Halogens

GCSE Halogens

Intermediate Halogens

General Properties

• Ionic radius increases down the group
• Reactivity decreases down the group
• Electronegativity decreases down the group

Fluorine

Fluorine is the most electronegative atom and it can therefore only have 1 oxidation state, -1.

This would give fluorine only the common oxide, while no oxoanions are possible.

Properties of halogens

Bond lengths and bond strengths

The smaller the bond, the stronger the bond, and it would require a lot of energy to break that bond. This is true for all molecules except for fluorine, since the bond is so small that the lone pairs would actually repel each other making it weaker.

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^–

Reducing Power

If fluorine is the most oxidising agent it will therefore mean that the Iodide ion is the strongest reducing agent. The extra electron on Iodine is far away from the nucleus and therefore it can easily be lost, making it a good reducing agent.

Solubility

All halogens have Van der Waals’ bonding and therefore these will not dissolve in water (although fluorine and chlorine will actually react with water).

On the other hand, iodine can be made to dissolve by the preparation of the tri-iodide ion, which is iodine joined to an iodide ion, to give I₃^–. since this has a charge, it will be able to dissolve.

Reaction with Alkali

Halogens react with alkali to give different salts, including the simple salt and oxo-salts. Since fluorine cannot have a positive valence, it reacts in a different way.

Fluorine + alkali

2F₂ + 2NaOH –> F₂O +2NaF + H₂O @ cold temperatures

At higher temperatures the oxide will react with more alkali to give:

F₂O + 2NaOH –> 2NaF + H₂O +O₂

Showing that F₂O is an acidic oxide.

The overall reaction would then be:

2F₂ + 4NaOH –> 4NaF + 2H₂O + O₂

Halogen + alkali

For all the other halogens the reaction goes like this:

X₂ + NaOH –> NaX + NaXO + H₂O

But the oxo-salt is unstable to give:

3NaXO –> 2NaX + NaXO₃

Which would result in:

3X₂ + 3NaOH –> 5NaX + NaXO₃ + H₂O

Chlorine

3Cl₂ + 3NaOH –> 5NaCl + NaClO₃ + H₂O

Bromine

3Br₂ + 3NaOH –> 5NaBr + NaBrO₃ + H₂O

Iodine

3I₂ + 3NaOH –> 5NaI + NaIO₃ + H₂O

Preparation of Halogens

All halogens (except for Fluorine) can be prepared using MnO₂ and sulfuric acids.

2NaCl + MnO₂ + 3H₂SO₄ –> MnSO₄ + Na₂SO₄ + Cl₂ (collected via downward delviery through H₂SO₄)

2NaBr + MnO₂ + 3H₂SO₄ –> MnSO₄ + Na₂SO₄ + Br₂ (collected via distilaltion)

2Nal + MnO₂ + 3H₂SO₄ –> MnSO₄ + Na₂SO₄ + l₂ (collected via sublimation)

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 + HBrà Br₂ if H₂SO₄ is concentrated

Nal + H₂SO₄ –> NaHSO₄ + Hl à 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

Test for halogens