Group 7 - Water treatment

Outlines how chlorine is used in water treatment and making bleach. Also covers disproportionation.

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  • Created by: Scott
  • Created on: 05-12-11 16:24
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Water treatment
Halogens form simple, non-polar molecules so dissolve readily in non-polar
organic solvents.
However, chlorine, bromine and iodine are sparingly soluble in water (a polar
solvent)
Chlorine also undergoes a very slow reaction with water forming initially chloric
(I) acid, HClO(aq) and hydrochloric acid (HCL):
Cl2 (g) + H20 (aq) HCl (aq) + HClO (aq)
Work out the oxidation numbers, what do you notice about Chlorine?
Chlorine is both oxidised and reduced in this reaction.
A redox reaction in which a molecule, atom or ion is simultaneously oxidised
and reduced is called disproportionation.
The chloric (I) acid then slowly decomposes to produce hydrochloric acid and
reactive oxygen atoms which kill bacteria:
HClO (aq) HCl (aq) + O (aq)
Formation of Bleach
The way in which chlorine reacts with aqueous sodium hydroxide depends on
the temperature. With cold (15 C) dilute aqueous sodium hydroxide a mixture
of halide (Cl-) and halate (Cl0-) ions is formed:
Cl2 (g) + 2NaOH (aq) NaCl (aq) + NaClO (aq) + H20(l)
This is another example of disproportionation where chlorine is both oxidised
and reduced. This reaction is used commercially to produce household bleach,
which is a mixture of NaCl and NaClO.

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