4.10 - Halogens in Oxidation State -1

• oxidation states of halogen ions

  -1: Chloride (Cl-), bromide

  0: bromine, chlorine (Cl)

  +1: chlorate(I), bromate (l) - CIO-

  +3: bromate (lll) - (BrO2)-

  +5: iodate(V) - (IO3)-

  t?iodate(VII) - (IO4)-
• Oxidation state of fluorine

  only forms oxidation states of 0 or -1
• halide ions


  ions of halogen elements in oxidation state -1

  all colourless
• test for halide ions - method

  all dilute nitric acid to remove ions that may interfere with reaction

  add silver nitrate (corrosive) to solution to produce a precipitate & then add ammonia solution to distinguish between precipitates that look similar
• half equation for precipitate formed during test for halide ions

  Ag+ + X- →AgX
• what does ammonia do in the test for halide ions

  reacts with silver ions to form diamminesilver (l) ions in a reversible reaction which lowers concentration of dissolved silver ions
• test for halides - fluoride

  no precipitate because silver fluoride is soluble
• test for halides - chloride

  white precipitate that quickly turns purple-grey in sunlight

  When ammonia is added the precipitate dissolves to give a colourless solution
• test for halides - bromide

  cream precipitate

  when dilute ammonia is added precipitate remains unchanged

  when concentrated ammonia is added precipitate dissolves to give a colourless solution
• rest for halides - iodide

  yellow precipitate

  when ammonia is added precipitate doesn't dissolve

  it's insoluble
• why are halides reducing agents

  they lose an electron in reactions so reduce what they react with & are themselves oxidised
• trend in reducing power of halides

  as you go down the group they become more powerful reducing agents because ions get bigger so outer electrons are further away from nucleus & there's more shielding so outer electrons are use strongly attracted to nucleus so are easier to lose
• reducing power of chloride ions

  weakest reducing agent

  strongest oxidising agent
• reducing power of iodide ions

  strongest reducing agent

  weakest Oxidising agent
• test to prove trend in reducing power of halides

  reaction with sulfuric acid

  reaction with silver nitrate solution
• reaction of halides with sulfuric acid

  all produce a hydrogen halide but what happens next depends on the reducing power of the halide

  reduction products: NaHSO4, ISO2, S, H2S
• reaction of sulfuric acid with sodium Chloride

  products: NaHSO4 (white solid), HCI (forms fumes in moist air)

  reaction stops there because chloride isn't strong enough reducing agent to reduce sulfuric acid - oxidation numbers of S & Cl stay the same

  type of reaction: acid-base reaction
• reaction of sulfuric acid with sodium bromide

  overall ionic equation: H2SO4 + 2H+ + 2Br- → Br2 + SO2 + 2H2O

  2 reactions because bromide ions are stronger Oxidising agents

  products of first reaction: misty fumes of HBr, solid NaHSO4

  products of second reaction: Orange vapour of Br2, co lourless choking fumes of SO2

  type of reactions: 1 is acid-base reaction, 2 is redox
• reaction of sulfuric acid with sodium iodide

  overall half equation: H2SO4 + 8H+ + 8I- → 4I2 + H2S + 4H2O

  4 reactions because iodide ions are very strong reducing agents

  products: dark solid forms which gives purple vapour on warming (I2), yellow solid May be seen (S), bad agg smell (H2S)

  type of reaction: redox
• properties of hydrogen halides .

  strong acids when dissolved in water

  colourless acidic gases

  react with moisture in air to produce misty fumes of acidic gas

  polar bonds but hydrogen fluoride forms hydrogen bonds
• reaction of hydrogen halides with ammonia

  NH3 + HX → NH4X

  acid-boase reaction that gives white fumes
• reaction of hydrogen halides with water

  HX + H20 → X- + H3O+
• making hydrogen halides


  ould concentrated phosphoric (V) acid to sodium halide

  NaX + H3PO4 → HX + NaH2PO4

  or add concentrated sulfuric acid to sodium halide, but this only works for fluoride & chloride because sulfuric acid is a strong Oxidising agent so some of bromide or iodide would be Oxidised to bromine or iodine

  NaX' + H2SO4 → HX + NaHSO4
• hydrochloric acid


  strong acid made from dissolving hydrogen Chloride in water
• hydrobromic acid


  Strong acid made by dissolving hydrogen bromide in water
• hydroiodic acid


  strong acid made by dissolving hydrogen iodide in water
• hydrofluoric acid


  Weak acid because despite strong ionisation the ions formed bind themselves together too strongly so not a lot of H+ ions

  made by dissolving hydrogen fluoride in water
• what type of reaction are halogen-halide reactions

  redox /displacement reactions

  more reactive halogen displaces less reactive halide

  what was displaced is oxidised, what caused displacement is reduced
• how do you know when a halogen-halide reaction took place

  there's a colour change
• what happens if bromide is displaced

  bromine forms

  in water solution mixture will turn yellow

  in hexane Organic layer turns Orange
• what happens if iodide is displaced

  iodine forms

  in water solution turns brown

  in hexane organic layer turns purple
• how can the colour change in halogen- halide reaction be easier to see

  shake reaction mixture with organic solvent like hexane

  halogen present dissolves in it I settles as a distinct layer above aqueous solution