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

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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.

1 Tests for Halogens
2 Hydrogen Halides
- 2.1 Test for Halide Ions
- 2.2 Solubilities of Silver Halides at room temp
3 Reactions Of Halide Salts with Concentrated Sulphuric acid
- 3.1 Reactions for Iodides
4 Oxidation Reactions of the Halogens
5 Comments

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 (Cl_2(aq) + 2Br_- -> Br_2(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.