6. halides

•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.