Properties of Group 7 Elements and Compounds

Group 7 elements become less reactive down the group
Group 7 elements react with metals to form ionic halides with an X- ion

Fluorine
Pale yellow-green gas, rarely seen as an element because it is so reactive. Reacts with almost anything.

Halides
Ionic halides typically have a halide ion with a 1- charge. In contrast to the reactive elements, halide compounds are mainly very stable.

Sodium chloride NaCl - common salt
Sodium fluoride NaF and tin (II) fluoride SnF2 are fluoride compounds added to toothpaste to prevent tooth decay
Crystals of calcium fluoride, CaF2, also known as fluoride and fluorspar, are used to make lenses to focus infrared light.

Testing for Halide Ions
The presence of halide ions can be detected with a simple tube test.

The unknown halide sustance is first dissolved in water.
An aqueous solution of silver nitrate, AgNO3 is added.
Silver ions, Ag+ from the AgNO3 react with any halide ions, X- present, forming a silver halide precipitate, AgX
The silver halide precipitates are coloured - the colour tells us which halide is present.

Sometimes it is difficult to judge the exact colour.

if you are unsure, add aqueous ammonia, NH3
different halide precipitates have different solubilites in aqueous ammonia - this confirms which halide is present.

Halide test results are:

chloride: Ag+ + Cl- ---> AgCl white precipitate, soluble in dilute NH3
bromide: Ag+ + Br- ----> AgBr cream precipitate, soluble in conc NH3
iodide: Ag+ + I- ------> AgI yellow precipitate, insoluble in conc NH3

This type of reaction is called a precipitation reaction. A precipitation reaction takes place in aqueous solution when aqueous ions react together to form a solid precipitate.

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Undergraduate Chemistry - Part 1 (The Periodic Table) Notiz am Group 7 Elements: Uses and Halide Tests, erstellt von siobhan.quirk am 20/05/2013.

Group 7 Elements: Uses and Halide Tests