6. halides
TRANSCRIPT
•Discuss the similarities and differences in the chemical properties of elements in the same group
Chem. Properties of groups Chem. Properties of groups3.3.1
Halides
When halogens react with metals, they form compounds called halides. Many naturally-occurring halides have industrial, household and medical applications.
caesium chloride
sodiumhexafluoroaluminate
titanium(IV) chloride
lithium iodide
potassium bromide
Halide Formula Uses
CsCl
NaAlF6
TiCl4LiI
KBr
Extraction and separation of DNA
Electrolysis of aluminium oxide
Extraction of titanium
Electrolyte in batteries
Epilepsy treatment in animals
Identifying halide ions
Halides can be identified by their reaction with acidified silver nitrate solution to form silver halide precipitates.
Silver chloride has a low solubility in water, so it forms a white precipitate: the positive result in the test for chloride ions.
KCl(aq) + AgNO3(aq) KNO3(aq)
+ AgCl(s)
potassium
chloride
silver chloride
+potassium
nitratesilver
nitrate+
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.
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.
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.
Hydrogen halides
The hydrogen halides are colourless gases at room temperature.
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.
Hydrogen halide Boiling point (°C)
HF
HCl
HBr
HI
20
-85
-67
-35
Halides as reducing agents
A substance that donates electrons in a reaction (i.e. is oxidized) is a reducing agent because it reduces the other reactant.
fluoride
incr
easi
ng
red
uci
ng
ab
ility
iodide
bromide
chloride
The larger the halide ion, the easier it is for it to donate electrons and therefore the more reactive it is.
This is because its outermost electrons are further from the attraction of the nucleus and more shielded from it by other electrons. The attraction for the outermost electrons is therefore weaker.