Trend in electronegativity of the Halogens

Fluorine is the most electronegative element, as we go down the group the strength in electronegativity reduces. The reduction in electronegativity can be accounted for in 2 different ways, the distance between the nucleus and the bonding pair electrons increases going down the group, this increased distance reduces the attraction between the bonding pair electrons and the nucleus. As we go down the group, we also increase the amount of shielding, with a greater amount of shielding there is a weaker force of attraction between the bonding pair electrons and the nucleus. A weaker attraction between the bonding pair of electrons and nucleus means less ability to pull those electrons to itself so less electronegative.

Trend in boiling points of the Halogens

The group 7 elements, ignoring Astatine and Tennessine, are all diatomic simple covalent molecules. They all contain no hydrogen bonding or permanent dipoles so the only intermolecular force they have is Van der Walls. As we go down the group the molecules have a larger Mr and larger amount of electrons, this can create a larger instantaneous induced dipole and therefore the melting and boiling point increases as we go down.

Trend in oxidising ability of the Halogens

As we go down group 7 the oxidising power of the halogens decreases, as an oxidising agent they are reduced and gain an electron to become a negative ion, a halide.

As chlorine has a greater oxidising power than bromine it is able to oxidised bromide ions in to elemental bromine.

This is a displacement reaction where an element is taking the place of a less reactive element in a solution or compound.

Trend in Reducing Power of Halide Ions

Going down the group for halide ions the reducing power increase, fluoride being the weakest and iodide being the strongest.

Solid halide salts can be reacted with concentrated sulfuric acid and the reactions demonstrate the changing reducing power of the halides.

Sodium Chloride and Sulfuric Acid

The initial reaction is a neutralisation, steamy fumes of hydrogen chloride would be produced as well as a white solid of sodium hydrogen sulfate. This is not a redox reaction. The chloride is not a powerful enough reducing agent to continue the reaction.

Sodium Bromide and Sulfuric Acid

The first reaction to occur is a neutralisation reaction similar to the reaction with sodium chloride but will this time produce hydrogen bromide and sodium hydrogen sulfate, steamy fumes and a white solid.

The bromide ions from the hydrogen bromide are a strong enough reducing agent to further react with the sulfate ion and will reduce sulfate to sulfur dioxide and produce bromine. This is a redox reaction. The observations of these two chemicals are red/brown liquid with brown fumes of bromine and colourless choking gas formed.

Sodium Iodide and Sulfuric Acid

As above the initial reaction is a neutralisation reaction, producing steamy fumes of hydrogen iodide and a white solid of sodium hydrogen sulfate.

The iodide ion being the strongest reducing agent of the three can reduce the sulfate ion to hydrogen sulfate, or sulfur while producing elemental iodine, the observations would be a colourless eggy smelling gas, a yellow solid, and a grey solid respectively.

 Testing for the Halide Ions

To test for a halide ion the solution is first acidified with nitric acid to remove some impurities and then silver nitrate is added. If Chloride ions are present a white precipitate forms, bromide a cream precipitate and iodide a yellow precipitate forms. The colours could potentialy be confused so dilute ammonia solution is added which will redissolve silver chloride but not silver bromide or iodide. Concentrated ammonia is then added which will redissolve silver bromide but not silver iodide.