Download 1.3.9 Group 7 Elements - uses and halide tests

1.3.9 Group 7 Elements: Uses and Halide Tests
Properties of Group 7 Elements and Compounds

Group 7 elements become less reactive down the group

They react with metals to form ionic halides (X- ion)
Fluorine is too reactive to study (anything in its way is spontaneously burned!)
Summarise Table 1 p94
Halide Ions (X- where X is any group 7 ion)
When any reagent is reduced, electrons are given to it. (
). The electrons are
given by the reducing agent, which is itself oxidised. (
)
The halides can therefore be classed as reducing agents as they could possibly lose
electrons and form ____________ atoms (which in turn pair up to form halogen
molecules):
Reducing Power of the halides ____________ down the group.
(So iodide ions are strongly reducing whereas fluoride ions are non-reducing)


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Sodium chloride is the most familiar halide
Sodium fluoride and tin fluoride are added to toothpaste to prevent tooth
decay
Crystals of Calcium fluoride are used to make lenses to focus infrared light
Identification of halide ions with silver nitrate
Silver chloride, silver bromide and silver iodide are all insoluble salts. They are
_________________ when an aqueous solution containing the appropriate halide ion
is treated with an aqueous solution of _____________________________.
The colours of the three silver salts formed with chloride, bromide and iodide ions,
and their different solubilities in aqueous ammonia, can be used as a test for the
presence of the halide:
Halide ion
present
ClBrI-
Addition of
AgNO3(aq)
Equation
Addition of dilute
NH3(aq)
Additon of
conc NH3(aq)
Water treatment
Chlorine and chlorine compounds are used in water treatment. For many years,
small quantities of chlorine have been added to drinking water and to swimming
pools in order to kill bacteria. (The concentration of chlorine in drinking water is ~
0.7mgdm-3, higher concentrations are used in swimming pools.)
Key Definitions
Precipitation Reaction -