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Revision:Chemistry 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)

A Level Chemistry Unit 1.7 - Group 7 (The Halogens).

Contents

Physical properties

The halogens are diatomic molecules \mathsf{(X_2)} that exist in group 7 (VII) of the periodic table. They have the following physical properties:

Symbol Name Formula State at r.t.p. Boiling point (°C) Atomic radius (pm) Ionic radius (pm)
\mathsf{F} Fluorine \mathsf{F_2} Pale yellow gas -188 72 136
\mathsf{Cl} Chlorine \mathsf{Cl_2} Pale Green gas -34.7 99 181
\mathsf{Br} Bromine \mathsf{Br_2} Brown liquid 58.8 114 195
\mathsf{I} Iodine \mathsf{I_2} Dark grey solid 184.0 133 216

They also have many common properties:

  • They are coloured.
  • They exist as diatomic molecules.
  • Melting points increase as you go down group 7.
  • They are toxic.
  • The reactivity decreases as you go down group 7.

Chemical tests

  • Chlorine – Turns damp litmus red, and then bleaches it.
  • Bromine – Bromine also turns damp litmus red and bleaches it but slower. When sodium hydroxide is added, bromine loses its colour.
  • Iodine – When starch solution is added, a blue/black colour forms.

Hydrogen halides

Hydrogen halides are very soluble in water, for example hydrogen chloride gas dissolves readily in water.

\mathsf{HCl_{(g)} \longrightarrow H^+_{(aq)} + Cl^-_{(aq)}} (in the presence of water)


When the hydrogen halides dissolve in water they form acidic solution due the high levels of hydrogen ions.

HCl dissolves well in water because HCl is a polar molecule as is water and so the intermolecular forces are favourable. Hence the HCl and H2O molecules will have permanent dipole interactions to such an extent the HCl dissociates.

Identification of the halides

Chloride Bromide Iodide
Addition of \mathsf{AgNO_3} White precipitate Off-white precipitate Yellow precipitate
Addition of dilute ammonia solution Precipitate dissolves No change No change
Addition of conc. ammonia solution Precipitate dissolves Precipitate dissolves No change

Reactions of halide salts with H2SO4

Chloride salts

Reaction \mathsf{NaCl}\mathsf{+ H_2SO_4} \mathsf{\longrightarrow} \mathsf{NaHSO_4}\mathsf{+ HCl}
Oxidation state -1+6 +6-1

Bromide salts

Reaction \mathsf{NaBr}\mathsf{+ H_2SO_4} \mathsf{\longrightarrow} \mathsf{NaHSO_4}\mathsf{+ HBr}
Oxidation state -1+6 +6-1


Reaction \mathsf{2HBr}\mathsf{+ H_2SO_4} \mathsf{\longrightarrow} \mathsf{2H_2O} \mathsf{+ SO_2} \mathsf{+ Br_2}
Oxidation state -1+6 +40

Iodide salts

Reaction \mathsf{NaI}\mathsf{+ H_2SO_4} \mathsf{\longrightarrow} \mathsf{NaHSO_4}\mathsf{+ HI}
Oxidation state -1+6 +6-1


Reaction \mathsf{2HI}\mathsf{+ H_2SO_4} \mathsf{\longrightarrow} \mathsf{2H_2O} \mathsf{+ SO_2} \mathsf{+ I_2}
Oxidation state -1+6 +40


Reaction \mathsf{8HI}\mathsf{+ H_2SO_4} \mathsf{\longrightarrow} \mathsf{4H_2O} \mathsf{+ H_2S} \mathsf{+ 4I_2}
Oxidation state -1+6 -20

As it can be seen, as you descend the group the halide ions become more easily oxidised. In fact \mathsf{Cl^-} ions are not oxidised at all with \mathsf{H_2SO_4}. Hence from the equations we can conclude that the reducing power of the halide ions increase going down the group.

The chlorate(I) and chlorate(V) ions

There are two oxo-anions of chlorine each with different oxidation numbers:

The chlorate(I) ion – \mathsf{OCl^-} (where chlorine is in the +1 state)

The chlorate(v) ion – \mathsf{ClO_3^-} (where chlorine is in the +5 state)

Disproportionation

This occurs when a species is simultaneously oxidised and reduced. Chlorine undergoes this in an alkaline environment:

\mathsf{Cl_2 + 2OH^- \longrightarrow Cl^- + OCl^- + H_2O}

The chlorate(I) ion also undergoes disproportionation but on heating:

\mathsf{3OCl^- \longrightarrow 2Cl^- + ClO_3^-}

The halogens as oxidising agents

As you descend group 7, the halogens become less effective as an oxidising agent. This property can be used in the extraction of bromine from seawater. If Chlorine gas is passed through seawater then the bromide ions are oxidised to bromine. Therefore Cl2 is a better oxidising agent than bromine.

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