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Revision:Chemistry Edexcel Unit 1.7 - Group 7 (The Halogens)

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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Chemistry Unit 1.7 - Group 7 (The Halogens)

Topic 1.7: The Halogens


ELEMENT STATE AT ROOM TEMP
Fluorine Pale Yellow Gas
Chlorine Greenish Gas
Bromine Brown Volatile Liquid
Iodine Dark Grey Solid


  • All halogens are diatomic molecules, and contain van der waal’s forces in between the molecules.
  • Melting and boiling temperatures increase with increase size of molecules.
  • First Ionisation Energy decreases as you proceed down the group
  • Halogens in positive oxidation states do NOT form positive ions as such, but are covalently bonded to more electronegative atoms.


Contents

Tests for Halogens

Chlorine

  • Bleaches damp litmus paper rapidly: if blue litmus is used, it briefly turns red before being bleached
  • Oxidises I- to I2
  • Oxidises Br- to Br2 (Cl2(aq) + 2Br- -> Br2(aq) + 2Cl-(aq))
  • Turns starch Iodide paper to blue/black colour

Chlorine will oxidise bromides to bromine, and iodides to iodine. In Both cases, the solution turns brown, though that of iodine is much darker, and if excess chlorine is used, solid iodine will precipitate.

The first of these reactions is used to extract bromine from sea water.

Moist Starch-iodide paper (paper impregnated with starch and Potassium iodide), turns blue/black with chlorine. The iodide ions in the paper are oxidised to iodine, and the starch then reacts with this to give a blue/black compound


Bromine

  • Bleaches Litmus paper but slowly
  • Liberate Iodine from Iodide solution
  • Turns starch-iodide paper blue/black
  • Bromine will bleach damp litmus paper, but much more slowly than chlorine.
  • Turns moist starch-iodide paper blue/black, same as the reaction with chlorine


Iodine

  • Has no effect with litmus paper
  • Turns starch solution blue/black
  • Is Purple in organic solvents without oxygen

Iodine will turn starch iodide paper blue/black, but will also do the same to starch solution, which neither chlorine nor bromine can do.


Hydrogen Halides

  • ALL halogens form covalent halides H-X, which are colourless gases at room temp and which give misty fumes in moist air.
  • Hydrogen Halides are all extremely water soluble
  • HCl, HBr and HI are all strong acids, and become stronger as the halogen atom gets bigger
  • \displaystyle \mathsf{HX_{(aq)}} + \mathsf{H_20} \longrightarrow \mathsf{H_30^+} + \mathsf{X^-} (X represents halogen)


Test for Halide Ions

  • Silver Flouride is water soluble, however, the rest of the fluorides don’t.

\displaystyle \mathsf{X^-_{(aq)}} + \mathsf{Ag^+_{(aq)}} \leftrightarrow \mathsf{AgX_{(s)}}

(X represents halogen)


Solubilities of Silver Halides at room temp

In order of decreasing solubility:

  • AgF (soluble)
  • AgCl (fairly soluble)
  • AgBr (not very soluble)
  • AgI (insoluble)


- Chloride Bromide Iodide
Addition of Ag+(aq) White precipitate Pale Yellow Precipitate Yellow Precipitate
Addition of NH3 Precipitate dissolves No change No change
Addition of Conc NH3 Precipitate dissolves Precipitate dissolves No change


Reactions Of Halide Salts with Concentrated Sulphuric acid

  • Sodium Chloride with conc. Sulphuric adic at room temp. gives HCl as Misty fumes, and sodium hydrogen Sulphate.


\displaystyle \mathsf{NaCl} + \mathsf{H_2SO_4} \longrightarrow \mathsf{HCl_{(g)}} + \mathsf{NaHSO_{4(s)}}


  • Both Bromide and Iodide Ions are stronger reducing agents than Chloride ions.
  • Bromide ions reduce sulphuric acid to sulphur dioxide


\displaystyle \mathsf{NaBr_{(s)}} + \mathsf{H_2SO_{4(L)}} \longrightarrow \mathsf{HBr_{(g)}} + \mathsf{NaHSO_{4(S)}}


\displaystyle \mathsf{2HBr_{(g)}} + \mathsf{H_2SO_{4(l)}} \longrightarrow \mathsf{Br_{2(l)}} + \mathsf{2H_20} +\mathsf{SO_{2(g)}}


Iodide ions are bigger than Bromide ions, and are even more reducing.


Reactions for Iodides

\displaystyle \mathsf{NaI_{(s)}} + \mathsf{H_2SO_{4(l)}} \longrightarrow \mathsf{HI_{(g)}} + \mathsf{NaHSO_4}


\displaystyle \mathsf{8HI} + \mathsf{H_2SO_{4(l)}} \longrightarrow \mathsf{I_2} + \mathsf{H_2S_{(g)}} + \mathsf{4H_20_{(l)}}


\displaystyle \mathsf{I_2} + \mathsf{I^-} \longrightarrow \mathsf{I_3^-}

Oxidation Reactions of the Halogens

  • All of the halogens are strong oxidising agents, the oxidising power decreases down the group due to the size of the halogen increasing.
  • Any halogen will oxidise the ions of those halogens below it in the group.
    • e.g. Chlorine will oxidise Bromide and Iodide ions, and Bromine will oxidise iodide ions.


Comments

  • Suitable for: A Level Chemistry, Unit 1, Edexcel Board.
  • Written by: RazMaTaz1
  • From here.
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