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11.2- The Chemical Reactions Of The Halogens
Trends in oxidising ability
Halogens usually react by gaining electrons to become negative ions, with
a charge of -1. The makes the reactions REDOX; halogens are oxidising
agents and are themselves reduced. For example:
Cl2 + 2e- 2Cl-
The oxidising ability of the halogens increases as we go up the group.
Fluorine is one of the most powerful oxidising agents known.
Halogens will react with metal halides in solution in such a way that the
halide in the compound will be displaced by a more reactive halogen (but
not by a less reactive one). This is called a DISPLACEMENT REACTION.
So for example, chlorine will displace bromide ions, but iodine will not.
Cl2 (aq) + 2NaBr (aq) Br2 (aq) + 2NaCl (aq)
Cl2 (aq) + 2Na (aq) +2Br (aq) Br2 (aq) + 2Na+ (aq) + 2Cl- (aq)
The sodium ions are SPECTATOR IONS.
The two colourless starting materials react to produce the red-brown
colour of bromine.
F- Cl- Br- I-
F2 - Yes Yes Yes
Cl2 No - Yes Yes
Br2 No No - Yes
I2 No No No -
In this redox reaction the chlorine is acting as an oxidising agent, by
removing electrons from Br- and so oxidising 2Br- to Br2 (the oxidation
number of the bromine increases from -1 to 0). In general, a halogen will
always displace the ion of a halogen below it in the periodic table; see
The Maelor School
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The extraction of bromine from seawater
The oxidation of a halide by a halogen is the
basis of a method for extracting bromine
from seawater. Seawater contains small
amounts of bromide ions, which can be
oxidised by chlorine to produce bromine, as
in the reaction discussed before and shown
Cl2 (aq) + 2Br- (aq) Br2 (aq) + 2Cl- (aq)
Extraction of iodine from kelp
Iodine was discovered in 1811. It was extracted from kelp, which is obtained
by burning seaweed.…read more