Group 7 Elements: Uses and Halide Tests

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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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