Halides are compounds formed between halogens (F, Cl, Br, I) and metals. They have many industrial, medical, and household applications. Halides can be identified by their reaction with silver nitrate solution to form precipitates of insoluble silver halides. The hydrogen halides are colorless gases with hydrogen fluoride having an unexpectedly high boiling point due to hydrogen bonding between H-F molecules. Larger halide ions are more reactive reducing agents as their outer electrons are farther from the nucleus and more easily donated.

1. 3.3.1 Chem. Properties of groups
Chem. Properties of groups
•Discuss the similarities and differences
in the chemical properties of elements
in the same group

2. Halides
When halogens react with metals, they form compounds
called halides. Many naturally-occurring halides have
industrial, household and medical applications.
Halide Formula Uses
caesium chloride CsCl Extraction and
separation of DNA
sodium NaAlF6 Electrolysis of
hexafluoroaluminate aluminium oxide
titanium(IV) chloride TiCl4 Extraction of titanium
lithium iodide LiI Electrolyte in batteries
potassium bromide KBr Epilepsy treatment in
animals

3. Identifying halide ions
Halides can be identified by their reaction with acidified
silver nitrate solution to form silver halide precipitates.
potassium silver potassium silver
+ → +
nitrate nitrate chloride
chloride
KCl(aq) + AgNO3(aq) → KNO3(aq)
+ AgCl(s)
Silver chloride has a low solubility
in water, so it forms a white
precipitate: the positive result in
the test for chloride ions.

4. William Fox Talbot
William Fox Talbot (1800–1877) was a British scientist and
mathematician. He was one of the key figures in the
development of the use of silver halides in photography.
A French scientist called Louis Daguerre developed the
use of silver halides on copper plates. These were effective
at producing prints, but could only be used once.
Fox Talbot adapted the process by
removing any unreacted silver
halide by washing with sodium
thiosulfate solution. This meant that
the print could be used repeatedly
in the way that photographic
negatives can be today.

5. Hydrogen halides
The hydrogen
Hydrogen halide Boiling point (°C)
halides are
colourless gases HF 20
at room
temperature. HCl -85
HBr -67
HI -35
Hydrogen fluoride has an
unexpectedly high boiling point
compared to the other
hydrogen halides. This is due to
hydrogen bonding between the
H–F molecules.

6. Halides as reducing agents
A substance that donates electrons
increasing reducing ability
in a reaction (i.e. is oxidized) is a
reducing agent because it fluoride
reduces the other reactant.
The larger the halide ion, the easier
it is for it to donate electrons and
chloride
therefore the more reactive it is.
This is because its outermost
electrons are further from the
attraction of the nucleus and more bromide
shielded from it by other electrons.
The attraction for the outermost
electrons is therefore weaker.
iodide