All halogens exist as diatomic molecules (F2, Cl2 etc…). There are some general trends you should know:
Boiling Point;
The BP will increase as you go down the group. Halogen molecules only possess VDW’s forces between the
molecules, so therefore the stronger the VDW’s forces, the higher the BP. A larger surface area and more
electrons give more and larger VDW’s forces, so the BP increases down the group
Electronegativity;
The power of an atom to attract the electron density of a covalent bond towards itself. The
electronegativity decreases down group 7 as Fluorine is the most electronegative element. This is due to
the atomic radius increasing, and more electron shielding, which gives less electrostatic attraction of
negative electron to the positive nucleus.
Halogen-Halide Displacement;
A halogen can displace a halide ion in an ionic compound if the halogen is more reactive than the halide.
Fluorine is the most reactive halogen, with reactivity decreasing down the group.
Oxidising power of the halogens;
Halogen atoms can take electrons from other species, reducing them, in turn producing a halide ion. The
halogen which can exert the most electrostatic attraction on an electron (Fluorine) is the strongest
oxidising agent (smallest atomic radius & least electron shielding). Therefore, the oxidising power will
decrease down the group.
Reducing power of the halides;
Halide ions can give up their electrons to reduce other species, in turn producing elemental halogen
molecules. The reducing power of the halide ions increases down group 7 as the outer most electrons are
least strongly attracted to the positive nuclei. We can observe these reactions with concentrated H2SO4
which is a strong oxidising agent.
As both fluoride and bromide ions cannot be oxidised
by the sulfuric acid, a simple displacement reaction
occurs where the halide ion is substituted by a HSO4
–
ion.
However, when sulfuric acid reacts with HBr or HI, it is capable of oxidising the halide ions, forming water
and the halogen molecule in both cases. However, in the case of bromide, SO2 is formed, whereas in the
case of iodide H2S is formed.
Reaction of Chlorine in water;
Chlorine reacts with water in a disproportionation reaction to form HCl and HOCl. The HOCl is a very strong
oxidising agent which is used to sterilise water as it removes electrons from bacteria, which in turn kills
them;
The :OCl-
ion is the incredibly strong oxidising agent, taking both
electrons and hydrogen ions from the bacteria to form water
and chloride ions.
Reaction of Chlorine in cold aqueous NaOH;
Chlorine again disproportionates in NaOH to form
NaCl, NaOCl and water. Again, NaOCl is used as bleach as it
contains the hypochlorate ion (OCl) which is a very strong
oxidising agent, removing electrons and hydrogen ions from bacteria to form water and chloride ions