Group 7 halogens notes

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  • Created on: 14-01-13 22:01
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Topic 11- Chemistry Revision- Group 7, The Halogens
Group 7 is made of diatomic molecules (F2, Cl2, Br2 I2 and At2).
PHYSICAL PROPERTIES:
F2, Cl2, Br2 and I2 are all gaseous halogens and vary in colour when at room temp;
Halogen: F2 Cl2 Br2 I2
Room temp: Pale yellow gas Greenish gas Red-brown liquid Black solid
T
They all have the characteristic "swimming pool" smell.
A number of properties of F2 are unpredictable as F2 is significantly smaller than the others in
that it has less shells, making the F-F bond very weak because the non- bonding electrons on
each F atom are very close together and therefore repel each other.
SIZE OF ATOMS:
The size of the halogens INCREASES DOWN group 7 due to more shells.
ELECTRONEGATIVITY OF THE HALOGENS:

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Electronegativity INCREASES UP group 7 due to the electron density of the covalent bond being
pulled closer to the nucleus as there is less shielding/ less shells in smaller molecules such as
F2.
This is more important in terms of electronegativity than the increased nuclear charge down
the group.
MELTING AND BOILING POINTS:
Melting and boiling points INCREASE DOWN group 7 because larger molecules have more
electrons and therefore have stronger van der Waals forces between molecules.…read more

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The Cl2 GAINS electrons as it is itself reduced.
DISPLACEMENT REACTIONS: A type of redox reactions:
These show the oxidising ability of halogens (ability to remove an electron/ gain one
themselves).
These involve a reaction between a halogen (F2, Cl2, Br2, I2) and a metal halide which is a
halogen and metal compound (e.g. NaBr).
When a halogen and metal halide react together, IF THE HALOGEN IS MORE REACTIVE THAN THE
METAL HALIDE, it will displace it/ replace it (i.e.…read more

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Halide
Halogen
KCl- Colourless KBr- Colourless KI- Colourless
Cl2 No reaction Orange solution Brown solution
(Br2) formed (I2) formed
Br2 No reaction No reaction Brown solution
(I2) formed
I2 No reaction No reaction No reaction
If there is a reaction, it means the halogen has displaced the halide, and therefore the
colour of the solution changes.…read more

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Cl2 (g) + H2O (l) <--> HCl (aq) + HClO (aq)
Aqueous chloric (I) acid formed, HClO, ionises to form chlorate (I) ions (ClO-).
HClO (aq) + H2O (l) < --> ClO- (aq) + H3O+ (aq)
In bright sunlight, chlorine and water undergo an additional reaction: Cl2 + 2H2O --> 4HCl + O2
Halide ions
Halides are the -1 ions formed by the halogens (because when they bond ionically with metals,
they need to gain an electron) .…read more

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Reactions with sulphuric acid
All halide ions react with concentrated sulphuric acid, H2SO4, to give a hydrogen halide as
a product.
+1 -1 +1 +6 -2 +1 +1 +6 -2 +1 -1
Na X + H2 S O4 --> Na H S O4 + H X
THIS IS NOT A REDOX REACTION BECAUSE NO CHANGE OF OXIDATION STATES- it is an
acid-base reaction.
Steamy white fumes of the HX are seen (e.g. of the hydrogen chloride).…read more

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BUT: what happens next depends on the halide.
Some halide ions are strong enough in reducing ability (HBr and HI) that they can reduce
the sulphuric acid, H2SO4, to water and sulphur dioxide.…read more

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Testing for halides
Halide solutions are colourless so it is hard to know what is what.
Therefore, you can test for a halide using the silver nitrate test!
1. Add dilute nitric acid, HNO3, to remove ions that might interfere with the test.
2. Add silver nitrate solution (AgNO3 (aq)).
3. A precipitate is formed of the silver halide.
Ag+ (aq) + X- --> AgX
*Note- AgF does not form a precipitate because it is soluble in water*
4.…read more

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