Group 7 – Halides and Halogens

A halide is a halogen ion – so for example I- is a halide and I2 is a halogen.

Reactions of Chlorine
Chlorine is used in drinking water because HClO kills bacteria. Chloric(I) acid also turns litmus paper red and then bleaches it white whilst hydrochloric acid just turns litmus paper red (PH-12).

Chlorine + Water Hydrochloric acid + Chloric(I) acid
Cl2 + H2O HCl + HClO
..0 ..+1-2 ...+1 -1. +1 +1 -2

Here chlorine has changed its oxidation number from 0 to -1 in HCl and to +1 in HClO. When oxidation and reduction of the same element occur in the same reaction it is known as disproportionation.

Another example of disproportionation is when Chlorine reacts with cold aqueous sodium hydroxide. The sodium chlorate(I) formed is used as bleach:

Chlorine + Sodium hydroxide water + sodium chloride + sodium chlorate(I)
Cl2 + 2NaOH H2O + NaCl + NaClO

When hot concentrated sodium hydroxide solution is used sodium chlorate(V) forms:

3Cl2 + 6NaOH 3H2O + 5NaCl + NaClO3
The Sodium chlorate formed is used as a weedkiller


Displacement reaction
In a displacement reaction the more reactive element ‘swaps’ with a less reactive one in a compound:

C12 + NaI 2NaCl + I2
Here the solution would turn brown because the iodine has been displaced.

Note: All compounds of halides: I- Cl- Br- form colourless solutions.

Cl2 + 2Br- Br2 + 2Cl-Turns orange
Cl2 + 2I- I2 + 2Cl-Turns brown
Br2 + 2I- I2 + 2Br-Turns brown

This table provides evidence for the reactivity of halogens. Chlorine is the most reactive as it can displace both bromine and iodine. Bromine is less reactive than chlorine but is more reactive than iodine as it cannot displace chlorine but can displace iodine. Iodine is the least reactive because it cannot displace either chlorine or bromine.

Halide Tests
AgNO3 and NH3 can be used to test for Halide ions: Cl- creates a white precipitate of silver chloride which dissolves in dilute ammonia, Br- creates a cream precipitate of silver bromide which only dissolves in concentrated ammonia and I- a yellow precipitate of silver iodide which will not dissolve in ammonia.