr(s) + 2H2SO4(l) ==> NaHSO4(s) + Br2(g) + SO2(g) + 2H2O(l)
  • 8NaI(s) + 5H2SO4(l) ==> 4Na2SO4(s) + 4I2(g/s) + H2S(g) + 4H2O(l)
  • NaF(s) + H2SO4(l) ==> NaHSO4(s) + HF(g)
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Identification of halide ions using silver nitrate

  • The test has to be done in solution
  • Solution is acidified by adding dilute nitric acid to the silver nitrate
  • The nitric acid reacts with and removes other ions that might give a precipitate with silver nitrate
  • F- = NO PRECIPITATE
  • Cl- = WHITE PRECIPITATE
  • Br- = PALE CREAM PRECIPITATE
  • I- = PALE YELLOW PRECIPITATE
  • The precipitates are the insoluble silver halides: AgCl, AgBr and AgI

Ammonia is used to confirm the halides:

  • AgCl + Ammonia --> precipitate dissolves to give a colourless solution
  • AgBr + Ammonia --> precipitate remains in dilute ammonia but dissolves to colourless in conc. ammonia
  • AgI + Ammonia --> precipitate is insoluble in ammonia solution of any concentration
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Uses of chlorine and chlorate(I)

  • Chloride is very reactive and undergoes reaction with water molecules forming a mixture of hydrochloric acid and chlorate (I)
  • Cl2 + H2O (http://alevelchem.com/img/arrow.gif) HCl + HOCl
  • Humans are tolerant of small doses of chlorine, whilst low conc. of chlorine kill small microorganisms, therefore water treatment plants add chlorine to kill micro-organisms and make water fit for human consumption.
  • When chlorine is bubbled through cold sodium hydroxide, a disproportionation reaction occurs and chlorine is simultaneously oxidised and reduced:
  • 3Cl2 + 6NaOH --> NaClO3 + 5NaCl + 3H2O
  • The sodium chlorate (V) produced (NaClO3) is a useful weedkiller and oxidising agent.
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