Intermediate Halogens

Halogens are the group VII elements, mainly Fluorine, Chlorine, Bromine and Iodine.

Going down the group some distinct properties can be seen:

1. The reactivity down the group decreases a lot, due to an increase in the radius of the atom.
2. Fluorine and Chlorine are gases, with Bromine being liquid and Iodine being solid.

Displacement Reactions

A set of reactions which are very important in the halogens group is the displacement reactions, where a reactive halogen will replace a less reactive one, such as the displacement of Bromide ions with Chlorine.

2KBr + Cl₂ → 2KCl + Br₂

Fluorine is never used due to its reactivity, and therefore Chlorine is the most reactive followed by Bromine followed by Iodine.

Bleaching action of Chlorine

Chlorine  bleaches organic colours by the process of oxidation in the presence of moisture and the formation of  Chlorate (I):

1. Chlorine first dissolves in water to give a mixture of Hydrochloric acid and Hypochlorous acid (chlorate (I)).

H₂O + Cl₂ → HCl + HOCl

2. The hypochlorous acid is very unstable and decomposes to give hydrochloric acid and nascent oxygen.

HOCl → HCl + [O]

3. The nascent oxygen oxidizes the colouring matter to colourless matter, resulting in bleaching

Colouring Matter + Nascent Oxygen → Colourless Matter

Oxidising Power

The higher the electronegativity of the element the higher the oxidising power. Therefore, Fluorine has the highest oxidation power with Iodine being the least oxidising. Fluorine can never be oxidised itself, and it only has two oxidation number, 0 and -1.

All halogens react with Hydrogen fo produce the halide, with Fluorine being an explosive reaction, Chlorine explosive in sunlight due to the formation of radicals, bromine being a simple reaction while heating is required with Iodine.

The oxidation power of the halogens can also be checked using thiosulfate ion.

S₂O₃^2- –> SO₄^2-

Cl₂ –> Cl^–

S₂O₃^2- –> S₄O₆^2-

I₂ –> I^–

Preparation of Hydrogen halides

All acids can be prepared via the reaction of their salt with sulfuric acid.

NaCl + H₂SO₄ → NaHSO₄ + HCl

NaBr + H₂SO₄ –> NaHSO₄ + HBr → Br₂ if H₂SO₄ is concentrated

Nal + H₂SO₄ –> NaHSO₄ + Hl → I₂ if H₂SO₄ is concentrated

In order to avoid oxidation of HBr and HI, H₃PO₄ can also be used.

2NaBr + H₃PO₄ → Na₂HPO₄ + 2HBr

2Nal + H₃PO₄ → Na₂HPO₄ +2Hl