Chemistry Unit 1.7 - Group 7 (The Halogens)

Notes on Group 7 (the halogens)

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Chemistry Unit 1.7 - Group 7 (The Halogens)
PHYSICAL PROPERTIES
The halogens are diatomic molecules that exist in group 7 (VII) of the periodic table. They have the
following physical properties:
Symbo Formul Boiling point Atomic radius Ionic radius
Name State at r.t.p.
l a (°C) (pm) (pm)
Pale yellow
Fluorine -188 72 136
gas
Chlorine Pale Green gas -34.7 99 181
Bromine Brown liquid 58.8 114 195
Iodine 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.
(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
Off-white
Addition of White precipitate Yellow precipitate
precipitate
Addition of dilute ammonia Precipitate
No change No change
solution dissolves
Addition of conc. ammonia Precipitate
Precipitate dissolves No change
solution dissolves
Reactions of halide salts with H2SO4
Chloride salts
Reaction
Oxidation
-1 +6 +6 -1
state

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Bromide salts
Reaction
Oxidation
-1 +6 +6 -1
state
Reaction
Oxidation
-1 +6 +4 0
state
Iodide salts
Reaction
Oxidation
-1 +6 +6 -1
state
Reaction
Oxidation
-1 +6 +4 0
state
Reaction
Oxidation
-1 +6 -2 0
state
As it can be seen, as you descend the group the halide ions become more easily oxidised. In fact ions
are not oxidised at all with .…read more

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