A-Level Chemistry OCR Notes

Google rating

4.8

Trustpilot rating

4.7

View All Resources

Group 7, The Halogens

The halogens exist as diatomic (X2) molecules
Down the group electronegativity decreases, as atomic radius and shielding increases
Down the group boiling point increases, as the molecules have a greater surface area and electrons for stronger London forces

The halogens have seven electrons in their outer shell (s2p5), so they need to gain one electron to obtain a stable octet, forming ions with a charge of 1-, this is shown by the reduction of chlorine:

Cl2 + 2 e^- → 2 Cl^-

A more reactive halogen will displace the halide ion of a less reactive halogen from solution
- If a solution of chlorine (Cl2) is added to a solution of bromide (Br^-), the chlorine will displace the bromine

Cl2 + 2 Br- → 2 Cl^- + Br2

Precipitation reactions with aqueous silver ions are used to test for halide ions. Silver halides are formed when AgNO3 (aq) is added to a halide solution:

Ag^+ (aq) + X^- (aq) → AgX (s)

Silver Halide	Colour of Precipitate	Addition of Dilute Ammonia	Addition of Concentrated Ammonia
AgCl	White	Dissolves to give a colourless solution	Dissolves to give a colourless solution
AgBr	Cream	Does not dissolve	Dissolves to give a colourless solution
Agl	Yellow	Does not dissolve	Does not dissolve

Use of Chlorine and Chlorate

Chlorine can dissolve in water to form hydrochloric acid and chloric(I) acid in a disproportion reaction (chlorine is both oxidised and reduced).

Cl2 (g) + H2O (l)→ HCl (aq) + HClO (aq)

Oxidation States: 0 -1 +1

Chloric(I) acid is an oxidising agent that kills bacteria and sanitises water
- Cl2 (g) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H2O (l)
Sodium chlorate(I) has a bleaching action which allows it to be an active ingredient in household bleach.
The advantages of using chlorine to treat water is that it prevents the spread of waterborne disease and sanitises water. However, it is added without direct customer consent, chlorine is a respiratory irritant and it could react to form chlorinated hydrocarbons, implicated in cancers.

Google rating

4.8

Trustpilot rating

4.7