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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).


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…

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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

This is more important in terms of electronegativity than the increased nuclear charge down
the group.


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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

These involve a reaction between a halogen (F2, Cl2, Br2, I2) and a metal halide which is a

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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…

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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 -->…

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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


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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.

-1 +6 0 +4

2H X + H2 S O4 --> X2 + 2H20 + S O2…

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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)).…


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