Sunday, March 1, 2009

igcse free chemistry notes -Halogens

Physical properties of halogens

The colour of the Halogens changes from a lighter colour to a dark colour as we go down the group and the melting and boiling points increase as you go down the group.

Fluorine at the top of the group is a yellow gas This changes to a yellow-green gas for chlorine, a red liquid for bromine and finally black solid for iodine at the bottom of the group.

Reactions of halogens with metalsMetal + Halogen --> Metal Halide

Iron reacts slowly with iodine to form iron II iodide. Iron reacts faster with bromine to form iron III bromide.
Iron reacts with chlorine reacts even faster to form iron III chloride.
Other metals like sodium also react.sodium + chlorine ---> sodium chloride

The reaction of halogens with hydrogenHydrogen reacts with halogens to form hydrogen halides. E.g.Hydrogen + chlorine ---> hydrogen chloride
H2(g) +Cl2(g) --> 2HCl(g) (a hydrogen halide)
Hydrogen halides are very soluble in water.
The gas hydrogen chloride forms hydrochloric acid when it dissolves in water. Other hydrogen halides like hydrogen bromide, HBr and hydrogen iodide, HI also dissolve to form acid solutions. An acid solution has a pH less than 7

e.g. 1. An acid turns universal indicator red.

Displacement reactions
Halogens like chlorine are very reactive and displace less reactive halogens like bromine from halides like bromides.
Potassium bromide + chlorine ---> potassium chloride + bromine
2KBr(aq) + Cl2(aq) --> 2KCl(aq) + Br2(aq)
2Br-(aq) + Cl2(aq) --> 2Cl-(aq) + Br2(aq)
Chlorine and bromine also react with potassium iodide showing that the order of reactivity is: Chlorine > bromine > iodine

Uses of halogens and halidesFluorine compounds (fluorides) are put into toothpaste and some drinking water supplies. Fluorides join with tooth enamel and make teeth resist attack by acid which prevents tooth decay. Chlorine is used to in swimming pools and drinking water to kill bacteria. Iodine is used as an antiseptic because it will kill the germs on the skin without damaging it.

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