Group 7, The Halogens
- The halogens exist as diatomic (X2) molecules
Halogen | Apperance |
F2 | Pale yellow gas |
Cl2 | Green gas |
Br2 | Red-brown liquid |
I2 | Black solid |
- Down the group electronegativity decreases, as atomic radius and shielding increases, reducing the force on electrons. So oxidising ability also decreases.
- Down the group boiling point increases, as the molecules have a greater surface area and electrons for stronger London forces
- A more reactive halogen will displace the halide ion of a less reactive halogen from solution
Halogen | Colour in Water |
Chlorine | Pale Green |
Bromine | Orange |
Iodine | Brown |
- Sodium halides will react with concentrated sulphuric acid.
NaX (s) + H2SO4 (l) → NaHSO4 (s) + HX (g)
- Halide ions react with silver nitrate solution to form a silver halide precipitate
Ag^+ (aq) + X^- (aq) → AgX (s)
- Dilute nitric acid removes any soluble impurities that could interfere with the test to identify halide ions
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 (disproportion reaction).
Oxidation States: 0 -1 +1
- Chloric(I) acid is an oxidising agent that kills bacteria and sanitises water
- 2Cl2 (g) + 2H2O (l) → 4HCl (aq) + O2 (g)
- 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